engine tuning help?
 

I have a ZDZ 40RV That I can't seem to get to run right.

Two problems. The first is the idle is very sensitive. It will idle high and then drop off suddenly to a very slow idle. I can't get the thing to run in between. I have adjusted the carburetor leaner and richer, even switched out the carburetor with a new one and it still behaves the same way. I then adjusted the timing and changed the spark plug and gapped it to specs. Still could not get the engine to idle right.

The second problem is when the engine is up to full rpms, when I cut the throttle the engine dies abruptly. If I reduce the rpms slowly the engine will remain running.

The engine is a newer engine, producing good power, transitions well from idle to full rpms, and compression is okay.
I have fresh fuel, checked the cylinder base gasket, adjusted the timing, and changed carburetor with one off of a good running engine.

I'm not sure where to go from here. The only other things I can think of is the rotary valve or ignition.

Where should I go from here?

Thanks

RE: engine tuning help?
 

You may be moving the needle too much when you move it?

The idle starting high and then going to a lower rpm is a sign of the low speed being too lean.

The engine quitting when you chop the throttle from full bore is also a sign of being too lean.

RE: engine tuning help?
 

ZDZ40RV of a newer kind, does this mean it's new or under warranty, then send it back.
Anyway I think the 2 problems have a common cause and it looks asif the fuel/mixture amount being supplied
to it isn't consistant with throttle setting wich might have 2 or 3 causes, 1) the carb, but that is unlikely because you already
replaced it and the problem aspect didn't change, or 2) ,the rotary valve or 3) always to consider
false air intake, so inspect the gaskets .

RE: engine tuning help?
 

How many flights are on the engine, is it possible it is inthe break-in stage? It may need more time before the needles can be set for fine tuning.</p>

RE: engine tuning help?
 

What kind of a throttle setup are you using?

While others have provided some useful information, gassers are extremely sensitive coming off idle. If you have a geometry issue with your throttle linkage or a poorly centering throttle servo, that can also cause the idle issues but I'm more inclined to think it's a low speed needle setting problem.

The other thing as mentioned, and I don't know how familar you are with gas engines, but once you get close on the needle settings moving them to their final position is more like you "think" about moving it... lol They are not like glow engines where you can move the needle a quarter turn or something.... ;)

RE: engine tuning help?
 

Thanks all for all the great advice, I went back and reset the needles to their stock positions, tighten up the throttle linkage, and the engine fired right up and ran fine. Ran it a second time, and it still does not idle right.
When I cut the throttle to an idle it runs at a fast idle for 5 to 10 seconds then drops to a dead slow idle. The engine just won't run between fast idle and a dead slow idle. I've adjusted the needles lean and richer with no luck.

Could this be an rotary valve problem?


Thanks again
Rick