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Rear Cylinder Miss Fire! (SOLVED)

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Rear Cylinder Miss Fire! (SOLVED)

PostAuthor: Quarterbiker » Wed Jan 01, 2020 11:25 am

After much trouble shooting and following symptoms from normal procedures of diagnosis i finally found the problem. A small leak in the fuel pump was allowing fuel to pool in the vacuum line that runs to the intake manifold this was causing a rich condition and miss fire at idle and low RPM. higher engine speeds the bike ran fine. This was a slow developing problem, i first noticed the bike was getting slower to start and the mileage was dropping then the last couple of days it became hard to start and noticeable the rear cylinder was not running so it was time to find the problem

I call this engine a Happy Motor because it normally jumps right to life!

These symptoms could also be a week coil, fouled or worn out plug, carb problem, ECU problem. tight or sticking valve, low compression, cracked manifold and a few other things. I checked all of those before finding the problem. This vacuum controlled fuel pump is a first for me and others that i asked, it never crossed my mind this could cause Only the rear cylinder to miss fire! It has 1 carb and a shared manifold! Anyway for those that experience this problem and cant find the answer it might very well be the fuel pump.

I purchased a 22 dollar one from Amazon.

During this head scratching experience i discovered my valves were a bit tight, so check them every 5k miles or so. Hope this helps!

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Re: Rear Cylinder Miss Fire!

PostAuthor: mark75 » Wed Jan 01, 2020 5:00 pm

Glad you found the issue. Thanks for sharing.

I am going to mark as "SOLVED" to help others searching.
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Re: Rear Cylinder Miss Fire! (SOLVED)

PostAuthor: abidali69 » Tue Jan 21, 2020 6:49 am


I Am facing very similar problem with my 250 rear cylinder. its slightly misfiring at idle, if plug wire removed from front cylinder; engine stall at idle on rear cylinder, rear exhaust manifold comes very hot with compare to front cylinder and definitely the engine power is compromised. Definitely in first its looks like rear cylinder is firing is retired, some timing or TCI issue, but I had checked everything a lot but failed.
I check the vacuum lines for fuel leakage and flowing in rear cylinder is making it richer, but I couldn't find any leakage. but after reading your case I am thinking about thoroughly checking fuel cork and fuel pump both.
what you advise.


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