Force 125 - Heavy Mid Range Shake & Shudder

[Water4Life]

New to the forum, hello all!

I just picked up an '88 Bayliner [see details below] with a 125 Force. Everything is in great shape - it was sold to me cheap because the seller's mechanic told him the motor needed a $500 part to maybe fix it... I am still trying to get in touch with the mec.

Here is what I have - it starts like a champ, idles well, once the RPM's are up and going it runs great; the issue is with the mid range RPMs and a whole lot of shaking/shuddering. I need to find a tach to put in, so for now I don't have specific RPMs to quote. But from just above idle, to just below getting up on plane, it shakes like it is misfiring or running w/o a cylinder. I have pulled the plug wires one by one, and can't find a noticeable difference. It is beyond just "running rough" it is into the "wow, that really shakes the boat" - thoughts? Any suggestion on a good Tach?

What have I done? Cleared out a bunch of old gas, sprayed the carbs down [haven't removed and torn them down yet], checked plugs [looked good/newer], used the inductive timing light to check that each plug wire is at least having some electrical activity, and played with adjusting the carbs mixing screw.

I've looked over countless forum posts, and I'm just not sure what to make of the shaking. I'd like to be able to cruise the bays instead of idle or being on plane. I have lots of comments on the electronics for this motor, and while I have plenty of mechanical experience, I am just not sure how to diagnose this if it is electrical.

Boat: 19' - 1988 Bayliner Capri Bowrider
Outboard: Force 125 - Model 1251X8C
Location: North Idaho

 

[SkiDad]

my 1st suggestion is to verify spark on all 4 cylinders. You can get a spark tester from any auto parts store for around 10-15 bucks. If you have spark on all 4 try a new set of NGK BHUX spark plugs. You can get those local as well. Some folks put the wrong plugs in these things.

after that I would go here: http://www.outboardignition.com/support/116-3301.pdf

 

[jerryjerry05]

Start it on the hose.
Make sure the prop is clear, put it in gear and check the prop for wobble?
Remove the prop and check for runout??
The shaft should turn absolutely straight.

Compression and spark checks?? Post the results..

 

[Jiggz]

I would check for plays in the driveshaft, forward and aft and sides. The shaking is probably caused by slipping clutch dogs. So to verify this, while in the water, rev up the engine around 3K~4K rpm holding it for a few seconds (you need to do this locally with the top cover off using the cam mechanism. This is to ensure the advance tower also advances timing. DO NOT JUST USE THE CARB TIE ROD. It does not advance the timing which is critical for this diagnosis). in neutral and see if the engine misfires. If it shakes, then it is a misfiring problem. If it doesn't then most likely it is a slippage in the clutch dogs.