I have a 383 in a '71 Duster and I can't seem to figure out why it sometimes backfires through the carb during hard acceleration. The motor was in the car when I bought it so I don't much about the internals other than it had a "soft rebuild" so I'm assuming it's pretty much stock and was just freshened up. I added a stock 4bbl manifold, a 650 CFM Summit carb, Mopar Performance distributor with orange box. New plugs, wires, fuel pump and fuel pick-up. Have tried different timing settings and adjusted the fuel level in the carb with little improvement. Suggestions needed! ](*,)
383 woes
- Thread starter mneal
- Start date
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OldmanRick
Well-Known Member
If you're backfiring at low r's on the intial hit, it could be the accelerator pump. That usually is from a lean condition. What have you tried as far as timing, as in numbers?
mischiefdart
Well-Known Member
Sounds like it could be lean under hard acceleration to me.
It will backfire at higher rpm when moving or just winging the carb. Didn't used to do it, in fact it ran great. If I bring up the rpm's slowly it's fine, but try and spin it up quick and it pops. The only thing I changed before this started was new plugs and changed out the Thrush glass packs for Thrush welded mufflers (the wife was complaining about the interior noise). It has headers with 2-1/4" outlets, I'm running 2-1/4" pipe all the way out. The exhaust dumps out in front of the rear wheel wells. I've tried timing anywhere from 0 degrees to off the scale with vacuum advance unhooked and plugged. Stock spec is 2.5 BTDC +/- 2.5. I'm wondering if the intermediate shaft is off one tooth. I've set the idle mixture by vacuum but it seems to idle more smoothly if it's leaned up a little.
demon seed
The Original Demon Seed
Have you checked the timing chain for stretch?
OldmanRick
Well-Known Member
I agree with demon seed. The fact that changing the ignition timing all over the scale made no difference, it certainly sounds like a valve timing issue.
67Dart273
Well-Known Member
1---Forget stock timing specs
2--The intermediate shaft has nothing to do with it, so long as you can set the timing, and don't have the vacuum can in an awkward position or jammed up against something.
3--Have you checked that the mechanical advance is working?
Set the idle timing to AT LEAST 10 BTC OR MORE if the engine will handle it on startup, and doesn't ping. You can also get a good idea from the vacuum gauge.
THEN see how much mechanical advance you have at high RPM.
I ALWAYS check the timing marks for accuracy with a piston stop. This is the ONLY accurate and practical way to check TDC.
You can easily "figgure" this by measuring around the balancer with a small tape, and work out how many degrees per inch that is, then mark the balancer for at least 40*. Then you can use dividers to cut that in half for 20 marks, and again for each 10.
MOST factory stock distributors in the "smog" era from 68 and worse, later, have long, slow advance curves which are NOT helpful for performance. But if something is wrong like a sticky/ rusted mechanical advance, it's all that much worse.
440outlawdart
Well-Known Member
dont use stock specs,sounds like a timing issue
OldmanRick
Well-Known Member
Fingers crossed that the above timing work will get it done.
But, if not, and trying not to be too gloom & doom, just one more thought......Has there been any noise from the valve cover area? Just wondering if a intake valve spring could be on it's way out.
But, if not, and trying not to be too gloom & doom, just one more thought......Has there been any noise from the valve cover area? Just wondering if a intake valve spring could be on it's way out.
I checked the timing chain when this all started and there is no slack in it. The distributor and ignition box are new, just installed it yesterday. The thing that bugs me is that it used to run great, makes me wonder too about a weak or broken valve spring or some other issue with an intake valve. Thanks for the insight everyone, I'll keep messing with it, may have to dig deeper.
To 67Dart273, I put your vapor return system on this car and it cured my vapor lock issue, thanks!
To 67Dart273, I put your vapor return system on this car and it cured my vapor lock issue, thanks!
Ok gents here's an update: bad spark plug! Even though they are new, been in just a month or so it just kept running worse and worse. Started pulling plugs and behold! Wet plug on #3. Changed it out, set the timing to 12* initial and Woo Hoo! Runs better than ever. Any recommendations for plugs on that motor other than the Autolites I'm running now?
Thanks for all the advice, it's good to know there are people willing to help!
Thanks for all the advice, it's good to know there are people willing to help!
txstang84
Well-Known Member
Odd-I never had any issues whatsoever with autolite plugs even in my blown 5.0...Probably just drew a dud. If the car did run ok before with them and it was only one plug, I'd say just replace the bad one and press.
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