Holley carb on 250 cyl. adjustment question

[T and S]

I have a 390 cfm holley on a 250 straight 6 engine, carb is new. Engine is fresh rebuild - 9.1 compression, mild comp cam, offy intake, 292 exhaust manifold(2.5 outlet), HEI distributor.
I have trouble with a stumble - letting the clutch out and taking off, it takes off ok, stumbles, then takes off again. I have checked and set float level, mixture screws are at 1.5 turns out, good idle.
It came with .025 accelerator pump nozzles. Have tried .031's and .035's, and have .037's on the way. Still has the stumble, maybe not quite as bad.
Anybody running the 390, have you had this problem, and what nozzles did you end up with ??
Kind of getting frustrated, even thought of trying an edelbrock carb.
Have heard it mentioned this could also be an distributor advance problem - any comment ?? Any help would be greatly appreciated. Thanks in advance !

[geezer#99]

What is your timing curve like?
What’s initial set at?

[T and S]

to be honest, I haven't checked it. I usually borrow a timing light,(haven't needed one till now) but I am buying one tomorrow.........

[68gmsee]

A timing light and a vacuum gauge are two tools I've used many times on these old trucks.

[geezer#99]

Originally Posted by T and S

to be honest, I haven't checked it. I usually borrow a timing light,(haven't needed one till now) but I am buying one tomorrow.........

Put your initial at 12 degrees.
Make sure you plug off the vac advance when you set it.

[T and S]

Bought a timing light. Also got a hold of my engine builder, he is having me drive it to his shop Friday morning and is going to help me out with it. He may strap it to the roller dyno for test and tune, I hope ....... I will let you know how it goes.

[T and S]

Did some research. Found a really good article from Super Chevy about vacuum advance, how it works, and how to tune. About %99.99 sure this is my problem. The HEI distributor worked fine on a stock engine, but not so much on my modified motor. I for sure am no expert on this sort of thing, and I am sure my engine builder is - he builds race engines for a living. Going to his shop Friday ....... http://www.superchevy.com/how-to/eng...r-drivability/

[mattfranklin]

For me, as long as I can hear it if it knocks, I advance until I can hear a light knock at WOT and then back it off just a little.

[T and S]

Success!!!!!! Took truck to Baldwins (engine builder) this morning. I was only there a hour or so. He checked the timing and advance, was dead on, working fine (so much for my guess). He took it for a drive, right away he said it had a "lean miss". Changed power valve from a 5.5 to a 5.0, changed main jets from .051's to .065's. Wow what a difference, runs really good now. Case closed.

[68gmsee]

Originally Posted by T and S

Success!!!!!! Took truck to Baldwins (engine builder) this morning. I was only there a hour or so. He checked the timing and advance, was dead on, working fine (so much for my guess). He took it for a drive, right away he said it had a "lean miss". Changed power valve from a 5.5 to a 5.0, changed main jets from .051's to .065's. Wow what a difference, runs really good now. Case closed.

It's good you found someone with knowledge of Holleys. There aren't many carb people left out there.

I for one have never worked on Holleys or owned one so I looked up the power valve and found a good article on how to select the right one for a given engine. Might help someone with similar problem.

https://www.racingjunk.com/news/2015...ey-carburetor/