When warm 454 Mercruiser Stumbles and Missed from 1500 to 2500 rpm

[johnrmclean]

I have been working on my boat which was not run for a couple years to get it back in service this summer. Its a 1994 454 with a quadrajet carb. Initially it would not run up over 3000 RPM. I went through a series of tests etc and eventually found the ignition module was bad. I replaced the module and now it runs great at idle up to about 1000 RPM and over 3000 but I have some issues between say 1500 and 2500. Not so much on acceleration but sometimes when accelerating on to plane I may get a backfire in this range but it pops out of the water and runs fine over say 3000 rpm. My biggest problem is when the engine is warmed up and I idle down off plane into the 2500 to 1500 range . The engine seems not to be running smooth and may stumble or may surge. If I go down below1500 to say it 1000 runs smooth. If I throttle back up over 3000 on plane it runs smooth again. Any ideas or advice what might be the issue?

 

[Scott Danforth]

pull the carb and go thru it. not sitting for a few years, I bet there is crud in the idle circuits

 

[Bondo]

Ayuh,.... Sure sounds like a carb problem to me,....

 

[NHGuy]

Yeah, at those rpm it's gunked in the primary circuit. You can take a shot at it with seafoam. With a very small amount of gas in the tank add some seafoam, 1 to 1.5 cans per 7 gallons. Then run it at the problem rpm to be sure the carb gets the cleaner into the problem circuit. In fact get the carb loaded up with cleaner infused fuel and then shut off the engine for 10 or 15 minutes to allow the cleaner to soak. Restart and give it a bit of a beating, repeatedly accelerating. If that doesn't work, it's time for a rebuild.

 

[johnrmclean]

thanks for the feedback everyone...