Hi
I have a NGH 17cc gas and am having all sorts of trouble with this engine!!!
What does it mean when an engine backfires, while running at full throttle.
Have this engine in a Focke Wulf 190 60 size, runs great when on a test stand, but all sorts of burbling and not running smooth in the plane when in flight?
Someone told me at the field I should attach a velocity stack to the carb, I just don`t know.
What is the reason for the scary backfires at full throttle?
Regards
dutchy50

My guess is the top end is too rich.
You dont need a velocity stack.The only thing that will do is reduce your fuel burn rate.

How did you run the engine on the test stand versus the airplane. is it upright in the test stand and inverted in the airplane?
As DirtyBird mentioned, it may be too rich inverted. You may need to check the fuel tank and fuel lines carefully too. It sounds like it is getting air bubbles in the fuel line from somewhere. Maybe the vibration is causing excess air bubbles to get picked up by the clunk. Or there is a subtle air leak of some kind.

It could be firing out of time. Might be the ignition breaking down at high load.

Oh yeah, I almost forgot. Do make sure that the spark plug cap is really on the the spark plug good. It actually is a twist lock affair. Usually once you get it on a locked, you have a hard time getting it off. But having the spark plug cap on loose is bad as it stresses the ignition coil and can damage it with excessive voltage levels. One thing to never do is see hoe long of a spark one can get off the coil, that typically destroys the ignition coil. The coil can die a slow death like that too. The ignition coil is built into the CDI module.

The other thing is the battery used to power it all with. If the battery can't provide enough current, it can cause the engine to run poorly too.

Also try another spark plug. It is possible to get a bad plug. It doesn't matter what brand either.

Then the timing sensor could be mis-adjusted. The sensor could be off thus causing the engine to not run well. That also leads into some cases where the sensor itself had problems and was defective too.