Weak fire...possible miss. Need Advice...

[justaZcarguy]

OK, here's the scenario, this engine is the one in the '74 260. It's been built from head to toe, L28 / N42 setup, bored, hemispherical pistons, cam, head is polished and ported, mikuni intake with triple mikunis, mallory dual point ign., headers, bla, bla, bla. The motor sat for about 1 year. Has been run hard for the last 40k miles or so. Compression on the number 1 hole (front of the motor, closest to the fan) is 150. The heat coming off the header at that hole is way cooler that the rest. Seems to miss. Have replaced the wires, plugs, cap rotor...

With the compression at 150 it doesn't seem like it could be rings. I know this is vague, anyone have a place to start on this?

The motor has plenty of power but the miss is driving me crazy, and no, it does not seem like a cam rough idle.

I can give specifics on the exhaust temps later but last time I checked it was 100 degrees difference or so. This was established by using an infared temp gun with a laser.

Advice? Places to start? If this is not enough info, I'm sorry, this is being composed at work and I need to hurry. Questions are welcome....

Just need a place to start on this....

Thanks all!

[hmsports]

Have you checked/adjusted the valves? Maybe an intake valve is loose and not letting in enough fuel on the intake stroke -- or maybe the exhaust valve is not letting the gases evacuate.

I had a similar problem once and when I took the valve cover off, one of the intake rocker arms had popped off -- very simple fix. Caused by soft valve springs and over-reving.

Maybe a long shot but check for blockage in the number one intake manifold runner.

Do you have the ability to do a leak down test? I'm not sure if that will show anything.

What about a burnt valve or maybe a leaking head gasket?

Just some ideas.

Rick

[2ManyZs]

I agree with Rick. Check out the possibilty of a loose valve and/or a worn out rocker. Only have to take off the valve cover and do a good visual inspection.

A leak down test would be a wise second choice if you see nothing out of the ordinary when you do the visual inspection.

One simple test and you will know whether you have a valvetrain problem or a more serious problem, eg. rings? If you pressurize the cylinder and can hear air escaping through the carbs, it is a valve problem. If you hear nothing there, take off the oil filler cap, if you hear air there, well, it's a little more serious......

If you hear the air through the intake at least you will know that it is only a head problem. You will then need to figure out if it could be a loose/improperly adjusted valve, a possible burned or bad seat, a bad valve stem guide, or possibly even a bent valve. Have you ever had the valve seats replaced with hardened steel seats? This would be one of my first picks for the trouble spot. Taking a head off is much better than tearing down the whole engine.......

Good luck and let us know what you find.