98 Force 120

I have a 98 Force 120. I had the top half rebuilt a couple years back. The compression in all 4 cylinders is fine. The problem is when I put it into gear, there is a vibration that you can feel throughout the boat. It also can come and go, but it is 95% always there. However, it might stop for maybe 5 seconds and then start up again. At full speed there is nothing noticeable but then of course the hull is hitting the water so i don't know. It is most noticeable just putting it into gear at idle. I have taken it to two marinas and demonstrated the problem. At first they say something is wrong. Then they (say anyway) that they dropped the lower unit and everything looks alright and not to worry about it. I figure it is something with the lower unit since it only happens when I put it in gear. Could I have a bearing that is sometimes slipping in in the case, by that I mean the whole bearing not just what is susposed to be spinning. I would greatly appreciate any suggestions.

Hook up the hose.Make sure it's getting enough water.
Then start it put it in gear and watch the prop and shaft for wobble.
Any wobble? Take the prop off and see if the shaft is bent.
Try a different prop.
Is the flywheel running true?No wobble? J

Does it vibrate in gear only in the water with a load on the prop or does it do it on the trailer also in gear with no load on the prop. This is an important question.

The shaft if true, it is not bent. I can try it again out of water on the trailer, but I am 95% sure it only viabrates in the water with a load. And like I said before, you can be in the water, put it in gear and it will sometimes stop vibrating for maybe five seconds or so and then start vibrating throughout the hull flooring again. But I don't think it has ever had the vibration except for when I put it in gear. If you are at idle, in gear, and feeling the vibration throughout the floor and then take it out of gear, the vibration immediately quits. Thanks so much for your time considering this.

A vibration only in gear under a load usually means a misfire (not running on all cylinders).

Thank you Chris. How would you test to find out which cylinder is misfiring? The only thing I could think of would be to attach a timing light to the wires one by one while I have it in gear in the water. Is there a better/ safer way? Also, would you still think it is miss firing if it still seems to run OK at full throtle? It use to top out at about 48 MPH and now it seems like it is topped out at about 43. However, I think the carbs need to be adjusted a little because when I push it all out it actually seems to lag a little at the top end like it may be too rich. But I would rather have it too rich than too lean because I'm afraid of it not getting enough lubrication. I can pull skiers with it. I have some trouble getting people up that weigh say over 220, but would it be able to do all that if it was misfiring? Also, about the damage to engines from ethanol, is there anything that can be done or added to the gas to fix that? I already had to have the top end rebuilt once because a marina gave me unmixed gas when I thought I was getting mixed. I can't afford that again. Thanks for your responces.

Docksidemarineservices said:

A vibration only in gear under a load usually means a misfire (not running on all cylinders).

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Misfire means that one or more cylinders are not firing due to lack of spark or fuel. You need to do a compression test and a spark test first. You also want to make sure you engine is reaching running temp. If it is not, you will condensate water in the combustion chambers and cancel the spark at the plugs.