Today i was at the field, and I started my .40 engine after two weeks ago having a crash. I check everything and it seemed to be OK . the first thing I noticed different was to prime it I had to put my finger over the carb.. Normally I put it over the exhaust pipe.

Then I notice the engine had some kind of run on knock, with the carb close the engine would continue to run for 10-15 seconds or so. I removed the top of the cylinder cleaned it as best as I can , and tried it again but it still had run on knock.

I removed the tank and re-seated the top and reinstalled it then flew the plane twice. When I landed the first time and restarted the engine it would cut off when I closed the carb. the second time I over shot the runway and hit some bushes. Not enough damage to call it a crash but I was very curious.

Can some one tell me what the devil is going on with this engine please ?

Yes, you will need an old preist and a young preist !

If it still is knocking then I would say you have a slightly bent connecting rod from the crash 2 weeks ago......and if you flew it that damage would probably be irreversable ,,it would probably just wear out a bit faster ,you may be able to get a new rod but it probably wont help if you've already flown it,, just go ahead and run it as long as you can.Rog

if putting your finger over the carburator and flipping the prop does not prime fuel than you have a BIG air leak between the carburator and the fuel tank. this includes the o ring that holds the carburator.

When I say run on knock I'm not talking about noise , I'm say the engine continuse to run although the carb. is fully close.

I just looked at the engine parts and I think I'm missing the Head Gasket.[:@].

never had to do this before . only over the exhaust pipe.

if the engine continues to run when the carb is closed that is mostdefinatly a bad seal at the carburator seat.

makes sense but what about the head casket, is this piece of rubber ? I dont see it if it is [].

some model engines do not seem to have them. it is usually in small motors like this, a small, super thin piece of plastic. contact the engine manufacturer and see if the engine is supposed to have one, (if you cant find it) and replace the carburator o-ring.

The head gasket will usually be made of copper. It's a thin ring about .010 thick. Without it, you'll raise compression, which will advance ignition timing, which will cause preignition, which will cause piston, rod and cylinder damage. Make sure it's there, and if it's not, get one.

cool this makes alot of sense because the engine was idling with the carb . closed and this was tripping me out. Thanks.

in largewr engines it is but i have never had a model engine who's head gasket was not that think plastic unless it is copper that appears and feels that way. the head gasket missing will not account for the idling with the carburator closed. in order to run you have to get air so if the carburator barrel is closed and it is still getting air, that it has to be behind the carburator. that's the carburator's seating. it neds a new o-ring.

Sounds like you definetly have air. I got a tip from Rossi. Take HI temp RTV, and spread it on the carb. Push Down while tighting the carb in place. Tighten all screws on the motor. There is on most motors a nut that hold the hi end needle in place on the adjustment. You can have air there and really not know it. Look close, and if you see as much as a drop of fuel tighten it as well. Be careful not to over tighten. Also the back plate is a area that can cause air. The tip from Rossi is a good one. Now I do this to all my engines. I have a 15 that has a crack on the motor wnere the carb goes. The RTV even stoped that from getting air.

Good Luck

Spaceclam is right.. Some engines do not have a head gasket,
at least my 46 la did not have one... Another right on air getting
into the carb keeping it running while the carb. is closed...