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misfire and high rpm


Guest petrolhead1989
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hey guys..


just recently, ive noticed an issue..

My bike misfires at higher revs..


When i say higher revs, i mean, real high revs.. I havent done much riding recently, especially not on faster roads where i need to thrash gears to pickup speed quickly..

But riding down the A10 yesterday, i kept in 3rd up close to redline, and it just misfired, and continued jumping and spluttering until i changed to 4th to lower revs..


Ive changed the spark plug, correct plug gap, still did it..

So remembering that my kymco is now 15 years old, i thought that the coil was breaking up and failing to charge up quick enough..

So i went up my local bike shop and replaced it.. Still does it..


Ive worked out that the misfiring mainly happens when the engine is hotter, like optimum operating temp..

When the engine is a little cooler, i am able to ride at higher revs, slightly higher, but still misfires..


its to the point where i can only get about 50-55mph out of my bike, because it misfires at the revs it needs to push it to top speed..


I had in my mind about lean fuel mixture (remembering back from my nitro rc days)

and yeah my fuel/air mix is slightly lean, But its been the same since ive had the bike..

nothing new has been changed recently, still the same exhaust and cone air filter, for the past year and a half ive owned it..


I have a brand new CDI unit in my shed, which i can try on it..


but finding the direct route to the issue would be alot better for me, rather than trial and error, as i dont really get alot of free time lately, with decorating my new home, and trips to addenbrooks to see my baby daughter..


any ideas?


cheers

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Is it a bit like this? https://www.youtube.com/watch?v=OAb7DrgguyY (watch revs from around 15 seconds onward)


If it is that was both coils breaking down and a dodgy HT lead (that visually appeared fine). HT leads are easy enough to test on a meter. Coils you can test on a meter but the results can be meaningless as they can test fine on the shelf but break down when hot.

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thanks for your reply mate.


But unfortunately, as stated above, ive already replaced the coil, and of course the HT lead too, but still exactly the same..


Ive stripped the carb this morning, cleaned it and rebuilt it, just to take that out of the equasion..

Ive got to pop to my mothers later, so i will test and report back


:cheers:

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Okay, replaced the spark plug again.. even side gapped the plug, to get maximum mixture burn

(a trick that racers have been doing for over 100 years)


replaced coil, and now replaced CDI..


and still does it!!!


need ideas guys

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What happens if you give it a touch of choke when you're giving it some stick?

 

unfortunately my carb is chokeless mate...

my bike starts first click, no matter what the weather, without a choke..

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What happens if you give it a touch of choke when you're giving it some stick?

 

unfortunately my carb is chokeless mate...

my bike starts first click, no matter what the weather, without a choke..

 

fuelling at statup and at full RPM are very different. it may be fine at startup, but too lean/rich at high RPM.

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thanks for that page :cheers:


I have been playing around with multiple sized jets, trying riding it with each one..

No change at all..


I adjusted the float bowl when i stripped, cleaned and rebuilt the carb

Still no change.


And as for fuel mixture screw, its exactly the same as it has been since it was brand new 15 years ago, and never had a problem.


Ive disconnected all ignition system, IE: cdi, coil, HT lead boot, and thoroughly sprayed contact cleaner in everything, all connectors are shiney and look new..

So that crosses bad connection out of the equasion :(


cheers

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valves cocked up with carbon deposits??

ignition timing slipped??

 

the engine has only done 1200 miles since brand new, so i really cant see that being the case to be honest..


and hmm, worth a look into the timing..


will check that tomorrow morning, and get back to you :cheers:

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  • 7 years later...

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