TY 250 A shop manual and questions????

[Tillerman6]

I have a 74 model 250A serial number 434-011251  and I need the shop manual for it.  That's the downside.  Now the good side is that I also have a 1975 B model shop manual if anybody needs one.?

Maybe we can trade?

 

I have been using the info from the B model shop manual to do all a the tune up and work on the ignition (Magneto and Points/condenser) up to a point.

I checked out the schematic for the B model and it seems to have a couple of things different about it that I can't find or explain on my A model.

#1 There is NO DIODE in my wiring harness now.  ( at least I can't find one)  I don't know if it has been removed, or it never had one? Normal for a 74?.

THe B model shop manual says the diode is to prevent the engine from running backwards.  Now I'm not sure if they really mean running backwards or just back firing, which my bike does A LOT!  (TOO MUCH).  Now on a 250 that is not a leg breaker, but I would be more than happy to make that problem go away.  If the diode was a modification or addition to the 75 model that fixed the back firing problem, then by all means where do I get one?

 

#2 the left side coils on my A model (there are two of these on this bike) do not resemble the photograph of the left side coil in the 1975 B shop manual. There is only one coil on the left side of the 1975 B model)  Is that right?

The TY 250B 1975 book shows only a single coil on the left side and I am wondering if: #1 Does it have a stronger spark?

#2 if there is any  reason to swap out my dual coils for that single coil on the 75 model? Has anybody done this?

So I'm wondering if the dual coils are the stock ones for this 74 model A?

Ignition timing- This bike is a points and condenser bike.  The magneto system seems to be very difficult to set up properly. This is mostly due to the fact that the magnet rotor wheel (flywheel has only 3 small holes in it for access to adjusting the points.  All the adjustments are done thru these holes since the flywheel must be in place to time the engine.  And the Yamaha parts are extremely expensive for me!  They want 50 bucks for the points and 30 bucks for a condenser or there abouts.  (any alternatives anyone?)

The Yamaha part numbers (if they have not been superseeded) are Points 434-81321-10-00  and condenser is 207-81326-90-00.

I am running OK right now, but I would like to have the next set in the box waiting for the next time I need them.

Since you are supposed to remove the head and set up a dial indicator on the piston, you have to remove the gas tank, seat, exhaust pipe and the magneto rotor too if you are going to inspect and or replace the points or condenser.  There has to be an easier way!

So what I did was I put marks on the flywheel and a mark on the side of the crank case to line up when the crank was at TDC with the factory spec of .122" BTDC.  I set this up perfectly and set the points gap for .016" while at TDC, but the engine would never start.  Kick back a lot, but never start!

My feeling is that this is too much advance, so I ended up fudging a bit and just eyeballed the setting when the timing was more like .080" BTDC with the .016" of gap.

The locals at the Yamaha shop say that this timing is not critical, so I am going with their advice for now.  The bike does start a lot better now, and even though it back fires aganst the kick starter, at least it does start and run pretty well.  Don't know if I need that diode?

Is there anyone on here that has a TY250A model with some experience with these problems or solutions?  I am all ears!

 

Thanks!  - Tillerman6

 

 

 

 

 

[feetupfun]

Standard points on Yamahas normally last decades in trials use. Two of my TY250s still have the original points and condensers and still work perfectly.

I wouldn't worry that your stator coil looks different to the photos in the B manual. A and later models ignitions all work great if they are undamaged and set right. Post up a photo if you still think yours isn't standard.

Yes the diode was an improvement that came in with the B model. Having one will not stop the motor kicking back when you are starting it because a diode will only stop the ignition making a spark if the engine is rotating backwards and a kickback happens when the motor is rotating forwards (too slowly).

Good that you set a timing mark on the flywheel. It makes it easier to set the timing compared with a gauge measuring piston position through the sparkplug hole.

Your Yamaha shop led you astray if they said that the timing wasn't critical. The timing is critical but the points gap is not critical. On a Yamaha you change the gap to achieve the right timing. Yes you can do it through that little hole. Set the clamping screw so it holds the points in place but not too tight that you can't move them to get the adjustment right. Once it is right then fully tighten the clamping screw.

The points should begin to open at the timing mark with the motor being turned in the running direction. To tell exactly when they are opening you can use your eyes and a torch, or a timing buzzer or a timing light. I usually set them with a buzzer, then run the engine and use a timing strobe on the timing mark to confirm that it is right. The ignition should work fine with a gap anywhere between 0.2 mm and 0.5 mm. I know Yamaha recommend a narrower range.

When you do buy points and condenser, don't buy cheap stuff or an old condenser. Also you can mount the condenser up near the HT coil and it will still work OK. That way the condenser will not get as hot.

The only workshop manual I know of for the A model is the owners manual and they do come up for sale. There are two parts manuals. One is just A model and the other is for A and B models. They also come up for sale now and then. The A motor is the same in many ways to the B. The place most people get tricked up when working on A models is that the clutch and primary drive is different to the B and later. There is nothing wrong with the A design it is just what Yamaha had in all their dirt bike engines at the time and the B is a later design.

If you are serious about getting rid of the kickback, you can fit a modern aftermarket ignition that has an advance curve. This gives you much less chance of a kickback at kicking speed.

[Tillerman6]

1 hour ago, feetupfun said:

Standard points on Yamahas normally last decades in trials use. Two of my TY250s still have the original points and condensers and still work perfectly.

I wouldn't worry that your stator coil looks different to the photos in the B manual. A and later models ignitions all work great if they are undamaged and set right. Post up a photo if you still think yours isn't standard.

Yes the diode was an improvement that came in with the B model. Having one will not stop the motor kicking back when you are starting it because a diode will only stop the ignition making a spark if the engine is rotating backwards and a kickback happens when the motor is rotating forwards (too slowly).

Good that you set a timing mark on the flywheel. It makes it easier to set the timing compared with a gauge measuring piston position through the sparkplug hole.

Your Yamaha shop led you astray if they said that the timing wasn't critical. The timing is critical but the points gap is not critical. On a Yamaha you change the gap to achieve the right timing. Yes you can do it through that little hole. Set the clamping screw so it holds the points in place but not too tight that you can't move them to get the adjustment right. Once it is right then fully tighten the clamping screw.

The points should begin to open at the timing mark with the motor being turned in the running direction. To tell exactly when they are opening you can use your eyes and a torch, or a timing buzzer or a timing light. I usually set them with a buzzer, then run the engine and use a timing strobe on the timing mark to confirm that it is right. The ignition should work fine with a gap anywhere between 0.2 mm and 0.5 mm. I know Yamaha recommend a narrower range.

When you do buy points and condenser, don't buy cheap stuff or an old condenser. Also you can mount the condenser up near the HT coil and it will still work OK. That way the condenser will not get as hot.

The only workshop manual I know of for the A model is the owners manual and they do come up for sale. There are two parts manuals. One is just A model and the other is for A and B models. They also come up for sale now and then. The A motor is the same in many ways to the B. The place most people get tricked up when working on A models is that the clutch and primary drive is different to the B and later. There is nothing wrong with the A design it is just what Yamaha had in all their dirt bike engines at the time and the B is a later design.

If you are serious about getting rid of the kickback, you can fit a modern aftermarket ignition that has an advance curve. This gives you much less chance of a kickback at kicking speed.

feetupfun,

  Thanks for the nice info.  And I guess your specifications are loose enough that my settings would be in the ballpark.  I neglected to mention that the reason I did initially start working on the TY  was an oil leak from the clutch control arm and another leak under the magneto at the left crank seal.  I ordered the factory parts and installed these, but the left crank seal being replaced was what changed the timing of the points and possibly the amount of air that was reaching the crank case.  There was an oily residue under the magneto area, and that is why I suspected the main seal.  But that was not the oil leak (gearbox) that had been p****** oil on my garage floor ever since I got the bike used.  Anyway, with those two things fixed, I was (now)  not getting spark enough to fire the engine.

I must have removed and replaced the fly wheel at least 6 times trying to get the points clean, cleaner, the gap set right and tearing the head, exhaust, tank, seat etc off the bike just to be able to take the TDC and BTDC measurements with a dial indicator.

I also was not completely sure if the double stator coils inside the mag should be on the left or right side of the crank because at one time I had these completely off the bike for inspection and I could not remember if I got them back on the right way or they were even the right parts to begin with. (used bike)

Then there was the condenser problem.  I had ordered a new points and condenser from England and what showed up was the wrong condenser because of the way the felt wiper arm was welded to the side of the condenser body.

So with the advice of the local Yamaha mechanic I re-installed my old condenser and re-soldered the three wires to it that come from the points, the black lead out to the spark coil, and the lead from the ignition coil inside the magneto. I was also not sure of the schematic because of the shop manual being from a B model.  The mechanic said he had never seen a condenser go bad.  I thought that was a little strange, but I am not a mechanic. The parts guy priced me out of the realm of easy reach for the new parts and said he did not stock them any way, so I had to try something different.

With the ball park approach and a couple of temporary felt tip marker lines from when the TDC and BTDC positions were taken, I could tell that the timing was too far advanced.  There was spark now, but the timing was so far advanced that all you would get was kickback. 

Then I remembered what the mechanic said "less advance gives easier starts and maybe not peak HP, but more advance gives harder starts and more kickback" 

So now I had the engine back together and between the BTDC and TDC ink marks on the flywheel there is about maybe 1.5 inches of distance. The ohm meter readings are VERY sensitive to any rotation of the magneto rotor (flywheel) so you have to go super slow from the BTDC mark CCW towards TDC with slow motion to look for when the readings go from  0.8 ohms to 1.7 ohms because the ohms reading will fluctuate on the digital ohm meter with the slightest wiggle of the flywheel.

So at a super slow CCW rotation you can eventually find the point at which the reading changes from .8 to 1.7 ohms on the black wire to ground and that is your opening position. Luckily, when that happened, I was between the .122" BTDC and zero TDC going by my crude marks on the flywheel. It was probably less than perfect and maybe closer to .080 to .100 of BTDC on the piston, but I was not in the mood to put the dial indicator back on to find out.

I could not use a timing light because the engine has to be running for that.  A simple light bulb and a battery will not show a noticeable difference between .8 and 1.7 ohms, so that won't work and a buzzer will not change it's tune much for that small difference in resistance either as far as I know. ??  But the factory shop manual shows a "points checker" which I do not have, and that sent me back to the ohm meter.  I think the best type of meter for this is an old school D'arsonval meter movement with a wheatstone bridge circuit that has a 1 ohm full scale setting.

I have one of those, but it is too big and fragile to really be of much use under a bike, so I put it back on the shelf.

 

So rather than trying to change the points gap again thru that tiny hole in the mag wheel ( it is shooting in the dark) I thought why not just kick it and see what it does.

I tightened down the adjusting screw and put the spark plug back in.

I also drained the float bowl and noticed some junk in the gas, but it could have been from the dust on the engine covers.

This time it ran but was running way too fast with no throttle applied.  

I was really relieved to hear it run at all, so I waited till the next day to play with the fast idle problem.  I had previously had the throttle cover/ kill switch opened up to inspect and clean up the kill switch and see if that made any difference to the .8 - 1.7 ohms on the other end of the black wire and I could never get a hard ground out of that kill switch, but I cleaned all the contacts inside and lubed up the throttle cable and cable housing while I was in there looking for something that could be causing the fast idle problem.

I remembered that I had put in a fresh left side crank seal and this could have changed the amount of air that was being ingested, so the carb settings were now looked at.

The manual says that the air bleed screw on the carb is the main thing that has to do with idle speed so I closed off most of the air bleed adjustment.  This and a little tweak of the slide (down limit) screw seems to have cured the fast idle along with some lube of the throttle cable and careful re- assembly of the cable to twist grip fit. 

So today it was idling well and I ran around the yard and grounds up and down some hills and put putted for about an hour.  There were no leaks and it was fairly easy to re start, did not overheat even though it was about 90 degrees today and had plenty of low end grunt even with a 14 -56 combination on the chain drive.

p.s. When I first inspected the points I noticed that there was a white looking spot on both sides and some pitting.  I suppose this could have been accelerated wear from the leak of the left crank seal.   Some fuel/oil vapors from the crank side of the engine were leaking out under the magneto for sure. This was cleaned up with some 400 grit paper and some electrical contact cleaner. 

So you are probably right about the points lasting for years - IF the crank  seals are good.

I would be more tempted to get the timing spot on, but if it is running fine (excepting a few kick backs) and with the history of the .122 timing not working well at all, I think I might have to leave well enough alone for now.

 

 

 

 

 

 

 

 

 

 

 

 

[b40rt]

To check when the points open you need a bulb / buzzer and a battery to supply power. You will have little success with power generated at virtually zero revs.

[feetupfun]

The setting of the points can be done with little effort by using a 12v battery and a 12v incandescent light bulb. Apply the 12v DC across the points and you will see a large change in brightness of the light as the points open and close. Yes a digital ohm meter would turn the job into a nightmare, unless you disconnect the condenser first, which is why people use a light bulb or a buzzer.

Fuel vapour can cause deposits on the faces of the points. Pitting of the points faces is usually due to poor condenser performance. A good test for the condenser is to look through the little hole in the flywheel with the motor running. If it is a bit like fireworks night inside there, then the condenser is not working well. Even with a good condenser there will be a little bit of sparking.

I use 21mm BTDC measured on the rim of the flywheel for my timing mark.

The air bleed screw should be somewhere between 1 and 3 turns out from fully in. The actual position of the screw is chosen to provide the nicest firing pattern at idle. The idle speed should be done by moving the throttle slide bottom stop. If the idle firing pattern cannot be made nice within the range I mentioned for the bleed screw, then there is still something wrong. Your air leakage at the LH crank seal sounds like it was very bad. Leakage there first becomes noticeable in the way the motor runs when there is a tiny oily witness at the outer lip.

You should be able to tell which way the stator plate goes on by the run of the wiring. TY250s have the points in front of the crankshaft. If you still have any doubts about your setup, take the flywheel off again and take some photos.

It won't hurt anything to run it a bit retarded if you are taking it easy, and it will reduce the likelyhood of kickback

[oni nou]

Setting the points can be done with little effort using a 12v battery and a bulb....the moment that the light dims is when the points are set to optimum.

Hopefully some one else will be along to reiterate this in a while.

[johnnyjazz]

i have the 250a manual and parts list as a pdf if youd like? pm me.  more than happy to share and put something back.  Ive learned so much from the awesome mechanical wizards and trials masters here!!

[Tillerman6]

Guys!

  Well I must have done something right with all the adjustments. The LH crank seal was definitely leaking and there was a few drops of mix under the magneto when the cover was removed, so I know it was leaking from the seal right there.  And that leak could have affected the carb air bleed setting, And the old setting was pretty wide open loose, so it might have been compensating for the bad LH crank seal leak.  I  just screwed it in most all the way and it caused the engine to run at a normal two stroke type of idle.  I also adjusted the slide stop screw out a bit and lubed the throttle cable and housing and the twist grip itself.  Now the idle is what I would call normal and it does not die if left for a few minutes, so I am super happy about that.  Also the engine will just about idle up a hill if you back off the throttle all the way. I love that "tractor" effect and the low end grunt.  This bike has the Boysen reed kit on it and they do work well at low revs.

 

Then last night I decided to take a small chance and take it out to my spot in the hills and ride it for a couple of hours to see how it performed.  And the more I rode it the better it got, so now it starts w2hen warm on the first kick.  It also stopped kicking back for some reason, so now I'm thinking that the carb float might have let too much gas down into the crank case and basically flooded the cases?  Otherwise why would running it for a few minutes make any difference to the kickback situation?

 

It does not smell like gas any more, so possibly the float was stuck open (dry) when it was sitting over the winter.

I appreciate you guys helping with my lack of understanding about the points timing.  I am on a steep learning curve in that respect because I can't forget my electronics training and the idea of the zero point eight ohms for closed and only 1.7 ohms for open. (if you are using an ohm meter to the black wire of the mag wiring harness to ground).  That is because the primary coil to ground is already at like 0.8 ohms or less and it is hooked directly to the points!.  But now I see how a different circuit could work.  Nobody explained the light bulb circuit yet and with just my doodling on a piece of paper it looks like you would have to disconnect the points from the normal wire leading to the mag and hook a new wire from the points to the light bulb center contact, then the shell of the light bulb to the + side of the battery. and the - side of the battery to ground.  Now this would work for the light bulb and points, but you would have to tuck a new wire under the flywheel to make it work for timing. Everything under the flywheel is already almost at ground to begin with if you leave it hooked up normally.  And the flywheel has to be installed for any of these measurements because the inside of the flywheel has the cam built into it.   This means removing the flywheel at least twice.

That being said, however, This new wire idea does have the advantage that it isolates the points from the almost grounded mag coils, so you could use a lamp or a buzzer no problem.

But that is still an awful lot of work just to get the points set.  I like the comment from one of the guys "set the points gap for .015 - .016 at TDC"   what could be easier than that?

I might swap the points screw out for a metric Allen head bolt next time I change the points out.  Much easier to hit the hole in an Allen bolt than the tiny buggered up slot of a 40 year old plain screw!   Any body know what the size and pitch are?

 

 

 

 

[Tillerman6]

13 hours ago, b40rt said:

To check when the points open you need a bulb / buzzer and a battery to supply power. You will have little success with power generated at virtually zero revs.

If you are using an ohm meter the power for the reading is coming from a battery inside the ohm meter, however the magnetic field and the coils of the mag will be jacking with the ohm meter reading if the flywheel even twitches.  This is especially true for a digital ohm meter. 

All that being said, nobody has yet explained how they hook up the light bulb and battery.  I think I have figured out a way using a new wire, but the flywheel has to come off to hook it up and also disconnect the normal wire to the points. So the flywheel has to come off at least twice for all that.

 

[Tillerman6]

19 hours ago, feetupfun said:

Standard points on Yamahas normally last decades in trials use. Two of my TY250s still have the original points and condensers and still work perfectly.

I wouldn't worry that your stator coil looks different to the photos in the B manual. A and later models ignitions all work great if they are undamaged and set right. Post up a photo if you still think yours isn't standard.

Yes the diode was an improvement that came in with the B model. Having one will not stop the motor kicking back when you are starting it because a diode will only stop the ignition making a spark if the engine is rotating backwards and a kickback happens when the motor is rotating forwards (too slowly).

Good that you set a timing mark on the flywheel. It makes it easier to set the timing compared with a gauge measuring piston position through the sparkplug hole.

Your Yamaha shop led you astray if they said that the timing wasn't critical. The timing is critical but the points gap is not critical. On a Yamaha you change the gap to achieve the right timing. Yes you can do it through that little hole. Set the clamping screw so it holds the points in place but not too tight that you can't move them to get the adjustment right. Once it is right then fully tighten the clamping screw.

The points should begin to open at the timing mark with the motor being turned in the running direction. To tell exactly when they are opening you can use your eyes and a torch, or a timing buzzer or a timing light. I usually set them with a buzzer, then run the engine and use a timing strobe on the timing mark to confirm that it is right. The ignition should work fine with a gap anywhere between 0.2 mm and 0.5 mm. I know Yamaha recommend a narrower range.

When you do buy points and condenser, don't buy cheap stuff or an old condenser. Also you can mount the condenser up near the HT coil and it will still work OK. That way the condenser will not get as hot.

The only workshop manual I know of for the A model is the owners manual and they do come up for sale. There are two parts manuals. One is just A model and the other is for A and B models. They also come up for sale now and then. The A motor is the same in many ways to the B. The place most people get tricked up when working on A models is that the clutch and primary drive is different to the B and later. There is nothing wrong with the A design it is just what Yamaha had in all their dirt bike engines at the time and the B is a later design.

If you are serious about getting rid of the kickback, you can fit a modern aftermarket ignition that has an advance curve. This gives you much less chance of a kickback at kicking speed.

Sorry that is a little confusing. 

On this engine the points gap translates directly to the "timing".  they are one in the same thing, so I agree with you when you say the timing is critical but I have to disagree about the points gap not being critical.  There is a direct relationship between the two.  If you change the points gap you are also changing the timing.

All that being said, the engine will probably run anywhere between .2 and .5 mm gap of the Points?  and if my math is working properly this translates to  .0078" to .0196" which is in the ballpark of what has been suggested locally at .0016".

Too bad that the factory manual has you taking the head off to get .122" of BTDC piston position.  This is not easy to do and it is even harder to get the points to actually open at this point because you are forced to work thru a tiny hole in the flywheel.

But if you could be so kind as to suggest a way to get a buzzer or a light to work in this scenario, I would be most interested to hear how it is done on this engine.  Since the primary ignition coil is almost a dead short to ground and the other end of the winding is hooked directly to the insulated side of the points, then both sides of the points are already almost at ground unless you do something to change that?

 

 

 

[feetupfun]

11 hours ago, Tillerman6 said:

Sorry that is a little confusing. 

On this engine the points gap translates directly to the "timing".  they are one in the same thing, so I agree with you when you say the timing is critical but I have to disagree about the points gap not being critical.  There is a direct relationship between the two.  If you change the points gap you are also changing the timing.

All that being said, the engine will probably run anywhere between .2 and .5 mm gap of the Points?  and if my math is working properly this translates to  .0078" to .0196" which is in the ballpark of what has been suggested locally at .0016".

Too bad that the factory manual has you taking the head off to get .122" of BTDC piston position.  This is not easy to do and it is even harder to get the points to actually open at this point because you are forced to work thru a tiny hole in the flywheel.

But if you could be so kind as to suggest a way to get a buzzer or a light to work in this scenario, I would be most interested to hear how it is done on this engine.  Since the primary ignition coil is almost a dead short to ground and the other end of the winding is hooked directly to the insulated side of the points, then both sides of the points are already almost at ground unless you do something to change that?

 

 

 

OK I'm gonna have one more go before I give up.

You don't need to take the head off to find top dead centre. Take the sparkplug out and insert something rigid that will reach the piston crown. There are special tools made just for this job but you can alternatively make something up. It is called a top dead centre tool if you want to go looking on the internet. It doesn't even need to have a measuring scale or a dial indicator because you can determine the amount of advance in mm around the flywheel rim using geometry, and I have told you already the distance is 21 mm. You just need to mark TDC on your flywheel so it lines up with something on the casing and measure 21mm anticlockwise from there and make a mark.

The only important things about the points is that they start opening when the piston is at the timing mark, they conduct electricity when they are touching and when they are fully open, the gap is not too big or too small.

To make a simple tool to show clearly whether the points are open or closed, use the points as a switch/contactor in a low voltage DC circuit that includes a DC supply and a light bulb. When the points are touching, the light will be bright. When the points are not touching, the bulb will glow less brightly. Disconnect the points wire from the HT coil before you start. It is at this disconnection point that I connect one side of my DC supply, if I am using the bulb method. The other side of the DC supply goes to somewhere on the motor casing.

The light bulb method is popular, as is using a cigarette paper between the points or using a buzzer or using a volt meter or using direct visual observation of the points. They all work

[Tillerman6]

6 hours ago, feetupfun said:

OK I'm gonna have one more go before I give up.

You don't need to take the head off to find top dead centre. Take the sparkplug out and insert something rigid that will reach the piston crown. There are special tools made just for this job but you can alternatively make something up. It is called a top dead centre tool if you want to go looking on the internet. It doesn't even need to have a measuring scale or a dial indicator because you can determine the amount of advance in mm around the flywheel rim using geometry, and I have told you already the distance is 21 mm. You just need to mark TDC on your flywheel so it lines up with something on the casing and measure 21mm anticlockwise from there and make a mark.

The only important things about the points is that they start opening when the piston is at the timing mark, they conduct electricity when they are touching and when they are fully open, the gap is not too big or too small.

To make a simple tool to show clearly whether the points are open or closed, use the points as a switch/contactor in a low voltage DC circuit that includes a DC supply and a light bulb. When the points are touching, the light will be bright. When the points are not touching, the bulb will glow less brightly. Disconnect the points wire from the HT coil before you start. It is at this disconnection point that I connect one side of my DC supply, if I am using the bulb method. The other side of the DC supply goes to somewhere on the motor casing.

The light bulb method is popular, as is using a cigarette paper between the points or using a buzzer or using a volt meter or using direct visual observation of the points. They all work

Feetupfun,

  Thanks for the clarification!  I was wondering because all the comments just said to hook a light across the points with a battery. They didn't say to disconnect anything.  Sorry if I got confused. 

As far as the 21mm measurement on the circumference of the flywheel-   I will look for the top dead center tool or make something myself. That sure beats removing the head and then I can put some permanent marks on the flywheel for the next time I need to service the points. Do you know if there were different versions of the outer flywheel ring weight?  I am including a picture of my flywheel for your inspection. 

I am sorry for the inconvenience and my ignorance of the situation.  My electronics training kept telling me that the light bulb was not going to see a .8 ohm difference when it was already at full brightness when the points were opening.  But if the points wire is disconnected first and another wire is hooked externally to the lamp circuit then you really have got something that is either on or off. No guess work then.

 

 

 

 

[feetupfun]

Yes that's an A model flywheel. The B and later model flywheels have a bigger external steel ring than the A model external steel ring.

[Tillerman6]

feetup fun,

  Thanks for all your help and patience!  I think my knowledge of this ignition circuit is about 3 times more accurate and easier to work on now as compared to the shop manual.

32 minutes ago, feetupfun said:

Yes that's an A model flywheel. The B and later model flywheels have a bigger external steel ring than the A model external steel ring.

So do you have this same flywheel?  If not I could extrapolate from the 21mm and your outside diameter onto this outside diameter and come up with an equivalent number.  It would be so much easier than doing the factory style setup.

 

 

[Tillerman6]

Finally got the 74 250A bike back together after the new cylinder and piston, but the kickback problem is still there and probably worse than before.  I noticed that the TY250B manual (Cycleserv- Australia) shows a diode between the primary coil and the high voltage coil,) page 97) but that publication does not show any part numbers for anything.  If anyone has any idea of what that part number is- I would like to hear about it.  The same manual says that the diode is to prevent the engine from "running backwards"  Right now the engine kicks back about 19 times out of 20, so this is getting old real fast. Once the engine is warmed up a bit, it will start with less kick back events, but still does it.  I do not think that the kick back is from anything hot inside the chamber because it will kick back when it is stone cold on the first kick.  Today before I tried to start it again I set the timing with less advance.  The points are opening now at .113" before TDC on the dial test indicator.Before that, it was set for .116", but it will kick back with either setting.  The piston is brand new and the cylinder head have been cleaned of any excess carbon deposits.  The plug is the B7ES specified with a .020" gap.

The kick starter lever pawl will engage the starter at a point just aft of where the kick starter is straight up, but the engine only spins over one revolution for one kick before the kick starter arm hits the footpeg.  Most of the time the engine will kick back before the stroke is even half way completed. I have tried indexing the kick starter arm further forward to give more spin to the crank shaft for each kick, but this is awkward because the arm wants to fold up under your foot.

So I think what is happening is that the plug is firing too soon.  The only way this can happen is if the magneto is generating an erroneous (and possibly opposite current pulse) thru the system. Just from the feel of the kick starter, it seems to happen when the piston is about an inch from TDC on the way up.  But while the points are being adjusted, and the crank is being turned by hand, of course there are  no indications of any reverse currents being generated by the magnets.

I made my own current source from a couple of AA batteries and an LED diode for a load.  There is a milliampmeter in series with the LED and this circuit is put in parallel across the points by hooking one lead of the ammeter to ground and the other end of the LED to the black wire coming from the high side of the primary coil.

With the 3 volt supply, this circuit runs about 50 milliamps with the points closed and 46 milliamps when the points open. The change is very precise as far as the relationship of the piston travel and the points opening, so that aspect is really not in question.  I did see where Feetup fun said to disconnect the HV coil from the primary coil during the time when the points are being tested - good call as this would increase the difference between the current readings  due to the points being open or closed.

I guess it's time to hook up my oscilloscope to the black wire and see if I can see any high voltage spikes or abnormalities that are being sent to the High voltage coil. 

The trick is going to be trying to figure out if there is more than one pulse per revolution reaching the HV coil, or if there are negative and positive swings in the pulses themselves.

It's a 2 channel scope and maybe I can hook something to the flywheel and get a "1 per" timing signal off that.  Then chan 2 will be a sample of the primary side of the HV coil. just spin the engine over with an electric drill.  If chan 2 is making more pulses than chan 1 then it proves that the HV coil is getting more than 1 pulse per revolution.