Could resistance cause significant backfiring? Resolved!

[QQBlue1]

Here's problem with my 1968 318 / auto: I'm trying to sort out a serious backfiring problem. At times it backfires so hard it sounds like its breaking and kills the car while it is at an idle. It will hardly make power when I put it in gear.

I've doubled checked to make sure I'm not pulling cold air where the long-tube header meets the exhaust.

I've put on a good replacement carb and it still backfired a bit, and cleaned out my original eddy 1406 carb. Vacuum seems good/normal. However, at idle it sucked the metering rods down out of sight. Seems like if the springs were right should be able to see them bounce a little? So maybe the engine is starved for fuel? Anyhow, when backfired again it blew one of the metering rods and springs right out of the carb.

The timing hasn't been touched, so I'm not inclined to tamper with it BECAUSE I just put on a new wiring harness and suspect the issue is there. I put in a M & H with new bulkhead connectors. It is wired to a Ready to Run MSD. I haven't by-passed the ballast because I'm a little confused as to how and why even though I've looked through the thread here. But I essentially wired it in the way I had it before, albeit I have a new battery and I pulled a hotter coil off my shelf. At idle (when it isn't backfiring) I'm getting the following Ohm meter readings:

Battery at rest 12.77 volts
at idle 13.30
at 2500 rpm 14.10 or so

Alt 13.8

at the + coil 9.7
Ign side of VR 11.8
field side 10.8

Also, I ran an additional ground from the + coil to the body and that started smoke at the ignition wire going to the Ballast resistor. Again, I think I should by pass the ballast, but I'm wondering if all the backfiring is possibly from a bad ground somewhere creating resistance?

 

[turbine68rt]

If the metering rod pistons are bouncing at idle, try using the next step weaker springs. They should be fully seated at idle.

 

[BLK 68 R/T]

A ground from the positive on the coil??? Any ground from either side of the coil is not good.

 

[BLK 68 R/T]

So the backfiring through the carb is intermittent and not all the time? Only when you go WOT fast or?

 

[QQBlue1]

A ground from the positive on the coil??? Any ground from either side of the coil is not good.

I quickly found out that was a bad idea based off some bad advice I got. The back firing is intermittent. Initially, it was happening when I let off the throttle. I replaced a bad plug wire the other day and thought that would resolve it. I would suspect a vacuum leak in the carburetor, or that it is running lean, but it just hasn't responded to those adjustments. The whole thing is puzzling.

 

[Rusty knuckles]

I'm not very familiar with the ready to run distributor. If it was a points ignition I'd say your condenser is bad. I'd certainly focus on ignition. Something interfering with the magnetic pickup in the distributor?

 

[BLK 68 R/T]

Did it have issues before you replaced the wiring harness?

 

[Geoff 2]

The Ready-to-Replace dist should have the full 12 volts, NOT be wired through the bal res.
The coil: if the coil is factory original, it needs the bal res.

If it is an aftermarket coil, it may...or may not...need to be used with a bal res.

 

[QQBlue1]

The Ready-to-Replace dist should have the full 12 volts, NOT be wired through the bal res.
The coil: if the coil is factory original, it needs the bal res.

If it is an aftermarket coil, it may...or may not...need to be used with a bal res.

Did it have issues before you replaced the wiring harness?

Blk 68 R/T - I did not have issues with the old, slightly modified harness, but it was really old, bulkhead was a mess etc., and to properly insure the car I needed to replace it. Anyhow, If I by-pass the B. resistor as you mentioned on my other thread, I understand that I should connect the double (blue & brown) wire with the blue wire plugged on the other side? That makes me nervous because the "blue ignition run wire" is connected to the "brown ignition start wire" . . . I'm guessing once the ignition is started and comes back to the run position, the initial surge of 12 volts reduces during normal operation at the ignition run position permitting the connected blue wires to create a harmless loop with fewer volts than what is needed for the ignition start position?

Geoff 2 - Are you saying I should have the full 12 volts at the positive terminal with the upgraded coil (with the ballast resistor by-passed)? With the old harness I used a factory style coil. Once the back firing started a pulled the up-graded coil off the shelf and replaced the factory style coil but have seen no change or improvement. I suspect I should get a new MSD 2 coil or its equivalent.

Thanks for helping me think through this. Whether this resolves the backfire, I'll at least know I have the wiring right.

 

[hunt2elk]

With a MSD ready to run distributor, you bypass the ballast resistor. There is also a specific Blaster coil they tell you to use.

 

[BLK 68 R/T]

Blk 68 R/T - I did not have issues with the old, slightly modified harness, but it was really old, bulkhead was a mess etc., and to properly insure the car I needed to replace it. Anyhow, If I by-pass the B. resistor as you mentioned on my other thread, I understand that I should connect the double (blue & brown) wire with the blue wire plugged on the other side? That makes me nervous because the "blue ignition run wire" is connected to the "brown ignition start wire" . . . I'm guessing once the ignition is started and comes back to the run position, the initial surge of 12 volts reduces during normal operation at the ignition run position permitting the connected blue wires to create a harmless loop with fewer volts than what is needed for the ignition start position?

Geoff 2 - Are you saying I should have the full 12 volts at the positive terminal with the upgraded coil (with the ballast resistor by-passed)? With the old harness I used a factory style coil. Once the back firing started a pulled the up-graded coil off the shelf and replaced the factory style coil but have seen no change or improvement. I suspect I should get a new MSD 2 coil or its equivalent.

Thanks for helping me think through this. Whether this resolves the backfire, I'll at least know I have the wiring right.

So here’s my thoughts - 1. Bypass the ballast like I mentioned in your other thread. 2. Install the correct coil as mentioned above. 3. Check and adjust timing, you changed your distributor so it only makes sense to recheck timing. 4. Check the gap on the new distributor.

 

[lewtot184]

If it's backfiring thru the carb that usually indicates two things; ignition timing is too late or lean condition with the carb. You may have both.

 

[BLK 68 R/T]

Blk 68 R/T - I did not have issues with the old, slightly modified harness, but it was really old, bulkhead was a mess etc., and to properly insure the car I needed to replace it. Anyhow, If I by-pass the B. resistor as you mentioned on my other thread, I understand that I should connect the double (blue & brown) wire with the blue wire plugged on the other side? That makes me nervous because the "blue ignition run wire" is connected to the "brown ignition start wire" . . . I'm guessing once the ignition is started and comes back to the run position, the initial surge of 12 volts reduces during normal operation at the ignition run position permitting the connected blue wires to create a harmless loop with fewer volts than what is needed for the ignition start position?

Geoff 2 - Are you saying I should have the full 12 volts at the positive terminal with the upgraded coil (with the ballast resistor by-passed)? With the old harness I used a factory style coil. Once the back firing started a pulled the up-graded coil off the shelf and replaced the factory style coil but have seen no change or improvement. I suspect I should get a new MSD 2 coil or its equivalent.

Thanks for helping me think through this. Whether this resolves the backfire, I'll at least know I have the wiring right.

Bypassing the resistor -
How to bypass the ballast resistor??

 

[QQBlue1]

So here’s my thoughts - 1. Bypass the ballast like I mentioned in your other thread. 2. Install the correct coil as mentioned above. 3. Check and adjust timing, you changed your distributor so it only makes sense to recheck timing. 4. Check the gap on the new distributor.

I have bypassed the ballast and put in a new Master Blaster 2 coil. The voltage readings are much better. Thanks for your help on that. I've been adjusting the timing a bit and thought I found the sweet spot, but it started backfiring again. Seems to be getting plenty of fuel. I've had the MSD for quite a while, perhaps it needs some attention . . . I'm probably down to the tinkering phase.

 

[QQBlue1]

Gentlemen - In the process of switching out the steering box, re-wiring the engine and headlight harness, upgrading the ignition and coil, bypassing the ballast resistor and rebuilding plug wires etc . . . I ended up mis-marking the #1 and #3 plug wires when I reinstalled everything. A simple mistake, but for an amateur a job like this requires a lot of paying attention . . . all this to say your advice and motivation is indispensable for keeping guys like me on the road and the learning curve. Thanks again and hopefully I'll be of use for someone in the future!

 

[BLK 68 R/T]

Gentlemen - In the process of switching out the steering box, re-wiring the engine and headlight harness, upgrading the ignition and coil, bypassing the ballast resistor and rebuilding plug wires etc . . . I ended up mis-marking the #1 and #3 plug wires when I reinstalled everything. A simple mistake, but for an amateur a job like this requires a lot of paying attention . . . all this to say your advice and motivation is indispensable for keeping guys like me on the road and the learning curve. Thanks again and hopefully I'll be of use for someone in the future!

That’ll do it…

 

[Geoff 2]

Happy days!

 

[kiwigtx]

The Ready-to-Replace dist should have the full 12 volts, NOT be wired through the bal res.

Agreed - Ready to Run distributor does away with the ballast resistor.

 

[451Mopar]

Make sure the carb is getting good fuel pressure and flow. It might be a lean pop.
I had a heck of a time tracing a fuel system problem back to a split in the rubber fuel line connecting the fuel tank to the hard line.
The carb would get just a small amount of fuel, and then she split line would start sucking in air leaning out the carb which would try to die when put into gear.
Usually before I start troubleshoot ignition, I will make sure the carb is good, and use an external fuel supply (electric fuel pump with a gas can.)
It just helps keeping me from chasing my tail looking at the ignition system, when the problem was fuel related.

 

[hunt2elk]

Make sure the carb is getting good fuel pressure and flow. It might be a lean pop.
I had a heck of a time tracing a fuel system problem back to a split in the rubber fuel line connecting the fuel tank to the hard line.
The carb would get just a small amount of fuel, and then she split line would start sucking in air leaning out the carb which would try to die when put into gear.
Usually before I start troubleshoot ignition, I will make sure the carb is good, and use an external fuel supply (electric fuel pump with a gas can.)
It just helps keeping me from chasing my tail looking at the ignition system, when the problem was fuel related.

OP got lucky and found he had a couple if plug wires swapped.