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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 31 Jan 2002
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carb trouble or not?

When being ridden my 3AJ stutters when I reach 6500-7000 rpm(right below the red line) and wont go higher. However when the bike is in neutral/ standing still there is no sign of stuttering and I can rev it into the red zone. (I wouldn't usually ride it in the red zone, I am just wondering if there is anything wrong). Someone said that it could be a rev limiter. I am about to leave on a long trip and would appreciate some help. I have recently cleaned the carb but didn't make any difference.

Thanks
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  #2  
Old 31 Jan 2002
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Hi John,

When it's really stuttering in high rev's, most likely it's the ignition. When it's not in "super" condition it has a hard job keeping up producing a nice spark at those high rev's and... just then your bike needs a good spark.
It's no real problem, just that things are not perfect. If you want "perfect", change all parts for new one's (spark-plug's, cable's and cap's, coil's,... and at last.. the unit itself)

Maarten
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  #3  
Old 31 Jan 2002
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Hi,

My 3AJ (with a 2KF engine) also stutters at high revs, but it starts already at 5000 sometimes. Under 'normal' circumstances (normal street use, no hard accellerating, changing gears at the right time, not faster then 130 km/hour) it almost never happens. But when going faster or under light load and higher revs it get worse quickly. Even strong headwinds seem to have a bad influence.
I changed almost all parts of the carburators, including the membrame, but no effect. All electric components are okay according to the specs.
Anybody any suggestions?

Peter

(I cant remember any stuttering before I change the 3AJ engine to a 2KF)
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  #4  
Old 31 Jan 2002
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Check and clean your air filter, check your inlet rubbers for leaks.
1990 xt600e 3uy American import
I have spent the last week adjusting the idle screw on the primary carb trying to get perfect running.At either extreme of screwed in or out it ran like shit.Too far in and it was cutting out in low revs,no power and lean running.too far out and power was affected also.I think I have it right now.Don't ask me how many turns out or in ...I just know that it is running better.
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  #5  
Old 1 Feb 2002
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thanks mmarten, but why does the bike stutter only when it has some load and not when I just rev it in neutral? How does the electrical system know the difference?

Peterkik- that's exactly what I'm experiencing, only a little less severe it seems.

Is this somehting that is likely to get worse or will it just stay like this until I change the components?
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  #6  
Old 1 Feb 2002
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mmarten- why would the engine only stutter when under load and not when I just rev it in neutral? How would the electrics know the difference?

peterkik- that is exactly what I am experiencing with my bike, only a little less severe.

Is this something that will get worse/ spread and eventually render the bike unridable or will it stay like this until I change the components?

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Old 1 Feb 2002
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I'm experiencing the stuttering now for over a year. Sometimes a little more sometimes a little less, but overall it doesn't get worse.
btw I also renewed the airfilter and the outletrubbers (engine side). The rubbers were cracked, but changing them didn't help a thing.
Idle screw is set according the manual. But now the weather is getting better (above zero), i'm gonna experiment with the idle adjustment again.
If I (ever) find the sollution I'll post i here :-)

Peter
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  #8  
Old 1 Feb 2002
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Hai,

When the bike is not under load (reving), there is much less (actually no) demand on the engine. Therefor the fuel/air-mixture is different (very lean) and ignite's easy. To keep it in high rev's (not wise) you don't need to open the throtle all the way.. it will just go there.

When you put the bike "under load" there is a real demand for power, mixture changes, become's richer and more difficult to ignite so every "mis-fire" is noticable. It just needs a better spark to ignite the fuel (on time).

Even if the spark looks good (when you take out the spark-plug) and the coils measure fine, when things get rough.. it's a diferent storry.

I hope this explains it a bit.

Maarten
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Old 1 Feb 2002
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the ignition point of xt600's ignition system is only influenced by the revs, nothing else!
it is *NOT* depending on the load (as it is in modern fuel injection systems)!
assumed that your ignition system is working properly (and if the revs are o.k. in neutral, it is), you should check your carb settings. probably the fuel/air mix is not ok. start adjustment at sec. carb. first change jet size there until you get an improvement when riding with max. throttle. then adjust needle position (standard position: 3/5).
fuel/air mix is also influenced by airbox, airfilter and exhaust system.
regards
klaus
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  #10  
Old 1 Feb 2002
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By the way, check out this topic, it might give you some idea's
http://www.horizonsunlimited.com/ubb...ML/000069.html

Maarten
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  #11  
Old 1 Feb 2002
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KlausXT- you say that the ignition is only influenced by rpm, and that if the bike revs in neutral without stuttering then the ignition is fine. But wouldnt the fuel/air mixture under load be more demanding to ignite than when it is in neutral? ie the ignition could be delivering a weaker than normal spark throughout the rpm range but it only becomes apparent when under load and at high (6500-7000) revs? Or have I misunderstood how things work?
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Old 1 Feb 2002
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ignition works like this:
separate coil in alternator (cables red - brown) produces AC current which loads a capacitor in cdi unit. pickup coil in
alternator gives information about revs and position of piston to cdi unit. cdi unit determines ignition point ->
capacitor discharges. this short burst of current through primary coil of ignition coil induces the high ignition current in
secondary coil.
this is the reason why spark is not depending on load (there is simply no indicator for this in cdi-unit), but only on revs
(the revs originally produce the AC current). so the spark is always the same - weak or strong.
if the alternator coil does not produce enough current the result will be a weak spark. this leads usually to bad
starting behavior of cold engine. indeed, THIS is the condition where you really need a strong spark, not at high revs
when engine is hot. here even a weak spark is enough to ignite the fuel mix (infact, sometimes it can explode even
without spark). critical point on high revs is the ignition time (position of crankshaft degrees where ignition takes
place).
an indicator for a strong spark is the length between electrodes which he can overcome.
there are special devices to measure this.
usual method to check coil in alternator is resistance measurement. but unfortunatly even a coil with good values can
produce weak sparks when engine is running (as described in the link which maarten has placed above).
so if you don't have such device only way is to change alternator against a functioning one. but as stated before, i
don't think it's an ignition problem.
one other thing came to my mind: check/change ignition lock or disconnect black/white cable from cdi to lock. if your
lock is faulty (black/white cable is connected to ground), there is no spark.
hope this helps for the first,
klaus
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  #13  
Old 1 Feb 2002
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thanks Klaus, that really cleared things up for me as far as understanding and diagnosing is concerned. Seeing as the bike starts fine. When the weather is cold (ie around 0 degrees or lower) it takes a bit of choke and throttle work in the beginning before the bike will run on idle but even then it will usually start on the first try.

The previous owner has drilled holes (three 7mm holes) in the air box and then closed them again, so maybe he has changed jets to accomodate the increase in air flow, that would make the current mixture rich.
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  #14  
Old 5 Feb 2002
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When you have such problems (stuttering) first thing to look is at FUEL LINE ! control and remove ALL fuel filters (even those inside tank) I went mad to find a solution, and even ordered a new CDI ....but just in time an "old hand" mechanic told me to feed the bike from a canister ..and the problem was solved : impurities in filters simply reduce the flux , that is enough for normal running, but not enough when the CV cuts in ...the effect is VERY similar to a ignition problem ...but is way less costly and easy to solve ...:-)
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  #15  
Old 11 Apr 2002
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JOHN-1
First of all why would you want to run a 3 AJ at 6.5/7000 rpm for any length of time?
Secondly, I had a similar problem when I first got mine,it would only run at 120kph for a few seconds and then cut out.
If you remove the main jet and jet housing from the left hand carb,you should find a small plastic filter on top where the fuel comes in.Mine was blocked with rust so I removed it and put inline filters in. Iv'e had no problems since.
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[This message has been edited by GRIFF (edited 10 April 2002).]
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