acdii

I have a 91 in my CG Chipmunk, set at about a 45* angle, and the idle is very harsh, it bangs at idle, and you can visibly see the cowl shake every time. It doesnt want to run right either, yet when I had it upright in a different plane it ran smooth and good. It appeared to be running good, so I taxied out and took off, got a few feet up and it quit. Needless to say I need to straighten the gear and fix the cowl now. I am running a 3 line tank, fill, feed vent, Vent attached to the muffler tap, feed to the carb, and the fill is plugged. I run this same setup on my LT-40 with an OS 52, and it works OK. I do have a problem with the OS, it sputters a bit at mid and high throttle, and if I get below a half tank, and idle for say a landing, there is a chance the engine will quit on throttle up.


The 91 tank is right in line with the high speed needle, and the OS needle is maybe a hair above tank center.(reason for it quitting when below half tank?) I have to measure it to verify. The Magnum ran a lot better in the other plane when upright. Any thoughts on what might be out of whack here?

wcmorrison

My Feedback: (2)

All things be equal, I would look at the tank. It almost sounds like a hole in a feed line inside the tank. This allows the carb to suck in unwanted air, makes the mixture lean and leads to detonation. And the lean situation is leading to over heating inside a cowl which leads to premature engine stoppage. So I think your engine is going lean because it has an air leak. Of course, the air leak could be on the engine too, so that needs to also be checked.

acdii

I found the problem. The glow plug center electrode was loose. It ran, but had low compression and it was loud on the power stroke, hence the hard idle. I replaced the plug and it ran perfect. I do think though the cowl wasn't allowing enough cooling. When I tuned it on the ground I had the cowl off, but when I flew it, the cowl was on and the engine quit after about a minute of flying. Needless to say it was not a pretty dead stick landing, the wind was against me and I couldn't get the plan back to the runway. Since there was a lot of dirt in the engine from landing in between rows of young corn, I took the engine apart and cleaned it. The crank was filled with fuel, and had some grit in it. Its all nice and clean and smooth now, so should most likely run good once I get the plane repaired.

Once I do, I will fly with the cowl off for a while and see how it goes, if it runs good then I need to figure out how best to get cooling through the cowl. Its on a CG Chipmunk, part of the head is exposed and the two openings on either side of the prop are open, and at the bottom rear it is all open, so I might have to baffle off one of the openings.