Ok so about a week ago i figured out cylinders 2,3,5,8 weren't firing by pulling the ignition wires and there was no difference in the idle. Checked Ignition wires and they are Sparking. Replaced all 8 spark plugs. Cleaned distributor(mallory Unilite) contacts.

Today I Drove it for the first time and it was CLEARLY Running on all cylinders, I hit the gas and it WENT. Everything was good and I drove it for about 5 minutes when it did the same thing. I pulled over and checked and sure enough cylinders 2,3,5, and 8 werent firing. Just to be sure I swapped the ignition wire from cylinder 4 to 3 and vice versa and the ignition wire is fine.

Any Ideas? Do I have to check the timing?
Thanks Guys, I can't think of any other possibilities

 

What do the new plugs look like? I'm not looking but, I believe that those cylinders run off the same "plane" in the intake. Look for something related to that. Maybe a carb issue.

Is this 2v or 4v?

 

It is a Dual plane Intake, and those are all on the same plane, So it must be either something blocking it or the carb barrels that give gas to that side have something blocking them. Unfortunetly I'm at work and cant check now!

 

Oh also Plugs looked fine, Inake is a Dual Plane Edelbrock performer 2V with a 4V Carter Mounted on top.

 

Another thing to look at is if it has an auto trans with a modulator that could be sucking up trans fluid and fouling those plugs. Look to see if it runs on that plane. Or it mat just be sucking air causing that plane to be lean.

 

Ok. Pretty sure the carb is fine, Ripped her apart and rebuilt it, everything is clean and set.

The Trans-modulator is indeed on that plane. Also after running it the plugs are no longer fine. The plugs on the bad cylinders are a bit tan, and seemed to be a little wet, didnt seem like oil...

 

Alright so I took the vacuum connection for the brake booster and the trans-modulator off and cleaned it out, made sure there were no leaks around it and put it back on. Took it out for a ride and it was running awesome again.

After about 5 minutes it began to bog down again!

So I guess this question is to Stepman- any idea how to make sure tranny fluid isnt sucked into the intake?

 

Check the fluid level. Is it dropping at all?

 

Ok Stepman, more info. Yesterday I drove the car and it was running awesome again, definetly on all 8 cylinders. it started bogging/running on 4 cyls about 5 minutes into it. So today I decided to Drive it with the vac line to the trans modulator disconnected. It started out fine, on 8 cylinders but about 2 minutes into the drive it was on 4 cylinders again, leading me to believe it cant be the sucking trans fluid problem. After I got home I pulled the plugs and they still looked the same, not wet or anything. Any other ideas? I'm thinking I have to pull the intake off...

And thanks so much for your help, If I can ever help you I Will GLADLY return the favor!!!

 

351c firing order:

1-3-7-2-6-5-4-8

Cylinders you 'appear' to have a problem based on information in post:

1-3-7-2-6-5-4-8

Have you confirmed the following:
Timing is set correctly from TDC
Alternator is outputing 14v or so when running
Voltage Regulator is working
12v to ignition box or petronix if you have
If stock have you replaced the points? Did you gap the points correctly?


Just because they sparked when you tested them doesn't mean it isn't giving you a weak spark which could decrease as you drive. When you have your 'problem' is you let the car sit untouched for 24-48 hours will it run just fine again for 2-4 minutes and then be boggy again?