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Have a 383 and running a 750 cfm vacuum secondary carb.

Motor is sputtering at idle up to 45 miles an hour. Someone said it could be loading up then burning all the fuel at once and thats causing it to sputter. Just put a new MSD Distributor in it because it was not advancing.

Company I bought the engine from recommended a 750 cfm carb but I have been told it is too much carb for a 383 and I should be running a 600 cfm. Any suggestions or personal experience with this?

Also what should I be gapping my plugs to? 50?

Thanks
 

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Have a 383 and running a 750 cfm vacuum secondary carb.

Motor is sputtering at idle up to 45 miles an hour. Someone said it could be loading up then burning all the fuel at once and thats causing it to sputter. Just put a new MSD Distributor in it because it was not advancing.

Company I bought the engine from recommended a 750 cfm carb but I have been told it is too much carb for a 383 and I should be running a 600 cfm. Any suggestions or personal experience with this?

Also what should I be gapping my plugs to? 50?

Thanks
Its definitely not too big of a carb, I will be using a 770 on my stroker.
 

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I gap mine to .045 but if you feel the spark isn't strong enough to burn the fuel you may want a smaller gap, say .035-.040.

I'd agree, a 750 cfm carb shouldn't make your car run like that even if it was too big, which I don't know that it is.

Did you have the same problem with the previous distributor? This may seem obvious but I have to ask, is the timing set correctly?

What do the spark plugs look like, do they look like its burning rich?

How old is the carb, does it need a good cleaning?
 

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i've ran

750 VS carbs on 283's and they did fine. U should have no problem on a 383 at all.

How old is this carb , how long if any had it been sitting and not been used , describe your ignition timing set up and valve adjustment personal procedures

did you , set the floats , set idle mixture screws using a vacume gauge , make sure there is no vacume leaks , and , are you using a vacume advance and is it connected to ported or full manifold vacume and do u use a PCV valve ????
 

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So is it cutting out or bogging? If its bogging than you have too much fuel going in....but if its cutting out than you dont have enough fuel.....Most likey you have too much so try going down a few sizes on the primary jets or change the throttle cam to a less aggressive one....or maybe even put a smaller squirter in.

Also has it been tunes yet? You will need a vacuum gauge for this.
 

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Maybe some dirt in the needle and seat? Retarded timing? 750 isn't big at all on a 383. I'd gap the plugs about .035"--I've tried as high as .060" with some ignition boxes, but never found anything.
 

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The carb is not to big I have Quick Fuel 750 on and works great. Maybe a bad power valve or wrong one in it. I would say .035 gap on the plugs.
 

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I was always told to take the displacement of your motor and double it. Then use that number in carb cfm for best performance if your motor is a mild build.
If your running a lot stouter built of a motor, then go no more than 100 extra cfm.

383x2=766

So a 750-850 cfm should run make a naturally aspirated motor run the best.
At least that's the rule of thumb I've always used.
 

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Discussion Starter #10
The carb has seen two summers with very little use. The fuel filter never had any crud in it so I don't think anything had made it to the carb. Floats are set, idle mixture screws are correct, no vacuum leaks that I could find, no PCV valve, running a HEI dist for ignition. MSD dist 2 days old.

Not sure if you would call it cutting out or bogging. At a steady speed it will pulse. When I went to hit the gas moderately it about stalled then took up as if I punched it.

Plugs were real black but they have seem to cleaned themselves up and seems to not be running rich nor super lean.

Could it be anything else but the carb?
 

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If it ran for two summers with those jets, I'm not so sure that's the problem.

If you don't think its the carb, remember that new doesn't mean good, new stands for Never Ever Worked. It almost has to be either in the carb or the distributor, but I'd bank on the carb.

Do you think the secondaries are opening properly? How fast do you have to go for it to pulse, any steady speed?
 

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no PCV valve
Probably not going to cure your problem, but Holley's are generally calibrated to have a PCV in the system--I know one engine builder that insists on having one even on race engines. If not a PCV, what are you using? Breathers? Or?
 

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well an easy way to tell if its the carb is to borow one from a friend that you know works and that will tell you weather or not it is in the ignition system or he carb.
 

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Discussion Starter #15
I thought they were opening properly, only when the engine has a load on it. The car can be going anywhere from idle to 60 to pulse. At a constant speed or even on acceleration.

I am just using 2 breathers. I don't have an open port for a PVV valve as I just did a power disc break conversion.

I am trying to locate someone with a carb I could use, but not easy to do.
 

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Add a T to the line and run a PCV. How much fuel pressure? Still sounds like fuel getting past the needle and seat to me, I once had a Holley that would load up and kill plugs because of dirt holding the needle open, but it could not be seen; take out the needle and seat, blow it off, re-install and be fine for a few miles. I had 3 filters in the system and could not get this carb to work. Turned me off Holley's for a long time; the next Holley I tried worked great for many years.
Where is the timing set? I don't believe you ever answered that question.
 

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I can't get over the pulsing. I'm trying to picture in my head whether it's an ignition miss or not. Do you have headers? If so you may want to try warming the car up then drip a little water on each tube to see if one is running cooler than the others, this might point you to a misfire in a particular cylinder.
 

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Not saying this is your problem but less than a month ago my dad got a NEW distributor for his car. You know right above the pickup coil where there is two circular rings of magnets with triangular shaped teeth? The inner ring was ever so slightly cocked and one side had clearance with the outer ring while the other side would actually touch. The car ran, but it ran horribly and we chased carb problems for an entire day at the dragstrip before we got it home and figured out the problem. This is prime example of NEW standing for Never Ever Worked.
 

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How does the car idle? Does it pulse at idle too? If you look down the carb while it idles, do you see gas dribbling out of the boosters?
 

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Kinda O/T,but is ok to run your PCV valve to the intake vs the carb?? I'm running my friend's Stealth carb that has no PCV outlet right now so I'm being forced to run without a PCV. I know this has been discussed to death,but I still wanted to ask.
 
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