Symptoms, symptoms, symptoms, I am spelling it 3 times correctly now. And what tests are best to determine if cam lobes are flattened? Thank you.
An exhaust lobe causes popping back through the intake. An intake lobe causes a miss. Sure fire way to tell is to pull a valve cover and watch the rocker action. It's easy to see one rocker moving less than the others.
Run a proper compression test. You should see a difference in the cranking pressure on a cylinder with worn cam lobes. This test, of course, assumes that the engine is in otherwise good condition. No collapsed rings, holed piston, etc. Bob
A friend called me years ago while tuning up his old '67 Chevy with a small block of some sort in it. He had it running fairly well and decided to do a cylinder power balance test by watching the tach at idle and pulling one plug wire at a time and tracking the RPM drop on each cylinder. A lesser, or no change of RPM in one cylinder would indicate a weak hole. Then he tells me that on seven of the cylinders pulling the plug wire drops the RPMs about 75 or 100 revs. But on one hole the RPMs actually pick up like 50 or 75 revs! He's puzzled and so was I for a little while. NOT firing that one cylinder made the engine run faster! Well after running that around in my giant brain for a couple hours the only thing I could think of that would cause that kind of behavior was an exhaust restriction in that cylinder. There wasn't any obvious valvetrain noise so we pulled the valve cover on that side of the engine and started it back up. Sure enough, the exhaust valve on that cylinder was barely moving compared to the others. The rocker and pushrod were OK but the exhaust lobe was wiped out. Which as it turned out was not too uncommon for SBC cams of that era. In another instance a coworker had a 68 chevelle with a 396 that he was regularly tinkering with. It had a mild cam and headers but he never felt that it ran as well as it should have. While looking under the hood one day after work I happened to notice that one of the header tubes near the flange was darker in color than the rest. With the engine idling we squirted a bit of water on each header pipe. It would evaporate almost instantly on every pipe except for the one that was darker in color. I told him my suspicions and a couple weeks later he pulled the intake manifold and found a few wiped lobes, including the exhaust lobe on that one cylinder. You can do the same thing by painting a white stripe on each header tube near the flange and watching to see if they all discolor evenly after running it a while.
You mean popping back through the carburetor, skipping, watching the rocker arms move one third of what they should, and valve clatter aren't enough?
Lopey idle like if it was a hot cam, pushrods coming out of the rockers, maybe bending after, and CATCHING ON FIRE.. Happened on a 454 I had that somehow didn't get broken in right..we did all the right things, we thought..but it just ground away. Also on the 302 I had in my truck..no fire on that one..
Missing,lifter noise,could just be a wore out lifter,what engine are you working on anyway? a roundeed out lifter bottom can give you the same synptoms.
This is a 350 Chev. Developed a backfire through the carb. Did everything several times relative to the accelerator pump, idle transfer holes are clear, idle transfer tubes are clear, idle mixture screws are not set too lean. (Rochester). Increased advance on the timing, backfire went away but became a hesitation. (normal acceleration from a stop). Timing is now advanced more than I would normally have set it. No vacuum leaks anywhere. Valves have been adjusted static and running several times. There is a random ticking (sounds like a lifter) that develops after the engine warms up. The ticking comes and goes, it is not constant. I just checked the lift with a dial indicator placed on top of the valve spring retainer. I am getting readings in the .311-.342 range, the cam (factory Chevy) specs are 450 lift intake and 460 exhaust.
The Jalopy Journal
