Mercruiser 3.0L running at High RPM

[Carter54]

Hello,

I have a 2000 bayliner capri 1750 with a mercruiser 3.0l 135hp engine. I recently had is serviced for a slow leak in the outdrive oil. As part of the service they pulled the drive and replace a couple of seals in the upper unit. The alternator belt was also replaced. Today was te first time in the water after the repair and the bot is running fine however it seems like the RPM's re much higher than normal. I am getting over 4K at 2/3 throttle. Did something not get put back on right? the prop is original and hasnt been changed or modified.

thanks Chris

 

[thumpar]

Re: Mercruiser 3.0L running at High RPM

4k RPM is not too high. You should be in the mid to high 4K range at WOT maybe even higher depending on your the spec. Does the speed seem right for the RPM?

 

[Carter54]

Re: Mercruiser 3.0L running at High RPM

I didn't want to push it to WOT which for this boat is 4400-4800. The speed seemed low that is my concern, at 25mph we were at 3k-3500 and at 30mph it went over 4k. Normally for this boat im just for 40mph at WOT.

 

[thumpar]

Re: Mercruiser 3.0L running at High RPM

Could be a spun prop hub if not getting the correct speed. Mark the prop and shaft and go for a quick run. If the lines move it is a spun hub.

 

[Carter54]

Re: Mercruiser 3.0L running at High RPM

I have a spare prop I can pull it and take a look. It doesn't feel like a spun hub it still has a good hole shot and acceleration seems smooth. It feels more like dropping a tooth or two on a sprocket of a motorcycle better acceleration but top end is lower.

 

[thumpar]

Re: Mercruiser 3.0L running at High RPM

Is there any chance the shop put on the wrong prop?

 

[Carter54]

Re: Mercruiser 3.0L running at High RPM

No there is a chip in it that I put there and the paint wear is the same. That is what it feels like though.

 

[Don S]

Re: Mercruiser 3.0L running at High RPM

Are you trimming the drive by trim gauge alone? If you had a spun hub in your prop, you wouldn't get much over idle.

 

[Carter54]

Re: Mercruiser 3.0L running at High RPM

I am almost positive it's not a spun hub. I will verify tomorrow.

 

[thumpar]

Re: Mercruiser 3.0L running at High RPM

Yea doesn't sound like a spun hub now. A bad coupler wouldn't make it that far either. I guess one thing you can do is calculate what speed you should be going with RPM and a GPS with a little slip added to see if it truly is missing some speed.

 

[NHGuy]

Re: Mercruiser 3.0L running at High RPM

Doe the motor sound right for the boat speed? If so they might have bumped the tach selector to the wrong position. That would make the tach read the wrong value even though the boat would go along as before. That's a stretch though since they should not have been around the tach for the work you had them do.
It is also possible they put a different ratio drive gear in when they did the upper seal. When doing the upper seal they would have removed the yoke. It has the gear on it's end and they might have changed the tooth count which changes the ratio. Whether intentionally or by accident would be hard to prove. Ask them why your boat revs way up now and didn't before.

 

[thumpar]

Re: Mercruiser 3.0L running at High RPM

They don't make a lower ratio than the L4 engine ratios. It could only be a higher ratio which would be backwards of what is happening. The tach idea might be a good one.

 

[Carter54]

Re: Mercruiser 3.0L running at High RPM

I took a look at the prop today and its definitely not a spun hub. I didn't think it was since I was able to get up to 30mph with no issues. there is a little more play front to back in the prop than I usually have when I install it but all looked fine there.

@NHGuy the engine did sound like the TAC was accurate that is what brought my attention to it in the first place. Its definitely running higher RPM.

I wont be able to get on the water again until next weekend but Ill record RPM at different speeds and post back. Plus ill run it wide open for a little and see what the TAC hits. I plan on calling the boat place tomorrow to ask if they have any ideas.

 

[Carter54]

Re: Mercruiser 3.0L running at High RPM

I had the boat out today. Its not as bad as I thought but it's definately running with higher rpm. WOT was running 5k it used to run 4.5-4.6k.

 

[ktbarrentine]

Re: Mercruiser 3.0L running at High RPM

I'd recommend backing up that tach reading with another tach, like a shop tach/digital tach. Would eliminate that doubt from the equation.
Digitach and GPS..... Love technology!