3.0 mercruiser running rough

[robbyrobs]

have a 1988 citation with a 3.0 mercruiser alpha one, had the marina rebuild the carb, tune it, new plugs, new wires, new cap, rotor, points, condensor, new coil, new shift interuppt switch, put a new fuel filter, and fuel water seperator on it, and the boat runs great at first, starts right up, runs great, but after an hour or so of operation it starts to stumble at higher rpms, starts around 2500 to 3k mark and won't go much past that. it doesn't stall, and you can kill the engine and it will start back up. also while it is stumbling it will go in and out, almost as if your going on and off the throttle. not sure what it can be, we put 93 octane in it and have probably ran close to 3 tanks in it so far this summer. any thoughs?

thanks

 

[myoldboat2]

Re: 3.0 mercruiser running rough

A year or two after I had a carb rebuild on my Merc 140 it started running really rough after about an hour of hard running. Turned out the choke was closing. Something wrong with the coil. Currently running without a choke, will probably install an electric choke kit. Did you check the angle of the choke plate when the rough running started?

My engine is older than yours, but another problem I had awhile back was with the resister wire. Rather that put a resister on the transom, Merc went with a resister wire. Those get old and corroded and develop too much resistance. As the wire got hot, an hour or two into running my engine would shut down. I no longer had points--I had gone to a Pertronix electronic ignition. The voltage going to the Pertronix was dropping too low because of the resister wire getting hot, and the unit would shut down. I replaced the resister wire with regular wire, tried a resister on the transom, but eventually removed the resister and just went with the other kind of coil. Runs great since.

Good luck with it.

 

[robbyrobs]

Re: 3.0 mercruiser running rough

yeah checked the choke, and it was fine, but it wasn't running either. not sure if i understand the whole resistor wire your talking about? i know i replaced the condensor under the distributor cap?

A year or two after I had a carb rebuild on my Merc 140 it started running really rough after about an hour of hard running. Turned out the choke was closing. Something wrong with the coil. Currently running without a choke, will probably install an electric choke kit. Did you check the angle of the choke plate when the rough running started?

My engine is older than yours, but another problem I had awhile back was with the resister wire. Rather that put a resister on the transom, Merc went with a resister wire. Those get old and corroded and develop too much resistance. As the wire got hot, an hour or two into running my engine would shut down. I no longer had points--I had gone to a Pertronix electronic ignition. The voltage going to the Pertronix was dropping too low because of the resister wire getting hot, and the unit would shut down. I replaced the resister wire with regular wire, tried a resister on the transom, but eventually removed the resister and just went with the other kind of coil. Runs great since.

Good luck with it.

 

[myoldboat2]

Re: 3.0 mercruiser running rough

The resistance wire is in the wiring harness. I can never find the parts diagrams on iboats--maybe somebody else can point you to it. Your wiring harness costs $200+, but I see on another site they have part # 25 in the diagram which is "84-94227A2 RESISTANCE CABLE KIT (TO REPLACE RESISTER WIRE)" for 22 bucks.

Not saying this is your problem, just a problem I had that kicked in after an hour or two of running. You should be able to troubleshoot it by measuring the voltage at the coil as the engine warms up over a few hours. If the resistor wire is bad the voltage will drop as the wire heats up.

 

[myoldboat2]

Re: 3.0 mercruiser running rough

btw, if memory serves me i think they design it this way so that the coil works at lower than system voltage--that way when it's time to start, they can provide full voltage through that "starter assist" wire or whatever it's called. i may be all wet on that, but if so somebody else will correct me.