Stihl 270 Air leak?

Fordsoll · Jul 13, 2023

I have a Stihl 270 Chainsaw I am working on for someone that is very annoying. I will tune it and get it to run enough to cut 4 pieces of wood and it seemed quite lean and the idle is really erratic it would be idling perfectly then rev up so the chain would move and turning in the idle speed screw would kill it. The carburetor is very clean with new diaphragms gaskets etc also have a new spark plug. I have tuned other chainsaws but maybe this one is different? The default settings are a quarter turn out on low and 3 quarters on high usually I start with atleast one turn and it should idle. I was thinking it could be an air leak because it has a plastic intake hose clamp and I cannot imagine that working well at all. And the compression is like 180 and the piston looks new.

Fordsoll · Jul 14, 2023

Also carburetor holds pressure well I am in the process of pressure testing it but i need to come up with something for the exhaust

Vintage Engine Repairs · Jul 15, 2023

I think you have come to your own diagnosis on this one, run a test and let us know.

Fordsoll · Jul 19, 2023

Its holding vacuum fine but not pressure. It is either leaking from the base gasket, clutch side seal or the intake boot I couldn't see well because it is dark outside and I had to dunk it in a bin of water to find the leak. Will hopefully try again tomorrow after work. I also found that the fuel line is ripped so that can probably also cause a lean condition now I know to pressure test the carb and then the fuel line while connected to the carb.

Lowhog · Jul 20, 2023

Fuel tank vent?

stihltech · Jul 21, 2023

Diagnosis from the outside in. Visual is very important. The idea is to look at everything as you are tearing down for the pressure and vacuum test. Ripped fuel hose is a good place to start.
Holds vacuum but not pressure? Most likely not seals but then again?
Sounds like a sealant failure cylinder to case.
Soapy water in a spray bottle is my go to. Let us know what you find.