DD 6V92 idle hunting and full-load problems

[Alaska]

I have twin DD 6V92 TIBs in my boat. They both were recently fully majored, with all internal components either reconditioned or replaced. My port engine has two problems, possibly related. It hunts at idle, the rpm cycling rapidly between 500 and 800 rpm. Before the boat was launched, we knew the engine hunted, but the Detroit Diesel mechanic thought there was a good chance the problem would self correct after the engine was operated at full load for awhile. The engine now has 80 hours on it and still hunts. This is a significant problem while navigating the narrow confines of our marina.

The second problem is that the engine will only reach 1820 rpm at full load, not the 2350 the companion starboard engine reaches. While trying to isolate this problem, the shafts and props have been checked and are in perfect condition. The engine was carefully aligned and is within .0002, very good for 2.5 inch shafts. The engine will reach 2515 rpm in a full no-load test with the transmission in neutral. With the shaft disconnected from the transmission, the engine will acheive 2510 full no-load in both forward and reverse. Clearly the engine's inability to reach its designed full load operational rpm of 2350 is caused by something in the engine and not the drive system. While running, if the starboard engine is run up to 2350, it will drag the port engine up to about 2050 rpm, but then if you reduce the starboard engine rpm until the engines balance, they equalize at about 1820. At this rpm, the boat is not fully up on plane and is not operating efficiently.

Thinking the governor might be the source of both problems, it was removed and checked. There appears to be nothing wrong with it.

One oddity about these engines is that they are both equipped with throttle delays and fuel modulators. I understand that it is uncommon for both of these control mechnisms to be on the same engine. Two different DD mechanics have counseled me to remove them. Johnson and Tower engineers suggested the same. I am beginning to suspect that the fuel modulator is causing the problem. Any suggestions or ideas?

[Genesis]

I'd back 'em both off and see if the problem goes away.

The hunting is almost certainly a governor or rack balance problem (tune-up issue), or something binding in the linkage.

[TedZ]

The hunting is a rack problem, no doubt. Set the valves and the rack after tapping the top of the rack where it rides on the shaft with a plastic faced hammer. A medium tap...don't bend anything. This is a must do on 6-92's and many other DD's or the rack adjustment is not where you think it is. Sounds crazy i know.

Power. Check fuel flow even if you fill a measured 2 gallon can from the return with a gallon and time it. Fix the fuel flow and you will fix the power.

Ted

[Alaska]

Thanks for the input. I also had a suggestion that there might be a fuel vacuum leak that manifests itself only while operating at high load. I will let you know what I find. Thanks.

[TedZ]

Could be. That would show up with a fuel flow measurement. Also, Racor makes a vacuum gage for their filters. Would show there, likely when running, definitely at shutdown.


Ted