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Hi!

I have a 3300 with a Bing carb. First flight after several months of dormancy turned up an idling problem. From a cold start, there were no issues. But aftert flying 20' I landed and turned onto the taxiway. Every 3-5 seconds the engine would drop from 1000 RPM down to 800-900, then cycle up again after a half second. It did this continually. On the takeoff roll, it hesitated once upon initially firewalling the throttle, but after that, it ran fine at anything above idle rpm. Landed again and the 200 rpm drop would occur again on idling.

My mechanic said it could be either something plugging up the idle jet (it's only .015") or an intake leak. I've removed and cleaned the jet. I'm planning on doing an induction leak test now, even though I haven't tested the engine since cleaning the idle jet. Before I cleaned it, I couldn't see anything partially blocking it, but with my eyes I could have missed that.

I plan to remove the carburator and hookup the blower end of a shop vac to the intake, then spray soapy water around all the connections, looking for bubbles.

Comments or suggestions?

Thanks,
Dan

Hi Dan,
Before you remove your carburetor you might try using a propane wand on your intake connections. If you have an intake leak and you introduce propane at hose connection points you will get an rpm increase. Obviously it takes two people but it does work. I had a stripped hose clamp on my 2200 Camit which you couldn't tell by looking at it and found leak with propane. There are several youtube videos showing how to make a cheap propane tester.

Also with setting for a while maybe a little condensation leading to a bit of water in your fuel. I would also check your distributor caps& rotors-I had one go bad at 100 hours and I know of another that both went bad at 100 hours

Thanks Loren!

Not a bad idea. I had just watched something like that where they used carb cleaner.

When I was trying different jets when I installed my CAMit 3300, one of the jets made it idle exactly like you mentioned. Sorry but I don't remember which jet or whether I was increasing the size or decreasing.

Ken W
Sonex 959
650 hrs
CAMit 3300 260 hrs

One more thing you might want to check. The RPM drop on a single mag is quite pronounced at low RPM, but very small at the recommended 1,500 RPM. Might have coil going bad, or loose wires or distributor cap problem.

David A.

Thanks guys. Was also wondering the same thing about the mag at low rpm