1976 Sherpa M159 325cc Jetting Problems

[insultaco]

Hi

My M159 is all restored with new seals, bearings etc, etc. The only moving parts not renewed are the piston/rings/bores (loads of compression and fine in my opinion) and the swing arm bushes (not a problem). Apart from that its all new.

I've fitted electrex ignition which is timed correctly for this model I believe (2.9mm BTDC) checked on a strobe and it starts on choke very easily.

Everything is very smooth and lovely, BUT, in order for it to tick over correctly once warmed up it has to have the mixture screw wound all the way in! If I try backing it out at all (1.5 turns out is standard) it coughs and backfires even at 1/8th turn out from fully seated it starts to react negatively. The carb is a brand new Bing with new jets etc.

I understand the different stages of the carburation cycle and the effect the various elements play in the transition from idle through to max. revs and have gone through each element of the carb confirming the settings are correct, so:

Pilot Jet is already 45, which should be 40, so it should run rich on this jet anyway, but it doesn't.

Needle is on the second clip from bottom (tried next one up with no benefit), the needle is standard for this model.

Main is 125, but I do not have a problem once above idle/low revs, so this is not expected to need to changing.

Floats are at standard height as per the manual.

With the pilot screw all the way in, it runs and ticks over fine, which suggests it is something to do with the pilot mixture in some way.

Can anybody help me with an obvious answer? Even those nice people at BultacoUK couldn't shed light on the matter. If the answer is to have the pilot screw all the way in then I 'll just have to accept it as it is and move on, but it is strange....

Thanks in advance wise sages

Mick

[pschrauber]

Hi

My M159 is all restored with new seals, bearings etc, etc. The only moving parts not renewed are the piston/rings/bores (loads of compression and fine in my opinion) and the swing arm bushes (not a problem). Apart from that its all new.

I've fitted electrex ignition which is timed correctly for this model I believe (2.9mm BTDC) checked on a strobe and it starts on choke very easily.

Everything is very smooth and lovely, BUT, in order for it to tick over correctly once warmed up it has to have the mixture screw wound all the way in! If I try backing it out at all (1.5 turns out is standard) it coughs and backfires even at 1/8th turn out from fully seated it starts to react negatively. The carb is a brand new Bing with new jets etc.

I understand the different stages of the carburation cycle and the effect the various elements play in the transition from idle through to max. revs and have gone through each element of the carb confirming the settings are correct, so:

Pilot Jet is already 45, which should be 40, so it should run rich on this jet anyway, but it doesn't.

Needle is on the second clip from bottom (tried next one up with no benefit), the needle is standard for this model.

Main is 125, but I do not have a problem once above idle/low revs, so this is not expected to need to changing.

Floats are at standard height as per the manual.

With the pilot screw all the way in, it runs and ticks over fine, which suggests it is something to do with the pilot mixture in some way.

Can anybody help me with an obvious answer? Even those nice people at BultacoUK couldn't shed light on the matter. If the answer is to have the pilot screw all the way in then I 'll just have to accept it as it is and move on, but it is strange....

Thanks in advance wise sages

Mick

Mmmh ...

Which Carburator model have you mounted to your Bultaco, just to make sure.

Have you cleaned your carburator very well, all pipe and tubes inside, there are no cleaning rag or kitchen paper / Kleenex, ... left overs somewhere, (this can happend).

Look at your slide inside the carb, still working proper, which means:

The slide or the cylinder where the slide sits inside the carburator is not worn out, If the slide is chattering around inside the carbbode it' time to replace, if there are sliding marks an the cylinder surface you might have to exchange the carb body too. (Does happen when the carb is as old as the bike, with a worn out slide proper jettting is a pian / will not success.

Is the carb needle nice and there are no signs of abrasion visible and the needle is not bent, if you see sliding marks or the needle is worn out and the needle jet too. If the needle is bent you have to exchange both parts too.

[1oldbanjo]

Something about the mixture adjustment seems backwards...or at least not matching the operation of a normal carb. I am not familiar with a Bing - but on a Mikuni and most other carbs if you start backing out the mixture screw the mixture will richen with each turn until you reach the maximum flow that the pilot jet of a given size will allow. Opening the screw allows more air to flow into the area around the pilot jet - which then picks up more fuel. My initial thoughts on any mixture screw/pilot jet adjustment is that if I have to open the mixture screw more than a couple of turns and you don't hear the engine idling mixture becoming too rich - then the pilot jet needs to be larger. If the idle mixture becomes too rich before you get a full turn then I suspect the pilot jet is too big.

Do you think you have an air leak somewhere in the intake/carb connection?

[bultacosteve]

Actually the proper term is not mixture screw , but "air screw" on the bing carb. So when turning the screw in it is actually richening the idle mix (more fuel less air) and backing it out is leaning the mix (more air less fuel). So if its running better with more fuel (screw in fully) obviously it is needing that fuel to run correctly so try a richer jet. As the above post mentioned make sure the carb is perfectly clean. Even new carbs should be thoroughly checked.

To clarify further a "fuel mixture" screw works the opposite way. When turned out its adding fuel making it richer , turned in it is making the mix leaner. The easiest way to determine what a particular carb has ( air screw or mixture screw ) is as follows. The air screw on a carb is always on the air filter side or rear of the carb usually near the bell mouth. A fuel mix screw is on the intake side of a carb usually pointing straight down but in certain cases could also be sideways . It is important to realize the difference between the two or your adjustments will be going the wrong direction.

Steve

[lowbrow]

I could never get the Bing to work in 1976 and swapped the carb for a 28mm square Amal which was very responsive to adjustment. Once heard the Bing described as 'a metered leak' and thought that spot on. On a more positive note, the main jet does effect slow running on these things. The float height is very important too, but I have no recent experience, thank goodness! I also believe Amal monoblock main jets fit and offer smaller steps between sizes. I once watched a practice session with Lampkin, Rathmell and Coutard where Reg May was swapping main jets to get the mixture right off the bottom.

[insultaco]

Hi

I rang BultacoUK for the next size pilot jet up, they said that the biggest they had was a 45 and that to go higher would be a fools errand (my words), they suggested that with the pilot at 45, compared to 40 as standard was not sorting the root cause, which seems a reasonable comment. The problem is I've tried all the things that are obvious:

1. I've blown out the carb with a high pressure hose, its very clean as far as I can tell.

2. The carb is new, so no clatter from the throttle and the needle is straight and new along with all the jets.

3. All jets etc are standard apart from the pilot.

4. I've tried moving the needle up one clip with no noticable improvement.

5. I've tried putting all the old carb jets etc on with no change (In fact worse as this has a 40 Pilot on it).

Questions:

1. If I move the float height up slightly will this have the effect of richening the pilot circuit?

2. If I were to change the Main Jet for a larger one, wouldn't it have the effect of richening the mid-top end of the mixture, which is currently OK?

3. If I were to change the needle from standard, what would I change it to?

Thanks again

Mick

[zarkoff]

Hi

I rang BultacoUK for the next size pilot jet up, they said that the biggest they had was a 45 and that to go higher would be a fools errand (my words), they suggested that with the pilot at 45, compared to 40 as standard was not sorting the root cause, which seems a reasonable comment. The problem is I've tried all the things that are obvious:

1. I've blown out the carb with a high pressure hose, its very clean as far as I can tell.

2. The carb is new, so no clatter from the throttle and the needle is straight and new along with all the jets.

3. All jets etc are standard apart from the pilot.

4. I've tried moving the needle up one clip with no noticable improvement.

5. I've tried putting all the old carb jets etc on with no change (In fact worse as this has a 40 Pilot on it).

Questions:

1. If I move the float height up slightly will this have the effect of richening the pilot circuit?

2. If I were to change the Main Jet for a larger one, wouldn't it have the effect of richening the mid-top end of the mixture, which is currently OK?

3. If I were to change the needle from standard, what would I change it to?

Thanks again

Mick

hi there there is one thing that does alter the mixture settings on a carb that no one has mentioned yet and that is the air filter, have you changed the filter in any way as it does make a lot of difference . i once changed the filter on a bike and it run like a dog !! just worth baring in mind.

[breeze]

Hi

My experiences are with Kart 2 stroke engines (25 years) but I have come back to trials with an SWM 320. To your problems. I would pressure test the engine first, if ok the next often missed area is the ignition, obviously you have a spark otherwise it would'nt run at all, but if the spark is not of a high enough voltage and duration you can be trying to compensate by carburation changes. Assuming ignition is 100% (do not just assume it is ok - check) then re check the carb. Have you checked that the new carbs needle jet tube is the same (size of tube will affect most of the range of slide position), you say you have moved the needle clip up, this will weaken the mixture, sounds like it needs to be richer. Have you checked that the slide has the same cut-away? All of this, re the carb is assuming a Bing is much the same as a Dellorto etc.

[b40rt]

hi there there is one thing that does alter the mixture settings on a carb that no one has mentioned yet and that is the air filter, have you changed the filter in any way as it does make a lot of difference . i once changed the filter on a bike and it run like a dog !! just worth baring in mind.

As the man says, if the air filter is not oiled correctly it will effect carburation.

[pschrauber]

Hi

I rang BultacoUK for the next size pilot jet up, they said that the biggest they had was a 45 and that to go higher would be a fools errand (my words), they suggested that with the pilot at 45, compared to 40 as standard was not sorting the root cause, which seems a reasonable comment. The problem is I've tried all the things that are obvious:

1. I've blown out the carb with a high pressure hose, its very clean as far as I can tell.

2. The carb is new, so no clatter from the throttle and the needle is straight and new along with all the jets.

3. All jets etc are standard apart from the pilot.

4. I've tried moving the needle up one clip with no noticable improvement.

5. I've tried putting all the old carb jets etc on with no change (In fact worse as this has a 40 Pilot on it).

Questions:

1. If I move the float height up slightly will this have the effect of richening the pilot circuit?

2. If I were to change the Main Jet for a larger one, wouldn't it have the effect of richening the mid-top end of the mixture, which is currently OK?

3. If I were to change the needle from standard, what would I change it to?

Thanks again

Mick

To your questions:

To question Nr. 1:

You should not change the float height in any direction there is only one position it works proper! Check out by turnig the carburator upside down on a table, then look at the arms of the floats that they are perfect parallel to the table.

Look up the instructions book for your carburator, (still not knowing which brand and model you have for the correct distance in mm and 10/mm.

See picture:

2.

The main jet controlls only 3/4 to full throttle so the effect for trials riding is very low, I would opt for the standard one, best what you can do

See picture:

3.

The needle, I would also not change the needle, take the standard one, mine is sitting in the second noth counted from up downwards.

The needle has three parts the tip with the notches, there should be 4 nothes. By changing the notch position it is possible to raise or to lower the needle in order to obtain mixture enrichment or mixture weakening over the range that is regulated by the needle taper, which is, - look up again to answer to question 2: between 1/8 to half of the throttle. Here again I would not change the needle if I do not have an real issue. You can get different needles with different cylindrial parts (look at C), this will affect the mixture strengh in the the first throttle valve movements up to about a 1/4 throttel, very useful for trialsriding, (therefor I asked also if the needle is in proper status befor too). If you change f. e. to a reduction in the diameter of this cylindrical part this will produce a mixture enrichment and by increasing the opposite. But befor you do any changes here first get familiar with the throttel response you have now and then first start changing and do some experimental stuff. (It's just my opinion you can still do what you want).

Edited March 4, 2011 by PSchrauber

[insultaco]

Gentlemen, once again your help is in-valuable and has givn me much to consider. I'm going to check the cut-away to check it's the same as original and also put the original needle jet in the new carb, because I think this may be the one that came with it from Spain. The needle is now the original, but I've had both on using 2nd notch from bottom and 3rd from bottom (no improvement).

If that fails, then I'm going to try every notch on the needle to see if that works.

The float height is bang on standard.

The ignition is a brand new Electrex and the timing is strobed to 2.9mm BTDC which is mid-point of what this Forum and BultacUK state as correct.

The intake has a high flow filter with new filter oil applied as per the bottle and was also my first thought, but according to BultacoUK it should not make that much difference to the standard high flow filter, it just saves the air box.

If all else fails I'm going to drill out the original pilot and get it richer if it kills me. It's just a bit too fluttery at the bottom end at the moment and its not what I need when i'm mid wobble anyway

Its the last little frustration on getting my project singing again after being purchased as a none starter in December. It had previously stood for at least 3 years (more probably).

Anyway, its still a lot easier to get around on than the Greeves TFS I've had on loan for the past 15 months.

I'll report back with the final solution.

Thanks again

Mick

[insultaco]

Sorted!!!!

Thankyou all, it was me being a dumb t@#t.

I'd taken the main jet and needle from the original and forgotten to pop out the needle jet. As soon as this was swapped over it started and ran like a proper motorbike, although I had to sort out the snagged throttle cable as well (which it had ran with all weekend and was apparently compensating for the jetting.. ), it was a wonder it ran at all.

It needs a good blow out down the lane to clear the cobwebs and a final little fiddle with the tickover and mixture screw and it will be as good as I need it to be and better than I can ever expect to master.

[1oldbanjo]

I love Happy Endings!

I do appreciate when the solution is posted so we can all learn something.....Thanks!

Now go ride!

Edited March 7, 2011 by 1oldbanjo

[pschrauber]

Sounds very good, sometimes small things can get you in much troubel.