Erratic idle, temps spike for no apparent reason, runs hot even though it's tuned rich, flames out for no apparent reason... Unfortunatly, a lot of the signs of an airleak are also common with an engine that is worn out.
Airleak locations other than the engine directly:
Header at the engine.
Header at the coupler to the exhaust pipe.
Hole in either fuel line or pressure line.
Crack/leaky seal on fuel tank.
Airleaks on the engine:
Loose glowplug.
Carb neck where the carb slides into the engine.
Carb neck sinch bolt.
Carb fuel intake (banjo fitting).
Carb LSN seal, HSN seal.
Cracked carb housing anywhere.
Cracked crank case anywhere.
Backplate where it meets the crank case.
Backplate bushing worn for the pull start shaft creating a sloppy fit.
Front main bearing.
Where the cooling head meets the crank case.
To figure out engine air leaks:
Buy some canned air or wd-40 if you don't mind a mess. Start it up, get it off the ground and running at a fairly high idle after running it long enough to warm it up to "normal" operating temp. If there's an air leak, the idle/RPM of the engine will go up/down significantly as you spray canned air around these spots:
Around the carb neck area.
Around the carb sinch bolt.
Around the base of the engine head.
Around the perimiter of where the backplate meets the crank case.
Behind the flywheel at the front main bearing.
Normally, if I suspect an air leak, the first thing I do is tear the engine completely down, clean all parts with DA (denatured alchohol). Completely disassemble the carb (take out all needles and any parts that come off/out), clean all parts with DA.
Then I inspect all o-rings on the carb needles for rough spots.
I check the crank case for any cracks near the mounting points and carb area.
I check the pull starter shaft in the backplate to see how much play there is. Normally there's a little and it doesn't harm much.
I lube the o-rings of the carb with
associated green slime before re-installing the old ones or installing new ones.
I place a light film of
RTV High temp silicone around the perimiter of where the backplate meets the crank case. I also put a light coat on the backplate itself where it meets the crank case before putting it together. Make sure to tighten the plate bolts in an x-pattern (top right, bottom left, bottom right, top left). This helps prevent warping.
I also place a light coat of
RTV High temp silicone around the carb neck where it goes into the engine and around the sinch bolt (after taking it out of the crank case completely) before putting the carb back in. Make sure to put the sinch bolt in first or depending on the engine, you may not be able to get it in after the carb is installed. Push down with some mild force on the carb so it seats well as you tighten the sinch bolt.
Then I install the engine and replace the fuel lines with new.
Sorry for the short novel...
