Vapor locking troubles

[sparkydave]

Hello all, I've been trying to track down a problem I've been having with vapor locking, and I'm kind of stuck. I never had this problem last year, but it seems very determined this year. Basically, it can't go very far before it decides to make an unplanned stop, and today I managed to get it to vapor lock within 10 minutes (!) of a cold start in my garage just by running it at 2500 RPM. Today I figured out that I've got no fuel up to the fuel pump inlet when it decides to happen. If I take the cover off the pump there is liquid gas inside it, but not enough to pump to the carb, and likewise the line up to the carb is dry as a bone. I was starting to suspect maybe since I had repaired the vent line to the vapor canister since last year that maybe the tank was building up a vacuum, but after checking at the vent line with a vacuum pump it seems that the check valve in the gas cap is doing its job. The fuel pump is only 2 years old. For that matter, today's experience with vapor locking was with the gas cap removed, so I'm ruling out that the gas tank is building up a vacuum. I'm starting to wonder if either the fuel pump isn't doing its job as well as it should, or I've got a pinhole air leak in the fuel line, but does anyone have any thoughts on how to prove either case? I'm reluctant to go add an electric pump to supplement the mechanical one.
-Dave

 

[Dave Russell]

A pin hole air leak on the suction side of the pump could certainly cause such a problem. It should be possible to temporarily replace the line section with a rubber hose & clamps. Air leaks anywhere on the suction side could cause problems. Check or bypass all fittings & connections. Occasionally there is a leak in the fuel pickup tube inside the tank which would show up anytime the tank was not fully filled. Not likely though.

Another check would be to see if the pump's valves hold a vacuum when it's applied to the intake side.
D