Sherpa T 250 Erratic Carb

[sherpat250]

Hi all,

Coming off of a no throttle condition, my engine goes through a brief "flat spot" where it nearly stalls. Advancing the throttle just a bit farther makes it suddenly 'spring' to life and it runs well from there on up to high RPMs. I thoroughly cleaned out the carburetor, even all of the little passages (all that I could find, anyway) and the bike continues to run erratic at low speeds. A possible clue: Adjusting the air mixture screw on the carb seems to make very little, if any, difference in how the engine runs. I thought perhaps the rubber insulator was leaking between the carb and block, so I tightly wrapped the joints with electrical tape--just as a test. It made no difference that I can tell. Anyone have any ideas of other things I can try?

Thanks,

Kevin

Rochester, NY

[john collins]

Just a quick one - as you do not give year?

If it is what I call the square bowl Amal - fitted 75 on - just check the overflow pipes coming from each side.

They are joined from side to side as standard BUT there is a slit in the tube - which in effect means they are two separate tubes with open ends. Sometimes people see the split and fit a new piece of pipe - if you have done this just cut pipe in half with a scissors and try bike

[dadof2]

Try raising the needle a notch or two. It sounds as if its just a shade weak just as it starts to come off the idle jet. If you can get one try a size larger idle jet. Perhaps go through your carb clening again with extra care on the slow running jet and passages to and from it. Check the number of turns ut of the slw running mixture adjustment screw.

Cheers

PS just had a thought - is your slide worn near the cut away, this cancause weakness just as throttle starts t open.

Edited June 23, 2010 by Dadof2

[sherpat250]

Just a quick one - as you do not give year?

If it is what I call the square bowl Amal - fitted 75 on - just check the overflow pipes coming from each side.

They are joined from side to side as standard BUT there is a slit in the tube - which in effect means they are two separate tubes with open ends. Sometimes people see the split and fit a new piece of pipe - if you have done this just cut pipe in half with a scissors and try bike

Thanks for the input. The bike is a Model 91, which I believe is 1972-74 time frame. It has a concentric carb, with no overflow tubes.

[sherpat250]

Try raising the needle a notch or two. It sounds as if its just a shade weak just as it starts to come off the idle jet. If you can get one try a size larger idle jet. Perhaps go through your carb clening again with extra care on the slow running jet and passages to and from it. Check the number of turns ut of the slw running mixture adjustment screw.

Cheers

PS just had a thought - is your slide worn near the cut away, this cancause weakness just as throttle starts t open.

Thanks, I'll give that a try. The needle does appear to be down almost as low as it will go, so there is certainly room to raise it. The slide appears to be nice and snug.

[swooshdave]

Tear the carb down and report back with needle jet, main jet and pilot jet numbers. They are stamped on each part.

Instead of tape or "snuggest" of the carb to manifold a better test is to spray carb cleaner on the joint will the bike is running. A change in rpm denotes a leak.

[sherpat250]

Tear the carb down and report back with needle jet, main jet and pilot jet numbers. They are stamped on each part.

Instead of tape or "snuggest" of the carb to manifold a better test is to spray carb cleaner on the joint will the bike is running. A change in rpm denotes a leak.

Hello,

I tore the carb down and found the jet numbers. They are as follows:

Needle Jet 106

Main Jet 160

Pilot Jet 20

[big john]

You say the mixture screw doesn't make much difference to fine tuning...it should!

Here's a thought...

When were the crankcase seals last replaced?

Air can be sucked into the crankcases if the magneto side is worn giving a weak mixture and sometimes a flat spot.

If, on the other hand, the clutch side wears, it usually fouls plugs by sucking oil from the primary chaincase.

If the seals are not spot on, the bike will run crap and no amount of rejetting etc will improve it as the seals will continue to degrade and make the mixture settings go off further.

I presume this has happened gradually and the bike ran fine before all this?

Big John

[overthehill]

If the carb is the best part of 40 years old the best thing you can do is replace it with a new one - the bores and slides wear, the needle and needle jet wear - you can get a new concentric for about

[swooshdave]

I thought the pilot jet to be too small. I'm running a 35 compared to his 20.

The lag from "no throttle" is just where the pilot jet is in effect. Lag could be that it's too lean.

[sherpat250]

Greetings all, I have an update to my situation with the Bultaco carb...This is an Amal 627(L), and I have discovered a lead plug that is very loose in the small hole at the right hand side of the air intake horn. I believe this plug is used to block off the unused side of the carburetor, depending on which side the adjustments are to be on. Looking at a detailed diagram of the carb, it appears that this passageway is in the same circuit as my pilot jet. Do you think that could be the whole problem? I'm thinking it could be introducing excess air into the pilot circuit and/or interrupting the operation of the pilot jet.

I appreciate all the suggestions given thus far. The carb is otherwise in good shape (tight slide, proper float operation, no gum build-up, etc.) so I would liketo fix it if possible, rather than buy a new one, but it's good to know that option exists if I need to go that route.

Regards,

[swooshdave]

Pretty easy to test. Plug the hole.

[sherpat250]

Pretty easy to test. Plug the hole.

I have done just that, using a small amount of J-B Weld sealer. I'll report back on how it works. I'm more inclined to think this is the problem rather than a jet sizing issue, because it ran fine before.

[swooshdave]

I have done just that, using a small amount of J-B Weld sealer. I'll report back on how it works. I'm more inclined to think this is the problem rather than a jet sizing issue, because it ran fine before.

Well?

[sherpat250]

Well?

Hi Dave & all,

Well, the saga continues...I plugged the hole in the AMAL carb (where the loose lead plug was previously), and it did help a SMALL amount. However, I still have the erratic idle, the air adjustment screw makes little or no difference, and there is still the "flat spot" upon acceleration off idle. I was dissapointed, because I thought surely this was the problem.

I'm now investigating a possible leaky crankshaft seal. I pulled the flywheel, removed the left side crank seal, and sure enough, it does show signs of leakage (some grease/oil outside the seal). I also noticed that the fine, circular spring that goes around the outer edge of the seal is broken. There are actually two seals, back to back, and only one shows the broken spring. These are 38mm diameter seals. So, how about it, Bultaco pros? Do you think I am on the right track in suspecting leaky seals?

I will get new seals and give them a try. It has been suggested by someone near me who handles Ossa parts, that the seals MAY be common between Bultaco and Ossa. Does anyone know if this is the case?

Thanks,

--Kevin