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.

 

[Captain Ralph]

In item 3 "but may vary with special engine applications" what are those special applications?

I had that same question many years ago from then, our senior DD guy.
He was not sure but did mention, a wood plant he worked at when he was young, the engine was brought down to a near dying idle when engaging/dis-engaging a manual gear to drive the PTOs and blades.

The only time my engines have "hunted" were in the first 30 seconds of cranking when very cold after sitting a long while.

Didn't mean hunting for idle.
You ever hear the engine want to rev up a little just for a half second, then return to good normal idle, when you return to neutral?
This is what I was trying to refer to.

 

[Captain Ralph]

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.

What does your manual say different from my manual regarding recommended Idle RPM?

 

[cww]

Idle is supposed to be 550 I thought. My boat with the OEM wheels can also be a handful with the amount of thrust produced at idle. I have had to explain more than once that I can't go any slower. It does 5+ knots at idle. There are a bunch of anchored sailboats that are liveaboards on your way out of the ortega river, and they all seem to be self-appointed traffic cops who complain up a storm about a 1' wake that didn't hurt anything.

My engines when the governors are set properly have maybe a 10-15 rpm difference in gear vs. out of gear, if even that much. If you're getting a 50 or 100rpm drop going into gear then your governors need adjusting, that's not normal.

 

[Captain Ralph]

There are a bunch of anchored sailboats that are liveaboards on your way out of the ortega river, and they all seem to be self-appointed traffic cops who complain up a storm about a 1' wake that didn't hurt anything.

You must be talking about the hypocrites at the ortega landings?
Splash some ammonia based special liquid at them.
International recognized sign language may be easier and faster.

 

[Capt Mitch]

Mine definitely idles just shy of 700 with a very slight change if any when in gear. Luckily my boat came with a DD service manual the size of an old telephone book. I think I have one maybe two more boat rides until I winterize. I'm gonna record the drive pressure at all rpm ranges next time I'm out.

Also Giving it a rev doesn't change anything. I'll peak at the linkage. everything is so butterly smooth on mine I didn't think of that.

I love everything analog but any recommend a digital display system they like? I'd like a small screen on the port side of the helm giving me digital read outs.

 

[Capt Mitch]

You must be talking about the hypocrites at the ortega landings?
Splash some ammonia based special liquid at them.
International recognized sign language may be easier and faster.

I'm from the Lake of the Ozarks and we have our fair share of wake whiners here. I agree some of the Sea Rays need to get up on plane or slow down, don't just plow. It's the people that buy a house two lots off some of the busiest spots on the main channel, then freak the f out when you come in making a ripple.

On my Carver 440 Aft I watched guy fishing in the first slip off the sea wall in a condo dock. He see's me coming, intentionally reeled in his pole, walked all the way out to the end of the dock, and dropped his line in the water as I idle by so he could throw his hands up like what the hell. My reply "If you don't like boats, go to the (insert frustration word ending in ing) library."

That's my rant on that topic. lol

 

[Captain Ralph]

I love everything analog but any recommend a digital display system they like? I'd like a small screen on the port side of the helm giving me digital read outs.

I think you may like the Aetna tachs.

https://www.aetnaengineering.com/aetna-engineering-products/

 

[PATRICK J RICCI]

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

detroit diesel has a 550 minimum speed engauged spec on that engine. first confirm the tach is reading correctly at idle and go from there. 600-625 should be good if the props are sized correctly. the engine should turn 2350 at wide open throttle. if the engine is running correctly and not turning up then the prop size either pitch or diameted is oversized and should be adjusted. idling too low can cause a stall to occur and in some cases the engine can kick backwards and restart and get full of seawater from breething in from the exhaust and exhausting out the air silencers. seen that happen many a time.

pat

 

[Briankinley2004]

Didn't mean hunting for idle.
You ever hear the engine want to rev up a little just for a half second, then return to good normal idle, when you return to neutral?
This is what I was trying to refer to.

Not yet

 

[drburke]

Mine idled high when I purchased it, and then I learned the previous "mechanic" did not properly set the idle machinery (it is a little complicated).

You should have a good DD mech fix it.

In my case, the high idle would drift downwards as I drove the boat, eventually going below 475 RPM and thus dying when I was maneuvering, which is hair-raising.

I would make sure the high idle is not to cover up another issue like broken buffer spring or other. preventing hunting and rolling, etc.

FYI.

DAN