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Vapor lock/vapor pressure

To: mgs@autox.team.net
Subject: Vapor lock/vapor pressure
From: gc <litfuse@isp01.net>
Date: Thu, 27 Jun 2002 08:41:19 -0400
I've been wrestling with the problem with vapor lock since the weather
turned warm.  When my carbs get hot, I get about a quarter mile before I
get the putter-putter.  As far as I know, we don't have oxygenated fuel
here in South Bend, except for clearly labeled gasohol (which is ethanol,
not methanol, btw).  What I do suspect is that other low-boiling fuels
fuels such as butane get blended in to gasoline due to their cheapness.
Butane comes bubbling out at elevated temps just like CO2 out of warm club
soda.  When I disconnect (carefully!) my fuel line from the front carb off,
I get a percolating burping squirting fountain out of the fuel line, and I
know it's even hotter in the carbs.  Bad gas? partly.
I got the bright idea to test fuel pump pressure.  Got only 1 1/2 psi,
dead-headed.  (Grrr!  S.U pump only 4 years old!).  Ergo my question a few
days back about expected pressure.  (My modern car doesn't care: it's fuel
injection pump puts out 40 psi and recycles fuel back to the tank.  It
loves the gassy gas.)
Before I bite the bullet and buy yet another S.U, I'm installing an
aftermarket fuel pump that really does put out about 4 psi.  My manual says
1.8 psi, other listers say it should put out 3.5 psi. I'm thinking I need
at least 3.5 - 4 psi to deal with modern fuels.
At that pressure, I hope the gasoline stays liquid.  I don't know about
TC/TD fuel pumps, but I suggest checking pressure at the carb and also
rigging up a crude flow restrictor like a pinchable hose to measure flow
vs. pressure on your pumps to determine if a restriction is in your line, too.
I also tore apart my S.U to see if there were obvious problems with valve
seating.  Nothing obvious.  Any suggestions?
Gary Cook

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