Backfire and stall at 2000 RPM when under way?

[IMPChris]

First question for here there are many people with massive amounts of information I hope maybe someone can help me. I have a mid 80's (pre '87) chevy 350 Mercruiser set up. It has Thunderbolt IV ingnition and runs with Rochester Quadrajet. The motor runs and starts fine when not in the water. I have now put it in the water and when under power the motor starts to backfire and stall at about 2,000 - 2,100 RPM. This is only when under way, the motor wil run smooth and long at any RPM while in nuetral. So far I have replaced the fuel filter, checked the cap and rotor, the alternator is charging, and i have put in a new thermostat. I am unable to tell if the problem is ingnition related or fuel related, and would be greatfull if anyone knew the parameters of the coil and ingnition wires while running. Any help would be greatly appreciated. If I have missed any information let me know and i will fill in the blanks.

 

[Haut Medoc]

Re: Backfire and stall at 2000 RPM when under way?

Welcome to iboats......
Backfire through the carb is generally either a lean fuel condition or a bad intake valve......
The fact that it runs well while not in gear is irrelevant, as it is not doing any work.....
I would reccomend a compression test for starters....​

 

[IMPChris]

Re: Backfire and stall at 2000 RPM when under way?

Thank you I will do that. There were electrical issues with this one motor only and not the other, do think the issue could be electricaly related?

 

[IMPChris]

Re: Backfire and stall at 2000 RPM when under way?

Thank you for your help. The compression turned out OK. The problem was a limited amount of fuel due to a partial blockage back in the fule line. Much build up in line, not sure if from new type gas or not. Thank you for the help.