Running rough at low RPM after warmed up
 

97 Gen VI 454 330HP Carb motor. My boat starts great cold and runs great then I go through a no wake zone for a few minutes followed by a 15-20 minute cruise at about 3/4 throttle. Now I come to another no wake zone and idle the boat through. Boat sounds like its running on about 6 cylinders from idle to 1200 rpm. Above 1200rpm it clears up and runs normal. What I have done so far, new plugs,used carb cleaner, checked choke after warm adjusted the air mixture screws to one and a half turns out and experimented with a half turn in either direction from that. It is the Weber Carb. Anyone have any experience with this problem? Suggestions on next steps in troubleshooting this? I was thinking new cap,rotor,wires and possibly coil?

when was the last tune up done?

Not sure. Just bought the boat at the end of the season last year. It has 310 hours on it. Thats kind of the reason I was thinking wires cap and rotor and coil. I did just put new plugs in it. I also did normal maint after I summerized it so It has a new fuel/water filter too.

Gonna throw this out there...others with much more understanding than me can confirm or deny.... I have the same issue with 7.4MPI's, and I'm starting to think it might have something to do with the notorious dreaded vapor lock tendencies of these boats/motors. Most link it only to heat soak after shut down, but mine definitely "lose" a couple cylinders during prolonged idle, only to recover completely with a return to running higher RPM's. It seems like the onset of VL (when fuel flow is minimal) which clears up when flow increases...???

Possible I guess. Would you be able to test this theory by unscrewing the gas cap while its acting up? Would that relieve the vapor lock if that is whats happening?

I WISH it was that simple but NO! There seems to be a couple of main approaches that involve installing a vent back to the fuel tank and/or Mercruiser's electric fuel pump kit. It's all about replacing vaporized fuel with liquid fuel. Again, I have no idea if this is related to your problem...just saw your thread while investigating my similar problem hoping to find answers. There are guys here that know a ton if you can get some replies! Good luck!

From what I have read so far I dont think vapor lock is my problem. It seems to be more of an issue with EFI models and not so much or carbed models. I also never have any trouble restarting when hot,it will always fire up just doesnt seem to run right till it gets above 1000 rpm when up to temp???

I'm going to lean toward a carb rebuild. If the old owner didn't use an ethanol free fuel or a treatment the needle and seats are probably worn.

Was it a salt water boat at some time ?

Just wondering if the exhaust may be leaking a tad and killing spark at low rpm.

Also, with today's fuels, always suspect carb problems.

I would call the seller and see if he had any issues like this last year. If he says no, I would be looking at the carb as a possible problem, but mostly I would say your problem is plugs or wires since you changed the plugs. Even if a plug wasn't cracked (very easy to do) when installing, you can have a bad plug out of the package, and I have had plug wires come lose after putting them on even though I knew they were attached good. Since you don't know how long it's been since a tune up, I would pull the cap and rotor to check those out, or replace with good quality replacements. If there is any question about vapor lock, when it has the problem just pull the plug off the top of the VST tank and it will relieve the vapor pressure into the bilge, just make sure your bilge vent is on. But it doesn't sound like the problem you are having.