350 Engine Backfires Through Carb

Alright, so I got a motor problem with my boat I thought you might like to try and figure out. Nothing is easy lol. Its like a puzzle. Here we go.

The motor I am working with is a GM 350 with an Electronic Distributor. It has the standard wiring (Old School) with Resitors, etc. I only have 60 hours on them since I completely rebuilt them.

It has been 6-7 months since I was last able to start and run my boat due to work, money etc. The last time I drove my boat it ran perfectly. I went down this past weekend and here is what happened.


Port Motor: Started and at Idle would have one single backfire through carb every 1-2 seconds at Idle. Nothing had been moved or touched since our last run.
Items I checked immediately:
- Distributor Cap – Cracks etc – Checked out Perfect
- Plug Wire – Burnt or Cross Firing – Checked out Perfect

Based on these findings I decided to pull the coil wire and turn the motor over. I got the same popping noise through carb.
Items I checked Immediately:
- Pulled all Plugs and turned over motor – No noise or back firing heard
- Checked Compression on All Cylinders
Left Bank: #1 – 145lbs, #3 – 145lbs, #5 – 150lbs, #7 - 142lbs
Right Bank: #2 – 150lbs, #4 – 145lbs, #6 – 155lbs, #8 - 150lbs

Here is the tricky part. When I put my compression tester in cylinder #6 and turned over motor I could hear the back firing noise. I could not tell where it was coming from though. I decided to plug cylinder #4 with a sparkplug and when I turned the motor over you could hear and see it back firing through the carb again (the linkage on the carb was moving when you would hear the back firing noise).

So based on all the information I have it has been really tough to come to a conclusion as to what is going on with it.

You would think that if it was a blown head gasket I wouldn’t have any compression, or if anything was wrong with the valve I wouldn’t have any compression. Nothing adds up since I have good compression on all of the cylinders.