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I have a holly 750 dual feed carb on my 350sb chevy. the engine has 202 port heads a 525 lift cam and an hei ignition. Is the carburator to big because it wants to spit and sputter at low rpms. Th only thing i didn't change wa sthe ignition system. i have new plug wires and plugs. but the cap and roter is off of a 305. could the cap and roter be causeing the problem.
I hope somone can help!!!
Thanks, Rocky
 

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there are a lot of things to consider with this question. I ran a 750 double pumper on a 350 chevy with no problem, a high compression 4-bolt main 2.02 heads Vette engine, backed up by a M-22 trans to a 12 bolt rear with 4.88 gears. You need to know what your car is doing. If you're working with a rear-end with highway gears and a stock trans better off with a milder cam and a little less carb. If you're running gears in the rear you're engine is turning r.p.m.'s much quicker which means it needs to be fed much quicker. So in a sense the whole drivetrain affects how you want to set up you're engine, or you can use the old high school mentallity that bigger and more is always better. Good Luck
 

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To half answer your question without going off into outer space, no your 750 carb is not too big, considering the fact that you have 202's and a .525 lift cam. I have pretty much the same motor only my cam has a split profile (.468 intake and .494 exhaust)I'm running a 625 Demon, and I'm having the same problem. It was worse until I stepped up to a hotter plug, maybe need to move up one more step, also on one cylinder I can here a rocker arm tappin' just a little so I'm sure the valves need to be checked one more time. I assume your distributor is an HEI, the cap and rotor should not affect it just because it came from a 305, unless it's just a bad part. Your problem, as well as mine, could be caused by lotsa things, maybe Roy or one of the other guys will read this and jump in here and solve both our problems.
 

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First thing I woud do would be to replace that used distributer cap and rotor with new ones, then re-chec your timing to be sure it is right on, and while there check to see if vacumn advance is advancing it is possible that you have the vacumn line on the wrong port on the carb, then adjust your carb out properly and with that cam I would say you need to set the idle a little higher, I know I had to do that on mine. Try that and see what happens.
 

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I'm so glad both you and Jooooooe have the same engine. I must have missed something though, I couldn't see from what you wrote if you both have 8.5 to 1 pistons or was it 11.0 to 1 pistons. Ah what's the difference anyway?
 

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A 750 is not to much carb,but where is your timing set? With the cam you are running if its to slow it's going to be lazy and stumble and do other weird things. Providing your ignition system is good, plugs ,wires,cap and etc.,I would try bumping the timing a couple degrees to see if it helps. I've got an idea you will need 10=12 degrees initial. Just an idea and something cheap to try.
 

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I would replace that HEI with a good aftermarket Accell or Mallory ignition system. I ran a 750 on a 350, with a good ignition system nd had no problems. Also you might want to check the RPM range of your cam and intake, sometimes that will cause a low end dead spot. :D

[ February 25, 2002: Message edited by: wrenchtwister ]</p>
 

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Hello Rock, Read your problem and it does sound as though you are still using astock ype setup on the distributor (if not pardon me) if so itdoess ound like maybe you should consider reworking the unit or going aftermarket. Simple task get rid of the vacuum advance in favor of the centrifuga. Truly it sounds as though your set up is being heldback by spark timing issues. HEI is good for street and strip applications. Find some help info on tricking the stocker before you buy new. hope this helps !
 

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Sounds like everyone has a good idea such as timing, and converting to cetrifical advance bedause of the vacum loss caused by cams which is what you might check out is the power valve and the accalerator pump. I have ran 780 Holley's on a lot of engines but never had a lot of problems that couldn't be fine tuned out
 

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Lets keep this simple, First of all there is no good reason to change the HEI , even with your cam you should have enough vacumn to operate the vacumn advance. Secondly The HEI does have a centrifical advance in it also, so you have already got that. Replace that used distributer cap with a new one and check and readjust your timing. then readjust your carb set the idle a little bit higher than when using a stock cam. Look at your carb installation instructions and be sure that the vacumn line to the advance is connected to the right port or it will not work properly. You can check your advance for proper action with a timing light. Make your timing adutments with the vacumn advance disconnected and the hose plugged, then hook hose back up and check for advance action.

[ February 26, 2002: Message edited by: Henry Highrise ]</p>
 

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try putting a 50 cc pump on the secondaries and maybe a bigger squirter and jet on 'em also but start with the pump. i had a 750 double pumper on a 305. after i did these changes it ran great
 

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750 IS NOT TO BIG FOR YOUR ENGINE, AND THE HEI IN GOOD SHAPE WILL NOT CAUSE A PROBLEM ALTHOUGH I PERFER LOCKING OUT THE VACCUMUN ADVANCE AND RECURVE THE MECHAINCAL ADVANCE TO BE RUN 10 DEGRESS AT IDLE WITH ANOTHER 25 DEGRESS OF MECHANICAL TO BE ALL IN BY 2500 RPM. MY SON RAN THIS SET UP WITH A 750 ON HIS 67 CHEVELLE WITH NO PROBLEMS,RAN 13.50 E.T. AND WAS VERY STREETABLE. MOST OF THE TIME YOU HAVE BOUGE OR SPITTING AT LOW RPM IT IS DUE TO NOT ENOUGHT FUEL WHEN THROTTLE BLADES OPEN AND LET IN MORE AIR, TRY ADJUSTING ACCELLERATOR PUMPS TO GIVE A SHOT FUEL AS SOON AS THROTTLE STARTS TO OPEN. I RUN AN 850 MECHANICAL ON MY 377 CUBIC INCH 66 NOVA DRAG CAR AND PLAN AN GOING TO 950 WITH ALCOHOL SOON.
 

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just a little back up of what others have said,I believe your problem is either adjustments, float hight or jet size...that is if you have determined it is a fuel problem.You might start by reading your spark plugs and take it from there . any help I can give you are welcome to...most of my 35 years messing with hot rods has been drag related ...for whatever that might be worth !?
 

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Thanks for all the help, the simple answers are always the best ones. When you have rebuilt the whole car from front to rear, you get so much going on that you could actually forget to put gas in it before trying to crank it for the first time and end up commiting postal retaliation on your dog because you can't figure out why it won't start :mad: . After your dogs funeral you calm down and ask yourself, "did I put gas in the tank?.. Oopps. Just to stay outta trouble, I was only kiddin' about the dog, that really wouldn't be funny.
 

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Floats could be set to high. That would make it spit and run rough.
 
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