fuel or electrical?

[ron_a218]

I have a '96 90hp 3cyl Merc' . At WOT it will run well for awhile then cut back very suddenly (about 1500rpm). It will then pick back up and run normally again. At idle I've checked spark and its ok and compression is ok. I decarbed it but haven't got to try it in the lake again. It seems to me like its dropping a cylinder, maybe a bad plug or coil? The voltage reg/rectifier and stator were replaced in '05. Does this sound electrical or fuel related, it happens so suddenly my thought is electrical but wanted some help on where to start. TIA

 

[timmathis]

Re: fuel or electrical?

Hard to guess at things like that ? check your tank vent.
Some motors have a limp mode I'm not sure what years have it. You will have to do some troubleshooting.
Do you have a manual?
Good luck, Tim

 

[Silvertip]

Re: fuel or electrical?

When it tends to die, push in on the key (activate the choke/primer) or squeeze the primer bulb a couple of times. If the engine picks up, its a fuel delivery problem. Could be a clogged pickup in the tank, air leak in the fuel line, or a weak fuel pump. Try running on a portable tank if you can round one up. That would eliminate the fuel line and tank as the problem.

 

[ron_a218]

Re: fuel or electrical?

I've checked the tank and the bulb stays tight. I have a piece of clear gas tubing going from the fuel filter to the first carb where I can check for bubbles and there are none. I do have a manual but not sure where to start. I hate intermittent problems.

 

[ron_a218]

Re: fuel or electrical?

I'm sorry if I led you to believe it was dying, its not, it only cuts back on power suddenly at open throttle, it's so sudden you feel like you've hit something!

 

[Texasmark]

Re: fuel or electrical?

Cutting back to me would suggest to me that you are dropping 1 cylinder. At high rpm's plugs are usually a culprit. That engine has a funny triggering circuit (I have one and just happen to be in that area of the service manual this minute) where the three trigger/coil modules are wired in series:

"#1 CDM (Capacitor Discharge Module....includes the coil that connects to the plug cap) stator voltage (trigger) return path is through either CDM #2 or #3. The return path for CDM #2 and #3 is through CDM #1......."

I think I would go after grounds first because high rpm's mean high triggering currents (fires more times per minute so current is higher) and higher currents through marginal grounds could build up voltages that could keep the triggers from operating properly.

Possibly the crud makes a resistance and the voltage builds up over this resistance (when that cylinder quits firing) and finally gets large enough to jump over the crud and work till it has to do it again.

I'd remove each module from it's mount and clean up the area where the screws touch the plate and the module (grounds). Clean up where the plate touches the block the same way.

The black wire is the ground return. follow it around to the 3 trigger modules and look for a place where it is tied to engine ground. .....remove that screw and clean all that and inspect all black connections between this ground and between the modules, carefully pulling and pushing the connectors in and out will be a good idea to insure that the contact is clean.

HTH

Mark

 

[ron_a218]

Re: fuel or electrical?

Thanks Mark , I'll check out the grounds this afternoon and probably change plugs when I can get some. Ron A

 

[rednecktech00]

Re: fuel or electrical?

My Alpha One Mercruser was doing a similar trick. Since I was dead in the water with nothing else to do, I tried to remove the gas cap. Couldn't do it without a wrench. Ran fine with the cap loose. Changed the vent before the next cruise. No more problems.

 

[Texasmark]

Re: fuel or electrical?

Hear you on what happened to you redneck. I have had similar experiences but when the vacuum built up in my tank the fuel pump couldn't pump and the engine just shut down completely and stayed there till the vacuum bleed off.

Ron said his an off and on thing so I looked elsewhere. '96 is old enough to have corroded grounds I'd think.

Mark

 

[ron_a218]

Re: fuel or electrical?

I cleaned and checked all the grounds and didn't see a problem. I mentioned to a friend that I thought it might be a faulty coil and he suggested I start it and run it after dark and that I might see spark jumping and sure enough it was jumping from one plug wire back to one of the grounds coming off the middle coil. There was also light or glow where each plug wire went under the little clamp that holds them close to the block. I assume none of these conditions are normal and I guess I need new plug wires. I guess my #2 wire was periodically jumping to ground and causing my problem. I guess I won't know till I get back out to the lake. Thanks guys for your help.