Gas tank vent/jetting

[Bart_]

I was just Sherlock Holmsing my friend's old Homelite. Cleaned the carb, new fuel line, fuel filter's good, impulse connections sound/sealed, carb kit. Tweaked the mixture screws all's good. On to the impulse pressure oiler. Remove some clogs, confirm rate valve, tight feed hose fit, but duckbill on impulse line seems stiff(er). Having no duckbills I spot the gas tank vent and say "comparison time". Gas vent seemed more springy rubbery not as plastic-ey, visually identical same PN's in IPL. Swap them. Oiler starts coming to life and jetting goes lean!

I know the metering diaphragm and it's lever spring are ridiculously weak, but has it been anyone's experience that tank venting actually materially affects their jetting?

 

[Reach]

Yes, if the tank does not vent properly it will run lean because the fuel is not flowing like it should. The carburetor is trying to pull a vacuum in the fuel tank.

 

[Bart_]

Agree with the reasoning. What gets me is the small differential of flow activation/onset delta pressure across two seemingly identical vent valves actually made a difference at the carb.

 

[Bart_]

I checked about 4 Stihl FSM's and all just said your tank vent "shouldn't build up vacuum" but none stated a performance spec or threshold of how much vacuum is too much. On a cooler day I read 2 psi to open a tank vent valve and inside the warm house I read 1 psi. The pliability of that valve is suspect.

Anyone have an actual vacuum tank venting spec in their FSM?

 

[Reach]

I checked about 4 Stihl FSM's and all just said your tank vent "shouldn't build up vacuum" but none stated a performance spec or threshold of how much vacuum is too much. On a cooler day I read 2 psi to open a tank vent valve and inside the warm house I read 1 psi. The pliability of that valve is suspect.

Anyone have an actual vacuum tank venting spec in their FSM?

I have never heard of a specific spec for vacuum, but it seems to me that it would be really low. Probably so low you could not measure it without high precision instruments, one psi seems like a lot of vacuum to me.