I was trying to tune my Poulan 25cc with XYZ electronic ignition yesterday. The engine is easy to start and responds well to throttle. With my Tower Hobbies tach and using a APC 16x8 prop, I recorded an idle of 2600 rpm and a top end of 7100 rpm. No matter what I did, I could not get it to idle any lower reliably. Is that idle the best I should expect from this engine? It seems fairly low when listening to it by ear. The top end is reall singing and is more than enough to power my Dynaflite SE5 so I am happy with the top end. I just have to get a low enough idle to be able to get the airplane to slow enough to land.

Thanks
Jim

Almost forgot, I am using a walbro WT 324 carb and I did open up the intake manifold a little. Other than opening up the muffler by removing the baffles and drilling two 1/2 inch holes in it there are no other mods.

can you supply a few pics on the mods that you did ??????

YOu have a vac leek some were. check it over good to make sure that you have it all sealed up.

Paul

No vac leak,you will only get better idle w/bigger prop,or a prop that loads more,good luck

Is your carb closing all the way??. Remove it and hold it up to a light, any light you see is an airleak and will prevent the engine from a slow idle.

Throttle will close all the way. I use it sometimes to kill the engine with my transmitter.

When I say it will not idle low enough, I mean that when I try to lower the idle, it will die. Or that it dies when come off throttle.

Jim

How's the ignition timing? Advanced timing can cause idle problems. It should be at 28 degrees. An ignition that retards the spark at starting/idle speeds gives the nicest idle. I don't know if the ignition you are using has advance/retard or not.

AV8TOR

I tried retarding the timing as I was tuning. I retarded back until it started knocking at idle but still would not idle any lower. I was just begining to wonder if this was as good as it got?

Jim

I can never seem to achieve the "1500 rpms" numbers they brag about in the magazines, but 1800 to 2200 is more the norm for me. 2600 seems a bit high. If you moved the timing until it knocked, I think you were advancing it instead of retarding... To retard, move the sensor in the same direction the prop moves. If you have a moveable magnet on a collar, then move it opposite the way the prop moves to retard.


AV8TOR

I was moving the sensor in the direction the prop turns, retarding the timing. Now that I think back, it might not have been a knock but maybe a rattle. Could have been coming from something other than the engine. When I was testing, I was pretty tired from spending all day finishing up on my Midwest Texan and tuning the quadra 35 in it.

Maybe I should give changing the timing one more time.

Jim

Another thing that will help the idle is a bigger and/or heavier prop. The APC props are heavy but efficient.

This all assumes that you have idealized the idle fuel mixture. Often times the idle mixture needs to be richer than ideal for a good throttle transition. (Walbro even mentions this in their literature.) What I often do is program some exponential into my throttle channel in the radio so that the throttle doesn't slam open so fast. Moving the throttle stick on the transmitter slower works as well. Then you can lean the idle to a better mixture which will also permit a lower idle speed.

AV8TOR

I can understand that. If I come down to idle slower, it will idle lower instead of dieing. So what you said makes sense. I have the timing set on it now probably a little less than 28 degrees BTC. It seems pretty low by ear anyway. May be an inaccuracy in the Tower tach that I have also. I think I will leave alone for now and fly to see how it does.

Thanks
Jim

It will probably slow down to approach and landing speed just fine being a biplane. If not, go to a larger diameter, lower pitch prop. That will help "brake" the airplane on approach.

Have fun, and let us know how it goes.

AV8TOR