Half power and really shaky

I cleaned the carburetor of a mid-80's 4-stroke 9.9HP Honda and, following this, it worked beautifully for a couple of week-ends.
Recently, I can only have it run at half power and it's very shaky.
Does that ring a bell to anyone?
I tried to play with the throttle directly in the motor just in case, but it makes no difference.

It may have jumped time. Check the timing marks to make sure all is good there.

If ok, pull the choke out a little to see if the motor picks up. There may be issues with the carb again.

Of course, drain the carb into a glass jar and see if there is water in the fuel.

Could also be a bad spark plug.

Mike

Really shaky....half power....two cylinder engine.....sounds like maybe you have one of those cylinders not firing.

Try this:
Pull your spark plug boots off and lubricate them with a TINY bit of dielectric (tune-up) grease. This makes it easy to perform the test I am about to suggest.

Start the engine and set the throttle somewhere well above idle. About 1500 rpm is ideal but if you don't have a tachometer, just raise the rpm up off idle a bit.

Then, using a DRY glove or a DRY rag....or both for insulation...pull one of the plug wires and see if the engine speed changes or not. If the engine seems to run pretty much the same with the wire off then that is probably the cylinder that is misfiring. If the engine stalls or changes in speed significantly, then that cylinder was firing and you simply need to try pulling the wire on the other cylinder.

If pulling the wire on each cylinder makes a big difference in the way the engine runs...then BOTH cylinders are contributing.

If one of the cylinders seems to be misfiring using this method then the simplest test would be to SWAP spark plugs between the cylinders and see if the problem follows the plug. If so...bad plug. If not...then it might be a bad plug wire...or maybe even a stuck valve or a timing issue.

Many times just doing the first part of this...removing, greasing and reinstalling the plug boots....makes the problem go away... Which, of course, indicates that there was simply a bad connection.

Good luck.

Thanks
Fantastic forum!
I pulled the choke a little and it worked better, so I opened the carburetor again and there was some new stuff in there.
Cleaned everything and it works much better.
These things are so sensitive, that's crazy

Yes...these little carbs will tolerate ONLY the best of fuel and the best of care. Don't fudge on putting clean FRESH fuel in a CLEAN tank and completely DRAIN the float chamber any time you won't be using the outboard for more than a few days. That keeps mineral scale from forming and keeps your HONDA outboard pretty much trouble free.

Glad you got it going.