Idle Issue

[broux2]

Boat: 1991 Malibu Sunsetter with 5.7L Mercruiser Ski.

I rebuild the carb over the winter and overall is running much better. The issue I have now is when slowly cruising around for more than a half hour the idle rpms are getting much higher to about 1100rpm. When I start the boat and warm up for a few minutes they are at 600rpm Any ideas what would cause this? Thanks..

Edited August 2, 2009 by broux2

[phataz5]

I have just done my carb on my 95 response and have the same probz..

[Baddog]

Mine's EFI so I don't know if my problem would apply to your engine but . . . . . I had to replace my engine water temperature sensor, which, as it failed kept telling the engine it was cold when in fact it was warmed up, resulting in a very rich fuel/air mixture (I.e. when starting cold) and the idle rpms were quite high. Also made for troublesome restarts when warm.

[skier92]

Idle and/or choke adjustments. (or idle mixture valve if there is one)

Edited August 3, 2009 by skier92

[broux2]

Does anyone have information on choke adjustments? The choke I have is the coil that is bolted on top of the engine.

I would imagine EFI information would not help at all in my case but thanks for the info.

[Green_Giant]

I would say some combination with the choke adjustment and the idle set screw. I haven't really seen the idle mix screws lead to a high idle, just a rich idle. As you screw them in to lean it out, it will start to stumble, but doesn't sound like your problem. My 90 supra had the same challenge, it had a Holley 600 w/elec choke, not sure what you've got.

You have your main idle set screw where the throttle cable hooks in. It could be set too high to compensate for a poorly adjusted choke (when cold), then when it warms up, you see the high RPM's. Try getting it up to opperating temp, make sure the choke is full open, and back off the main idle stop screw down to 650 or whatever is spec for your motor. Then after your motor is fully cold, make sure the choke is fully closed. May take some trial & error to get it adjusted right, and this all assumes your choke is properly functioning in the first place.

[68Slalom]

Sounds to me like a small vacuum leak that develops after it gets warm. You wouldn't notice so much cold because the motor is running rich, you can try spraying WD-40 around the carb. base and around the area to see if the idle comes down. You basically sealing up and checking for leaks, you can also use propane if you have the proper hose and valve.

Edited August 3, 2009 by 68Slalom

[broux2]

Thanks for the replies. Looks like i have some things to try...

[broux2]

so the choke looks to be functioning fine. Today I took the boat out for a while and was putting around for about an hour. After about 30 minutes the rpms started to go up when putting back inn neutral. I kept putting around for a while longer and the engine started to sputter as we were moving. I hot the gas to go faster and it sputtered pretty bad for a few seconds and then took off. I slowed down for a few more minutes and the same thing happened. Any thoughts.

[Baddog]

so the choke looks to be functioning fine. Today I took the boat out for a while and was putting around for about an hour. After about 30 minutes the rpms started to go up when putting back inn neutral. I kept putting around for a while longer and the engine started to sputter as we were moving. I hot the gas to go faster and it sputtered pretty bad for a few seconds and then took off. I slowed down for a few more minutes and the same thing happened. Any thoughts.

Sounds like intermittent crap in your fuel lines somewhere.

[Woodski]

What type of carb? There are two basic carbs on these boats, either a Holley or the Weber (same as a Carter AFB). Both operate very differently so troubleshooting can be very different, most Mercruiser's I have seen have the Weber carb. If so, a Carter rebuild guide will show you how it operates, Edelbrock also sells these carbs under their name.

Ideas mentioned are good things to verify. The sputtering on acceleration ciould also be the accelerator pump. Either the diaphram is bad or there is dirt somewhere in the circuit not allowing a good fuel discharge from the nozzles when you crack open the throttle.

When you rebuilt the carb did you check if the throttle blade shafts sealed correctly, weren't too worn, that would be a source of an air leak that could contribute to your problem.

[broux2]

so it is a rochester 4mv carb wth a divorced choke. throttle blades were good when rebuilt a few months ago. Accelerator pump is new also. I guess it could be crap in the lines but what explains it happening around the same time every time i go out. It is only after the engine gets completely warmed up. Carbs are so frustrating.

[Baddog]

It's been so long since I worked on a carb but, when it goes whacky on you is the choke seeming to re-engage causing ti to run crappy? Could be related to the water temperature sensor malfunctioning.