carb help

[buddha823]

my 72 has a built 383 (I bought already built) I was using a holley 750 that came with my truck and it ran alright. I cleaned that carb and put in new gaskets. but a bunch of corrosion was already damaged the wall of the carb... so I bought a new edelbrock performer 750 and truck runs 95% better but still sputters a bit when rolling on gas but not when I floor it. any ideas? I don't really know how to tune carbs....

motor is bored .60 over has aftermarket cranck rods and pistons...AFR heads port polished whole 9.... I don't have the exact specs for everything in front of me on cam (520/580 I think) size and port size cc's all that...

please help I don't know much but am trying to learn..

[cool68]

looked at holley.com or youtube yet?

[71K20chevy]

Sounds like either an ignition problem or vacuum leak to me. That's what I'd check first..

[buddha823]

Originally Posted by 71K20chevy

Sounds like either an ignition problem or vacuum leak to me. That's what I'd check first..

ignition problem? I have a digital 6al an it doesn't seem to be flashing like a problem is happening.


I took the Holley off and YouTube doesn't seem to be help....


the vacuums I have hooked up are break booster and vac to trans... should there be more?

[Casaba]

Off idle bog... That can be a major pain! Do a google search for that term and you should find plenty of threads on the hotrodders forums

Also look here:
http://67-72chevytrucks.com/vboard/s...d.php?t=389084

[71K20chevy]

Originally Posted by buddha823

ignition problem? I have a digital 6al an it doesn't seem to be flashing like a problem is happening.


I took the Holley off and YouTube doesn't seem to be help....


the vacuums I have hooked up are break booster and vac to trans... should there be more?

No vac advance? In theory that would cause a bog or hesitation on part throttle acceleration, but it would have no effect on full throttle acceleration.

[buddha823]

Ok ill look into that

[davepl]

You absolutely need vacuum advance. Sounds like you need a bigger accelerator pump shot to me, though I've never had to change it on one. I'd fix the vacuum advance first and see where you stand.

Remember, the OEMs were insanely cheap. If a part wasn't needed, it didn't have it. So if a truck came with a part, you need it. That goes for vacuum advance, fan shrouds, thermacs, heat risers, chokes, manifold stoves, and all the other stuff people remove. Unless driveability isn't a concern.

[engineer_gregh]

Edelbrock's are really sensitive to fuel pressure. I'm running a 750 on my big block. It never idled correctly and sputtered sporadically under full acceleration. I realize those are not the same problems you have but are some what similar. I replaced the off the counter mechanical fuel with an Edelbrock fuel pump and saw an immediate improvement. Idles really well and accelerates with zero problems. I've read many posts on this forum about fuel pressure and Edelbrock carbs. Keep us posted on what you find that fixes your problem.

[C. M. Wolf]

Have you tried correctly adjusting the accelerator pump lever on the new carb? (It should be a warm engine to rule out the question of it being the choke).

It's generally the 1st thing overlooked when replacing carbs..

IHTH

Michael

P.S. ...And yeah, Edlebrocks tend to be a bit picky to fuel pressures. It's my understanding they tend to be jetted slightly higher than stock engines run well on. (Pardon my lack of a better detailed answer, I'm Holley fan mostly.)

[piecesparts]

How about something that you might not want to hear. You might be trying to overfeed your engine. The thought process, that many carb designers will tell you, is that you have to have the right amount of air flow through the venturis to make a carb work right. If you are trying to run a 350 or 383 engine in a truck and you are driving it on the street, that 750 CFM carb will react different in that heavy truck compared to a lighter vehicle, like a Camaro. You have to keep the air flow up to get the carb to work right. A slightly smaller carb will serve you better than a larger unit in daily driving, mostly due to the air signal that it is getting from the amount of flow not volume of air that is going through the carb. A 750 works really well at higher RPMs, but a 650 will provide more drivability in a heavy bodied truck. All of this will also depend on what cam you have in that 383. The more that you have for cam capability, the more response you will need on acceleration from a stopped condition. Then again if your cam is to much, you may be already in the transition area of the carb, just to keep the engine from killing itself at a stop light. (Setting the idle too high to keep it from bogging down) That is a totally different thing to deal with and it will be looking at a different stall speed on a torque converter. I gave up my 750 carb and settled on a 650 carb to get my 383 to work smoother. I also installed a 2400 stall converter and now things work great. This did limit my highspeed capability, but enhanced my take off and the application of torque at a stop.

If you are driving at high RPMs, then a 750m will be more efficient, but the 650 is a smarter selection for daily driving. You can ask techs at Holley, Edelbrock, or any of the other manufacturers and they will tell you about the same thing. Been there, done that.

[Daman]

All Edelbrocks are easy to adjust the accelerator pump. It also has three settings on all of them. Just pull the "S" hook and put it in the next hole. Start with the top hole