Idle rpm for 50 MY with 8V71n's seems high.

[Capt Mitch]

New owner to a very original very well kept '68 50MY. Being my first diesel boat, I didn't notice it, but my dad pointed out the boat seems to idle high at 700rpm. In gear after it gets momentum it moves about 6mph. The speed doesn't bother me as handling the boat in close quarters isn't a problem. It just almost almost sounds like the choke is on if it was a gas boat.

In my head I was wondering if it needs that much RPM for the transmission pumps to be happy? I can see a situation where lower rpm and the transmissions slip into gear causing more clutch wear. I'm gonna dig thru the manuals(if they were any older they'd be on roman scroll) I'm just wondering what others idle at with naturals. Thanks in advance for any input.


very seasoned captain
total hatt newb

[Pascal]

700 rom is a bit high, I think I should be 600. But any diesel with big wheels will gather momentum quickly if left in gear. Typically when maneuvering you put the boat in gear for a second or two and don’t leave in gear much longer

[Capt Mitch]

700 rom is a bit high, I think I should be 600. But any diesel with big wheels will gather momentum quickly if left in gear. Typically when maneuvering you put the boat in gear for a second or two and don’t leave in gear much longer

My tachs have been a little finicky the last couple of times. I'm about to head to the boat. I'll double check to see exactly where they're at.

Thank you for the reply.

[SEVEN]

Mine sits at 500 RPM and seems perfect.

[Captain Ralph]

550 RPM idle in neutral. Around 520 RPM in gear.

You want to check your clutch oil pressure when the idle is lowered. Could be one of many reasons the reason the idle is high (if not a tach error).

There are two screws to adjust while setting a Detroit idle.
Idle speed and idle buffer. Not one with out the other.

Check the manual or find a real Detroit tech to verify the tach and adjust your idle.

[JuiceClark]

Yep, just over 500 in gear feels and sounds right.

When the engines have been running, sometimes the idle hangs-up there around 650...and that bothers me. With a rev or two in neutral, they come back down. Does yours come down when you give it a vroom-vroom in neutral?

[Briankinley2004]

My 12-71's idle at 650-670. Mechanic verified with digital tach and didn't seem too concerned. Old school DD guy. Comes down to about 550-575 in gear.

[Captain Ralph]

My 12V71TIs, idle as I recommended. Clutching in and out around the docks do not drive the governors crazy and clutch pressure stays around 200PSI.

BTW, From the V71 DD manual;

[Briankinley2004]

In item 3 "but may vary with special engine applications" what are those special applications?
My governors are not crazy that I know of. The only time my engines have "hunted" were in the first 30 seconds of cranking when very cold after sitting a long while.
Going to check my clutch pressures next time I run and when shifting to see where that is.
Also may need to check cable travel but that would seem odd to be keeping both engines idled high.

[rsmith]

I’ve got the original operating manual for 71 series there is a big warning about operation below recommended Idle RPM. Says will result in improper oiling of the cylinder walls and can result in engine damage.