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Running rough below 3000 rpms

  • Pete Christensen
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4 weeks 1 day ago #17297 by Pete Christensen
Pete Christensen created the topic: Running rough below 3000 rpms
Doing compression test all 80/79 except cyl 3. 912 is early 80 horse under 1200 hrs. Runs just fine. Cyl 3 was 80/76 last year and similar for the last several years. Plugs all looked good. So I ran the engine for several minutes and retested. NO Joy. Went flying for an hour, retested, NO Joy. On some of the tests I could hear the oil tank bubbling. On others I'm pretty sure I heard air hissing in the exhaust. My wimpy compressor kept cycling on and off so hearing the noises was hit and miss. I believe I need to take the head off next and inspect the valves. Maybe remove the cyl and piston?

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4 weeks 1 day ago - 4 weeks 1 day ago #17298 by Pete Christensen
Pete Christensen replied the topic: Running rough below 3000 rpms
Engine has been running great. Flew last weekend and then Wednesday was going to fly. Started up and it was sounding rough at idle. I warmed up a minute and advanced the throttle and it continued rough until it got to 3000 rpm. Let it warm up some more then ran it at 3000 rpm for a couple minutes hoping to clear up the problem. No joy. I ran it at high idle and turned off the fuel valve. After a while it smoothed out just before it was starved for fuel. Opened the fuel valve before it died and it ran smoothly for a few seconds before it began to run rough again. Sounds like a carb problem where maybe one carb is getting too much fuel. Mag check at 3k is normal. I dropped the float bowels and the floats looked to be floating normally. Fuel looked clean. I had run on this tank for a couple hours Saturday. Gascolator had clean fuel. Suggestions? Why would a good,running engine start running rough after just a few days?
Last Edit: 4 weeks 1 day ago by Pete Christensen.

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4 weeks 1 day ago #17303 by Roger Lee
Roger Lee replied the topic: Running rough below 3000 rpms
Symptoms may point to a clogged idle jet. Since you have to drop the float bowl weigh the floats. Don't just look at them. They should be under 7 grams. You can buy gram scales on Amazon or in town for $10-$15. When it is running rough at low rpms it is on the idle circuit and may be starving for fuel or if a carb is flooding due to a heavy float it will stumble at low rpms. Pull the idle jet and clean it with a piece of wire down through the center. Use an 18 ga. wire and strip it back 3" and then peel back only 1 strand to go down through the center of the idle jet.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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3 weeks 4 days ago #17345 by Pete Christensen
Pete Christensen replied the topic: Running rough below 3000 rpms
Thanks Roger.

Turns out it was the float needle and float arm. Had a wear mark on the float arm causing it to stick.

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3 weeks 4 days ago #17347 by Roger Lee
Roger Lee replied the topic: Running rough below 3000 rpms
Good deal. Now it's time to fly :)

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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3 weeks 2 days ago #17371 by ENRICK FOURNIER
ENRICK FOURNIER replied the topic: Running rough below 3000 rpms
when did you perform a 200h inspection of the carburator

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3 weeks 2 days ago #17372 by ENRICK FOURNIER
ENRICK FOURNIER replied the topic: Running rough below 3000 rpms
when did you perform a 200h inspection of the carburator am dwith type of fuel are you running,

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