'70 402 fouled spark plug - Why?

[mosel70]

1970 C10, 402 with 454 heads. This spark plug is out of firing sequence #3 (second from front/drivers side). First plug is after about 100 miles. Second plug is after about 1500 miles (really gunked). I did a compression check and it was 115 psi dry, 114 psi wet. All other cylinders were 106 to 115 dry btw. Don't know how many miles are on the engine. It runs good outside an idle overtemp I'm dealing with (getting large CFM electric fans).

Would that kind of a fouled plug be a valve issue?

[PGSigns]

Looks oil fouled. Also looks like the end has been painted and that is not helping it fire. I would put a new plug in the hole one heat range hotter and see what it does. And it you painted all of them put a new set in it.
Jimmy

[68gmsee]

You don't mention if any of the other plugs get fouled but if this is the only one, it may be worn valve guide or the valve stem seals allowing oil to seep in the cylinders.

Depending on the engine year it may have "O" ring, umbrella or positive seals. I've had them basically become brittle and fall apart when touched. If you have worn valve guides or bad valve stem seals you may see a puff of smoke come out of the exhaust when you first start it after sitting for some time.

Another thing, the compression seems low so you may also have worn rings or sticking rings but do the valve stem seals check first. It's easier and cheaper.

[Ironangel]

^^^Agreed with both the above posts^^^ What's the casting number on those heads? With those compression readings I'd be inclined to think the 454 heads could use a valve job. If those heads are 118cc or 122cc smogger heads your killing your compression ratio. A dry compression reading that increases significantly when a wet reading is taken is indicative of worn rings. Low readings that remain low wet or dry indicate a valve problem. As oily and carbon fouled as those plugs look with as few miles on them I'm thinking those heads need some attention. If it were me, I'd get rid of those heads and either find a set of good closed chambered oval port heads or a set of aftermarket oval port heads with something in the 100cc combustion chamber size. Need more info, base timing? Head casting #'s? Anything done to the lower end when the heads were swapped, etc...

[mattfranklin]

Nice looking truck. Beautiful engine, too.

I'd agree with the other guys on valve issues and putting in a fresh set of hotter plugs.

That compression is pretty low and it making it wet with a squirt of oil doesn't help it, then gotta be valves. ** Or valve adjustment? ** Checking valve adjustment is pretty quick and easy compared to the other things.

[mosel70]

Number on both heads is 336781. Looks like date stamp is 16 78. 454 heads according to http://www.nastyz28.com/bbcmenu.php

I was told PO put a 3/4 cam (old term) in and that's all I know. I assume (correctly?) that 106-115 on compression test is due to the big cam.

I appreciate Ironangel's advice on what heads to get. I think I'll just pull/clean that spark plug frequently until I'm ready for new heads or a valve job as a minimum. The other spark plugs looked really good except for one that had a little wet oil.

Thanks to y'all for the help!

[Ironangel]

Right on, that sure is a purdy big block! The 336781 heads are good, made from 1973-85 for 454. They are an open chamber 113cc head comparable to the 049 heads everyone raves about. The 049's were used on the 402 and the 454 from 70 - 73 and then the 781's. Both have great potential if you ever want to stick a set of domed pistons under them, they flow better than closed chamber and allow a wider and taller cam selection. Heres a pic of my 69 L-34 396

[PGSigns]

You could always just pop a pair of valves seals in the cylinder. Put a set of umbrella seals in it. All you need is a way to put air in the cylinder and simple valve spring tool. They are like $20 from the parts store. I us the hose from a old compression gauge with an air filling on the end to pump up the cylinder. Pull the rockers, put the air on it and then hit the retainer with a hammer and a 3/4" socket. That loosens the keepers. Pull the spring and pop the seal on it the valve and back together.
Jimmy

[68gmsee]

^^ As mentioned above re. valve seals. It's a lot cheaper to remove the valve covers and put seals on the valve stems for the time being.

I used a small pancake compressor to install umbrella seals on my 69 until I had time to rebuild the heads. The oil would slowly seep down the stem and coat some of the plugs after sitting for a while. Only took one morning to do.