Erratic RPM's and sound at higher throttle

ab1

Petty Officer 2nd Class
Joined
Mar 2, 2010
Messages
111
I just replaced the lower unit on my 93 Johnson 150 faststrike. Everything went well and I put it on the muffs just to make sure it runs, pumps water, and shifts. First thing I noticed when I started was the RPM's jumped up to around 2000 and were erratic, bouncing from 1000 to 2000 or so. It seemed to settle down quickly and I left it running for a bit, shifted once or twice, and headed to the lake. Water stream was strong. The lu was used but had low hours due to a powerhead blowing. Water pump only had about 3 months on it so I left it alone. I previously decarbed and put new plugs and it was running strong.

Out to the lake and it ran well. A couple of times at start up it would jump to 2000 or so for a minute and then settle down to around 800. I seemed to have good power. On my way back to the ramp, I noticed it was running at a higher rpm than it generally does for cruising speed (maybe 30 mph but creeping to 4000 rpm's where I'm usually at 45+). I gave it a bit more throttle and it was making a sound like the prop was loosing bite. Very subtle "spinning" sound. Shut it down, sat, stated back up and same thing. I thought my prop was spun, but I've been searching posts and think it could be timing? Ignition? When starting, sometimes it goes right to 800 idle other times it jumps higher and bounces.

When the lu blew, it broke the drive shaft at the pinioin bearing and spun up to very high rpms before I shut down immediately. Checked compression and it's good at 90 all around. New lu went back up fine. I did have my wife rotate the crank clockwise so the splines would align. Any thoughts on what I'm dealing with?
 

ab1

Petty Officer 2nd Class
Joined
Mar 2, 2010
Messages
111
Re: Erratic RPM's and sound at higher throttle

Any thoughts guys? I just checked the lu fluid and it has no water so I believe the new lower unit is in great shape and sealed well. It shifts fine.

I'm really puzzled why it now has an erratic idle as well as a noise when I try to accelerate past 3500 or so. I don't konw how to describe it but it's a spinning/whirling noise. Could old drive shaft breaking have caused something else in the motor to go bad, or messed up the timing?

I would really appreciate some opinions before I start searching for things. Any quick things I can check to get on the right track?
 

ab1

Petty Officer 2nd Class
Joined
Mar 2, 2010
Messages
111
Re: Erratic RPM's and sound at higher throttle

Figured yesterday wasn't the best time to post with some of our resident experts out for father's day. Hope you all enjoyed the day!

I could really use some help on this issue if possible. We are trying to get out this week but I'm considering cancelling if I can't get it running right. Don't want to cause any damage after shelling out for a new lu.
 

blimp

Petty Officer 1st Class
Joined
Sep 30, 2009
Messages
237
Re: Erratic RPM's and sound at higher throttle

just a wild guess here, but i would be suspicious of the lower crank bearing. drive shaft breaking can cause some pretty big forces, especially if the broken part of the drive shaft was unbalanced and spun at a high rpm. maybe try getting a stethoscope and listening while you turn the flywheel with ignition off.
 

ab1

Petty Officer 2nd Class
Joined
Mar 2, 2010
Messages
111
Re: Erratic RPM's and sound at higher throttle

Would the lower crank bearing cause it to idle high when not in gear? I can see how this might be causing the issue/noise when I'm at throttle.
 
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