mercruiser 140 with issues

[James D. Evans]

84 Mercruiser 140...

Starts, idles, and will run perfectly fine for about 30 minutes, full throttle or part throttle, but then will start cutting out an back down to about 3500rpm, and continue to get worse till it won't run over 2000rpm. Will run fine up to 2000 rpm, anything more it just wont take. You can hear it in the exhaust cutting out. Even when it is acting up, you can shut it down, and it starts with the bump of the key, just won't run over 2000rpm. Does not shut down, die, etc, Have put a jumper wire to coil from battery, with no change in running condition. Anti-siphon valve removed with no change in how it runs either. Fuel pickup is clean and clear. Not overheating, runs about 140 degrees. No unusual noises, other than the exhaust when it starts acting up.

History on this boat.

Head replaced 11/2013. Points, condensor, and coil replaced this weekend. Sourced a ic14sb coil from Napa, which is a true 12 volt coil for points. Use them on old ford tractors as well, and I have 12 volts going to it, not using the resistor wire. Wires, plugs, dist cap, and rotor button replaced 10/13. Timing at 6 Btdc, and dwell at 30.4. Drained fuel tank, and installed all new filters, boat does have a water separating filter also. Running non ethanol, 87 octane fuel. Battery/Alternator good. Cleaned all grounds, and made sure the big plug was seated correctly and clean.

 

[Maclin]

Could be fuel tank venting issues. When this happens try loosening the fill cap, listen for rush of air, and see if it can gain the RPM's back.

 

[dubs283]

Sourced a ic14sb coil from Napa, which is a true 12 volt coil for points. Use them on old ford tractors as well, and I have 12 volts going to it, not using the resistor wire.

dont know what kind of coil that is but you should be running no more than 9-10 volts to the coil with a points ignition

sounds like the points are glazing over/burning and the condenser is breaking down causing weak spark

 

[boozer1966]

I have a 1984 Regal with a Mercruiser 140 (serial #6853440) with very similar problems. Mine will start and run great for about an hour, then it develops a miss that appears to be clearly electrical. (It becomes much worse if you hit the trim button when it is happening.) I have replaced the entire ignition system to include Pertronix electronic ignition. I had a bad spark plug which was making the problem worse. After replacing all of that, the boat runs much, much better, but after a period of time (heat) the miss returns. I have also taken apart and cleaned all of the wiring connectors I can find. The biggest difference for me came with installing the Pertonix (with coil) and new plugs and wires. Best of luck.

 

[James D. Evans]

I used to be able to make mine miss/skip when I hit the trim button, but the new alternator cured that.

 

[James D. Evans]

Update, FIXED!

Cleaned all connections with a wire brush on a dremel. Installed electronic ignition, resister between ignition switch and coil to keep the amps down at the electronic module, new rotor, cap, wires, and plugs. Reset timing, and adjusted the idle after it was verified to run correctly. Ran 1/3 tank of fuel yesterday without a hiccup, about 4 hours run time...3 of which was towing. The skip/miss/whatever when the trim was used prior also disappeared.

 

[kbibeau1]

I am also having the same problem. I was just curious if you bought the electronic conversion kit that uses the same distributor or if you went with the full kit that comes with the distributor and all. thanks

 

[James D. Evans]

Purchased the Kit with NO distributor.