backfire through carb, need help
 

I am almost embarrassed to ask this, but i really am at my whits end here. Here is what i have, new 383 stroker motor, full roller, Edlebrock aluminium heads, new msd distributor, msd 6al box, Fitech fuel injection, initial timing set at 12 degrees and total at 34 degrees. motor starts right up and idles great, slow roll into throttle, it runs great, stab the throttle, and poof up through the carb, sometimes even killing the motor. i have adjusted the valves several times, check timing several times and still this issue exists.
Any help here would be great.

Re: backfire through carb, need help
 

Try more timing!
Bump it up to 16 initial.

Or have you tried increasing your accelerator pump setting?

Thanks, I did add some timing into it and it still had isssues, not tried the adding fuel on pump setting yet
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Re: backfire through carb, need help
 

Have sufficient fuel pressure for the injection?

It shows on the FCC that I do.
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Re: backfire through carb, need help
 

Typically lean surge is evidenced by a backfire out the carb while rich is out the exhaust.

On a higher mileage motor can be signs of a valve or seat issue. Or a valve misadjustment.

A/F go lean on throttle transition?

That is exactly what I was thinking. It appears the vacuum is staying steady, I will check to verify, and if I slowly roll into it it re a fine, but from a straight hard on throttle, it backfires through carb. This is the 2nd motor this has done this. I had a rebuilt 350 originally that did the exact same thing. Out of frustration and pure want fora 383 full roller, I changed it. Only things that remained the same are fitech with FCC and all new MSD ignition, including 6AL box.
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I did order a brand new Quick fuel 680 vacuum sec and electric choke just to in case I get to frustrated and want to check a normal carb. Can always resell one or the other
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Re: backfire through carb, need help
 

Update:
really frustrated here, here is the full background:
Started with new 350, mild flat tappet cam, Edelbrock RPM aluminium heads, RPM intake, complete new MSD Ignition - 2 wire Distributor, 6AL box, a blaster coil and Fitech Fuel Injection. With this set up, i was having backfire issue when you quickly hit the gas. At idle, it is great, or slowly hit the gas it is ok, but jam the throttle from an idel, boom. With this said, i decided after this that i wanted a good reason to get my ultimate motor, a 383 full roller stroker, so i build a 383 stroker, a Howards full roller mild cam, along with the above mentioned Aluminium heads, intake and MSD ignition and Fitech. With this setup i again had the backfire issue when i quickly hit the gas. At idle, it is great, slowly getting on the throttle it seems to be fine, but jam the throttle from an idle, boom up through the carb.
So out of alot of frustration, i adjusted the timing, from 10 - 16 btdc, i removed the Fitech and set a carb i know is good on it and same issue. i have checked and re-checked the spark plug wires, and i am now going to try to remove the MSD and try to install my old HEI and see what happens.
Im just so frustrated at this point. Any and all ideas are appreciated!!!!

Re: backfire through carb, need help
 

vacuum at idle is 12 to 13", steady, slowly get into the gas and it dips and then jumps just as it should.

Re: backfire through carb, need help
 

howards roller cam:
intake duration at .50 lift = 219
Exhaust duration at .50 lift = 225
intake lift = 510
exhaust lift = 525
lob separation = 110
hydraulic lifter roller cam good from 1500 - 5600 rpm

Re: backfire through carb, need help
 

I think I would measure the voltage to the distributor.
You may or may not know HEI needs a solid 12 volts to run best. If you used the original distributor power wire on the truck it won't do. It is a resistor wire dropping the volts down to 8 or 9 or so for the old points type ignition.
If supply voltage is good I'd switch back to the HEI distributor and give it a try. Just because the MSD unit was new doesn't mean it can't be bad.

I did measure the voltage. Here is where it's interesting. The truck is a complete frame off, all new wire harness, but still I tested the voltage and it was great. I talked with MSD and they said a minimum of 10 volts at both main power and switched which I have 12v at both, of course the one only when the key is on.
So, now I'm too the ignition. I disconnected the entire MSD, wires and all and now going to try that. I also have another set of good wires I'll try also.
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Re: backfire through carb, need help
 

So I am at that point where frustration takes over, so I stopped, took a minute to think and here is what I am doing.
I took off the Fitech and all of the MSD ignition components. I then did a compression check, here are the numbers:
1 - 180#s
3 - 195#s
5 - 192#s
7 - 190#s

2 - 180#s
4 - 190#s
6 - 192#s
8 - 180#s

The motor has about 30 min. on it. So these numbers may get a bit closer to each other, but otherwise they look good to me.
I was able to get an brand new MSD single wire HEI Distributor for cheap so I have that to install and an old Edelbrock 650 that I know is good. I will install these and try to develop a base line, get it running good then slowly replace/introduce the MSD 6AL box, and then the Fitech.