Rear Cylinder with a Miss

[pitbull pat]

On my 02 pitbull w 107 I have a miss in rear cylinder. Have new plugs, switched plugs, and even switched wires directly from coil as both cylinders fire at same time and the rear cylinder miss persists. . When the bike is running at idle and I pull the rear plug wire the bike still runs off front and there is no miss in that cylinder that cylinder. when I pulled back plug wire the bike misses and dies. Clearly the back cylinder is not hitting all the time (the bike still runs strong as fuck on the street). I am lost and what thoughts I do have aint good (valve, lifter...) Anyone been through this and any ideas?

By the way, as for carb I increased jetting from 29.5 Intermediate to 33 (always had to run w choke on a bit) and main from 70 to 76. I also rebuilt carb a few months back. Problem don't seem fuel related as the front would be effected also it seems.

 

[Jersey Big Mike]

Do you have compression releases on that engine -- have you checked them to make sure back one is staying completely closed? Just a thought.
Only other things that come to my mind are ugly and should effect while running on road.
I'm sure a more knowledgeable engine guy will speak up.

 

[Rottweiler]

When you switched plug wires you did it at the coil? So you can rule out plug wires? You removed the carb so check for air leaks also.

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[Patrick]

Left plug wires on coil and moved front to rear..so cannot be wire..it sounds like spark issue but system checks out....wlll do compression test next ...baffeling...

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[Patrick]

No compression valves

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[Sven]

"the bike still runs strong as fuck on the street"

If that's the case, it's not compression, not fuel, not spark, but an air leak. My thinking is at idle, vac is like E. It finds the shortest path. So if there is an air leak at idle, air goes in sooner that fills the void. Less vac pressure at the jet holes because it's before the jetting-equals less pull from the jet holes. Pop is lean. Misfire is lean. Spark to flame front is more a jump to the next fuel molecule, not like a domino flame front.

Spray the intake where it meets the head. Cold, the gap is bigger so with brake clean, spray it on a cold engine that idles. Hot, it closes the gap some. Alex, I'll take misfires for 200 dollars.

 

[Patrick]

Both cylinders test at 160..so will try break clean..used wd40 but that aint much...put new iridium plugs in..after bigger jets some black smoke ..thinking i need a 32 intermediate

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[Rottweiler]

I never had good luck with iridium plugs.

 

[HMAN]

Shot in the dark.....cracked coil? Loose coil wire?

 

[Rottweiler]

We has a air leak between the carb and rear cylinder.

 

[Patrick]

The new plugs helped and my coil does have a crack at mount that does not seem to effect anything..as i can switch
Plug wires at plug and bsck cylinder still has issue...
Replacing coil also.

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[Patrick]

What are you guys seeing for compression...i was 20 # higher if i rolled.over more but books generally say 4 revolutions..

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[john sachs]

What are you guys seeing for compression...i was 20 # higher if i rolled.over more but books generally say 4 revolutions..

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Don't care what the book says. Crank engine, with the throttle wide open, until the gauge stops moving. That's the compression that's in that cylinder. Any other way, you're pissing in the wind.
John

 

[Patrick]

Got it thanks.

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[dingo]

I had a similar problem......

Electrical Issue - Pulsing Tach and Loss of Spark to One Cylinder

Having an electrical issue and wondering if anyone else has experienced or has suggestions to resolve: 06 K9 - 16K miles - installed one of the early Low Down Chop Shop WirePlus units 3 years ago - no issues since install - original speedometer - original ignition module - original crank...

www.bigdogbiker.com

See post #10 for my fix