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The TTR250 FAQ web pages -> TTR250 -> Repairs and maintenance -> Misfiring in top gear
Post Info TOPIC: Misfiring in top gear
katana


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Misfiring in top gear
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I am busy reading the repairs thread - finished the carb one earlier.  I have an odd issue.  It only happens in 6th gear, usually while overtaking a car. no

I will be going strong, when all of a sudden I get a splutter in the revs, almost like running out of fuel.  When I change to 5th, I can rev strongly with no hesitation or splutter.  I cannot get my head around whether the bike is running lean or rich at that time.  The throttle will be pinned.  If I close the throttle slightly, the revs resume as normal, but obviously I lack the acceleration needed to complete the overtaking maneuver.  

My reasoning is hitting a block because at max throttle there might be insufficient vacuum to suck fuel for the revs required.  Then again, it might be over fueling.

 Any ideas?

This could be linked to my starting problem.  Choke does nothing.  The only way I can get the bike to start is a number of squirts with the accelerator pump and then the bike will run for a couple of seconds.  I have to repeat this process until the bike idles by itself.  Another carb strip is due, but some direction could help a lot.

Cheers 



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Pendo


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Check the fuel cap, blocked breather tube causing a vacuum in the tank?



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TTRfan


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Have had similar symptoms. Turned out to be one of the camshafts was a tooth out on the timing chain.

Unlikely I know but might be worth checking...............

Brian



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katana


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Will do. When I checked the clearances when I bought it 1000km ago, they were spot on though. Even the cam positions were correct. Strange it only happens in 6th though.

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petenz


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weak fuel mixture... only shows up when the motor
is under high load.... lower gears freer reving it dosn't show up...
take it for a run and do a plug chop when it happens...


...

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katana


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petenz wrote:

weak fuel mixture... only shows up when the motor
is under high load.... lower gears freer reving it dosn't show up...
take it for a run and do a plug chop when it happens...


...

 Makes good sense.  Bigger main jet then?



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TTRfan


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Or raise the needle wink



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katana


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After my airbox and exhaust mod, I had to lower the needle to get rid of over fueling. I can only drop the needle if I get a smaller main jet.

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JOHNYBOY


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you can do a fuel chop to see how running full throttle in a high gear press kill switch at this moment pull in clutch pull up remove spark plug be very carefull not to burn your fingers if you take a look at it see how its burning the fuel too rich or too lean you can now make carburettor adjustments to suit hope this helps obviously every thing else has to be in sync such as tappets set correctly newish plug no air leaks fron inlet stub manifold etc etc john

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katana


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Thanks John

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