Backfire through carb

[Csand714]

Hey everybody, hoping someone with a little more knowledge can help me find a solution. Working on a Mercruiser 350 5.7 I’ve been working on and having some issues.

My main question, will a bad Thunderbolt IV ignition module cause a backfire through the carb at idle?

The engine starts flawlessly, runs and idles but at around 160-190 degrees on muffs at idle it will randomly backfire through the carb and stall. At that point it’s difficult to get restarted and will backfire and stall almost immediately. I’ve had the heads rebuilt recently due to water in the oil, new ignition sensor, distributor rotor, coil, plugs, and wires. Timing is set to 8 degrees.

My only idea is carb or the ignition module getting hot and sending bad signals? I have a Quadrajet 4 barrel that was rebuilt a little while ago. Adjustments to the fuel mixture screws don’t seem to have any effect and manual choke plate opens smoothly. I’m leaning towards buying a new carb as this one could have permanent damage from overheating in the past?

 

[alldodge]

Always good to list what motor your working on. Your previous post list a 1984 350

My only idea is carb or the ignition module getting hot and sending bad signals? I have a Quadrajet 4 barrel that was rebuilt a little while ago. Adjustments to the fuel mixture screws don’t seem to have any effect and manual choke plate opens smoothly. I’m leaning towards buying a new carb as this one could have permanent damage from overheating in the past?

The module isn't causing the backfire. If the distributor cap is corroded up this could cause some issues.

If timing is set correct and the valves have also been done correctly. Valves are adjusted for zero lash and then 1 additional full turn. Zero lash is where there is no up/down movement of the push rod.

If all set correctly then I would see a carb issue

 

[Csand714]

Thanks for looking out for us rookies alldodge. I did the valves myself and set them to zero lash with a half turn not full turn and have brand new distributor cap, wires plugs and coil. I’m leaning towards either water in my fuel or carb issue. Potentially my carb may be warped and running lean due to overheating in the past? If that’s the case would you recommend a new Quadrajet or switching to a Holley 4175 or edelbrock 1409?

 

[alldodge]

Most go to the 1409, but the Q-jet is a carb that is one of the easiest to modify if needed, not that the 1409 is hard

I would get another 1/2 turn on the rockers

 

[Csand714]

I’ll give that a try today and see what happens also going to check my compression. Don’t really want to switch to the 1409 because of the need for new intake manifold or adapter plate.

 

[Csand714]

So just following up, turned out the carb had a clogged idle pick up tube and that’s why it was backfiring once the fast idle and choke finished their functions. Good news!