Anyone ever hear of the timing jumping on a new motor?

[sdkar]

I installed my new engine and it ran fine.  I have never removed the plug wires since new.  During trying to time the engine I had a huge backfire.  Now I have to set the timing to where the vacuum cone hits the carb just to get it to start.  The engine timing acts like it needs to move even more further toward the carb just to run right.  This is a brand new crate motor.  I don't think I could jump the timing chain on a new 502 with no miles on it and only about 10 startups.  It ran great before the backfire.  Anything I should check?

[Wallace]

You wouldn't think a new motor would have such a problem, but anything is possible.  I once had a similar problem and it turned out to be the dirve gear for the distributor.  Might be a place for you to check.

Wallace

[sdkar]

Here is the latest.  Brand new ZZ502 with brand new Holley 850 cfm Model 4150 (part # 12366996) that supposedly has blow out protection on the power valve.  While trying to time the engine, I had one very loud back fire followed by a couple of minor ones.  The wax in my ears fell out on the first BANG.  The engine ran horribly immediately and the plugs were badly fouled.  Finally, the engine would not even start and would just sputter and backfire.  I cleaned the plugs and put them back in and was able to get the engine to run again, though still rough.  As stated above, the power valve in the carb has the blow out protection.  How good is this protection.  Could the power valve still be messed up and need to be replaced?  How do I check the power valve to make sure this is the problem?  Finally, any other ideas on what could be wrong and what I should check or try to do?

Thanks guys,

Steve

[Wallace]

Steve,

Power valves don't like backfires.  If it ruptured it will need to be replaced.  It is found on the metering block between the front bowl and the body of the carb.  Check the valve to be sure that air will not pass through the diaphram.

Wallace