383 backfires

Here is the setup

383 rebuilt with 906 heads, ported some
comp cam- intake 504 lift / exhaust 514 lift
intake 274 duration at 50 and exhaust 282 at 50
9.5 to 1

new holley 750 vac. secondary carb
m1 intake
tri y headers
MSD 6al with pro billet dist.

timing has been tried from 8* BTDC -16* BTDC at base timing-

Full advance has been set from 30*-38*


starts fine idles nice. revs well- under any load- (foot on brake) give it some gas it backfires through the carb.

the car is gutless, will not burnout- even in water. what gives? what could be the issue?

Years ago I had a cam go flat in a 318. The first clue I got was it started popping through the carb as soon as I put any load on it. If you’ve verified timing and believe the carb is in working order, you might want to pull the intake and put your eyes on the cam lobes.

Good luck! 🍀

BTW: 274 & 282 duration at .050 would be ultra radical race only type of camshaft dimensions. I’m thinking yours would be seat to seat numbers.

i believe you are correct on the cam at .050... those might be advertised numbers.

was going off memory.

I have pulled the intake and the spun the motor over several times and all cam lobes look good, plus there is not metal in the oil.

thanks for the thoughts!

If it's not flat it's probably not center lined correctly. Or plug wires are crossed?

Well that’s good! Next thing that I’d look at is cap, rotor & plug wires. (That’s actually the first thing I should have thought of actually. Haha). Resistance in the secondary ignition would possibly cause misfires that may backfire through the carb. The coil could also be something to look at.

The first roller cam I installed was ground 8 degrees retarded, that rascal wouldn't fall out of a tree with tractor hook onto it
Did you degree the cam? If not do that next
If so did you degree it on the correct lobes
Please don't ask me how I know to do that now The first lobe on the drivers side on all Mopar BB wedge motor is the number one exhaust lobe, not the intake lobe
Gen 2 426 hemi motors are the opposite, first lobe on the drivers side is the number one intake lobe

I think you need more timing if your cam specs are correct. Try 20 degrees initial and 38 total.

Problem since day 1? started after a part or adjustment change? In addition I would do a compression test & for sure you need more initial. Holler how it goes.

i would check for vacuum leaks, double check timing and ignition wires etc.
then jet the carb up 6 steps, reset idle mix when it's warm and then see if it works better.

I have checked for vacuum leaks, did not find any.
Checked the firing order countless times. Replaced wires with new MSD wires,

Originally the Engine ran fine. Then the cam bolt loosened up and the cam gear slipped bending valves. Had the heads rebuilt, put in new cam from comp cam. Was degreed in with dual idler floating gear drive. Had smaller 650 vacuum secondary carb. Started this issue. Thought it was a lean issue, went to new bigger carb.

Still had the problem. Pulled the gear drive and installed a timing chain. Degreed it. The car starts right up, idles beautifully. Just gutless and then backfires.
Passes a leak down test.

A backfire through a carb is ignition while the intake valve is open. Correct? I verified by eyeball with the distributor in and intake off that there should be no ignition happening with intake open...watched cam lobes.

I wondered if it was a bad grind on the cam.

I just pulled the top end apart and will be installing new heads, 440 source stealth heads.
New gaskets everywhere too. Thinking I am this far in I should degree the cam again...

backfire through the carb could be timing issue.
could also be a lean backfire...

a larger carb isn't exactly a "richer" carb. there is metering in the carb that dictates fuel mix. that's why i would jet it up and see if the lean backfire goes away. it's a pretty quick way to rule something out.

years ago i rebuilt a holley 600, put it to baseline and when the motor was cold it was undriveable. backfire through the carb from a stop or even a roll, any throttle change basically. i ended up jetting it up 6 steps and it ran great. but the reason i needed to go so far from baseline was because it had a slight leak at the baseplate/body gasket. i figured it out by spraying carb cleaner on that gasket when it was idling.

I'm going to assume that since the OP has said he "degreed" the cam that it was done with a degree wheel and the person doing it knew what they were doing....... And it's not just a matter of verifying the dots are lines up.

I'd do a compression test.

How's the lifter preload?

Jetting in that carb?

Check for rotor to cap phasing.

Check the pick up coil wire polarity.
I've seen both msd distributors and control boxes that had the polarity reversed from what it was supposed to be.

Yes we degreed the cam in with a degree wheel. I think I knew what I was doing at the time. LOL.

Can't do a compression test now the heads are off. - But we did one a a year or so ago... the car has been sitting, thats when we did the leak down test too. comp was even in all cylinders.

Lifter preload felt good on the pushrods. was able to spin them by finger on base circle, but couldn't move them under full lift.

So if the MSD did have reversed polarity what would happen????

Jetting is whatever it came with.

While the heads are off, have the springs tested. I would think that bad springs allowing valves to bounce could cause what you’re describing.

If the polarity of the pick up coil wires are backwards it puts the rotor phasing way off, and it can fire to the wrong terminal.

It can run like what you're describing.

If its a non-digital box, then the proper way for the wires to go is whichever has the timing retarded(compared to when the wires are swapped).

I have a jumper wire I use on the dyno that has the male/female distributor connectors on it, with bullet connectors in the middle of the jumper so I can easily swap the polarity.
It's come in handy a few times.

That would be too funny if that was the issue.

the MSD came from one car and went into this car. it ran fine in the other, in fact the entire motor came from that same car- prior to the cam fiasco the motor ran very well in the other car.

but the MSD was wired in at a later point- it was supposed to be a temporary thing. LOL. you know how that goes... I will have to check that out.

Distributor - Blk/Org is +, Blk/Vio is -
MSD box - Violet is +, Green is -

I had these connections reversed when I put my car together. Idled fine, timing looked good but would pop & bang when I tried to make a pass.

I had a similar experience a lot of years ago.We took the engine going 11.20s right out of my car and put in in a friends gutted Duster hoping for a 10 second time slip. We used the engine as it was, including the dist and carb. We used the stock coil that was mounted in the Duster and didn't give it a second thought. Sounded good in the garage. Would not run a lick under load. Eventually put it back in my car and it ran just fine.

A friend had a similar problem and we spoke via phone back and forth on what to check. We got to the point of checking the timing chain to see if it jumped a tooth. Didn't hear back for a few days, he finally admitted that he rechecked the plug wires again and found two that were switched.

the engine is torn down to short block, checked cam timing. it is .5 * degrees off of cam card- should be 112* it is 111.5*

Cam is correct-

checked MSD wiring- that is correct

verified with 2 people of firing order multiple times- correct
****

Will now reassemble with new gaskets, heads and see how it runs. If I have the same issue i will replace the MSD and coil. Cant hurt to do plugs and cap and rotor first.

Is the polarity correct from the mag pickup in the distributor to the MSD box?

You verified this by swapping the two wires and saw that the way you have it has the timing retarded compared to when the wires are the other way?

Or you just know the colors are what they're supposed to be?

I've seen both the distributors and control boxes have the polarity of those wires be incorrect.

i just verified the MSD by colors. this box was in another car running normally. The distributor is another story. I have other one to try out once the motor is back together.