I put a post in a different forum about a recient mod I did to my 427. The mod included going from a 650 to a 770 Holley carb and changing from factory side pipes to headers and 3 inch tubes.
One of the resulting problems was a backfire. From idle to 2500 rpm the car seems to run fine. Over 2500, it backfires while holding a steady speed, decelerating or even accelerating. The number 3 plug continously fouls ( I have changed the plug and wire numberous times ) It also does something else...it constantly blows the PCV valve out of the valve cover.
I finally had some time to work on the car tonight after a club meeting. I dug out a compression tester and checked number 3 cylinder. It had 120 lbs of pressure. I thought this was ok till I checked number 1 and it read 190 pounds. AHAAA said I. Actually I said something much more colorful but this IS a family forum. I removed the valve cover and had my wife crank the engine while I watched the valve train. The fourth rocker arm from the front of the engine was not moving at all. Now I remember reading somewhere which this is, but a function of age keeps me from remembering. Is it the exhaust or intake valve?
Anyhow, I took out the trusty socket set and started cranking on the rocker arm fastener until the spring seemed to compress the same distance the other springs compressed. Yes, I actually do know how to do a valve adjustment by cranking down the nut till it taps and then back off the adjustment. The three inch tubes and extremely nonrestrictive mufflers will not allow that. Just no way to hear it.
I put everything back together and started the car. It no longer seem to have a steady backfire. I don't even get one if I slam it up to 4500 or 5000 and let it backdown. Problem fixed??? I will try to get it out tomorrow for a quick ride and see how it acts.
Anyway.....I can understand the backfireing and plug fouling if a valve was stuck partly open but not if it is closed.
Any thoughts on :
1. Why did it backfire
2. Why is the PCV valve being blown out of the valve cover (never did that before the mods)
3. If I have the adjustment too tight, will I screw up the cam lobe
Thanks in advance and VERY sorry for such a long post.
G.

That would be an exhaust valve. If that valve is closed than the pressure builds in the cylinder causing the back fire when the intake valve reopens, sometimes it will end up blowing the head gasket. That may be why your compression is low. Did you recheck it afterwards? I tend to think it is because that would could be why the PCV valve is blowing out of the valve cover.

|UPDATED|9/7/2003 6:14:47 AM|/UPDATED|

Correct on the backfire because of pressure being released by the intake valve. I'm not crazy about the way you set your valve lash. I've always read and have always set mine with the valve closed on the backside of the cam lobe engine not running. Turn the rocker arm nut until all slack is removed and there is a light grab on the pushrod while trying to twist it between your fingers, once your to this point give it another half turn of the nut. I have a concern that when you first took the valve cover off there was no movement. When did it just start doing this? If it was all of a sudden and you had to really make a big change to get the valve to open then I would say the lobe is starting to go away on the cam and its only going to get worse. If you over tighten it several things can happen.

One - bend the pushrod.
Two - Hang the valve open and the piston comes up and smacks it doing major damage.
Three - Whip the cam clean putting lots of metal through out the engne.
Four - Break the lifter by putting to much stress on the plunger.

Best way to see if the cam lobe is gone is to use a Gauge to measure the lift and compare it to the cam specs. Sometimes you can remove the push rod and pull the lifter from the bore and use a bright light to inspect the lobe.

Sunday morning I rechecked the compression and there was NONE. So I started to loosen the nut till I managed to get up to 180 lbs. I took the car out for about 30 minutes. The backfire is gone on acceleration and steady speed but still there on deceleration. I spoke with the guy who did my heads for the 454. He seems to think that it is now a function of a lean carb. he is going to look at the car Friday morning.
I think the 4th valve is the intake side of #3.

You may have burned the exhaust valve

The carb was adjusted, float reset and vacuum advance disconnected. Timing set to 36 degrees. Car runs great at both high and low RPMs. I can pull out in 4th gear and experiance no pinging from the engine. My dyno test from a past year shows no power increase over 4000 rpm. The car now actually feels like it makes power all the way up to red line. At 3000 rpm ( 70 mph ) I can punch it and get pushed back into the seat. Still backfires when I take my foot off the gas but I think that is the header gasket. If you hold your hand over #3 cylinder, you can feel the exhaust coming out. I will pull the header and replace the gasket when I get a chance.
Thanks for the input.