5000 RPM max for 92 90hp VRO?

[tfbrown1270]

Just put the boat in the lake for the first time, couple of things came up but I'll address them on separate posts, this one is about the max RPM. This motor was just rebuilt, new pistons, rings, rebore etc

I ran double oil (VRO operating and 50:1)...pretty smoky!..and after 4 hours of varied running through the RPM band I tried a few forays to WOT and managed only 5000 rpm max (it has a teleflex tach)...
I used the trim to try numerous trim angles and managed to get -5100- at the point of aeration but that slowed the boat by a couple mph and I could HEAR that it was trimmed too far up so I lowered it just enough to get a good bite and it settled in again at 5000 rpm....I believe I should see 5500 rpm....
a few stats
-prop is a 13 1/2 X 17 aluminum in perfect shape (the Force had the same 13 1/3 X 17 prop)
-the cavitation plate is about 2 inches above the bottom of the boat
-the boat is a 16 ft 1989 Princecraft aluminum shallow V with a pretty flat bottom from about 1/4 of the way back from the bow. It is a light boat.
it managed to get to 38 mph max

*****now this engine is a replacement for a Force 85 that ran like crap and it managed to get the boat going 40 mph*****

Somethings wrong...but I don't know what?

 

[ob]

Re: 5000 RPM max for 92 90hp VRO?

I agree. As propped I'd think that engine would rap up past 5000 pushing the rig you describe and as the engine is mounted to transom.Have you insured whether the carb flaps are opening fully horizontal at wide open throttle position?Did you inspect the condition of the plugs after the initial double oil run in?Are you running the recommended Champion plugs?One other item.Insure that the timer base(just under the flywheel and forward) advances all the way to its stop at full thottle.

 

[tfbrown1270]

Re: 5000 RPM max for 92 90hp VRO?

OB I appreciate the reply and insight...
I noted the plugs were Champion QL77... can't recall the rest... and I pulled the plugs all were wet with oil but definitely not fouled.
I'll have a look at timer base as well, fwiw I noticed a link (thin bent rod extending towards the back of the engine on the right side) from the top of the throttle arm that was not locked in place.
I don't know what position it started from since the lock nut wasn't jammed up against the back of the throttle arm but I locked it where I found it (the end of that rod was just about even with the front of the throttle arm).
I noted that the arm can be moved forward/aft with a knurled wheel built into the throttle arm, CAN ENGINE VIBRATION MOVE THAT ARM and change the position?...if so maybe thats the culprit.
Also it ran great above 2000 rpm but idled poorly (rough) and stalled repeatedly. Idle on muffs was about 900 rpm, in the water 500 rpm!.
The boat is at the lake (2.5 hours away) so it'll take a couple of days to check these things so please keep the ideas coming so I can look at avariety of things.

 

[StratosRob]

Re: 5000 RPM max for 92 90hp VRO?

I agree the johnson 90 should push your rig to 5500 with no problem. I have a 1989 90Hp johnson almost the same set up 19p prop and I get to 5500 with no problem. Good idea to take a look at the carbs. Where the carbs rebuilt when the engine was. Keep us posted

 

[tfbrown1270]

Re: 5000 RPM max for 92 90hp VRO?

The shop tells me the carbs were rebuilt at the same time.