[Highway Star]

Thanks for the insight Randy, but the car has had an HEI all along. It receives its power through an American Autowire classic update kit and seems to work well.

Idea that just dawned on me: Could my 1" four-hole phenolic spacer be the cause of the stumble?

Very likely not. There is only one way to find out. Let me tell you this though. I have a 4 hole 1" phenolic spacer on my 350...quick rundown...

~10:1 355
cam: 221/221 .470/.470 108 LSA
edelbrock rpm
holley 80457-2 65 primary jets, long black VS spring, stock secondary metering plate, stock squirter.
HEI 18° initial/38° total and 10° vac
headers
2000 stall/TH350

The 4 hole spacer made it more responsive.

The floats should be set so that fuel level is RIGHT below the sight holes...so that when you lean on the car, it just dribbles out. Are they set like this? Have you had it completely apart? Ethanol fuel leaves some awfully gummy reside inside a carb.

 

[rkd]

Have done some of this with that same and similar Holleys, in search of gas mileage.

Make sure the accel pump lever moves immediately with the throttle linkage. I am still on the stock orange cam, I think. Never had to change squirters, or have not yet anyway.

I set the idle mixture screws with a vacuum gage, then set curb idle.

Timing on my 350 needs to be 16 deg base. No vac advance at the moment.

In two of my escapades, going down 2 sizes on main jets, from 66 to 64 caused a mother of a lean bog and poping when trying to accelerate. Putting them back as before fixed it. A 350 with that cam probably wants 68s or bigger.

Got both of mine to drive nice for years. I also have gone with a lighter spring in one for the secondary, but that should come later.

 

[MadmanMark]

I would strongly suggest getting your timing set right before playing with the carb. Timing by ear/vacuum doesn't typically get you close. Get a light, make sure it's somewhere around 36 max when you rev it up (with the vacuum advance plugged). Let it idle down and see where the idle timing is, but the 1st thing is to make sure your max advance is right at WOT.

 

[matt2491]

Thanks guys. I'll put a timing light on the sucker today. :thumbsup: I'm getting a hunch my vacuum advance isn't working or something related to ignition.

Also, to be clear, this carb does NOT have adjustable floats! Its supposedly preset by Holley. There are two flush plugs where the adjustment screws would be.

 

[Dave69Z]

I have a 350 330hpo motor and on nthe dyno it only liked 32 degrees total.

 

[Highway Star]

I have a 350 330hpo motor and on nthe dyno it only liked 32 degrees total.

This only helps if you list out the combo. Does it have vortec heads by any chance? bet it does...

 

[Rokker]

Usually idle is best with 30-40 degrees of timing. With 20 degrees of timing in the distributor and no vacuum advance you might be seeing 40-50 advance above 2500rpm..

 

[Scotch]

You can give it a bit longer (fatter) pump squirt by adjusting the linkage to the accelerator pump. Keep giving it more fuel until the stumble goes away. Normal deal.

If this is the ONLY issue with the engine, then only the accelerator pump needs to be messed with. You've taken the time to get your timing and jetting right everywhere else through the rpm range- don't mess with them.

 

[matt2491]

Well I've never really addressed the vacuum advance tuning, so I might as well see whats going on with it. I just installed a new MSD Street Fire HEI with the adjustable vacuum canister. How do I correctly tune it? I've heard that you are supposed to back it out until it stumbles or pings under light throttle, then turn it back in one turn... but I'm not exactly sure...

 

[Dave69Z]

Yes it does. Vortec heads, holley 650dp, edelbrock performer rpm intake, lunati voodoo 268 cam, Holley 110gph fuel pump, david unified small body hei conversion distributor with taylor 8mm wires. carb has 31 squirters and stock cams. Cant remember jets for sure(67/71?)but they we changed them from stock and it ran worse so i put them back to stock and went up on the power valve to a 8.5. Found the secondaries were too far open and closed them back to holley specs.

 

[Bowtie-72]

I had a very similar problem (ok at normal throttle but at full pedal it had nothing) with a very similar engine except I was using a Holley 80508S carb which is adj floats but vac sec and no sec metering plate. After doing a lot of the same stuff as you and looking all over for a solution, I ended up actually converting to EFI. (Bear with me).... After the whole engine side conversion, I dropped the tank and found this:

after all that, my fuel sender was bent. Had I looked into that first, I would have saved a ton of dough and time.