>coil problems are one possible source ... they tend to have
>an 'edge' to them because the coil tends to be arcing or not.
>It's sort of a binary (off-on) type thing, although arcing CAN
>happen in such a way that it abruptly REDUCES the voltage
>rather than cutting it off completely, in which case your
>engine would lose power, but might not stall out completely
>until another more serious arc develops.
>Swap out the coil and see if it helps.
It is a definite possibility -- I'll get a coil to swap and give it a try.
I've also experienced weak and rough engine operation in more modern
vehicles when the distributor cap developed arc trails across the plastic. I
inspected the inside of the '51 M38's cap and it looks like new -- no sign of
arcing where it ain't supposed to be.
>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.
I wonder if surplus or NOS condersers are a good idea. Capacitors do not age
gracefully. Fresh is defintely better, as long as it is the right value and
fits mechanically
>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...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.
The engine compartment hasn't even had time to get hot when this problem
strikes. Hmmm...
>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.
While it was still running rough after a stumble and stall-fest, I did a test
where I connected a fuel hose stuck into a can of gas on the floor of the
garage to the intake of the fuel pump. Did not make a difference, so I
discounted the possibility of further problems with the fuel filter, tank,
and lines.
>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...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.
Don't know if the cap is vented or not, but I guess if I catch it in the act
I could just reach out and unscrew the cap and see what happens. I hadn't
thought about the fording vent line for the tank being clogged, but I think
my gas-can test rules both cap and vent out. But I guess it could be a
combination of factors...
>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).
The filter is in the tank itself -- I inspected it, but really couldn't tell
if it was passing fuel well or not. It is a cylinder of porous material thru
which the pickup tube sucks fuel. But the engine performed poorly on the
test can of gas, so I thought it was probably OK.
>Another possibility is a clogged carburetor fuel bowl vent
>that allows pressure to build up inside the float chamber.
I think the vent is clear. If I plug it temporarily, the engine gradually
gets rough. But, on the other hand, perhaps something is intermittantly
blocking it while driving. Now that I know what it is, I am gathering the
parts to reconnect it to the carb air intake tube as it should be.
>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.
Mine doesn't have one, nor does the TM show one. But maybe it needs one!
>Many exhaust manifolds also have a heat riser valve that
>diverts exhaust gas against or through part of the intake ...
>if the heat riser valve isstuck open under high-heat
>conditions, it can easily overheat the intake manifold, which
>in turn overheats the carburetor.
I suspect the M38 uses the temp sensing spring which works against a
counterweight. It flips freely by hand...but don't know if it is in the
right position or not.
>Another possibility, of course, is a marginal fuel pump.
I bought a suitable container today to hook up a gravity gas feed direct to
the carb. If I can catch it in the act, this test may eliminate the pump as
a suspect, as well as reconfirming the rest of the fuel system is OK. Or not
-- we'll see.
>Is your engine recently overhauled?
>I've seen almost the exact same symptoms on an
>aircraft engine with overly close valve guide clearance.
I don't think the engine has been overhauled recently -- the guy I bought it
from would surely have bragged about it when trying to close the deal!
>Hope that gives you a few things to check out.
Definitely -- thank you for all the good ideas and detailed explanations.
Lots of possibilities...
Steve Johnston
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"Jeepobsession" -- a new fragrance by Cavin Willys.
Smells like 90-wt gear oil leaking onto a garage floor.
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