I recently installed a rebuilt 351W with a crower cam. I set the distributor to 1 at TDC on the compression stroke. To get it started my friend and I adjusted the dist slightly. We ran the truck for about 15-20 min with no problems. Two days later we started it up again and set the timing. We set the wrong timing, but it was running and idling with no problem or noises. We drove it to the end of the street and stopped. While sitting there and checking the tranny fluid level a knock sound started. We shut off the truck and now it seems to back fire through the carb every time we set the distrubutor back to 1 at TDC on the compression stroke. We cannot get it started again without the seeming back fire to through the carb. The only other thing that changed is that I changed the base plate on the carb because the throttle stuck on the old carb. I set the idle mixture screws to the same as the plate that ran two days before. We are out of ideas of what the problem is. The truck ran two days ago. Does anybody have any suggestions on what the problem might be?

 

Was the timing chain replaced when the cam was installed? It sounds like it jumped time to me.

 

Sorry, forgot to mention that it is a gear drive.

 

I'd pull the valve covers and check for proper valve train operation. If for some reason (broken rocker/bent pushrod?) the exhaust valve isn't opening it can make some noises like that and the built-up cylinder pressure that can't get out the exhaust will pop back through the intake.
Hook up a vacuum gauge looking for a pulsating needle. If everything is smooth, after setting the timing, the idle mixture will have to be adjusted to the engine demand, not just the same as the old throttle plate.
Just my 2¢
Greg

 

Thanks, we have the passenger side cover off now but really only paid attention to the valves on piston 1. My friend put a vacuum gauge on when it was running before there was a problem and said it was only pulling like 5psi. I'm not sure how old ALL of the hoses and connectors are, but I want to replace all of them. However he suggested figuring out the back fire problem first. Anything else to look for besides checking the valve train? From my understanding of everything it must be a problem there but I'm wondering if anybody else knows of other things.

 

If the reading is steady, the valve train should be OK. Only 5 (inches of mercury) would indicate either a really bad leak or retarded timing. Either can cause backfiring. If the carb is set too lean (same as a vacuum leak) you'll have problems also.

 

The valve train was the place to look. Several rockers had loosen their bolts. Once they were tightened the problem went away.