507F90C Force Outboard

[Bob La Londe]

1990 50 Horse Force 2 Stroke.
A while back I trashed the lower unit on my Mercury so I threw this old Force back on my boat until I could get a new or rebuilt lower unit. It ran great first time out, but the tach did not work. Better top speeds than I had ever seen on that motor on that boat before running two up.

When I got in I checked the connections on the back of the tach and found that a couple of the wire terminals were touching. I corrected the problem, and the tach began to work erratically.

The tach finally failed, and the motor is running on one cylinder most of the time. After it warms up the second cylinder fires, but misses erratically and often enough to noticeably slow down the boat.

Now I am wondering, if the problem is more likely a bad tach or a bad ignition winding or a bad coil? Any suggestions.

I do have a manual for this motor so "go buy a manual" would not be as helpful a suggestion as it might be in other circumstances.

 

[Frank Acampora]

Re: 507F90C Force Outboard

Typically, ignition problems get worse with warm-up so that confuses me a little.

It would be wise to closely examine the plugs and plug wires to eliminate an intermittent short. Don't forget to check the plug boot clips. Broken terminal inside may cause problems. See: It takes a hotter spark to fire an engine at start and while cold, so a marginal spark could cause problems.

Not too familiar with that engine, however, at this point, I would completely disconnect the tach at the engine terminal to eliminate any problems with tach or wiring as a source of trouble.

Then, if that is the electronic ignition with a cd module, disconnect the stop wire at the engine terminal to eliminate stop circuit and switch as a source of trouble. CAUTION: the engine will not stop running with the wire disconnected. If it should be magneto ignition then the points and condensers need to be checked.

If those check out, then it is more likely that a bad cd box or coil is causing the problem. The stator would be the least likely problem since there is basically nothing to wear out with age. Yes, they do go bad but less frequently than other conponents.

Also, do not discount the possibility that it may be fuel related. Too rich would usually give a slow but steady run, however, too lean could cause spitting, backfiring, and missing.

 

[Bob La Londe]

Re: 507F90C Force Outboard

Its pretty much the bottom cylinder not firing. I guess its time to get out a meter and see what I can find.

 

[Frank Acampora]

Re: 507F90C Force Outboard

If it's electronic, switch the coil wires. If the bottom now fires, then the CD box is bad.