Weird misfire

[-83MONTESS-]

So I just got my new TH400 in and finally got a chance to take it down the road to test it out. Shifts and drives fine but I noticed that I have a weird misfire. Its only evident when im trying to maintain a certain speed or RPM. If I accelerate or decel it's fine but anytime I hold the same speed for more than a second or so I get a bad misfire. There is no throttle hesitation or stumble and no backfiring.

I didn't notice it with my Quadrajet that ran pig rich. A few months ago I swapped on a Proform/Quickfuel 750DP. I got it from a buddy cheap, cleaned it up and swapped it in. Ran fine in the garage with minor mixture screw adjustments but I haven't been able to drive it until now. I played with the timing a little bit but it really didn't change anything. Primary jets are .077"s and secondaries are .084"s. Judging from my plugs it is still running pretty rich as they are still wet. Any ideas as to what is causing this miss?

 

[GuysMonteSS]

A misfire might be ignition related.
Check that all ground wires are clean and tight.
Check all spark plug wires.
On a big block its pretty easy to get # 5 & # 7 mixed up.
If you have an ignition box make sure that all the wires and connections are tight.
Guy

 

[pontiacgp]

After you try what Guy suggested if it is still there I would try a different ignition module for the misfire. What size are the idle air bleeds on the carb?

 

[-83MONTESS-]

I couldn't tell you the air bleed size. I do have 2 plug wires that touched the header on break in and melted the boot. I wrapped them in electrical tape and put insulating sleeves over them. I made sure all the wires were tight and they aren't mixed up either that was actually the first thing I checked.

One thing I forgot to mention is a few weeks ago I was playing with the timing and had a backfire through the carb that killed the engine. I started it back up and it seemed like nothing changed so I didn't think much of it. Could a blown power valve do this?

 

[-83MONTESS-]

I have an MSD 6al box. All the connections are tight but I haven't pulled the distributor cap to check on the ones inside the distributor.

 

[pontiacgp]

I couldn't tell you the air bleed size. I do have 2 plug wires that touched the header on break in and melted the boot. I wrapped them in electrical tape and put insulating sleeves over them. I made sure all the wires were tight and they aren't mixed up either that was actually the first thing I checked.

One thing I forgot to mention is a few weeks ago I was playing with the timing and had a backfire through the carb that killed the engine. I started it back up and it seemed like nothing changed so I didn't think much of it. Could a blown power valve do this?

the newer holleys don't blow the power valve as easily as they did. If the power valve was blown you'd know it

 

[pontiacgp]

what distributor are you using

 

[autonaut]

Measure the resistance (ohm) on those spark plug wires. All of them.

It's an easy job and it will help you knock out that part of the equation if further troubleshooting is required..

Dont know much about that holley design so i can judge if the carb could be the sinner here. But it doesnt sound like it.

 

[truracer20]

Check to make sure your distributor isn't bent. Look at the pickup coil and see if it has an even gap the whole way around.

 

[pontiacgp]

I like it that you are now talking about a small misfire when maintaining a speed, that means you are driving your car...