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 Running rich on one cylinder at idle 
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Joined: Thu Oct 23, 2014 8:40 am
Posts: 36
Post Running rich on one cylinder at idle
Fellas. I’ve got a motor I aquired has bps cam, q pipe, mikuni and iron mountain heads that have 150 hours on em. I have been trying my hardest to figure out my idle problem. Try any and all jet sizes and have determined the carb is not clogged in any way. Currently running 25 pilot, 165 main, accelerator pump nozzle is operating and pointed in the right direction. I’ve got valve lash set to .006 exactly how wildly fast shows in his video.

At idle I only run on one cylinder (the one that has the hose from the mikuni on it) and it seems to run rich. And when cold as it warms up I get excess fuel in the pipe until it warms up. Also the other cylinder doesn’t appear to have excess gas or run rich. As I accelerate I run on both cylinders. My next steps are a leak down and compression test.

What could/would cause this scenario? Leaking intake gasket, head gasket or leaking valves??? I’m planning to take off the heads check the valves then lap/cut accordingly.

Thanks in advance!


Thu Nov 28, 2019 8:16 pm
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Joined: Fri Jan 10, 2014 1:16 pm
Posts: 2107
Location: Gladewater TX
Post Re: Running rich on one cylinder at idle
Your exactly correct with your possibilities. Run a leakdown


Fri Nov 29, 2019 11:37 am
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Joined: Thu Oct 23, 2014 8:40 am
Posts: 36
Post Re: Running rich on one cylinder at idle
Thanks Will I’m going to tear into it today! I’m hoping to get it through the season without wrecking anything then get the heads done right by you!


Fri Nov 29, 2019 12:36 pm
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Joined: Thu Dec 08, 2011 5:50 pm
Posts: 30
Location: DFW
Post Re: Running rich on one cylinder at idle
Mine is doing almost the exact same thing but only has 10 hours or so on the motor after a rebuild. I found a tight intake valve tonight but I can’t run it after dark here in the drive with all the kids getting ready for Christmas. I’ll try it again tomorrow to see if adjusting the valves helps out. This thing is frustrating the crap out of me.

I just went through the carb and soaked and cleaned it. Have new jets so it appears to be all clean inside now but my port side cylinder is filling up and dumping gas out the exhaust joint. I thought I had a bad coil but the spark tester shows a good and solid spark so it appears to be something else.

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Tue Dec 24, 2019 10:30 pm
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Joined: Thu Jul 04, 2019 3:08 pm
Posts: 7
Post Re: Running rich on one cylinder at idle
I had a 670 Predator build that was giving fits with a TM34 Carb, came with a 50 pilot Jet, dropped all the way down to 25 pilot Jet. It would load up terrible at Idle after 15 minutes the plug on the left side viewed from intake side would completely foul, right side was always good. Sometimes I could open it up and after coughing and puking for 60 seconds it would clear itself, other times I would have to change plug. Ended up being the float needle, it had been changed to the 1.5 for pressurized system but was leaking by. Brand new lines and 2 fuel filters. Ran great on the original 2 barrel carb it came with.
I will be watching for that same problem with the TM38 on my 35hp Vanguard upgrade.


Thu Dec 26, 2019 9:10 am
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Joined: Fri Jan 10, 2014 1:16 pm
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Location: Gladewater TX
Post Re: Running rich on one cylinder at idle


Thu Dec 26, 2019 12:13 pm
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