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.

 

[Bart_]

What makes this more interesting is that I saw a jetting change caused by difference of 2 or 3 psi vacuum vs guessing 1 psi and as your saw heats up (the gas tank too) it changes to net pressure. So you can imagine that carb jetting is not as precise as we might think. That's neglecting hot day/cold day air density too. FSM's all said carb test must be able to hold off about 0.8 bar (0.8 atm) inlet pressure so mfr's are certainly technically aware of this.

For scale, 1 atm is approx 15psi is 30" mercury so you can find any of the three scales on standard pressure/vac testers and 1 psi isn't by any stretch difficult to measure even on the cheapest gauge.

I found one aftermarket duckbill that specs 0.15 to 0.7 psi (presumably at room temp) fwd drop but no one else publishes a spec, none of the oems.

edit - watched a bunch of youtube vids for accidental info and sure enough spotted a few gauges on certain style vents that bled down to definitely the smaller part of 1 psi. There's likely been design changes to the valves through the years. 60's/70's duckbills may just suck to a certain degree. doh

edit 2 - jigged an old duckbill up to my bench pressure gauge inside the house and got .75 psi flow tapering to 0.25 psi at flow stop. Maybe not so bad (except for cold days). Hopefully new replacement duckbill valves perform even better.

 

[Bart_]

Tested the old duckbill back to back with one aftermarket, same geometry, probably different rubbers and clearly old vs new. Old (warm, bit of recent flexing) 3/4 psi FWD bleed down to 1/4 psi, new 1/4 psi FWD bleed down to 1/8 psi!! much better, just now to see if it survives gas/oil environment. And one other new one yet to come in for testing. I can accept 1/8 psi looking like zero on a small pressure vac dial without zoom in.

 

[Bart_]

Other style - shorter, looks like hot plier squeezed tubing vs taped to tip - scored even slightly better. Little less than 1/4 psi to start and decayed to 0.1 psi ! I've got a big absolute gauge and I actually saw a difference in barometric pressure between test days. Weather's changing.

 

[Bart_]

Plumbed up a crankcase pulse experiment today. First try duckbill filling an airtight about 750 ml brake bleeding jar - couldn't build any pressure. Sacrificed the long below-brake-fluid-surface tube to shorter (to keep the duckbill in open air) and decreased the air volume to about 200cc by filling 2/3 jar of water - still no pressure buildup. Filled the jar right near the top with water, duckbill submerged and voila - at idle the duckbill never opened but 3/4 wot and a stream of bubbles shot out and I could build 2 1/2 psig in the jar. It took quite a few seconds to build up. I surmise I didn't see the buildup before with a larger air volume because it was just happening so slowly with the tiny tapped crankcase air pulse flow. Granting 1/4 psi fwd drop, the crankcase was doing 2 3/4 psig net. I realise it's asymptotic pressure rise/target just like a cylinder compression tester, but these rock n roll numbers at least let me know what I'm dealing with.