i have a mildly built 355 aluminum heads, edelbrock rpm intake, msd ignition, headers, mild cam, 6'' rod, hei dizzy...... i bought the summit 750 dp mechanical secondary carb and it runs horrable... it smokes, smells of raw fuel to the point my eyes are on fire, has no responce, and slower than the edelbrock 750 i had on there prior.


Is it too big and im sure this questions has been asked a million times im sorry for making it a million and one.

i have an air fuel gauge hooked up and it about peggs out lean when i stomp it and it about dies before picking its lazy *** back up. i tried messing with it and im about convinced i bought a carb way too big.

Even driving down the road at cruise speeds it would break up at light throtle... please someone help i really dont want to go back to a 650 to be disoppointed again! Thanks alot- chris

 

For a mild cam you might be more happy with a 650

 

Have you made any attempt to tune it? Or stuck float valve maybe? Vac leaks? A 750 would be large for a mild 350, but tuned right it should run fine.

 

If you haven't consulted the Holley tuning sticky at the top of the page, you might consider it.

JamesC

 

750 is big, but should work just fine. The lean spike when you punch it is the pump cam/pump arm/shooters. Sounds like they need some adjusting. The horrible idle sounds like your ignition advance (at idle) is way too low causing you to open the throttle blades way too far just to maintain an idle.

Tell us about the timing curve, what shooters are in the carb, what pump cam and what is your idle vacuum?

 

Just curious what is your base timing and max timing? Also are you using vac advance?

 

base timing is at 12 full advance is 36 @ 3,000 im running a standard HEI dizzy with accell supercoil. as far as squirters im not sure at the moment, had an older gentelmen tell me today than the squirters are too small causing me to have a lean condition under a hard throtle....... he said i prob have 25's in mine and recomends 31's. so now im headed to summit.com where the carb is from to order some 31's.


vac at idle is 12 max @1050rpm
im going to move my base timing at 16-18 and still 36 total @3,000 rpm stand by for updates..... hope to get my camaro fast once again!

 

You might want to look at the carb before you buy. Summit's website doesn't say what the primary squirters are, but it says the secondary squirters are .031.

It also indicates it has a secondary power valve. Hard to imagine why you would need that. You might want to plug the secondary PV and jet up the secondaries 6-8 jet numbers.

 

i have thought about that, i called summit tech and they said the primarys are .40's and secondarys are .31's like you said above.... to me .40 in primary is WAY too big. im going to down jet the primarys and see what happens. if that dose not clear it up im going to put a powervalve block off in the secondary side of the carb. just wanting to get this car right to run it this weekend! thanks alot for the help everyone i will keep post alive and let everyone know what i find to fix it! also both power valves are 6.5's currently fyi

 

12 initial is good (16 to 18 is better)...without vacuum advance. Once the vacuum advance is hooked up, you should be somewhere in the 45 to 50 advance range. Is that close to your setup? If it is, you should have more than 12" of vacuum at a 1000RPM idle with a mild cam. It should be closer to 20".

 

i have a lock out hei dizzy and im just set at 36 @3,000 sorry for that confusion.
I changed my primarys from a .40 to .31's and the secondaries from .31 to .37 it runs MUCH better and stronger but still have a small burp off the line, im told now from a friend i should up the jets in the primary side of the carb and that should clear up my issue.

 

im ordering the whole jet assortment from summit so this weekend when it gets here ill go inside the carb again and see if that wont clear up the hesitation im feeling now

 

Upping the jets isn't the way to fix a small "burp at the line". This really sounds like a pump shot issue. Make sure the actuator arm is lightly against the pump arm. Also, make sure that when your linkage moves, it is actually moving the pump arm. I had a problem with an older Holley once where the spring was worn out. When the linkage moved, instead of moving the pump arm, it was compressing the spring instead.

Also, locked out timing is fine for the strip, but not so great for driving around town and idling. You really want more than 36 degrees for those situations. That's where vacuum advance comes into play.

 

pancherj, i hear what your saying and ill look into it this weekend when i take the car out again, as far as my timing yeah i know vac advance would be more practical for the street but ive never had any of the issues im having now until i switched carbs... now mind you when i switched my squirters all around it made a MASSIVE difference but like i said i still have a little burp that ill have to try and work out, jets are ordered so once there in ill play around a little more!

 

Well just an update for everyone,
after buying the whole jet kit from holley, new powervalves, primary and secondary squirters i finally gave up on it and spent the money and bout a holley dual feed single pumper 670 street avenger carb.

I am happy with its AMAZING response off the peddle and drive ability and the no more "bog" as i described before, only issues im having now is its not as powerfull as the 750 was. would it be a good idea to go up in primary and secondary jets to make it what it was with the 750??? Again the car runs fantastic now its just a little slower than before.

 

At steady cruise if you feel it surge slightly its on the lean side as far as jetting

You can even check your AF ratio in the driveway at say 2200 2500 etc

May be a little light on fuel youll have to play with it.
How do your plugs look?
What does the float level look like?
Nothing is perfect out of the box they usually need a little tweak here and there.

Absolutely. $5 part woulda cured that carb.

 

As far as a surge goes it may have an ever so slight one at crusie speed and that makes me belive i may have to step it up a bit jet wise, the vac secondaries seem to open too soon under med throttle ( feels like its on nitrous when they open lol no joke)

I have not checked fuel level yet but i will once the weather cools down (its 101 right now it NC)
And have not pulled plugs to check yet either
Idleing it gear is a bit rough it wants to stall out but in park its fine (750 did the same thing... any ideas there?) thanks for the feedback!

 

i just this to help me fiddle with my carb
http://members.tccoa.com/392bird/tuning.htm

 

No matter what carb you stick on it, or how much you want to get out of doing it, you're going to have to dial it in.

With this new Avenger, it sounds like you now need to play with the sec diaphragm spring. At least it's quick-change.

 

I bought the same summit 750 cfm carb (electric choke) for my 87 T/A and am having alot of problems with it. Bogs really bad, dies while test driving and my plugs look HORRIBLE! They are completely black and almost look burned. As well as alot of backfiring, almost lost my eyebrows the other day. :/

I may have chosen too big of a carb for my set up. I have a stock 350. The carb came with a summit intake manifold and open element air filter. But so far thats all ive done too it. Along with hedman longtube headers and 2 1/4 in straight pipe exaust.
Any advice on how i can fix this problem?
Thanks, Matt

 

That carb is big very big for a stock 350. It sounds like you have some timing issues as well check that first and call summit Try to return or exchange it

 

Awesome, thanks for the advice. Il do that first thing in the morning.
the reason i may be having timing issues is the fact that it has no timing gauge and no visible timing marks. I have to time it by ear. All it has is a line across the vibration damper on the crank.

 

Heres some decent advice about that Holley. You have 12 inches of vacuum at idle, so the power valve needs to be replaced with a 10.5. Put a 50CC squirter in the back. Summit sells a full holley kit that comes with power valves of different sizes, jets, new float pins, e.t.c 750 Double pump on a mild cam is a bad idea but you can still make it work if you lean it out. if you go 2 jets lower in the primaries, you have to go 1 size lower in the secondaries. You can follow that procedure or 1 to 1. Proform sells a real nice adjustable metering block(s) you can throw on there that you can tune the jets with the turn of a screw and I loved them. Holleys are great carbs in my opinion, they are just a pain in the *** to dial in. Go with a 750 vacuum secondary so you can have the carb for when you want to go bigger with the cam and or the heads.

 

Food for thought...a 650cfm dbl pumper can handle 550-625hp...so no matter what you do...a 750 is too big.

A 570 or 600 cfm carb would make it drive a thousand times better. Even a qjet would be great. But take my advice...the 750cfm is too much.

Ive run 10.20s with a 650 holley...and a qjet in the same week..with 600hp. If youre racing..run a super sucker carb spacer to axd some low end...but for street you need a smaller carb!!!