Ok.. I'm not a novice.. I know my way around engines. But this one is driving me insane. Chevy Nova, 230 inline 6. Single carb. Here's how the exhaus sounds like:

putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT putt putt putt putt putt PHUTT

...you get the idea.

Did a power balance test, pulled a plug wire off one at a time while it was running. #6 plug wire had virtually no impact. Pulling any other wire made a significant change.

Compression pressure even with #5. Leakdown, while not great, was even with #5. New plugs, wires, cap, rotor. Replaced good points and condenser with a Pertronix pointless kit. No change.

New rings and pistons about 2 years ago. New bearings and re-seal this summer. Runs richer now, I had intake air leaks. But the engine ran like this before the re-seal and bearing job too. I leaned it out a little, no help. I have an O2 bung on the exhaust but it's pointless to try and read mixture if #6 isn't contributing.

I loosened the #6 valves a little, no change. (Hydraulic lifters).

I'm about to remove the intake to look if something got in there at some point. But I doubt that's it.

I would hate to pull the head off, but I might.
Then I could upgrade to a better head gasket for whatever future use I might come up with (evil grin)

I don't say this often, but I'm stumped. Hoping to hear from someone who's had similar problems.

Jan

What is your cranking compression?

Camshaft is on it's way out?

Not the same scenario as yours, but I had a miss @ different times,pulled out all plugs, all looked great & even,put them back in, get a miss @ different times, pull the plugs agian, all looked great, as I was reinstalling a plug, I felt a click in my hand. Hold the plug upright, then hold the plug down, "click" well the damm center porcelian (SP) collar would slide down & cover up the center electrode, causing an intermitant misfire.

What are you using for your ignition?, Points,HEI. worn out dizzy?

MBHD

I'll get better numbers later on but I got roughly 130psi on both #6 and #5. (WOT, almost cold). My leakdown tester isn't reliable as far as % numbers go so I won't quote numbers, but they were the same between #5 and #6, more or less. The gauges would still show a difference if there was one.

Good idea about the cam. I'll try to measure and see if there is a difference in valve lift. This engine suffered from low oil pressure when I got the car. It has had the distinct PHUTT for a long time. Nothing seems to make it go away.

Plugs are brand new, and I even replaced the #6 with another brand new one.

Worn out stock distributor with a Pertronix pointless kit. Those have has worked well for me in other engines, distributor wear no longer affects timing. I did try with points and condenser first. No difference.

Is the miss there all the time? Have you had a vacuum gauge on it? Been burnt before checking for cyiinder leakage with shop (90 + psi)air pressure. Sometimes a valve will show a leak using lower air pressure. We have a EVAP tester at the shop that I have used to put smoke & nitrogen into the spark plug hole & watch the smoke come out past a valve that "didn't leak".
Running compression test another thing that might help.

I'll check vacuum too. Our smoke tester is out of smoke... for the past 2 weeks. Nobody stays on top of supplies and shop tools. Oh yea I'm a mechanic. I don't get much time to work on this car, and half the time I forget to bring my tools home :P

the "miss" is there solid as rock. Never changes. The plug does fire. More or less the same color as the others. I can't put road miles on the car the way it is so it takes a while to get any coloration on new plugs.

A few questions..

What is the compression on 1-4 ? same as 5&6?

Does it change as the engine heats up?

Wonder if you got a sticky exhaust valve?

Are all the intake leaks fixed?

It will be several days before I get a chance to get my hands on it again. Good ideas from all. I'll check them all. Thank you!

Very little progress:
I got the side cover off and as I turned the engine over by hand, the #6 lifters moved as much as the #5 lifters, just by looking at them. Did not measure anything, I was in a hurry and it was getting dark.

I did buy a new radiator and an electric fan. Different story. They sold me a 3 row rad, when teh car has a 2 row. As a result, it looks like the original fan would interfere with the radiator. What better time to convert to electric fan and free up some much needed ponies

I did get one thought pop in my head earlier today: if the valve guides (esp. exhaust) were real worn, it could leak or run weird like this when it's actually running, but seal OK when turning the engine over by hand? I.e. leakdown test. Or seal ok when turning over with the starter, i.e. compression test. I'll repeat both tests later. Maybe a "slow" sticky valve could behave the same too.

I see a head pull in my future.