Engine stumble/rough idle at light acceleration.

[Mikehess73184]

Hi everybody, looking for some help because I’m not sure what else to try. I have a 1970 f250 with a 390 manual transmission with T18. Engine was recently rebuilt and I put the engine together. Engine idles great in nuetral but as soon as I put tranny in 2nd gear and accelerate, it stumbles and is rough. Stumble will clear up when I put my foot in it.

Engine specs:
FE 390 with Manuel transmission T18
Petronix 3 distributor and coil
1406 eldelbrock carb with electronic choke
RV can not real aggressive
20hg of vacuum at idle
New plugs gapped at .045
Initial timing ive tried at 12 and 16.

Things I’ve tried:
Confirmed TDC with tool
Played with initial timing 12 and 16 degrees
Cleaned 1406 carb
New accelerator pump
Moved accelerator pump to top richest hole
Checked float level
Changed step up spring to silver/plain which is 8hg
Changed jets and rods to make it 8%richer. 12% was too much
Checked all over for vacuum leaks
Vacuum advance appears to be working
Plug color looks good
Adjust idle mix screws to highest vaccuum
Cap and rotor has less than 1000miles on it

Not sure what else there is to do. I’ve read just about every forum related to this issue and they all say the same things that I’ve already tried or replaced. Starting to think maybe not a carb issue? Possibly cam flat or valve clearances too tight? Valve covers coming off next. Any ideas would be appreciated. I am no expert and most my learning has been from these forums and YouTube

[sargentrs]

Try setting your timing to max out at 38* at 2500 rpm. Let the initial fall where it wants to. Leave vacuum advance connected while doing it. See if that accomplishes anything.

[1972hiboy]

sometimes when setting the idle mixture screws to highest vacuum I find it steers me into the lean relm. If I suspect this I grab a can brake clean and puff a little bit into the carb, If idle rpms increase, your still on the lean side. I repeat this until i reach the tipping point of when I spray a puff engine rpms drop. That gets you pretty darn close.

[Mikehess73184]

Thanks for the reply I’ll try adjusting timing and messing with idle mixture when I get off work tomorrow

[Mikehess73184]

So I’ve always set my total timing with vacuum advance line plugged. You’re saying set total with it connected?

[sargentrs]

Once you pass higher rpm's the vacuum advance is all in and doesn't do anything anyway so it doesn't accomplish anything to plug it off.

[greyscar]

The Idle screws are just Venting the Idle Circuit on the Edelbrock allowing more or less Gas to flow. If it's Stumbling at the hand off of Idle Circuit to acceleration it's probably not getting enough Squirt from the accelerator pump and momentarily running out of Gas. You moved to the Highest Hole, you may need to Step up size to a bigger Squirt. When you Put your Foot in it the airflow by the Ventures Take over. Accelerator pump is just a Momentary Squirt of gas to get you to the Ventures effect. I think the Factory pumps on Edelbrocks is 35cc and I think you can get a 50cc, hope this helps.