Two technical issues:

[Todd Hedges]

1) How do I knoe if I damaged the "pop-off" valve on a Holley 4150 hp? While starting the car for the first time since the re-build, the timing was off and it back-fire through the carb. What kind of damage could have been done?

2) While trying to set the timing to 36 degrees at 3,000rpms, the timing would jump from the top of the index pointer to the bottom of the pointer. I estimate that jump to be 7 to 9 degrees in variation. What causes that condition? Would the multiple spark feature of the MSD 6AL cause that? According to the MSD info, there are 3 sparks per cycle for each cylinder while under 3,000 rpms. Would that cause a mis-reading?

I wish I were more engine worthy.
Someday I might be...

Thank you for any help.

Todd Hedges

[Hal Copple]

for one or more decades, Holley's have a built in back fire protector in the base, so it makes it unlikely but not impossible to blow out the power valve. It is a small "ball" that seals under back pressure.

The MSD fires multiple sparks under 3000 rpm, but i run an MSD, and it times just like any other car. I wonder if you are getting some sort of misread from an adjacent plug wire or something. Why don't you be absolutely sure you have no cross firing, perhaps you can clean out your distributor cap or something.

My timing light works just fine and normally off the number one plug wire. It is the front one, on the passenger side of the motor, that is number one cylinder.

[George Snyder]

Todd,

Just put your fingers over both air tubes. If the engine shuts down, then the power valve is blown.

You may ahve a bad timing light.

[CSX 4027]

I have seen rubber cams, deflecting timing covers, separating balancers, loose distributor hold downs etc. It can be a real chase.

[flipper35]

I am not sure on the dist you have, but some have a loose plate in the advance assemble that allows too much play and the timing fluctuates all over. SBM distributors are notorious for this.

[decooney]

Todd,
among other things mentioned...here are a few additional things to check on your ERA with FE and MSD setup...

- make sure the distributor pickup (epecially if its and MSD distributor hooked to an MSD box) wires are not unshielded or running adjacent to any spark plug wires. ERA uses a specific routing in their MSD equipped harness to avoid this. Even so, I would be sure to shield the pickup wire or it will cause the MSD to misread and cause a slight misfire at varying RPMs.

- on an FE, make sure #7 and #8 spark plug wires are not touching each other. If they are close, move them as far apart as possible.

- also, my friend had a bad pickup coil in his NEW MSD distributor that cause his MSD box to misfire or not fire once hot. You can check for the specified resistance at the leads if it's an MSD distributor. $17 and some silicone to replace. It was like 400 or 900 ohms I believe - cant remember...

[David Kirkham]

We had that problem (wandering timing marks) until we bought a Snap-On timing light. All problems then went away.

I am not saying all of the other cool advise here is wrong...as a matter of fact...I am now adding it into my aresenal on knowledge. This site is great.

David