TY 250 A Crankshaft Repair/ Advice?

[feetupfun]

54 minutes ago, Tillerman6 said:

 

I still did not hear from you about how you knew that the weights of the con rods are different between the 74 and 75's?

 

Is it riding weather over there now? Having any fun at all?  Probably still no trials meets any time soon?

 

Stay safe!

I did answer. I made two posts. Maybe you only saw the second one.

Riding weather where I live is absolutely fantastic. Having lots of fun test riding and practicing where I live. I've been concentrating on getting a 1968 Bultaco going as well as it can. We are in the middle of the trials riding season here but there is still a couple of weeks to go before trials competition starts up because of social distancing requirements. Preparations for the first competition events post-lockdown are in progress.

[section swept]

The last crank that I rebuilt needed a hydraulic press rated at 40 tons! This started the crankpin moving with the usual and expected load clang...scattering two onlookers... and from then on in until pin release the pressure was showing at 25 tons. Crankshaft realignment using vee blocks and dial gauges on a surface table, but some get a good compromise using eye and copper (dead blow) hammer to nudge one flywheel half a tad or more on the crankpin.

Reasons for flywheel half misalignment can be down to sudden shock being transmitted to the drive side of the crank. Air gun/ rattle gun tightening of crank nut. Excessive leverage against either side of the crank when tightening either the flywheel or drive nuts, over-torquing without using the correct flywheel locking method. A good reason to not lock the engine by the con-rod. 

Bouncing or smacking one half of the crank assembly to bring it into tolerance is a standard practice, its quicker than setting up on a press which could cause too much movement. Welding the crankpin in place is a belt and braces protection against high engine rpm sudden seizures spinning one flywheel half on the pin. 

[Tillerman6]

On 5/8/2020 at 11:04 PM, feetupfun said:

As a matter of interest in the vibration stakes, did you know that when Yamaha produced the second and later models TY250, they used a lighter conrod design, compared with the A model. This had a noticeable effect.

I have a few bikes with TY250 engines and the one with an untouched, original A model engine in it has enough vibration at one particular RPM that the vibration transferred to my body causes loss of vision. This doesn't happen when trials riding because the RPM never stays constant long enough and I'm standing up, but when I'm trail riding in 4th or 5th gear and find that particular RPM and am sitting down, the vision thing happens. When I do the same sort of riding on my other TY250 powered bikes, I don't get the vision effect. The other motors are TY250D  with original conrod and TY250B model with original conrod.

You might check your TY 250 A model if you still have it and see if the magneto wheel is wobbling with the cover off.   I got about .010"  of side to side wobble on mine and that ended up requiring a teardown to "parade rest"  All the gears and guts are all in small piles on my workbench right now.  - not fun.  When the crank was out of the engine it turned out to be the culprit, although I can't say if I caused the problem or it was already "spun" when I got the bike. I will also try and test the trueness of the magneto wheel itself when I get the crank back from the rebuilder's shop.

Right now there is still a problem with some of the parts suppliers because of the Covid-19 situation.  For what it's worth, the stuff on Ebay is pretty much un-affected by any delays.

It probably won't affect you if you are not in the States, but Partzilla is reporting delays.

 

Question- Do you think it's possible to add a diode to the wiring harness on my 74 250A like the ones on the 75 and later models?  I am still worried about the backfiring /kickback situation I had going on before the vibration problem showed up.

 

 

[Tillerman6]

On 5/25/2020 at 2:46 AM, section swept said:

The last crank that I rebuilt needed a hydraulic press rated at 40 tons! This started the crankpin moving with the usual and expected load clang...scattering two onlookers... and from then on in until pin release the pressure was showing at 25 tons. Crankshaft realignment using vee blocks and dial gauges on a surface table, but some get a good compromise using eye and copper (dead blow) hammer to nudge one flywheel half a tad or more on the crankpin.

Reasons for flywheel half misalignment can be down to sudden shock being transmitted to the drive side of the crank. Air gun/ rattle gun tightening of crank nut. Excessive leverage against either side of the crank when tightening either the flywheel or drive nuts, over-torquing without using the correct flywheel locking method. A good reason to not lock the engine by the con-rod. 

Bouncing or smacking one half of the crank assembly to bring it into tolerance is a standard practice, its quicker than setting up on a press which could cause too much movement. Welding the crankpin in place is a belt and braces protection against high engine rpm sudden seizures spinning one flywheel half on the pin. 

I am assuming that since we are in the Yamaha section of the forum that there is some chance that you may have re-built a TY 250A?  IF so, are there any tricks to getting everything re-assembled properly? I will have the Tusk puller and probably warm up the cases in my old barbbeque  and freeze the crank with some dry ice before I try to put things together, but if there was anything unusual or noteworthy that you remember about your experience, I would love to hear about it.

 

 

[Tillerman6]

On 5/7/2020 at 5:55 AM, greychapel said:

Allen millyard has some videos on his you tube channel showing how he splits and builds up crankshafts for his multi cylinder specials

I watched those and I was amazed. If I can even get my engine back together properly and it runs, I will be very happy. Thanks for the "link"!

[feetupfun]

2 hours ago, Tillerman6 said:

Question- Do you think it's possible to add a diode to the wiring harness on my 74 250A like the ones on the 75 and later models?  I am still worried about the backfiring /kickback situation I had going on before the vibration problem showed up.

Yes of course you can fit the diode. The rest of the TY250A ignition is the same as the later models. However the diode is there to stop the motor running backwards. It won't stop it kicking back which is caused by firing prematurely with the crankshaft turning in the forwards direction but too slowly. The tendency to kick back is made worse if the spark timing is a bit too far advanced.

If you want to minimise the possibility of a kickback then use an ignition that has an advance curve.

[section swept]

3 hours ago, Tillerman6 said:

I am assuming that since we are in the Yamaha section of the forum that there is some chance that you may have re-built a TY 250A?  IF so, are there any tricks to getting everything re-assembled properly? I will have the Tusk puller and probably warm up the cases in my old barbbeque  and freeze the crank with some dry ice before I try to put things together, but if there was anything unusual or noteworthy that you remember about your experience, I would love to hear about it.

 

 

That sounds like a good plan. Just ensure that the crank is exactly centred in the crankcases. When the engine is running there will inevitably be a slight amount of movement due to hear expansion and contraction, Yamaha design allowed for this but if you are worried you could always use roller bearings as opposed to ball bearing mains on the drive side. Leaving the crank in the freezer is a better bet than using dry ice, less shock to the crank. I believe the factory assembly method is to fit everything at room temperature. As soon as you fit the heated crankcases and frozen crank together there is going to be some fairly rapid expansion taking place so just be sure you have everything carefully lined up with no score marks in the bearing housings....oil the bearing outers before assembly. Believe it or not, some works motors had the bearing housings sized so that only a slight pressure was needed to push the crank mains into the casing, thus stripping and rebuilding was mush less fraught!

[Tillerman6]

9 hours ago, feetupfun said:

Yes of course you can fit the diode. The rest of the TY250A ignition is the same as the later models. However the diode is there to stop the motor running backwards. It won't stop it kicking back which is caused by firing prematurely with the crankshaft turning in the forwards direction but too slowly. The tendency to kick back is made worse if the spark timing is a bit too far advanced.

If you want to minimise the possibility of a kickback then use an ignition that has an advance curve.

So what are the options?  It seems pretty iffy to get the timing perfect trying to set the points with a feeler gauge and just a tiny access hole in the magneto wheel, and I am not that strong to give it a mighty kick thru. My chainsaw does the same thing.  Thanks very much for the reply!

 

[section swept]

I had a 325 Sherpa T that never ran backwards, my mate had a 325 Sherpa T that was virtually identical and it ran backwards on a number of embarrassing occasions. The reason my 325 never misbehaved was that the ignition timing (cb ignition) was set as Sammy Miller suggested, instead of the 2.5 mm btdc it was set at 3.25 mm btdc this made the engine feel more docile and softer in sections. Easier to ride if you will, thank you Mr Miller. My mate’s bike was always set to spec and hence various trips into people in reverse or backwheel up a tree scenario....he never changed to Mr Millers timing suggestion...silly boy!

With the Yamaha TY250 ignition set to spec. you get a peppy engine....something some riders didn’t like. Set the ignition slightly more advanced and there is less chance of kicking, spitting back....worked on my TY250 E. As for starting if you pussy foot about it will bite back, wear stout footwear or better still trials boots and go at it like you mean business, be positive and think about what you are doing. As for the chainsaw thats not going to be more than 110 cc and the recoil action should negated any kick back.

Most Twin shock single cylinder two strokes have their ignition timing around about 2-2.5 mm btdc and most will kick if you kick start in a half hearted way. The closer to tdc the more likelihood of kick back.

Edited May 28, 2020 by section swept
added info

[feetupfun]

6 hours ago, Tillerman6 said:

So what are the options?  It seems pretty iffy to get the timing perfect trying to set the points with a feeler gauge and just a tiny access hole in the magneto wheel, and I am not that strong to give it a mighty kick thru. My chainsaw does the same thing.  Thanks very much for the reply!

 

I already said you could fit an electronic ignition with advance. This will give you less advance at kicking speed.

If still using points, set the timing with a strobe light. Be patient. Lie the bike on its side. Wear a head light that shines from between your eyes.

[feetupfun]

4 minutes ago, section swept said:

I had a 325 Sherpa T that never ran backwards, my mate had a 325 Sherpa T that was virtually identical and it ran backwards on a number of embarrassing occasions. The reason my 325 never misbehaved was that the ignition timing (cb ignition) was set as Sammy Miller suggested, instead of the 2.5 mm btdc it was set at 3.25 mm btdc this made the engine feel more docile and softer in sections. Easier to ride if you will, thank you Mr Miller. 

With the Yamaha TY250 ignition set to spec. you get a peppy engine....something some riders didn’t like. Set the ignition slightly more advanced and there is less chance of kicking, spitting back....worked on my TY250 E. As for starting if you pussy foot about it will bite back, wear stout footwear or better still trials boots and go at it like you mean business, be positive and think about what you are doing. As for the chainsaw thats not going to be more than 110 cc and the recoil action should negated any kick back.

Sorry but to reduce kickback while kickstarting you should run less advance not more advance.

[Tillerman6]

5 hours ago, section swept said:

I had a 325 Sherpa T that never ran backwards, my mate had a 325 Sherpa T that was virtually identical and it ran backwards on a number of embarrassing occasions. The reason my 325 never misbehaved was that the ignition timing (cb ignition) was set as Sammy Miller suggested, instead of the 2.5 mm btdc it was set at 3.25 mm btdc this made the engine feel more docile and softer in sections. Easier to ride if you will, thank you Mr Miller. My mate’s bike was always set to spec and hence various trips into people in reverse or backwheel up a tree scenario....he never changed to Mr Millers timing suggestion...silly boy!

With the Yamaha TY250 ignition set to spec. you get a peppy engine....something some riders didn’t like. Set the ignition slightly more advanced and there is less chance of kicking, spitting back....worked on my TY250 E. As for starting if you pussy foot about it will bite back, wear stout footwear or better still trials boots and go at it like you mean business, be positive and think about what you are doing. As for the chainsaw thats not going to be more than 110 cc and the recoil action should negated any kick back.

Most Twin shock single cylinder two strokes have their ignition timing around about 2-2.5 mm btdc and most will kick if you kick start in a half hearted way. The closer to tdc the more likelihood of kick back.

I hear what you are saying about kickback, but it doesn't make sense to me.  Let's say the piston is on the way up and it's half an inch from the top dead center and the plug fires.  What's it going to do at low rpm?  it wants to reverse course because of all that pressure when the gas ignites.  Your foot is now indirectly fighting with the piston because you have the starter gear engaged and you are forcing the piston UP.  But most likely the explosion will be more powerful than your leg, so you get a little bite from the kickstart lever. 

So timing is everything and if I am not mistaken, moving the timing so that the spark happens at or just slightly after TDC will reduce the chances of a kickback.  Especially if the piston is already on it's way back down the cylinder and the flywheels are moving in the same direction that the  kickstarter would spin them. 

Anyway that has been my experience with engine timing. Last time I did that I think I was at 3.5mm BTDC and it was kicking back about every 10th kick.  It had lots of power, but you were always wondering if this was going to be the next episosode or not.  Then I decided to try to set it for 2.5mm Btdc and that reduced the kickback tendency by about 50%.  it also reduced the available power maybe 20%. in my estimation.  So I put it back to 3.5mm BTDC, but on that very crude points system this is only a best guess.  I'm probably going to retrofit an electronic ignition as soon as I get the hot setup from you guys.  

That way you get a system that has automatic timing advance and you are now rid of the fiddling with points and condenser.

But just so that i don't offend anyone I want to explain my definition of "advance".

To my way of thinking, more advance to the spark timing means that it happens earlier in relation to Top Dead Center.  I think this is a universal idea as far as I know.  Maybe somebody has another viewpoint, and I am not saying that they are wrong, but just that's what advance means to me.  I've never had my engine run backwards under any circumstances except for the times it kicked back and that would only amount to maybe a half revolution in the wrong direction. 

So does anyone have any good or bad things to say about the elelectrex brand electronic ignition systems from Speed and Sport? or is there another brand that is better in some way?

And please forgive me for not sticking to the stock points and condenser.  I think I'm all done fighting with that. Let's move on.  There is a good video (if old one) about the electrex system being installed onto a TY 250 with a blue painted frame on You Tube.  They did not finish the install, but got the major components mounted. It would be nice to hear about the reliability of that system if anyone knows about that system from personal experiance?

 

 

 

 

 

 

 

 

[feetupfun]

OK my experience with electronic ignitions for TY twinshocks is with three types:

Trails and Trials UK

Rex's Speed Shop

Electrex World

I've got two bikes with the Trails and Trials UK ignition and never had a problem and neither of them has ever kicked back. I also have a TY250A that still has the standard ignition system working perfectly and it kicks back a fair bit when I'm knackered during a hot trial but not when I'm fresh. Both the bikes with the T&T UK ignition run very well. One of them was fitted to allow the timing to run only a small advance at low RPM to tame the motor response down at low RPM yet still run well through the RPM range. The other T&T UK system was fitted because I was having problems with the original points ignition cutting out unexpectedly.

I've ridden a friend's TY250A that had been recently rebored and had a Rex ignition fitted. It was similar performance at low to mid RPM compared to my TY250A that has the points ignition system but from mid-rpm to high RPM it had noticeably more power than mine.

I helped a friend to fit an Electrex World system to his TY175 and was amazed to find that the mounting location for the HT coil was beside the airbox. This means that the HT cable passes over the carby and past the fuel hoses. The bike ran fine with this ignition. They may have found a more suitable HT coil by now.

[tony27]

I have am electrexworld ignition in my 250C, works well & easy fit but you're best to set the timing with a strobe light as per there instructions. 1 thing I did find was that the hole in the centre of the stator plate didn't clear the seal housing & I've had to have some minor turning done to correct this, did a 3 day event with it before I had the chance to sort that including checking the timing & it was great apart from some pinking climbing hills between sections due to being over advanced

[thai-ty]

On 5/25/2020 at 4:46 PM, section swept said:

The last crank that I rebuilt needed a hydraulic press rated at 40 tons! This started the crankpin moving with the usual and expected load clang...scattering two onlookers... and from then on in until pin release the pressure was showing at 25 tons. Crankshaft realignment using vee blocks and dial gauges on a surface table, but some get a good compromise using eye and copper (dead blow) hammer to nudge one flywheel half a tad or more on the crankpin.

Reasons for flywheel half misalignment can be down to sudden shock being transmitted to the drive side of the crank. Air gun/ rattle gun tightening of crank nut. Excessive leverage against either side of the crank when tightening either the flywheel or drive nuts, over-torquing without using the correct flywheel locking method. A good reason to not lock the engine by the con-rod. 

Bouncing or smacking one half of the crank assembly to bring it into tolerance is a standard practice, its quicker than setting up on a press which could cause too much movement. Welding the crankpin in place is a belt and braces protection against high engine rpm sudden seizures spinning one flywheel half on the pin. 

Concur with all the above, but especially, puleeze, please please, inexperienced people - a rattle/air gun HAS NO PLACE NEAR A CRANKSHAFT.

In fact i don't like them on any engine/clutch/gearbox internals.

You Tube can be a great source of education, it also has a fair few numpties who should not be let anywhere near a set of spanners.

Regards,

engine builder for 35 years.