35HP Mercury intermittent performance issues

[zzbb]

1989 Mercury 35HP 2 stroke
Take it to the mechanic it runs great. Go out to fish it dies.
Here's what happens. Take off from the boat ramp, it runs perfect, and I mean perfect, for anywhere from 5 minutes to all day. But all of a sudden it can't get full throttle and won't go back to idle, it may stall and it runs rough, and seems as if only running on one piston. So then it won't give full power, won't idle in gear.
If it stalls it starts right back up.

It may run great the next trip, or it may run rough from the get go. It will not repeat this behavior at the mechanics, nor if the mechanic is out on the water with me.

Here's what we've done:
Rebuilt carburator, new fuel pump, replaced fuel filter, checked gas line, checked gas, replaced spark plugs.
Fuel delivery seems ok.
Tested all ignition voltages, in tolerance. (Ever so slight variation in ohms in the spark box but mechanic said it should be ok.)
Mechanic checked the new spark plugs, tested good.


Can any of the ignition or electrical components ever work intermittently? Mechanic is baffled, says if it was broke it would be broke, not intermittent.

 

[zzbb]

Re: 35HP Mercury intermittent performance issues

I can't figure out how to edit my thread, (it won't do anything after I click save) so I'll try replying to my thread:

EDIT:

Haven't had any answers yet, but a lot of views, so presumably other people have had this problem? Here's how things have changed since original post:

We bought a new sparkbox and put it in. (This was the only place electrical test was not perfect.)
There was a big improvement, and the symptoms have now changed:

Motor runs much much better at idle or trolling speed, seems to be using all the gas, instead of sputtering at slow speed. The first day we tested it, it ran full throttle whenever we wanted.
The second day we tested it, new symptoms showed up. Now it runs fine at trolling speed, runs good at full throttle, but GETTING to full throttle is problematic. It just doesn't seem to want to get going, it gets there, but takes longer to speed up than what we know it should be doing.

Don't want to keep replacing parts......we did find two wires going to the rectifier that may need to be replaced, we will do that but can't see how that would affect throttle. Rechecked all wiring for corrosion or breaks or loose connecitons.

Any other ideas? Coils next? One plug always looks like it is cleaner than the other after running.

 

[Jripp]

Re: 35HP Mercury intermittent performance issues

I seem to me having the same issue with my motor. Did you ever find a end cure for your problem?

 

[achris]

Re: 35HP Mercury intermittent performance issues

Jripp, :welcome: to iBoats.

As this poster hasn't been seen since June last year, I suspect he won't be answering your question any time soon.

To get the best results, please start your own thread. Remember to include all the details, including the model, year and serial number of you engine...

Thanks....

Chris......