120 Lehman wont idle below 1030

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Greatlaker221

Greatlaker221

Senior Member
Joined
Apr 14, 2014
Messages
253
Location
USA
Vessel Name
Tortuga
Vessel Make
Hershine 37
I am looking at a boat with a 120 lehman While starting the engine on the hard, it runs smooth really at any rpm. Trouble is it wont idle down below 1030. Removed the throttle cable, backed off the low idle speed screw, same thing. Too high to shift, so sea trial is out til problem is resolved.

The boat has sat on the hard for a year. 400 hours in last 25 years, 1500 total hours, 1980 vintage. injector pump oil is clean and full, but maintenance schedule unknown. Injectors, valve service also unknown. I called Brian Smith and have a working theory, but I wanted input from the group, Thank you in advance for your advice.
 
Greatlaker221 said:
I am looking at a boat with a 120 lehman While starting the engine on the hard, it runs smooth really at any rpm. Trouble is it wont idle down below 1030. Removed the throttle cable, backed off the low idle speed screw, same thing. Too high to shift, so sea trial is out til problem is resolved.

The boat has sat on the hard for a year. 400 hours in last 25 years, 1500 total hours, 1980 vintage. injector pump oil is clean and full, but maintenance schedule unknown. Injectors, valve service also unknown. I called Brian Smith and have a working theory, but I wanted input from the group, Thank you in advance for your advice.
Click to expand...

Have the RPMs been confirmed with a strobe tach? If so then I'd say it's a injection pump issue.
 
Yes, an infra-red tach. The boats tachometer was plus 150 rpms.
 
Just to confirm, does that mean the true RPM is 1030 or is that the reading from the boats taco?
 
From the infra red tach, 1030rpm. Confident it was accurate. The boats tach was 150 rpms higher.
 
Fuel system all cleaned up from tank pickup to injector pump?

Bypass lift pump with a can of fresh diesel up on deck straight to on engine fuel filter....or past if you rig another (unquestioned) fuel filter.

Also I think one time I fiddled with the low speed screw and the RPMs went up when the arm was allowed to go back too far...but that episode was strange...I had high idle for a couple starts...then it just seemed to go away.
 
Last edited:
The fuel had water in it. Drained engine filters, broke into fuel line just before engine filters, pumped clean fuel from jerry can of new clean fuel with fuel line bulb thru filters, and bleed the injectors. Got it running again. again, runs great, just not below 1030 rpm on infra red tach.

I noticed like you did, when you pull back on the throttle lever past its natural resting point, it does speed up a little.
 
Air in the lines someplace? That's my first thought. I'd go thru the bleeding process carefully by the book.
 
Just to be clear...she runs smooth...but you just can't get it below 1030.

I would suspect the injector pump as Cpt.Bill11 suggested....

What's the exhaust smoke look like????

Did you let her warm up all the way????
 
isn't normal idle RPM about 675 on this engine?
 
If pump had water go through it, it could have scored one of the plunger/barrels, preventing it from rotating all the way when governor pushes the rack.

Could also be sticky from sticky from sitting.

There is a cover (I think there is) that if removed you can access the rack and move it manually.

Or start it up and alternately work the throttle and shudown lever. This will stroke rack between its limits and if sticky tends to free it up. If you do this, have a method to choke off air intake as a binding rack can cause a runaway.
 
I think you are on point. There was water in the fuel and it sat for a year. I have removed the pump and will send it out for service. Thank you all. Now to replace the fuel tanks that are letting the water in and are shot. :facepalm:
 
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