I just finished doing a valve job on my counter rotating Merc 351 engine and when I started it up I got popping through the exhaust and carb. The engine ran fine last fall except for #3 cylinder had 25#'s of compression. I checked to be sure the centrifugal advance was working and it was. I then advanced the spark and the exhaust popping stopped but the intake popping got worse. I suspect I needed the increased advance for the exhaust but once the centrifugal advance kicked in it was too much for the intake. Did I jump a tooth on the timing chain? How does that happen when the engine was fine last fall? The timing was off when I checked it with a timing light. I did not remove the distributor when I did the valve job. The plug wires were checked three times.
any Suggestions??
Thanks

 

My first suggestion is that you may have a valve or two adjusted too tight. What lifters are using? How did you set the valve lash on those lifters?

The next suggestion is to double check your firing order.

 

Indeed, it sounds like it's a valve problem, specifically something not closing.... A quick simple check would be to run another compression test. If you're low on the one cylinder, that could explain it. Also, you could pull the wires off one at a time to see which one makes the problem go away. If there's no spark, there's no backfire, and that should tell you which one is causing it as well. If you can disconnect any one cylinder and it still backfires, it could be that the timing chain is a tooth off, affecting all cylinders.

Those are my best guesses. Let us know what you find.

-Joe