Engine Stumble At WOT

[threewood]

First, this may be fuel or electric related or a combo of both. But the issue is related to temperature.

440, 10.4:1, solid lifter cam degreed, Holley SD intake, Edelbrock 800cfm Thunder, 3/8" fuel line from tank to carb, Edelbrock mechanical fuel pump, Pertronix Ignitor III, Flamethrower III coil, ballast bypassed. 21* initial, 34* total, vacuum advance hooked to manifold vacuum.

I have had a stumble when nailing the throttle. No issue with idle (900 rpms) or cruising. When I plant the pedal and get the down shift it will wind up for a second or two then stumble. If I get out of it and back in it will go a little higher rpm but the stumble is still there. If I stay in it sounds like it may be pinging. Pump shot is good and off idle response is crisp.

Thought it could be due to crappy stock pump so I upgraded to the Edelbrock mech which is made for this carb. Plus I rerouted the fuel line away from the headers. Also installed a 1/2" phenolic carb spacer. Still stumbled. Remounted the coil to the inner fender, same stumble. Checked all plug wires, no arcing at idle.

It does this during the day, 104* and sunny. When I drive the same route at night, no sun, 90* it runs like a raped ape. No stumble and I can stay into it without issue.

Where can I start checking? Can pump gas be sensitive to temp without vapor locking? Never had an issue with hot restart. Bad coil is my best guess so far but I need to call and get specs for testing it.

It feels like it could be fuel related. If the ignition cuts out under load, fuel still pumps and when the ignition starts up again wouldn't this cause some popping as the extra fuel lights off? I am half tempted to run an electric pump at the tank through a regulator to see if it helps. Only corn gas available here.

I might try and swap in a points distributor, stock coil and ballast resistor to see if it fixes it. Everything is new which is frustrating to diagnose.

 

[747mopar]

What your describing doesn't sound like a fuel/heat issue, sounds more like timing. If you have a large fuel pressure gauge you can tape it to your window and plumb it into your line. I suspected a weak pump and doing this proved I was right. Try advancing it a bit and see what happens. My engine likes 30 initial and 40 advanced, not so much as a hiccup ever.

 

[threewood]

What your describing doesn't sound like a fuel/heat issue, sounds more like timing. If you have a large fuel pressure gauge you can tape it to your window and plumb it into your line. I suspected a weak pump and doing this proved I was right. Try advancing it a bit and see what happens. My engine likes 30 initial and 40 advanced, not so much as a hiccup ever.

But would a timing issue be temperature dependent? It doesn't stumble when it is cooler outside. Also, with the heat and 91 octane I would be hesitant going more total with my compression. Starter does not kick at 21*, plus it registers 12" vacuum.

I will try to test fuel pressure.

 

[oldbee]

Put a pusher pump at tank like ur thinking sue be my guess.

 

[threewood]

Update. I hooked up a fuel pressure gauge and drove it to work in about 103 degree temps. At idle in gear, 5.5 to 5 psi. At cruise, 4.5 to 5 psi. At moderate acceleration it was dipping down to 3 psi. At wot it dipped down to 0 psi. If I feathered it, the psi would creap back up. So it seems as if my mech pump is not keeping up.

 

[threewood]

Drove it tonight in much cooler weather. At cruise, the needle was steady at 6psi. At wot it would drop no lower than 3psi which explains why it runs better at night. Definately fuel related.

I have a Holley black fuel pump and regulator ordered so I can hopefully get the car 100% in all weather.