My 1834cc VW engine will not maintain takeoff power for more than one minute. After about a minute the rpm drops from 3500 to about 2800 and runs rough. If I throttle back to 1500rpm for a short time I can again achieve full power for a little bit then the drop in power occurs again. What could be the problem?
First place I'd look would be fuel system: Insufficient fuel pressure (whether vertical head for gravity feed or fuel pump pressure). Blocked fuel filter, carburetor inlet, tank vent, kinked fuel line, float needle, float level. A distant second thing to check would be an overheating magneto.
Is this a new problem? If so, what changed since it ran right?
I had a loss of power once that was due to one of the intake manifolds not seated properly.
Jerry Folkerts
Before my first flight I really never did hold it at full power for very long. On the first flight after about a minute in the air I started to loose power and thank God I was able to get back to the runway. So, I really never experienced the problem until the first flight. I have readjusted the carb thinking it was running too lean because of the high EGT temps. But, the problem still is there. I am running it up on the ground now. I will have to recheck my flow rate on the gravity feed system. Thanks for your pointers.
You did do a fuel flow test with the airplane in a nose high attitude and a low nose attitude? The FAA has published standards on how do to these test.
Tony
Thanks, that is what I am going to do tomorrow.
My initial thought was a venting issue. Make sure you run the fuel flow test with the fuel caps and everything on.
Ron Wanttaja
Okay, thanks for that I will leave the fuel cap on.
Well, it looks like the flow rate is not my problem. I was looking for at least 6 or 7 gph of flow and I have 15gph. I just drained 2 gallons in 8 minutes.
Rough running at WOT after a few minutes combined with high EGT would lead me to ask a couple questions.Originally Posted by LAWise
1-What is the CHT doing ( I.E. poor cooling causing an issue by getting into knock, but this usually reduces EGT )
2-Verify the valve lash, lash going tight will cause those symptoms
3-Any evidence of exhaust valve sticking? It does not take much to hang things up and cause missfires.
Being that your issue is follows a thermal change not an RPM change you can probably rule out things like valve spring surge. Ignition might be an issue to look at as has been mentioned. I assume you have verified spark timing and its not retarded?
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