Re: Vapor lock - help please


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Posted by Alan Bowes on May 14, 1998 at 12:42:35:

In Reply to: Vapor lock - help please posted by Russ Cornelison on May 13, 1998 at 01:27:01:

Hi Russ,

Here are a few thoughts on the vapor lock issue.

Assuming that it IS vapor lock:

1) If you're using winter gas in the summer, it can make a difference, because winter formulations are more volatile. Use new gas from a reliable source.
2) Make sure the fuel pump heat shield is in place.
3) Make sure the fuel lines are not routed too close to any exhaust components.
4) Buy some "firesleeve" material and slip it over the fuel lines. Available from Aircraft Spruce and Specialty. Check their website and order a catalog.
5) Make sure the thick fiber gasket is in place between the carb and the intake manifold to help avoid percolation in the carburetor.
6) For summer use, you might want to close down the manifold heat riser valve (under the carburetor) to help avoid percolation.

Here are some other things to consider:

1) Make sure the fuel tank filler cap is properly vented so that vacuum does not build up in the fuel tank.
2) Replace the rubber fuel lines. This is especially important on the suction side of the fuel pump, since an old rubber line can delaminate and collapse inside,
blocking the flow. This might be more likely when the rubber is warm and softer.
3) A weak fuel pump can be a factor. You might want to test its output, but BE CAREFUL not to burn your place down while testing it.
4) Debris in the fuel lines or carburetor can do some pretty strange things, and not always in a consistent manner.
5) A clogged fuel pickup in the fuel tank can be a factor.
6) A clogged or improper carburetor float bowl vent can cause some interesting problems.
7) A sticky carburetor float valve could be a possibility.

If the fuel pump output is adequate and the float valve is not sticking, then I'd say that vapor lock is a good possibility, and shielding the pump and lines would
probably help. The fuel generally remains quite cool until it passes close by exhaust components. Shielding the pump and lines is usually the best solution,
short of re-routing lines.

Vapor lock is actually a less common problem than most people think. It's just that high heat can often cause other things to fail or malfunction. I'm not saying
that it isn't vapor lock, but there are a number of possibilities.

Vapor lock typically causes an engine to stop if a vapor bubble makes its way into the fuel pump and renders it incapable of developing enough pressure to lift
the fuel in the line above it and push it into the carburetor (or reduces its pumping efficiency to the point that the carburetor bowl runs dry before the pump can
supply enough fuel). Or when there is a large enough bubble(s) in the fuel line before and/or after the fuel pump that the carburetor bowl runs out of fuel before
the pump is able to pump the bubble through the carb and start supplying fuel again.

A good strong fuel pump provides at least some insurance against vapor lock.

Vapor lock does not occur as frequently in the line between the fuel pump and the carburetor, since the fuel is typically under a few pounds of pressure, raising
the temperature required to make it flash into vapor.

Hope this is of some value to you.

Regards,

Alan




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