6-71 dd 325 hp irregular minimun

[robymayway]

only one little technical question:
the right side engine, when it is at the minimun speed and gear in neutral position, the rpm engines goes up and down.

Left side engine is OK

engines have 2100 hours
What can be happened?

 

[MikeP]

That is referred to as "hunting" and is fairly normal on a COLD detroit when it is started in cool temperatures. It usually doesn't last more the 10-15 seconds. If it continues or occurs on a warm engine it could be because the idle speed is set too low or the engine is in general need of a tune up. Check the specs for your idle speed and ensure the idle is correctly set. If this does not solve the problem, a tuneup is in order which will resolve the problem assuming there is no abnormal wear or other related issues.

 

[REBrueckner]

Above post is right on...
I assume this is cold start...right?? like 60 or 70 degrees F. ...not after the engines have been running and are warmed up??? Be sure to state conditions of operation when posting engine questions....

To eliminate the cold start hunting if you wish, speed up the engine via throttle RPM a 100 or 200...NO racing a cold engine!!...and after maybe 5 or 10 seconds let her idle again and likely she'll be smooth....in any case, if that's all the irregular behavior you notice, it's nothing to worry about....

Another way to eliminate hunting and cold start wear and tear is to warm your engines first...either keeping some heat in the engine room or best of all, via block heaters in the engines themselves...helps reduce starter wear, battery strain, and improves combustion and oil lubrication....

When resetting idle speed, if you do get around to that, if that's necessary, be sure your transmissions engage into gear smoothly..too fast and they "clunk" into gear...I hate that.....about 500 RPM is smooth, 600 or 700 RPM is too high and will "clunk"....

 

[Genesis]

Tune-up time.

A good tune-up should take care of this, assuming its not just for a few seconds on a cold start (if so, its normal.)

There are a half-dozen things that can do this, all of which a tune-up will fix (or detect and require a part replacement - e.g. a "sticky" injector rack)

 

[REBrueckner]

After tune up of both my DD engines a few years ago, my starboard engine hunted just a tad more than before the tune up when cold...but nothing significant in either case....
the guy who did the tune up has been doing my engines for twenty years...so don't let anybody fuss with your engine if that's all you notice...it's unimportant.

 

[spartonboat1]

If cold, other past posts have indicated using 1,100 rpm to warm faster and avoid fuel wiping down the cyls. Also, it brings your oil pressure up. I usually start at off idle, maybe, 800-900 rpm, observe the exhaust water flow, then come up to 1,100, then again check water at exhausts again. I hold the 1,100 until the temps come off cold, up to just about 100 degrees. Old timer DD mechanic said 'always' check the telltale at the exhaust.

This is on a DD 6-71N, 310hp. 1,100 should be ok, as that is the old rpm for gas engines for the warm up idle. When they start the DD's in the ferry across the river from me, for the first time in the spring, they start cold (40's) and just windem' up. But they probably rebuild every few seasons.

 

[robymayway]

Hi friends!
I apologize if my answer are not so clear...
thanks to all for your reply! All really interesting!
ciao

 

[Will]

If it only hunts slightly warm or cold, ignore it. However, if it hunts dramatically, the engine could stall when you put it in gear. Most often, this can be resolved by adjusting the buffer screw on the governor. It is located on the side of the governor, near the top, opposite of where the governor is attached to the cylinder head and is the only screw with a lock nut. With the engine at full operating temperature, loosen the lock nut and back out the screw until the engine hunts. Screw it in until the hunting stops and tighten the lock nut. If the screw is too loose, the engine hunts. If it is too tight, the engine may idle faster out of gear than it does in gear or worse, you may not be able to shut the engine down. There are those who feel that this adjustment should only be made as part of a full engine tune up. However, I have found a need for slight adjustments of the buffer screw although rare, is not all that uncommon.
Will