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1989 Mariner 3cyl 75hp loses power/noisy at 4500 rpm

S

sdbasstracker

New member
Motor ran crappy/idled rough last year (2012) and finally just would not start at all last fall. Mechanic finally got to it this spring. No spark, so replaced switch box and stator, did a tune-up, replaced fuel filter and lower unit maintenance. Motor now starts great, idles great, accelerates smooth to about 4500rpm then gets rough/makes a "knocking" noise and won't go any faster or higher rpm. Probably worth noting that 1st run off the dock it gets to about 5000rpm before acting badly, then after being shut down for fishing and restarted to make subsequent runs it gets to 4000-4500rpm before behaving badly. Prop and kit was replaced-which did make shifting into gear and accelerating smoother but did not really address the rpm problem. I am obviously not a mechanic but I would like to be able to talk to my boat mechanic about this in a constructive way. Anybody know what is causing this issue and how to fix it?
 
The "knocking" is worrysome.

Can you tell where the knocking is coming from?

Engine, lower unit, in the housing between the motor and the prop etc??

There are quite a few turning/moving things on a motor that could cause a knock. Since it sounds like it runs ok through most of the power band my first thoughts are a bent shaft - either drive shaft or prop shaft (think of an out of balance tire on your car - you often don't notice it driving around the city but get out on the highway and crank her up and you practically get shaken out of your seat).

Also, you state that the prop/kit was replaced - why? hit something or grounded her?? maybe bent the propshaft in the process....

Somewhere to start..
 
Thanks for the reply! To my ear it sounds as if the noise is coming from the engine. If I move the controls to full throttle the knock does stop but it will not accelerate. I don't have a mechanics ear but it sounds like its struggling to get fuel or something. Mechanic said the prop was rubber mounted and the rubber had hardened over time-I think he was hoping that the prop was slipping (seemingly an easy fix). I do not suspect shaft damage: ran good to full speed before dying and only had trouble after stator/switch box/tuneup. Also prop was cosmetically in great shape with hardly a nick on the edges. If you suspect my mechanic is not up to the job feel free to say so......I don't get that vibe as he comes well recommended from a couple of friends who are his customers. Any more ideas? I love my boat but putting $1200 into it without result has seriously tested my affections!
 
My 1989 Merc 3 cyl 90hp had a knock right before the fuel pump died.
They are simple diaphragm pumps that can develop a hole in a diaphragm or stick a rubber check valve when they get old. Rebuild kits are under $15 and have plastic check valves and retainers instead of rubber.
 
Here is a new update to an old thread:

I have been occupied doing everything but fishing since my last post but I did get on the water with my boat and mechanic before freeze up last fall so he could hear the problem first hand. He worked on the boat with the following results: compression was good, carbs gone through and no problems noted, the high-speed timing was found to be 6 degrees off. He adjusted the timing to the correct setting, winterized and stored the boat for me in the heated shop. I just got the boat home yesterday when it was 65F; now its 10F and blowing a 50mph blizzard out so I
 
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