That stumble and then an immediate run fine condition screams fueling to me…slow flow fueling… You may have what I call a lazy float. One that has become stiff and remains near its top floating level…only allowing fuel to eek by slowly into the bowl.
Just a thought. I’d be hot on the trail of fueling…whether that be in the bowl, thru the filter, or out of the fuel pump… verify, verify, verify. Bet it lies in there somewhere. Know how to set up a “nurse tank” ? That would quickly eliminate from the pump back to tank… then you can go from pump to carb…
The condition you describe will not be electrical component heating either because you are well under the time needed for “heat soak” of those parts.
Fuel baby… fuel…that’s where I’d be … even more so with the nasty fuel we have today…collapsing rubber lines, gunking up, all sorts of fuel mayhem can be unleashed on a truck your age not prepp’d for it.
Engine always starts fine from a cold start and idles for an hour never showing any change or symptoms. The truck apparently has to be driven for the symptoms to appear.
The weird noise from the front of the engine — which seemed to occur coincident w/the adverse symptoms — isn’t related to the water pump as I had speculated prior. Instead the radiator fan blade is interfering with the top radiator hose slightly. It’s only heard after the engine warms up b/c the top radiator hose expands slightly then. Creates sort of a card-in-the-spokes sound. After correcting that blade/hose interference there’s no unexplained engine compartment noises.
Ignition timing is spot on at idle and advances smoothly as expected w/increasing engine rpm. Confirmed w/timing light.
My current theory – given the truck “has to be driven” relationship — is this problem is related to the transmission. Possibly a failing vacuum modulator.
Haven’t taken fan belt off b/c noise is now explained, see above.
Choke: The choke plate starts out nearly closed on a cold start and opens gradually to full open over the course of 5 minutes to full open. It’s full open after the engine has stalled.
Fuel pressure: Haven’t checked pressure, but verified fuel pump flow is correct while cranking, and the fuel level in the carb’s fuel bowl is correct immediately after a stall.
If the problem was fuel , what would explain always finding the correct fuel level in the carb’s fuel bowl, even immediately after the engine stalls? It seems like the only way it could be fuel is if there’s a problem likely a clog, in the carb itself. But then it would be hard to explain why it only happens when the engine is warm.
What would explain it? SLOW FLOW… Like a float that is partially stuck closed…at the top of its float level… Or debris in the lines caused by ethanol poisoning…slowing flow. A clogged filter, a lazy fuel pump…
Check… its so easy to check… just check it off the list brother. I understand how you can out think yourself with an issue like this…and that almost always begins by stating what you think it is NOT…and then not verifying that thing and moving onward to other items you do give leverage toward.
Every time I have said what it wasn’t and moved on…I eventually came back to (begrudgingly)…only to find I should’ve just verified its integrity from the get go. (This was when I was a younger troubleshooter)
Verify, Verify, Verify. Its the way things get fixed.
You know that theory I had about the transmission vacuum modulator? Well, you can scratch that idea off the list … lol … I disconnected the VM’s vacuum lines, drove the truck around the block, same problem, engine performed perfectly until it warmed up, then it stumbled and stalled. This experiment wasn’t a total loss though, I discovered if those vacuum lines aren’t delivering vacuum the transmission won’t shift out of first. That knowledge may come in handy someday.
I took your advice HB and moved on to testing a fuel-related theories. Since the fuel bowl has always full (i.e. at the spec’d level) when inspected after an idle stall, fuel supply problem to the carb seemed unlikely. However there is one part inside the carb that behaves differently when the truck is being driven vs idling. Power valve. The PV opens only during acceleration. So I removed the carb from the engine and popped the bottom tray off the PV port. It was full of fuel. No good. Not supposed to be any fuel in there at all. Sure enough the PV’s diaphragm had split and was leaking fuel directly into the intake manifold when the the accelerator pedal was pressed. I found the old carb rebuild kit parts and luckily I put the original PV in there. Replaced the PV w/the original, followed by a test drive, voila, problem solved! I’m giving you the check mark for the solution HB, good job
I rebuilt the carb 3 1/2 years ago and replaced the plugs at the same time. Truck is lightly used so not many miles on the current plugs.
Hey I will take that @George_San_Jose1 … even if the precise issue differed a bit. My aim was to get you thinking of fuel.
In a way that diaphragm breach would rob the normal vacuum from the regular circuit of the carb… and upon application of the accelerator…more vacuum would be created with no fuel accompanying it…
Just curious, are you calling the accelerator pump the power valve? I don’t recall any of the two barrel carburetors having a power valve. That was mostly on the Holley 4 barrels, but it has been a long time since I worked on one. Last one I had was a 78 Mercury Zepher Z7 with the 302.
No. The Autolite 2100 has both. I’m referring to the power valve. Sometimes this part is called the economy valve, same thing. For example Google “autolite 2100 diagram”.
