Does your engine actually stop running? Or is this just a re-start problem? In
either case, it sounds like it's probably heat related.
Vapor lock is a good possibility, although there are a number of other
problems that tend to mimic its behavior. High heat can often cause other
things to fail or malfunction. I'm not saying that your problem isn't vapor
lock, but there are a number of possibilities.
For example, percolation in the carburetor could be a culprit. When you shut
off an engine, certain parts of the engine will actually start to heat up
before eventually cooling down. The carburetor is the best example of this.
When the engine is running, there is a lot of cooling air passing through the
carb, but when you shut off the engine, the heat from the manifold seeps into
the carburetor. If it gets hot enough, the fuel in the bowl can percolate or
flash into vapor, which can cause re-start problems. Similarly, the fuel pump
may heat up for a while after shutting off the engine, since there's no air
passing over it or fuel passing through it to cool it.
There is even the possibility of a heat-related ignition problem, though this
is less common.
Vapor lock typically causes an engine to stop if the fuel in the lines or pump
gets hot enough to flash into vapor, and a vapor bubble(s) 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 lines 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 stock fuel pump provides a little bit of insurance against vapor lock,
but no guarantee.
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,
which raises the temperature required to make it flash into vapor. An
auxiliary fuel pump installed back by the fuel tank would put the fuel under
positive pressure between the fuel tank and the fuel pump, which would tend to
reduce vapor lock considerably, but it should NOT be necessary unless
something else is causing the fuel to heat up too much somewhere on its way to
the fuel pump. To me, an easier and more elegant solution would be to cure the
temperature problem, rather than installing an auxiliary fuel pump.
Note: One possible problem with using an auxiliary fuel pump is that you can
potentially develop too much line pressure at the carburetor float inlet. Most
pumps are designed so that the pressure they develop is a function of
differential pressure between inlet and outlet. If fuel is already under
pressure at a pump's inlet, it will usually produce more pressure at the
outlet. If the pressure at the carb float valve rises to over 5 or 6 psi, I'd
say it's getting a bit high, which will affect the fuel level in the bowl to
some degree and might even cause flooding if the pressure is high enough that
the float can't fully seat the float valve.
It routinely gets over 100 degrees F. here in the summer, and I don't recall
having a vapor lock or percolation problem on any of the nearly 40 vehicles
I've owned. However, I've fixed the problem a number of times on other
peoples' vehicles. Most of the time it was related to fuel lines in close
proximity to exhaust components, a missing heat shield, an overheated engine,
hot engine compartment due to an engine conversion, etc.
Again, I think the best solution to vapor lock is to limit the fuel
temperature in the fuel lines and the temperature of the fuel pump. To that
end, here are a few thoughts on avoiding vapor lock (assuming that it IS vapor
lock):
1) Avoid using winter gas in the summer. Winter gas will encourage vapor lock
because winter formulations are more volatile. Use new gas from a reliable
source.
2) Make sure the fuel pump heat shield is in place (very important).
3) Make sure the fuel lines are not routed too close to any exhaust components
(very important). For that matter, if the fuel lines are too close to the
engine block, it could also contribute to the problem.
4) Buy some "firesleeve" material and slip it over the fuel lines. This is
fiberglass-and-silicone sheathing. You can get it from a number of sources,
including racing shops and mail-order outfits like Aircraft Spruce and
Specialty. Check their website and order a catalog.
http://www.aircraft-spruce.com/main.html
A couple of other heat-related things to check:
1) Make sure the thick fiber gasket is in place between the carb and the
intake manifold to help avoid percolation in the carburetor.
2) 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. Also make sure that the vent line is properly
connected and that it is clear.
2) You might want to replace the rubber fuel lines. This is more important on
the suction side of the fuel pump, since an old rubber line can delaminate and
collapse inside when under negative pressure, 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) Last, but not least, a sticky carburetor float valve could be a possibility
(this is a pretty COMMON problem, and it can be related to heat, especially
with older neoprene-tipped float needle valves.
Regards,
Alan
DARYL LEHMAN wrote:
> A short time ago I noticed some talk on "vapor lock" and a NAPA part that
> cures this. I believe my M-37 has this. The symptoms are as follows:
>
> Starts cold 100% no problems. Run it for a short time so it gets warm try
> to restart w/ or W/O any amount of choke it just grinds. Sometimes starts
> to turn over sometimes not. Let it sit with the proper amount of cursing
> and usually fires up the second or third time.
>
> If my diagnosis is wrong let me know. Thanks in advance for the advice
> and information.
>
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