8V92 only getting to 1700 RPMs

This is actually for my parents' boat. They have twin 8v92's with 735 HP, vintage 1985. The port engine will not get above 1,700 RPMs. Boat was hauled a couple of weeks ago. New bottom paint, cutlass bearings, and both props were changed out because the props that were on there needed reconditioning. Prior to the haul, the engine would not turn up past 1,700 RPMs, so it's not the props. We've checked the throttle cable and it is pushing the throttle to the limit. Fuel filters (primary & secondary) have been changed.

Upon planing, you get the usual smoke, but it clears up once on plane. The starboard engine was just rebuilt last year, after a local yard "blew it up". Port engine was only running around 1,700 RPMs at that time and we were going to do more investigation, but weren't able to accomplish this last season as the other engine was being overhauled.

It's been a busy summer, so we're finally getting into it now.

Any help or suggestions would be greatly appreciated.

First check - is the tach reading correctly? There's no way our 53MY/8V71TIs would plane with one engine only spinning 1700. Then again you have nearly 2x the rated HP so maybe so...

Will the engine turn rated no-load RPM at WOT? This would be the next check, to be sure a governor problem is not causing it.

If fuel supply is verified not to be a problem, then I'd do a complete tune up to ensure everything is set correctly. It could simply be a matter of linkage adjustments to ensure all the cylinders are getting proper fueling at the same time.

Prior to performing a tuneup on our boat, I found exh port temp differences on one engine varying nearly 100 degrees; after careful tuneup, the differences dropped to within 20 degrees. The linkage adjustments/clearances described in the shop manual are not as easy to attain as it sounds. It requires care and several rechecks because you can affect the linkage on an injector you already set if you are not careful on the succeeding adjustments.

Did the engine ever go over 1700? If no, it could still be the props need some pitch removed.

If yes, I'd look for a bad injector.

It's a 55' 1985 Ocean SS. The boat fully loaded weighs approximately 55,000 pounds. When accelerating, the port throttle has to be advanced to the pins to get 1,700 while the rebuilt engine only had the throttle about 2/3 of the way.

They've had the boat for over 10 years and used to get 2,300-2,350 RPMs out of both engines. They've stuck with the original prop size of 28x32 three bladed on 2" shafts. This problem popped its head up a couple of seasons ago, but was not able to be troubleshooted because of the starboard engine rebuild. When the engines were working "properly", the boat would stay on plane at around 1,600 RPMs. Remember, the aft running section on this model is almost flat, making it easier to plane than the hull shapes of a Hatteras.

It feels like the engine is bogged down as there doesn't seem to be the acceleration compared to the rebuilt starboard side. Once on plane, however, the exhaust does clear up, so we don't think it's a bad turbo because that would leave a telltale sign.

Mike, what would cause the engine to go out of tune? I'll go poking around in the engine room to see if there is a fuel shutoff that may be partially closed. What is the max rpm for no load on an 8v92 and we can check that as well.

Thank you for your quick replies.

Max no-load should be on the little plate attached to the engine. If you can't find it, I'd say it's probably 2450 or 2500rpm.

The only thing that would really cause it to go out of tune is normal wear - which wouldn't usually cause a sudden drop in the top RPM, or, if the last tuneup was badly done, perhaps some linkage adjustments were not properly torqued and something loosened, causing the settings to go out.

Definitely do the-no load WOT check first.

Does it turn up max no load (neutral) RPM via throttle controls??? If not, will it turn up max no load with throttle control removed???

check the modulator. It limits rack travel until boost comes up. It is either sticking or out of adjustment.

The most simple answer is that it's starving for fuel at high rate fuel use loads. Check your RACOR and if that's already been replaced, replace your final fuel filter on the engine. Those little final filters frequently cause this exact problem and it's SO easy to fix. Just ask Angie. Obviously, it could also be a small air leak in the fuel supply system, like the RACOR lid, a fuel supply line connection or the pickup tube.

Doug Shuman

Put a fuel pressure gauge on the fuel minifold after the final filter. Put a gauge on the air housing to read turbo pressure. Take the boat out remove the buffer screw and run it up. See if you have fuel pressure if you do then use a pencil to push in on the fuel rod from the buffer screw hole to see if the rack is at fuel fuel. If it is look at the turbo boost pressure one of these checks will point out the problem.

Brian

Larry,

When you coming back to Jacksonville ???

River City Brewery keeps calling me as you must of left a huge tip the last time you were here.....

Just give me an advance notice so I can be off work........