Need help with missfire under load on 340

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71DodgeDemon340

71DodgeDemon340

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Hey everyone ive been having this issue for awhile but today it seems worse, 340 with 4 speed. It will rev up fine sitting still but under load at above around 1500-2000 rpm you can hear popping coming from the exhaust and feel like its surging, swapped the chrome box for a orange box i had same issue. Timing is 16 btc at idle, all in at 34-36 with vac advance disconnected, battery voltage is good off at 12.76 volts. alt putting out 13.6-14 volts. Plugs, cap and rotor are only about a year and a half old. I can especially tell when i go to take off it wants to buck and fall on its face. Cruising at around 2000 fells normal until i give it more throttle then it pops and crackles from the exhaust. Any thoughts? I appreciate it.
 
Does this occur at part throttle only? In other words, if you stand on the gas hard when it does it, does it stop doing it?
 
RustyRatRod said:
Does this occur at part throttle only? In other words, if you stand on the gas hard when it does it, does it stop doing it?
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ive only went wot once while i had it out and it still does it, doesnt seem as bad but its definitely not happy. Before i would only have this issue for about 5 or so miles than it seemed to run good, today is the first time ive had it run this poorly
 
71DodgeDemon340 said:
ive only went wot once and it still does it, doesnt seem as bad but its definitely not happy
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Normally, part throttle breaking up is secondary ignition. Rotor, cap, wires and plugs. They usually clear up when the gas is poured to it
 
It doesnt sound bad at all when car is sitting still and you rev it up, sounds good and no popping or crackle from the exhaust
 
Possibly pull the valve covers and check valve springs. I broke the very top of a valve spring and it did the same thing. Kind of a shot in the dark!
 
abodyfan said:
Possibly pull the valve covers and check valve springs. I broke the very top of a valve spring and it did the same thing. Kind of a shot in the dark!
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thats what im hoping its not i did think about that, wouldnt a broken valve spring do it without a load on the engine?
 
When my 340 did it, it wouldn’t miss cold. It was the very top of the valve spring that broke. As it warmed up it would start to miss.
 
abodyfan said:
When my 340 did it, it wouldn’t miss cold. It was the very top of the valve spring that broke. As it warmed up it would start to miss.
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mine usually does it cold, before it would do it for the first 5 miles or so and then almost seem like it went away, today it continued to do it.
 
Check from carbon tracking in the cap, check plug wire resistance and check coil secondary resistance. I just went through it on my Dart. It was all new when I put the 340 in it.
 
abodyfan said:
Check from carbon tracking in the cap, check plug wire resistance and check coil secondary resistance. I just went through it on my Dart. It was all new when I put the 340 in it.
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What do you mean by carbon tracking? Also how do i check the coils secondary resistance?
 
To check coil secondary connect your dvom probe to the coil positive and the other probe to the coil wire output. Check for carbon tracking in the cap. Ive learned the hard way never assume that new parts are good
 
abodyfan said:
To check coil secondary connect your dvom probe to the coil positive and the other probe to the coil wire output. Check for carbon tracking in the cap. Ive learned the hard way never assume that new parts are good
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on the 20k scale im getting 4.95 - 4.98 which i assume is 4900 ohms
 
Google it I believe any thing under 11 ohms is bad.
 
Valves sticking due to springs or carbon or The dist. hooked to the wrong vacuum. Early cars had constant vacuum at idle and later had vacuum at part throttle. Just something to check is pull the vacuum hose off of the distributor. I have seen this at times.
 
Oldmanmopar said:
Valves sticking due to springs or carbon or The dist. hooked to the wrong vacuum. Early cars had constant vacuum at idle and later had vacuum at part throttle. Just something to check is pull the vacuum hose off of the distributor. I have seen this at times.
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i just did this earlier and capped off the carb port and the vac advance port and it still is acting the same
 
Pulled all the plugs and you can tell its running rich which is no surprise because it always has, but i noticed that the plug from number 1 had a crack on both sides. I split the porcelain off where it was split and it looks clean, no carbon tracking.

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Also when i pulled the plug wire off the socket stayed on the plug, not sure if when it was crimped it cut the plug wire in half or broke it and allowed it to pull apart. They came pre crimped
 
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