F head compression

I rebuilt my F head about 5 thousand miles ago. Runs fine except fouls plugs and could have the carb adjusted better. I let a friend drive it the other day and of course it seems different now that it's back home. Backfires and plugs are fouled looking.

I checked the compression and got these numbers.
1. 120
2. 120
3. 106
4. 120

Put a bit of oil in #3 and reading jumped up to 120. Means leaky rings or something right. What would you do about it.

It's 8.5% off which is within acceptable range of 10%.

Pull #3 piston and re install a new set of rings?

The engine was honed at the machine shop at time of rebuild and looked smooth and "crosshatched" with the hone.

I tried to take care in not having the ring gap align at all.

So what would you guys do?

 

Is all the plugs fouled, or just #3?

 

all of them look dark. I'm pretty sure it's a carb problem, carburetor is weber right out of the box 6 months ago. I never even turned a screw adjustment. Just started it and it ran and sounded ok. Maybe not as much HP as I thought it might generate but it was tolerable. Now I might have to tweak the lean/rich setting a bit.

Dang Carburetors are the "Debil"

 

Dark and fouled are two different things. Dark comes from the fuel/air mixture, and indicates that you are running rich. Fouled comes from oil, and looks tarry or more shiny. Adjusting the mixture is a minor concern compared to oil fouling.

 

Dark then, definitely not wet and oily.

Would water in the gas cause backfiring? Every since my friend drove it, it seems to back fire a bit. He lost the gas cap in a very light rain.

 

Backfire/pop through the exhaust? Maybe just too much fuel. I'd adjust the mixture and see if it goes away.

 

That's what I'm gonna try. Let me get the tire back on first just in case the dam thing tries to run good for a few seconds... I'd hate to have a tire off during those brief moments!R)

 

Does anybody think I ought to add some of that oil additive that supposedly improves compression? Supposedly fills in gaps in cylinder and rings?

 

no, just snake oil in my opinion.
you might pull it out and inspect the rings.
maybe one broke upon install ????
could be several things...