Universal 25MXP low rpm issue

[FLAPILOT]

So I just purchased a Catalina 30 with a Universal 25MXP.

Right off the bat I messed up. My first day I forgot to open the engine through hull. I ran it for about 20 to 30 minutes like that. I didn’t see any glaring issues after that but later that day I did find a fuel leak at the RACOR fuel filter housing.

So I replaced the fuel filter housing and new filter, bled the system and everything seemed fine after that.

I ran the motor at about 2100 RPMs for about 3 hours straight and then stopped to take a break. Upon continuing I noticed that the engine would not go higher than 2200 rpms. Prior to this I know it would easily run in the normal range up to 3000 rpm or so. When I got to the dock I changed the oil and replaced the impeller. (Even though the impeller wasn’t damaged at all.) and still the engine won’t go over 2200.

The previous owner stated that he typically only ran the motor at no more than 1800 rpms.

Its been my understanding that 2100 is a pretty good starting point for motoring. But the engine starts right up and seems to run fine, no thick white smoke out the back. I also checked the throttle cable and it doesnt seem to be hung up or jammed. So I’m at a loss, any ideas?

 

[Stu Jackson]

So I’m at a loss, any ideas?

Look at the stop assembly, upper starboard side of the engine. There are set screws there.

Engines 101 - The BIGGEST & BEST collection of M25 Series Universal Engine Information on the Internet, plus some M35, too This includes a link to the Critical Upgrades topic which has more engine information please read it too.

Diesel Engine - c34.org

 

[kloudie1]

Running without water cooling the exhaust may have melted the inside of the exhaust hose and restricted exhaust enough to reduce max rpm.

 

[captcoho]

Don't know about the fuel leak, may have been coincidence, but checking and changing out the impeller paramount

 

[Hayden Watson]

You could have caught something on your prop or shaft that is increasing the load to restrict your max rpms. FWIW, I have used 1900 rpms for my preferred speed for decades with no problems. I have removed my mixing elbow several times for inspection and have not had any deposits.

 

[FLAPILOT]

Running without water cooling the exhaust may have melted the inside of the exhaust hose and restricted exhaust enough to reduce max rpm.

But the boat ran for three hours without issue. Wouldn’t a restricted exhaust be noticed much sooner?

 

[FLAPILOT]

Don't know about the fuel leak, may have been coincidence, but checking and changing out the impeller paramount

Yep, I did change it. Still the same issue

 

[JoeWhite]

Sounds like fuel starvation. Is the fuel path to primary filter partially blocked?

 

[FLAPILOT]

Sounds like fuel starvation. Is the fuel path to primary filter partially blocked?

That I’m not sure of. I guess that’s the next place I have to check.

 

[RoyS]

Following up on what Hayden suggested: prop could be fouled. Reach down to your prop shaft while in neutral and try rotating the shaft in both directions. It should turn easily. You might try an optical tach to verify that your on board tach is reading correctly. Carefully inspect your throttle at the engine end to determine if the lever there touches the end stop in full throttle position. Do not alter the end stop device setting just make sure that the lever reaches it.

 

[jonelli]

I know this is old now but I’m just reading this and didn’t see any follow up. I had a similar issue and tested all the stuff everyone is mentioning but in the end I found the problem to be a crappy diy throttle cable end crimp. The cable strands were held together with zip ties at the motor lever and we’re sliding out so I could never full throttle it. I found some cable crimp parts to fix it and it’s all good now.
my guess is that because they don’t make perfect length throttle cables anymore, you have to custom cut them now to fit. And if you don’t have the proper end crimp fittings, we’ll, people do makeshift solutions. They work for a while then fail.

 

[FLAPILOT]

I know this is old now but I’m just reading this and didn’t see any follow up. I had a similar issue and tested all the stuff everyone is mentioning but in the end I found the problem to be a crappy diy throttle cable end crimp. The cable strands were held together with zip ties at the motor lever and we’re sliding out so I could never full throttle it. I found some cable crimp parts to fix it and it’s all good now.
my guess is that because they don’t make perfect length throttle cables anymore, you have to custom cut them now to fit. And if you don’t have the proper end crimp fittings, we’ll, people do makeshift solutions. They work for a while then fail.

I never have figured this out and it’s been driving me batty for quite a while now. Mines not the throttle cable, I’ve inspected it backwards and forwards, it moves the throttle to its full forward and aft stops.

 

[jonelli]

I'm no expert, but here's what I've learned...
These motors are simple. Fuel, Electricity, Air/Exhaust and it 'should' run fine.
When I've gone slow or low RPM, I've had stuff on the prop/shaft or had a fuel issue.
Check the fuel shut-off cable. It's designed to shut off the fuel. Maybe it's not opening all way up when "open"?

I've never had exhaust flow affect my RPM's, but when I decided to check it, I found my riser needed replacing badly, so there are plenty of projects to do.

 

[jssailem]

Recent exploration regarding Diesel engines in boats suggested that running your engine at less than 75% of Wide Open Throttle can be hard on your engine (Killing your engine with kindness).

Running the engine at Low RPM's can foul your cylinder walls (cylinder wall glazing) reducing compression. Low compression will produce unburnt fuel.

The Perils of Chronic Under-Loading | Steve D'Antonio Marine Consulting

Things to look for:

• Compression test your cylinders
• fuel film on water from exhaust.