HELP! One month old Merc 5.7L runs rough when hot

[twarner4574]

My 1 motnh old remanufactured Merc 260 runs fine when cold, but if left idle, once warm, idles extremely rough,
and it starts to die out when attempting to drive after warm up. This began when throttling up after getting on plane and the motor
backfired once or twice. I've replaced the fuel water separating filter and the inline carb filter(Rochester Quadrajunk that was rebuilt last year)
Pulled a spark plug out too and it looks good. When running, the tach rpm jumps around as well. Any thoughts? I could use as much help as possible.
This of course had to happen just in time for the 4th of July weekend. Wtf.

 

[alldodge]

My 1 motnh old remanufactured Merc 260 runs fine when cold, but if left idle, once warm, idles extremely rough,
and it starts to die out when attempting to drive after warm up. This began when throttling up after getting on plane and the motor
backfired once or twice. I've replaced the fuel water separating filter and the inline carb filter(Rochester Quadrajunk that was rebuilt last year)
Pulled a spark plug out too and it looks good. When running, the tach rpm jumps around as well. Any thoughts? I could use as much help as possible.
This of course had to happen just in time for the 4th of July weekend. Wtf.

Sounds like your not getting enough fuel. When cold the choke plate is partially closed and allows more fuel to come in, once warmed up the choke opens and there is less fuel. The Q-jets are good carbs and pretty simple but are problems when adjustments are not correct and/or dirt in passages.

If the motor ran fine last year, I would start by checking the fuel filter contents. Dump the contents into a gallon zip lock to see if there is crud or water. If this is good, put a fuel pressure gauge between the pump and carb using a Tee fitting. Pressure should be between 4 to 7 psi

 

[twarner4574]

Thanks for the advice. I'll give that a try.

 

[twarner4574]

Hey there AllDodge. I just wanted to let you know that we discovered what the problem was so you can share it with whomever else may have this problem in the future. It turned out that the issue was a loose ground on my Thunderbolt ignition module. It wasn't allowing all the cylinders to fire. Once we tightened down the ground the boat began to immediately run normal and strong. It seems that the expansion and retraction of the components as a result of hot / cold on this one month old motor contributed to the bolt creeping out and coming loose. Thanks for the help though!