I have spend the last weekend looking for the cause of the following problem: accelerating to 3500 - 4000 rpm in fourth or fifth gear the engine starts stuttering. That is, it does not run smoothly anymore. It seems that it is possible to go above this speed and that the problem is less than.
I have checked the carberetor and checked the resistances of the ignition coils and the coils inside the engine. All seems fine.
Would there be anybody who knows the cause of this?

Auke

I have to add: there is no sidestand switch on this bike, so that cannot cause the problems.

Auke

Another addition: I checked and cleaned the carburettor, float level, needle positions etc. I lowered the needle of the secondary carb, the create a leaner mixture. This helped a bit, up to now no dangerously white spark plug. Also not total solution to the problem. Position is now in the middle (third notch of the five). By the way, it is the 55W model, so no membrane but a kind of metal piston.
I do not dare to lower the needle more, would this possibly be the solution?

Auke

hello a white spark plug and the stuttering indicate a lean mixture i had a similar problem with my aus xt.either raise the needle or use a larger main jet[my solution]. alean mixture will cause overheating.a cooler plug may also help.i hope i'm in time

txs Ben

I still have to test this solution. I wonder how come the mixture can turn out to be too lean when using the standard jets etc. Would that mean that something else causes the problem? Leaking inlet duct, worn carburettor, etc

I'll keep you posted.

Auke

Hi,

This sounds like a weak spark under load, or possibly bad carburetion.

I'll just list some of the usual causes/cures for this problem:

Start with:
NEW spark plug(s)
NEW Spark plug wires and caps
NEW or washed airfilter

Check intake manifolds/hoses for leaks, ANY sign of cracking replace.

NEW needles and needle jets - should be replaced every 25-50,000 km anyway, as they do wear and cause slightly rich mixture/bad fuel mileage. Your improvement from lowering the needle suggests that this may be a good place to start. "...Would that mean that something else causes the problem? Leaking inlet duct, worn carburettor, etc..."

Yes. The only time you should change the needle setting in the normal life of the bike would be if you modified the engine/exhaust etc, or were at extreme altitude. If just changing the jetting/needle position etc cures the problem, you haven't found the REAL problem yet.

Are you SURE all the passages inside the carburetor are clean? They can be very difficult, you need a powerful cleaner to do the job.

If all that fails, you may need to take it into a shop and have the coils and related items load tested. They may be fine in a static test, but under load break down.

Hope that helps, good luck!

------------------
Grant Johnson

Share the Dream!
at: www.HorizonsUnlimited.com

More news from the front:

- Lowering of the needle to standard position has not totally solved the problem, I
- When lowering the needle I also replaced the plug with the standard type, this led to a too white plug. Although the bike has 65000 kms. and the carb looks 'used' enrichment seems not the problem. I have put the needle back on the 4th notch from top, ehere I found it.
- The problem only occurs at a certain engine load, see earlier e-mail, above and below engine runs fine.
- Resistance check of the pickup and source coil indicated source coil (brown and red) OK, pickup coils (red/white, green/white) too high resistance, although not more than 50 ohms. So what does this this say?

Txs for all the advice and I will keep you posted.

Auke

hi auke,
first check/change the intake manifolds/sealings. if there is a leak the carb cannot provide a proper mixture.
second, as grant already mentioned, checking coils only by resistance measurement is somewhat tricky. you have to consider that the coils are contaminated by hot engine oil while engine is running. if the isolation of the coils is faulty, errors might occur only under certain circumstances.
easiest would be to change the whole left engine side-cover assambly against a functioning one (hope you know somebody who is able to help you, you can use as well any 34L, 43F and with some mods 2KF/NF). you can change also the cdi-unit but in most cases errors result from faulty alternators.
if the alternator is faulty, coils can be rewinded. in germany we have some workshops who are specialized on this job and they perform a pretty good work, better than original with one year guarantee.
and last: there are also reports about loosened parts in the silencer which can cause similar problems.
good luck!
klaus

Any luck sorting out your problem? I have
much the same situation on my 1986 xt600s-
stuttering at around 4000rpm in top especially if I grab a handful of throttle.
I'm wondering if the Supertrapp exhaust on my bike
(On it when I bought it)is messing up the mixture?

Status at the moment is still no luck. I removed the alternator / ingnition coils from the engine and have them send to a company that is can test it simulating the conditions inside the engine, so heating it to 250 degrees and ´loading´ the electrics as in the engine when running. This should give an 80% indication whether coils are faulty.

Intresting: they represent an English company that produces imitation ignition coils for this type; they can also repair CDI units for about 130 EUROs, a lot cheaper than buying a new one. The only thing they cannot repair or supply aftermarket is the pickup unit.

Look at www.carmo.nl

Keep you posted!

Auke

more news:
I had the source coil checked, diagnosis was ´not in a very good shape´. I had it rewinded. Engine runs better now, however problem not totally solved.
My next step is to have the CDI unit repaired / revised. I´m hoping thus that the problem was a combination of a weak CDI unit and source coil.

Auke

Success at last!

I installed the new original coil and pickup set from Yamaha and the problem is now gone. Note that the original coil and pickups give exactly the same resistance values as the new one. So, without 'dynamic' testing it is not always possible to diagnose the ignition.
Apparently the pickups were not OK.

Auke

congratulations!
keep her running!
klaus

And there I am again, 3000 kms into Libya and the problem is back. Have taken averything apart and checked, replaced CDI box and will now have flown in new coil and pickup set again. What am I missing here on the technical side?

Auke

strange!
did you check all the concerned wires and their insulation?
klaus