Check the distributor breaker plate for slop. It should pivot when vacuum is applied to advance but should have no other slop causing dwell to change.

 

How did your plugs read? If you have a carb it could be leaking at the rear affecting those two cylinders? Need more clues.

David

 

How did your plugs read? If you have a carb it could be leaking at the rear affecting those two cylinders? Need more clues.

David

I wrote the wrong cylinder number in my original post, it's #5 and #8 not 4. The plugs are new, after running they are still white and dry. I swapped the a plug wire from one of the firing cylinders to the dead holes and it make no difference? The wires are new.

If the exhaust valve was not opening would I still have compression?

I am going to check the distributer for slop in the bushings and advance plate
but really don't think that is my issue.

 

Also have a look at the intake gaskets...and for vacuum leaks.

 

check your firing order . i believe the 2 small block firing orders both end in 4 and 8. wires could be switched at the distributor or plug! 1-3-7-2-6-5-4-8 and 1-5-4-2-6-3-7-8 i think

 

check your firing order . i believe the 2 small block firing orders both end in 4 and 8. wires could be switched at the distributor or plug! 1-3-7-2-6-5-4-8 and 1-5-4-2-6-3-7-8 i think

I checked 351W=1-3-7-2-6-5-4-8

Thanks

 

+1 on the vacuum leaks. #4 and #8 are the rear cyls. There usually is a vacuum port at the rear of the intake. If you have a leaky brake booster or other vacuum accessory it would make the rear cyls lean. Try pinching off the vacuum hoses and pcv hose and see if it smooths out.

 

Wild guess, possible broken valve springs, would still have cranking compression, but no power output.

Vacuum leaks, spray carb cleaner at #5 & #8 intake ports.

Check distributor cap for carbon arcing to ground. I doubt a carbon between 5 & 8, cause you have #4 between them.

Arcing between 5 & 8 plug wires??

If its a dual plane intake, with engine running, spray carb cleaner on the side of the carb that feeds 5 & 8 into that primary venturi. Bill

 

New cap and rotor, plugs & wires didn't cure it. I've checked several times for vacuum leaks and found none. I am going to plug the brake booster hose off next. I will check the springs also.

 

Are both cylinders dead if you raised the rpm to say 2500? If they work ok then the mains are coming in on the carb.

Keep in mind that one side of the carb feeds the two center cylinders on one side and the two end ones on the other. The other side of the carb feeds the opposite or remainder of the cylinders.

Could be that one side of the carb has a 'smigley' or piece of dirt in it and cannot feed the two end cylinders #5 and #8 as they are the farthest away.

Worth checking. In essence one side of the carb (mixture screw ) will be dead.......Just throwing ideas out.....John

 

Are both cylinders dead if you raised the rpm to say 2500? If they work ok then the mains are coming in on the carb.

Keep in mind that one side of the carb feeds the two center cylinders on one side and the two end ones on the other. The other side of the carb feeds the opposite or remainder of the cylinders.

Could be that one side of the carb has a 'smigley' or piece of dirt in it and cannot feed the two end cylinders #5 and #8 as they are the farthest away.

Worth checking. In essence one side of the carb (mixture screw ) will be dead.......Just throwing ideas out.....John

John,
It has an 1850 Holley carb. I have the carb off now checking the idle circuits for what you suggested. I was almost sure that’s what it was because as
you said it seems to hit on all 8 when the rpms are up but when I pulled the front bowl and metering block I found no obstructions in them or the base plate. I blew it all out and installed new gaskets. I will reinstall it tonight and cross my fingers.

Usually I find a carbureted engine with points or Duraspark ignition very simple to diagnose and fix problems but this one is giving me grief.

Thanks for the ideas.
Gregg

 

Good luck with it Gregg and keep us posted.

Make sure you take out the idle mixture screws and blow compressed air into them . This will force air out the Idle air bleeds within the airhorn. You can also blow it out the reverse as well.

regards....John

 

John,
One of the idle circuits must have been plugged as it hits on all eight now! I will convert it to Duraspark now that I know I have no mechanical issues.
It still needs the idle mixture adjusted but I ran out of time last night.

Thanks,
Gregg

 

That's great Gregg. The Duraspark is an easy conversion.

Glad you have the carb issue straightened out. Adjust your mixture screws for maximum vacuum while in drive and then set your curb idle screw. You should be good to go....

Glad to help........John