RPM for Sport Jet 90?

[RedBarron55]

I have my Hobie Jet Fisherman running pretty good now and I have a question for someone who knows more about the Sport Jet than I do.
I observe a WOT of about 48-4900 rpm with a slight miss.
I wondered if the slight miss might be the RPM limiter doing it's work.
At the dock I pulled the black and yellow wire from the limiter and ran the engine up tied t the dock and got 5800 rpm.
I can buy 5200, 5500, and 5800 rpm limiters and I wonder what would be the pros and cons of higher rpm and hp?
I usually cruise at 4000 - 4200 so the higher limit would make little difference, but more power would get on a plane better and help get up if we decide to tow tubes or skis.
As an aside the engine seems to be happy at 3-5 mph trolling 1100-2000 rpms and will get into some very shallow water without stirring up weeds and sand/mud.
The funny looking intake grate seems to do a great job pulling water from the front of the boat and not the bottom.

 

[Bilmon]

Manual lists 4700-5300 WOT and 950-1100 idle so you could go up a little http://www.boatinfo.no/lib/mercury/manuals/90-120_sportjet.html#/0

 

[Cobra Jet Steering LLC]

A slight miss at high speed may be fuel delivery or fuel contamination take a close look at the spark plugs to see what they are telling you also any ignition ground wires, brain coils, etc. should be cleaned and the wires checked for internal corrosion even the main one on the engine block

 

[RedBarron55]

It runs smooth at high rpms without the REV limiter in the circuit so I think that the interrupting the ignition at the limit.
I wonder if doing the JB weld thing to tighten up the clearances a little might increase the loading and limit the rpm to 5500 or so and be more efficient?
Since the 120 has a 1:1 ratio vs the 1.14:1 of the 90 would this mean if the 120 turned 5000 the 90 would be putting out the same power at 5700? This would put the impeller turning at the same speeds, taking into account the differing drive ratios.
What would be the max RPM for a Force 90?
I think that I will choose a limiter to protect the engine at a higher RPM, but use the higher RPM to get up on a plane at WOT and not add to cruise or normal operations.

 

[Cammcrobb]

I have my Hobie Jet Fisherman running pretty good now and I have a question for someone who knows more about the Sport Jet than I do.
I observe a WOT of about 48-4900 rpm with a slight miss.
I wondered if the slight miss might be the RPM limiter doing it's work.
At the dock I pulled the black and yellow wire from the limiter and ran the engine up tied t the dock and got 5800 rpm.
I can buy 5200, 5500, and 5800 rpm limiters and I wonder what would be the pros and cons of higher rpm and hp?
I usually cruise at 4000 - 4200 so the higher limit would make little difference, but more power would get on a plane better and help get up if we decide to tow tubes or skis.
As an aside the engine seems to be happy at 3-5 mph trolling 1100-2000 rpms and will get into some very shallow water without stirring up weeds and sand/mud.
The funny looking intake grate seems to do a great job pulling water from the front of the boat and not the bottom.

Can I ask what blk and yellow you disconnected? There are 2 xonncections

 

[RedBarron55]

The Black with yellow stripe that goes to the square box that is the Rev Limiter. That Blk/Yel wire is the kill line that when grounded kills the ignition. The Rev Limiter "reads" the RPM from the brown trigger wire.

 

[Cammcrobb]

So I unplugged the blk/ yellow and it fired up right away, but wouldn't turn off until I reattached, I'm trying to bypass to see if the limiter is my issue

 

[RedBarron55]

You unplugged the wire that goes to the ignition switch. There should be a wire that just comes from /to the square (ish) rev limiter. That would leave the black/yellow wire that goes to the front ignition and dead man switches in the circuit. One goes to the CDI (or CDM) ignition, one to the front and one to the rev limiter, if memory serves.

 

[Cammcrobb]

I unplugged the blk/yellow that directly goes to the rev limiter and no stop, is that the wrong wire?