Hi all !
I have a ASP 80 fs 4stroke which runs(ran)like a clock. After a dead stick flight I have been unable to run this motor again. Lots of dismantling , cleaning setting of timing and valve clearances, still no joy.What I have discovered is that if I connect the exhaust pressure pipe to the crankcase vent, the motor will run.This is obviously bad practice and cannot be left like that. It appears to me that there is not enough exhaust pressure to force the fuel to the carb.
No blockages , kinked pipes etc
Please help if you have experienced something similar.
Thanks
Spyrat

Sounds like a leak in your fuel line or an air leak in your engine. 90% chance of it being a bad fuel line, probably cut or split where it joins the brass tubing in your tank. The backplate is the first thing I'd check if all the fuel lines are good.

As jester stated , start with the fuel tank and work your way forward. If you had any nose overs , the clunk might be stuck forward . Before you pull the tank , you might try holding the fuse nose up and try shaking the clunk down. Had this happen a few times to me,on different planes. But if the fuel lines have any age on them, I change them out anyway.

Thanks for your prompt reply, but have removed tank and checked clunk, and changed fuel tubing .No improvement.
I appreciate your input
Spyrat

Hi!
First of all...Have you mounted the tank according to "the tank mounting" rule????

Wild guess here - have you had a look at the sleeve and piston to make sure it's not worn out - maybe due to heat which made your engine quit?

The plane has flown many many times with the tank where it is , had the tank out to check on clunk but replaced in original position. Will not run on the bench either.<div>thanks Korps, that is my next move. I have already ordered the spares from the UK</div>

You could try putting a couple drops of oil down the glow plug hole and reinstall the glow plug. Turn it over, if the compressoin gets better , it's the ring and sleeve, if it doesn't, you may have a valve leaking.