Spark plugs keep fouling and Missing under a load?

[Billys44s]

Hey guys

So I’m having a problem with my 85 C10. I got a crate 350 from auto zone and I’ve only driven it a handful of times and I keep running into this problem that when I give it throttle it has a miss and when I drive and punch it hard, it misses pretty bad and I can’t figure what the cause of it is. I’ve been through 6 sets of plugs because they all keep fouling out and I can get it to idle ok but it has a slight surge at idle. I downgrade the carb from the recommended 670 to a 650 and I’m still having the same issue.

 

[fast 99]

After installing new plugs does it miss right away, or does it occur after some distance of driving? If it occurs right away would suspect an ignition problem. Can you provide pictures of the bad plugs. Are they oil or fuel fouled?

 

[Billys44s]

They normally are worse than this I cleaned the plugs up earlier but this is about 7 minutes of run time on them, it idles fine but then you give it throttle and it starts to miss real hard

 

[Bextreme04]

They normally are worse than this I cleaned the plugs up earlier but this is about 7 minutes of run time on them, it idles fine but then you give it throttle and it starts to miss real hard

Those don't look fouled. I would start with a compression test. Autozone rebuild is sketchy at best... you could have all kinds of wierd mismatched components in there. I would start with a compression test... they should all be within 5% of each other and more than ~150psi. Then I would look to make sure that my cylinder head casting numbers matched. Then I'd be looking at intake manifold or vacuum leaks. Then I'd be making sure the distributor is in decent condition. Have you already properly set timing and carb idle mixture?

 

[Matt69olds]

No wonder!it’s a plug fouler, it has a Holley.

Actually, the plugs don’t look too bad. If it’s missing under throttle, start with a through inspection of the ignition system. Remove the distributor cap and rotor, look at the tiny wires that go to the pickup coil. They are known to break causing intermittent no start of misfires.

start with the basics. Is the float set correctly? Is the choke opening all the way? Is the timing correct?

Run a compression test, hopefully the results are even. Run the test both hot and cold, ideally with all the plugs removed and the throttle wired wide open.

 

[fast 99]

Check for steady vacuum, possible broken valve spring, tight valve, flat cam, ect?

 

[bookit]

Check for a vacuum leak, at the carb base, egr mountiing, and manifold to cyl head, Also check for plugwire crossfire between cyls 5&7

 

[Ricko1966]

Okay tonight when it's dark spray the wires and cap with salt water in a spray bottle. Watch for lightning,I'll bet you have a bunch.