390 miss fire

[mrtleavitt]

Cylinder #4 on my 390 has a small missfire. The pushrods are all good and the Edelbrock cam and lifters were installed maybe 10,000 miles ago. All new plugs, wires, cap, and rotor and the problem wasn't fixed, so I figured it's probably either the rings or something in the heads. I pulled the spark plug and it was a little more black than the rest but it wasn't wet. If my rings were worn there should be oil on the plug right??? So I'm thinking it's probably a valve that is messed up a bit. Is my reasoning on the right track or is there another factor that I might be missing?

[Jake11]

Yup. Compression. Do you have any. If not wheres it leaking? Sunk valve?

[fomocoguy]

Either buy or rent yourself a compression tester (if you don't have one) and check it. Then sqiurt some motor oil in the cylinder and check it again. If it goes up significantly, rings. If it stays the same, valve. Check all your cylinders while your at it to be sure it's low in the first place.

[mrtleavitt]

Ok, I checked all the cylinders and they all checked out ok, about 130 for all of them. What else could I look for? Cam and lifters is all I can think of now, right?

[FORDification]

But what was the difference between the wet and dry tests? Since you've got an oil-fouled plug, we need to figure out where it's coming from.

[mrtleavitt]

It's not an oil-fouled plug. It looks like it's just not burning all the gas in the chamber. The wet/dry test was fine. Plus my exhaust is really gassy smelling and I used a emmision diagnostic tool at my brother's shop to adjust my carb so I know that it's tuned well. So it's definitly something with not burning enough of the gas.

[Jake11]

You got compression, ok. Maybee theres a vacumn leak thats making that cylinder missfire. Like a hose/gasket leak.
Also, make shure that the spark is going to the cylinder. Take the plug wire off, stick a screwdriver in the boot and lay it
on top of the motor. Run it and see if it makes sparks. Also, could just be a bum plug. It happens. Jake

[mrtleavitt]

Thanks for all the advise guys. I can't find any vaccum leaks, plenty of spark getting to the cylinder (have an MSD box), on my third plug now and still the same. I'll probably just bite the bullet this coming week and tear into the motor to check out the cam, lifters, and heads. It's due for some new gaskets anyways.

[FORDification]

Before you do that, why don't you invest in a set of plugs that are one heat range higher?

http://www.ngksparkplugs.com/techinfo/s ... country=US

[mrtleavitt]

Do you know one heat range higher in motorcraft for the sp420? I was figuring if my multiple spark discharge MSD ignition can't burn the extra fuel then a hotter plug probably won't do it. It's getting a great initial spark then about three smaller ones in between each major spark. I was doing some research and I don't think a hotter plug would do it. The main idea for a hotter plug it to keep it from fouling, and that's not my problem.

[Jake11]

Try this. Swap the plug from another cylinder and see what happens. Quick & easy.

[mrtleavitt]

Already done and no change. My bros 390 works great so I swapped his plugs and wires and no change in performance.