350 issues. . . . . . .

  HELP I have this 350 that has a problem when you hit the gas hard off idle. It has a vic jr intake, holley 750 vac sec. a mild cam and 2.02 heads. The rear gears are 4:11 also motor has headers and 3 in dual exhaust. I have good fuel pressure, timing is at 10 degrees and 32 total timing I believe, ignition is summit brand with the coil and rev box. When i hit the gas hard off idle it pops and dies. It will backfire out the carb. Ive had 3 different distributors and 3 different carbs on it all with no combo working out......      

You need to tune the carb. Have you tried a 50cc acc. pump? Have you aadjusted or changed the cam on the carb?  

yea ive tried adjusting it, and just put another 750 that i know is good off another car and it did the same thing. It fires up fine, idles good but when you hit the gas hard off idle it actually fires out the carb and dies? I just put a summit brand ignition in it with mechanical advance could that be it, its not advancing fast enough or too fast?  

Whats your timing set on? Did you adjust the pumpcam on the carb? What size squirters are you running? Have you tried changing the pumpcam? Most of the time that problem is in the carb.  

Idle RPM too low,when you stab it low vaccuum causes no fuel to move to the cylinders. Vic Jr. manifold doesn't help either. Go to 12degrees/36degrees for timing. Bring the RPM's up higher prior to launch. Either with a higher stall converter or letting the clutch out at higher RPM.  

Adjusting the pumpcam or changing,adding 50cc acc pump and changing squirters[if needed] will help with that problem.Whats the cam specs  

Im not sure on the cam specs as i bought the motor used with low miles. I havent messed with my carb cam but ive had 5 different carbs on it and they all so the same thing. Ive had everything from a q-jet, 650, 750 and 750dp....all act the same. Timing is set at 12 and 36 degrees idles at about 1000. it doesnt have that crazy of a cam and the thing pops and fires out the carb on the secondaries side when you stab it????????????  

Sounds like you need to tune your carb to the engine. I'm running a 750cfm vacuum secondary on my 406 thats pushing around 500hp and I dont have that problem. Why? I tuned my carb to the engine. You may need to pay someone to tune your carb for you.  

yep, you havent stated if you tuned the carb or not. you cant just throw a carb on a motor, set the idle and be done. it needs to be tuned for your specific motor and driving style... with a vacuum guage connected to a vacuum port at idle, it should pull 15-20 merc. vacuum. why is your car idled at 1,000 rpm? should be about 650-750 for auto and about 600 for standard trans... also, are you SURE its a 350? someone could have sold you a 283, 305, or 307, in which case a 750cfm is really too big, for a mildly 350 that would even be a little big. give us the numbers off the back driverside of the engine block behind the driverside head to make sure what you have.  

Its a 350, I ran the numbers before I bought it. Idle was at around 700 but i bumped it up just so it would run half ass decent off the line. I never tuned the carb, but have had 5 different carbs on it and they all act the same? Id think at least one would act a lil better even without tuning? They all fire out the carb and then it dies. Once it gets past that point it runs pretty hard, and it seems to start and idle good?  

Your not listening. I'm done with this thread  

Hi there,
   You have tried 5 carbs without success???
   Have you got a vacuum advance on your dizzy??
   What fuel pressure are you running at 1000 & 2000 rpm??
   What timings are you running??  This is a real hit and
   miss project. Have you Ever had this motor running sweet
   at any time?? Be systematic and jump one hurdle at a time
   These guys are trying to help and would like to see you
   get it right without a great lot of drama..All the best
   from Skyeking....  

tuning the carb is a very important part of the process. You will never get it right if you dont tune the carb. 5 carbs in hopes that one will work right without touching it. That is a big waste of resources.  

have someone tune the carb for you. its more than just throwing one on.  

I did have the car running sweet with the current carb, i changed the HEI to a Summing knockoff of MSD's AL ignition system. Distributor has mechanical advance. I guees i left out some info and sorry for that. I also swapped the 1.5 rockers for 1.6 rollers at the same time as the distributor. The care doesnt have as choppy of a idle which is weird, i thought the longer rockers would give the cam more lift. Honestly the car is acting like when I had the vacc advance line from HEI hooked to the wrong port on the carb. Now with this new mechanical advance dist would I change the spings? Timing is set to 12 and 36 total. I am listening and appreciate the help, should have gave all the info to start!