Misfire when light throttle and sparks fouling - Help needed

[360MX]

Good day all!

Dealing here with a frustrating problem on the MX Dart "super bee", first some context:

Car: Dodge Dart super bee (MX) 1978.
Motor: 360 C.I. rebuild to original standard specs (rebuild in 2019). Edelbrock AVS2 carb no more than 2 years. Edelbrock dual plane intake, HEI conversion using 4 pin module, MSD2 coil, new electronic distributor, new spark plugs wires, "fresh" set of champion 12y sparks, everything listed is fairly new and in working conditions.

Problem:
I had a problem with the engine kind of misfire (erratic) when give it light throttle, if you hit the gas the car just pick up an go no issues, it is only on low rpm let's say 1,5-2,0k rpms, past that point the response feels immediate, when I plug the vacuum gauge with all tuning tricks on the avs2 mixture screws adjustments I can get 15 psi but not static little erratic kind of bouncing up and down, but close to 15.

Last but not least I found out that sparks plug came out foulued with carbon (dry), but not all of them only 4 of them in a 1-1 pattern (1 cylinder good and the next 1 faulty) and this happened with the rest of cylinders 1 cylinder good and the very next one faulty. Distributor is brand new (6 months) I already verify gap between reluctor wheel and adjust to 0.008. spark is strong from coil and on to the plugs. Last weekend I disassembled the AVS2 carb just to find a shiny clean interior with no evident obstructions or anything abnormal.

Thank you for reading, any suggestions are welcome. =}

 

[Camtron]

I’ve mixed up the 5 and 7 plug wires and had a similar issue where it would sputter and backfire out of the carb at idle and run seemingly well under load.
Maybe a bad/dirty accelerator pump in the carb. When the engine’s off and the air cleaner’s removed, are you getting good streams of fuel when you hit the throttle on the carb?

 

[AMC Diplomat]

Purely a guess, but are the spark plugs torqued to spec and have you tried a different cap and rotor?

 

[Sub03]

I see you have checked your carb, but is it possible you have an issue with fuel/air on one side of your intake?
I mean with a dual plane intake, one side of the carb is feeding half your engine (1-4-6-7)
and the other side is feeding the other half (8-3-5-2).

A shot in the dark from me, but with every other spark plug fouled that was my first thought.

 

[Aspen500]

A shot in the dark here. I had an '81 D150 that ran fine, except on low rpm, light load (say up a slight grade). It would have a light misfire. Had brand new Champions put in just before I bought it. Anyways, after going through everything else, replaced the Champions with Motorcraft. Fixed.
Fast forward a few years, bought a '96 Dakota, which also had new Champions, and the exact same problem as the D150. Swapped the Champions for Motorcraft and problem solved.

My point is, if all else fails, try a different brand of plug and see if it helps. Doesn't have to be Motorcraft, I worked at a Ford dealer at the time so......

 

[360MX]

Thank you guys, for your comments really appreciate it.

Well, I try another set of plugs (Autolite-65 gapped at 0.35) slightly improvement on misfire however is still showing some on low low RPM´s 1,500 -1,800 range.

Purely a guess, but are the spark plugs torqued to spec and have you tried a different cap and rotor?

I try another rotor, same results, cap looks fine no noticeable failures.

I’ve mixed up the 5 and 7 plug wires and had a similar issue where it would sputter and backfire out of the carb at idle and run seemingly well under load.
Maybe a bad/dirty accelerator pump in the carb. When the engine’s off and the air cleaner’s removed, are you getting good streams of fuel when you hit the throttle on the carb?

I double check firing order its in the right position. About the accelerator pump: only thing that i notice is that the spring its not 100% straight it has a slight curve right in the middle, could this cause problems with the mechanism?

I see you have checked your carb, but is it possible you have an issue with fuel/air on one side of your intake?
I mean with a dual plane intake, one side of the carb is feeding half your engine (1-4-6-7)
and the other side is feeding the other half (8-3-5-2).

A shot in the dark from me, but with every other spark plug fouled that was my first thought.

Thats interesting, sparks that came fouled are: 3,4,7 & 8. the rest came out clear this with the fresh set of autolites, and after a 40 min driving downtown and highway.

Still looking to a solution on the fouling sparks pattern I really can figure it out.

Any other comments and solutions are welcome.

 

[AMC Diplomat]

Thank you guys, for your comments really appreciate it.

Well, I try another set of plugs (Autolite-65 gapped at 0.35) slightly improvement on misfire however is still showing some on low low RPM´s 1,500 -1,800 range.

I try another rotor, same results, cap looks fine no noticeable failures.

I double check firing order its in the right position. About the accelerator pump: only thing that i notice is that the spring its not 100% straight it has a slight curve right in the middle, could this cause problems with the mechanism?

Thats interesting, sparks that came fouled are: 3,4,7 & 8. the rest came out clear this with the fresh set of autolites, and after a 40 min driving downtown and highway.

Still looking to a solution on the fouling sparks pattern I really can figure it out.

Any other comments and solutions are welcome.

Try a different cap. My blind guess is the cap isn't perfect. Or the rotor and cap don't fit well together. Because 8-4-3 are fouled and so is 7, which is on the opposite side of the cap, I'm thinking there is something wrong with the cap or the distributor.

Torque spec for the spark plugs, off the top of my head, is 30 foot pounds.

 

[LSM360]

I’ve mixed up the 5 and 7 plug wires and had a similar issue where it would sputter and backfire out of the carb at idle and run seemingly well under load.
Maybe a bad/dirty accelerator pump in the carb. When the engine’s off and the air cleaner’s removed, are you getting good streams of fuel when you hit the throttle on the carb?

I did this too once and would've sworn there was no way I did it; I was wrong!

 

[Camtron]

I did this too once and would've sworn there was no way I did it; I was wrong!

I go back and check 5 times now, like a bad OCD tic every time I disconnect the plug wires; fool me once, lol

 

[Hayzoos]

Because 8-4-3 are fouled and so is 7, which is on the opposite side of the cap,

Good catch on the pattern. Another thought along those line is distributor shaft play, specifically wobble back and forth between 7 and 8 towers.

My master mechanic uncle used to stress when troubleshooting reaches the stage of replacing with a know working part, new does not mean known working.

 

[M_Body_Coupe]

What's the timing like? By that I mean: initial, followed by how many degrees advance at what RPM?...also, what is the VAC advance set up like?

The firing order on a SB Mopar is: 1-8-4-3-6-5-7-2, you found 8-4-3-7 to be fouled...seems like there is a bit of a sequence match here.

Assuming a dual-plane intake here and looking at a Performer RPM intake I see:3-2-5-8 are fed together, as are 1-4-7-6, so the fouling you are seeing doesn't seem to indicate a fuel distribution problem.

I honestly wonder if the VAC advance can is simply coming on too early, where basically at very slight part throttle you are still showing good vacum and if the VAC can is configured for very low vacum it might cause it to dump all that advance in causing the surging. On the other hand, if you are applying a larger throttle that causes your vac signal to drop (as it should) and the VAC can does not react.

 

[Aspen500]

Good catch on the pattern. Another thought along those line is distributor shaft play, specifically wobble back and forth between 7 and 8 towers.

My master mechanic uncle used to stress when troubleshooting reaches the stage of replacing with a know working part, new does not mean known working.

Way back in '83 I had an instructor at tech school always say "new just means new, it doesn't mean good" How true that is nowdays, unfortunately. When I was a Ford tech, many times the service manual would say "substitute known good part". It did not say new part.
Sorry for getting off topic of your thread 360MX.

 

[AMC Diplomat]

I was thinking about it today and if all else fails you could try an old engine trick and use spark plug non foulers

 

[360MX]

Thank you, guys, for all your comments and sideline conversation, really appreciate it, any ideas are welcome.

After not finding any other solution, I order another "new" remanufactured distributor Cardone brand I'm waiting on it. to swap the "new" Duralast that I am currently using.

I was thinking about it today and if all else fails you could try an old engine trick and use spark plug non foulers
View attachment 52327

Interesting I never hear of this piece, ¿How does it works?

What's the timing like? By that I mean: initial, followed by how many degrees advance at what RPM?...also, what is the VAC advance set up like?

The firing order on a SB Mopar is: 1-8-4-3-6-5-7-2, you found 8-4-3-7 to be fouled...seems like there is a bit of a sequence match here.

Assuming a dual-plane intake here and looking at a Performer RPM intake I see:3-2-5-8 are fed together, as are 1-4-7-6, so the fouling you are seeing doesn't seem to indicate a fuel distribution problem.

I honestly wonder if the VAC advance can is simply coming on too early, where basically at very slight part throttle you are still showing good vacum and if the VAC can is configured for very low vacum it might cause it to dump all that advance in causing the surging. On the other hand, if you are applying a larger throttle that causes your vac signal to drop (as it should) and the VAC can does not react.

To be honest i do not know exactly timing figures I do not have a timing lamp, what i did is to adjust timing using vacuum gauge I know is not the best way, but it seems to work, except for the problem already described the car runs great on high rpms and on idle. I also suspect on vac can but when unplugged it does the same thing (slightly misfire on low range)

This weekend a local mechanic takes a look at it and he comment that Harmonic balancer may be affecting timing causing the problems i present .... =0 is that even possible? he swears it happened this a couple of time before.

Thank you!

 

[Aspen500]

It could affect the timing reading if the outer ring has slipped, puting the TDC mark in the wrong place. What may look like 10° BTDC, maybe is actually 5° ATDC (just a random example). It'll cause an incorrect degree reading when using a timing light, but won't directly cause a misfire.

 

[M_Body_Coupe]

...To be honest i do not know exactly timing figures I do not have a timing lamp, what i did is to adjust timing using vacuum gauge I know is not the best way, but it seems to work, except for the problem already described the car runs great on high rpms and on idle. I also suspect on vac can but when unplugged it does the same thing (slightly misfire on low range)...

OK, good to know b/c you could turn the timing down a tad, so w/o the light hooked up you basically adjust for a slightly lower vac signal, and test with that setup.

....take it for a ride and see if the hesitation is still there, if gone, then the culprit appears to be the mix of idle & mech advance at that particular RPM, and possibly the fuel richness, meaning: too lean, in combination with aggresive timing, will cause the sort of symptom you are seeing.

 

[AMC Diplomat]

OK, good to know b/c you could turn the timing down a tad, so w/o the light hooked up you basically adjust for a slightly lower vac signal, and test with that setup.

....take it for a ride and see if the hesitation is still there, if gone, then the culprit appears to be the mix of idle & mech advance at that particular RPM, and possibly the fuel richness, meaning: too lean, in combination with aggresive timing, will cause the sort of symptom you are seeing.

I think you nailed it

 

[AMC Diplomat]

Interesting I never hear of this piece, ¿How does it works?

You put it in the spark plug hole and then put the spark plug inside it. It keeps the end of the plug from fouling if there's an oil leak in the combustion chamber or blow by in the engine

 

[Justwondering]

Reminds me of George Carlin.

You know you have problems when you start buying 'stuff for your stuff'.


JW

 

[PursuitSpecial]

I've seen this problem before. Some of the Edelbrock performer dual plane intakes have an inner bolt hole drilled all the way through into the runners, and if you have the bolts in the outer holes of the AVS2 the inner bolt hole is a big vacuum leak to one side of the intake, which will send every other cylinder lean.