need some mechanic advice please

I am going to try to give as much info as I can so this might take a while. I have a 1971 chevy 350 in a 1964 bel air wagon. When I first got and changed the spark plugs #8 spark plug was black and oily and it still gets black and oily to this day. Compression test on all cylinders are 140 to 150. Hooked up a vacuum gauge and it shows that I have a tiny miss at idle actually its a tiny stumble. When I push pedal and hold a higher idle vacuum gauge stays steady but you can still hear a light stumble in the engine. It never misfires. The engine drives great no hesitation and no weakness. So heres the kicker. After coasting and then reapplying the gas it will puff one shot of smoke out tailpipe. Not at first, only after driving awhile after the engine gets really warm. On a hotter day it will do it sooner. I am pretty sure its blue and it smells like tar so i think its oil. No smoke at start up of a cold engine. I changed the valve stem seals but I still have the problem. I have been thinking now it might be the valve guides on #8. But it only happens after the engine get real warm. It used to only happen occasionally but now the engine is starting get hotter and its happening more often. Oh and on cold days it doesn't even happen. I flushed radiator and put in a new water pump. So went back to spark plugs for some signs. Passenger side 2468 plugs. 2 is good 4 and 6 are white and 8 is black. So a friend of mine said this is a sign of intake manifold leak. 4 and 6 are running lean and oil could be getting passed to 8 to make it black. So is it possible that its not the valves or rings but the intake manifold gasket? Engine gets hotter and make the intake gasket leak bigger? Anyway that it where I am leaning unless anyone has had this problem and fixed it. Oh and this weekend hot day I drove about 20 minutes fwy and parked facing down a hill and smoke started gushing out the right side exhaust and it kept on smoking till I turned it off. If I keep the engine cool and under 195 to 200 it will never smoke. So if I pull the 180 thermostat and get a cooler one or don't put one in at all it will never smoke. This obviously isn't the cure but it won't smoke if I do this. Also oil is clean and coolant is clean and never a sweet smell out the exhaust. I also checked the seals after it was puffing all that smoke and they are fine. Wow I hope someone can help me narrow this down?

Thanks,
Vance

 

You would think that if it was a vacuum leak you'd get some roughness and stumbling.

 

Sure fire, quick way to tell if it is an intake manifold gasket leak: Take an unlit propane torch, turned on low, and run ti along the edges of the intake manifold. if the idle increases, you've found a leak. To me, though, it does not sound like a gasket leak. You said the the plugs on that side of the engine were white, grey, grey and black. That sounds like the front one is running lean, the middle 2 are about right, and the back is running rich. How are the plugs on the other side? If they are all even, and about equal to the middle 2 on the problem side, then the carb is adjusted fairly well. it also depends on which intake you are using. I would give serious thought to checking the EGR system, too. It could well be malfunctioning. That will cause all sorts of fun! You might also think of a leak down test.

 

Your #8 oil ring is probably broke.
Had a 301" sbc [283 + .125] as a kid, and had the same problem, but with 12 1/2 compression, fouled the plug.
C-43 AC truck plugs, 140 thermostat, and 50W racing oil kept it down, but that was back then.
That oil for high mileage motors works pretty good, now, and don't know if those plugs are still available - the electrode was twice as big, for idleing a long time.
Try a Accel coil - might burn them clean.

 

I agree with 2dr...I think it is a ring problem on number eight cylinder.

 

broken oil ring,cracked skirt,,or even a broken ring land will cause what you are describing,,,good luck and please post your findings,,,cheers

 

hate to argue with my fellow wagonites....but ...if its got good compression?....then its probably just valve guides .

notice how i said 'just'....thats so it doesnt sound expensive

 

Valve guides were my first thought too. Easier to see on first fire up.

 

Cracked or broken oil ring or #2 compression ring or burned ring landing. Cracked cylinder maybe?. If you changed the valve seals and the compression is good I can't think of anything else it could be?

 

Have you checked for a bad plug wire? This will definitely cause a plug to foul. Might be a dirty contact in the distributor cap. With that compression, It is pretty safe to say that cylinder should fire. the smoke when accelerating after coasting does point toward bad valve guides and or seals.

 

A broke oil ring won't affect compression, just oil control!

 

tis true...tis rare...but tis true

 

Actually, if the oil ring is broken and the cylinder walls have excess oil on them, it might raise compression in that cylinder especially when cold.

 

whats kind of oil are you using? how many miles on the engine?

 

Thanks for your input guys. New problem to add. Oil pressure runs at 50 and then when hot and at idle about 20. I have a ticking noise coming from number 8 and I think it is a lifter. It ticks just as loud after driving so its not exhaust manifold. So I ran some seafoam in oil and it still ticks. I pulled off valve cover to maybe adjust the valve and I only have oil coming out of 1 push rod. Another one has a tiny bit. I can see down the holes in the head that the lifters are spinning and there is oil all around them but nothing is making it up the rods except for the one. I also pulled all the rods and made sure they weren't clogged and they weren't. I actually just read that someone had the exact same problem and the pickup tube was clogged on the oil pump.

Thanks,
Vance