popping back thru the carb???

[madmaxx57]

ok got a 470hp 383...howards roller cam, msd streetfire, air-gap..cars runs pretty dam good but started having a problem with it popping back thru the carb on a rev from idle..carb is a proform 750 dp..we put a 50 acc pump on it..no luck..jetted up too...my buddy is a really good carb guy and he's running out of tricks to try..sooooo we took a good known 850 put that on..same issue....the 750 under load pops at anything under 1800 the 850 pops under load at anything under 1600...i'm thinking this isnt a carb issue..timing is set 8 deg...this car runs hard all the way to 6k no problem...late last yr it did it to me once in the box and made for a bad burn out..it seems worse this yr..no other issues holds 17-22 in of vac. at idle..not sure where else to look..valves are adjusted correct....ignition???..plugs look good..ngk #7's..converter seems to stall about26-2800 on the foot brake..soooo what do you guys think this might be?..thanks for the help!
max

 

[83StreetMachine]

You said the valves are adjusted correctly but did you take the valve covers off and watch all the rockers/valves while turning the motor over by hand to make sure the valve train is moving smoothly on all cylinders??

How much timing advance is the motor showing right before the pop thru the carb? Throw a timing light on it and see, maybe its too much advance too soon...

Have you tried disconnecting the vacuum advance on the distributor and revving the motor to see if it still pops?

Have you also checked for vacuum leaks?

If all checks out there, I would change out the ignition module in the dizzy.

If you still have the same issue change out the entire distributor with another one.

Hope this helps! Please keep us updated with your progress!

Good Luck!
Ed

 

[bowtie81]

Do you have the right gear on the dizzy for running a roller stick?
X2 on the module aswell

 

[gutlesscutlass]

im no pro but ive heard a bent valve can cause this

 

[madmaxx57]

ok this is what i learned today!...didnt have much time...i pulled the vac advance off and plugged..and i have no pop but i have hesitation..hopefully tomm. i can put back on the 750 and see if i still have the hesitation..or better/worse..i did increase timing from 8 -11..that seemed to help a little...i'm thinking about swapping out the whole distr. just to see what will happen...the cam is a cast roller cam so why would i need anything special for a gear? no vac leaks...keep the ideas coming!..lol i will update as we go!..thanks all!!

 

[83StreetMachine]

The hesitation may be just because the motor is only seeing 8-11 degrees of advance and it wants more.

BUT before you pull that distributor have a buddy come over and help you at least visually inspect the valvetrain. You want to rule out a bent valve before you keep chasing your tail.

Pull the plugs out of the motor and remove both valve covers. One of you start turning the motor over by hand while the other looks at each valve on each cylinder to make sure every valve is opening and closing smoothly and fully.

Good luck!

 

[madmaxx57]

ok i've learned some more!!..put the 750dp back on..now with the distr. plugged..i had the pop again..but no flames!!....sooooo..my carb guy stoppped last night on his way back from the track..and fixed it in 5 min.!!! he increased the squirter size on the primary side from 38 to 48..wow what a diff!..now off idle has no hesitation under load or free rev..wow what a diff!!..the throttle response is incredable..even he was shocked at the diff....the reason we only are at 11 on initial is i cant consistently find 93...so running 91, 11 was considered safe?...i cant wait to get it to the track in a few weeks!..so that what i learned..i would like to thank bowtie, gutless cutless for helping..i'm no tuner by any means..but watching one that really knows his stuff is awsome... thanks again guys!