Did you disconnect the fuel from the carburetor to do the test? If so, do you have a drain plug in your carburetor float bowl? If so, redo the fuel flow test through the bowl drain, the blockage could be in the carburetor. It could be the float needle, and some carburetors have an inlet screen that could be clogged.Originally Posted by LAWise
The CHT is up to about 340 degrees when I start to loose power. Excuse my ignorance but when you say "valve lash" do you mean the valve clearance? I just adjusted the valves to .006" on all valves when the engine is cold. As far as "valve sticking" I do not know how to test for that. What would I have to do to keep the valves from sticking if that is what is happening?
Hello,
Yes lash is sometimes called valve clearance. CHT seems reasonable. Valve sticking is evidenced by scoring marking on the valve stems when viewed through the exhaust port. I think some basic history on the engine would be good as there can be a multitude of things causing trouble here. High crankcase pressure caused by a blocked breather system can cause oil consumption that will raise EGT and cause the engine to run rough and lose power, incorrectly installed piston rings can do the same. Without a complete history its just shooting in the dark here.
-Aaron
The history of the engine is that I bought the engine as a new kit from Great Planes. It only has about 26 hours on it. I built the engine. I do have a background in mechanics, however, that does not mean I know it all. I know I still have a lot to learn. The engine has a Compufire electronic ignition and a Aerocarb carburetor that has no float system. No starter (I have to hand prop it). It usually starts on the second or third blade.
Ok so did you verify all the clearances and such when you built it, or did you just assemble the parts? I am thinking especially valve to guide clearance and piston ring end gaps at this point.
What was the measured compression ratio of the engine?
Is the ignition timing verified to be correct at full RPM?
Is the Cam Timing verified to be correct?
Aerocarbs are pretty simple and provided there is fuel flow running at WOT should be no issue.
What fuel are you running?
What does your breather system consist of?
-Aaron
How about your exhaust system ? Is the system built for this engine ? A restricted exhaust flow will cause a rich mixture and reduce power output.
I was talking this over with my dad and he brought up vapor lock isues that were common on the vw engines. How is your gascolator and fuel system isolated from engine heat?
Going back to the original post: Full power for awhile, then it fades. Full power recovers after low power for awhile. This sounds like a fuel delivery problem; the engine uses up the available fuel and power fades, then at the lower power setting the engine draws less fuel so it recovers, etc. Many of the suggestions don't explain the recovery after a short period of low power running. Anything temperature related would be on a longer time cycle.
Ron mentioned venting problems is a post above. Note that it is possible to have a venting system that feeds fuel just fine at low airspeeds, but not well, or not at all, at higher speeds. Some of the old "blast tube in the fuel cap" type, when put on backwards, would run just fine until there was a little airspeed.
BJC
Good point. My T-Craft had one of those caps with the forward facing tube on the wing tank. Even if it was pointed forward, if the locking tabs inside the cap weren't tight the low pressure above the wing would draw more air past the leaking gasket than the tube could provide, and it wouldn't feed. Took quite awhile to figure out what the problem was.
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