low engine rpm

Christian Williams

Christian Williams

E381 - Los Angeles
Senior Moderator
Blogs Author
Herb,

My diesel book ("Troubleshooting Marine Diesels", Peter Compton) says that if the rpms get within 10 percent of expected, the most likely cause is overloading.

In order of likelihood: dirty prop, dirty bottom, prop shaft binding, wrong prop, wrong transmission ratio.

Or the cause may be poor fuel supply, in order of likelihood: injector defective, air in system, contaminated fuel, water in fuel, control cable restricted, extra washer under injector, injector pipes leaking, lift pump defective, injector pump contaminated, fuel tank vent restricted,

With dark exhaust smoke: restricted air intake, restricted exhaust

Personally, when I run into the problem of something falling ten percent short of expected, I have learned to do nothing for a while. I learned it from my wife, who says it is why we are still married.
 
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HerbertFriedman

Member III
Christian, thanks for the info, lots to check. I dont think the problem lies in the drive train, prop, dirty bottom etc. because I cannot get the engine to rev beyond 2500 rpm a the dock in neutral, hence I changed the exhaust riser elbow, it was not changed for 18 yrs and the engine had 1200 hours since the last replacement. Since changing the elbow did not solve the problem, my concern turns next to the fuel system, i.e. injectors.

But I do like your advice about waiting a bit, my wife thanks you as well.
 
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Pat O'Connell

Member III
Hi Herb Hi Christian
Thanks....I am suspicious that you guys know something about boats. I'm positive that you know a lot about marriage! :<{)))
Best Regards
Pat O'Connell
Ericson 28+ Universal 5411
 
Ian S

Ian S

Member III
Another good reply from Christian. Break it down into order of likelyhood. Trouble shoot accordingly. I confess I never considered the tach being off!
He's only getting 2500 in neutral so that eliminates any loading issues. Possibly max throttle position adjustment. Check the paint on the locknuts. You will be able to see if they have been moved / adjusted. Generally this is nothing that is adjusted / moved in the life of the engine. it is set at the factory and thats it. most are safety wired. For other owners I would not be so quick to go after exhaust issues. It takes a really clogged exhaust to hurt performance. These tiny displacement engines produce little exhaust and you can get all the flow you need out of 2" exhaust with a 300+ cui. engine so bear that in mind. Remember as well that just BC the valves were adjusted does not mean they were set correctly. Takes all of 15 minutes to confirm the clearances. I still insist a compression check is in order. Keep us posted hopefully you will get this resolved without incurring to much expense.

Capt. Ian
 
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HerbertFriedman

Member III
Ian, I did check out the throttle cable. I loosened the check nut and backed off the adjusting screw about one turn and saw little difference it the rpm, maybe 2550 but still way short. Time to call in the experts and have a pro mechanic check out the engine. I did have he valves adjusted about 3 years ago but this low rpm problem just started recently so I think the valves were adjusted OK at that time but the adjustment could have slipped. Will let you know what the mech finds out.

Just for the record, what should a normal M25XP rev up to in neutral?
 
Ian S

Ian S

Member III
In neutral I would not have a guess. way over red line especially since these are pure mechanical diesels and have no rev limiter that i know of. well over operational rpm. the maximum figures stated are what the engine should rev to under normal loads at hull speed. back to the throttle. to be clear i am not speaking about the cable in any way. the cable operates a bell crank that translates that movement into lateral movement to a rod with two ball joints. that actuates the fuel timing pump. there is an idle stop nut and a max throttle stop nut. these are the components I'm referring to.

Capt, Ian
 
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