Ignition capacitors can also break down in the heat, giving you a much cooler, weaker
spark. A new capacitor is very cheap and it's good insurance.
Here are several other things to check out:Classic vapor lock is a very good possibility.
Vapor lock is nothing more than pockets of vaporized gasoline that become trapped in the
fuel pump. These bubbles prevent the pump from being able to develop the necessary
negative pressure on the tank side and the positive pressure needed to pump the fuel up
and into the carburetor. This vaporization is caused by some combination of heat, pressure
drop in the fuel lines, and fuel volatility. It typically happens on warmer days and in
slow speed or high load conditions when there is a lot of heat under the bonnet. In the
first place, the lower-than-atmospheric fuel pressure on the suction side of the pump
makes it easier for the liquid fuel to change to a gaseous state. It is even easier when
the fuel lines get hot. A further factor that could exacerbate the problem is a clogged
fuel pickup, filter, or other restriction in the fuel line between the tank and the fuel
pump, in which case the pump has to suck harder, creating even more vacuum and increasing
the liklihood of changing the liquid fuel to a gaseous state. Fuel tanks are typically
vented in such a way as to help prevent a vacuum in the tank and lines. If you have a
faulty cap or if a vent line is blocked, this can also build up a greater vacuum in the
fuel system. Yet another factor is gasoline volatility. Winter fuels are formulated with a
higher RVP (Reid Vapor Pressure) than summer fuels. A lower RVP is less volatile and less
likely to cause vapor lock. Some brands are less volatile than others. So, how do you fix
vapor lock? 1. Use a summer gasoline with an RVP of 7.0 or below. 2. Make sure the fuel
tank cap and vent(s) are in good order. 3. Make sure the fuel filter (if any) is clear
(especially if it happens to be on the suction side instead of between the pump and
carburetor). 4. Make sure the fuel lines, especially the rubber ones, are in good
condition. 5. Try wrapping the fuel lines with high-temperature insulating tape where they
pass near exhaust pipes or within the engine compartment. 6. Make sure the fuel lines do
not pass too closely to the header, tailpipe, or muffler. The fact that it runs OK again
after it's been sitting for a while is a typical symptom of vapor lock, since the fuel
vapor bubbles will change back to a liquid state when the fuel cools.
Another possibility is a clogged carburetor fuel bowl vent that allows pressure to build
up inside the float chamber.
Rubber fuel line components on the suction side of the fuel pump are a possibility. Old
rubber fuel hoses can delaminate internally. When they get warm (hence more flexible) it
is possible for the inner lining of the hose to be sucked together, restricting the flow,
kind of like a drinking straw collapses when you try to suck up a strawberry.
Another possibility is fuel percolation within the carburetor. I can't remember the
configuration of the M38 carb/manifold, but many carburetors have a spacer block between
the carb and manifold that is made of an insulating material. Some people leave these off
to help eliminate carb icing in cooler weather (68 degrees F or below...due to a
temperature drop of some 40 (give or take a bit) degrees through the venturi). Then,
particularly in the summer, the fuel literally can boil in the carburetor.
Many exhaust manifolds also have a heat riser valve that diverts exhaust gas against or
through part of the intake manifold to aid evaporation when the engine is cold and in
cooler weather. Sometimes these are manually settable, and sometimes they are operated by
a diaphram controlled by a temperature-sensitive ported vacuum switch...or by a flat coil
spring that unwinds with temperature changes. At any rate, if the heat riser valve is
stuck open under high-heat conditions, it can easily overheat the intake manifold, which
in turn overheats the carburetor.
Another possibility, of course, is a marginal fuel pump.
Some vehicles with fording kits had sealed gas caps on the tanks (not sure about yours),
and used a separate vent system for the tank. Any changes or problems with this system
could be a cause.
Is your engine recently overhauled? I've seen almost the exact same symptoms on an
aircraft engine with overly close valve guide clearance. When it got hot, the valves
started sticking and the engine would lose power and stall. The sides of the guides were
not reamed uniformly and when it was hot, the valves would come back down, but wouldn't
quite close all the way. One of them finally stuck hard and caused a rocker arm boss to
break right off. I seriously doubt that this is the problem, but it is at least a slight
possibility.
Hope that gives you a few things to check out.
Have fun,
Alan
(Salt Lake City, Utah)
> >After about 10 minutes of normal suburban driving the engine begins to lose
> >power, sputtering down to almost nothing. The throttle action seems "mushy",
> >not responsive. With a few pumps I can rev it back to life a few times, but
> >finally it stalls entirely. Then I can crank and restart the engine a few
> >times, make a little more progress down the road, then the cycle repeats.
> > Finally, the engine dies and refuses to restart till I leave it sit five
> >minutes or so. Then I can restart and nurse the sputtering engine home.
>
> You know, I have had several situations where every expert around was
> convinced I had a carb problem. It turned out to be the ignition coil. As
> they get used they warm up and any internal shorting seems to get worse.
> Coils only last so long, period! Just my opinion, your milage may vary.
>
> je
>
> ===
> To unsubscribe from the mil-veh mailing list, send the single word
> UNSUBSCRIBE in the body of a message to <mil-veh-request@skylee.com>.
===
To unsubscribe from the mil-veh mailing list, send the single word
UNSUBSCRIBE in the body of a message to <mil-veh-request@skylee.com>.