Sport jet 120 wot at 3800 rpm will not plane.

[Naliahs]

I have a F 16 the rayder with a sportjet 120. I am trying to figure what is going on with it. It initially was not runny. Took out the carbs and cleaned them. Now starting and idling welll however when I am wot the engine climbs up but stops at around <a href="tel:3800-3900">3800-3900</a> rpm. The boat is unable to plan and feels bogged down. I took apart impeller and shaft no twigs or blocking. Thought it could be cavitation took of ride plate and resealed it tight!!! Rebuilt fir pump, new spark plugs, compression is 135, <a href="tel:145 145 150">145 145 150</a>. Good spark in all cylinders. Not sure where to go next? It starts right up however engine feels bogged down. I was thinking of rebuilding the carbs? Any thoughts would be greatly appreciated.

 

[southkogs]

Got ya' down to one topic ... let's keep it as just one. No need to post multiple times on the same issue.

Welcome aboard.

 

[Naliahs]

I posted it on my phone and kept reposting it. I guess thank. Couldn’t find how to delete

 

[southkogs]

No worries. The forum software can be a bit flaky on any platform.

 

[racerone]

Check flywheel key.----Check reed valves.

 

[Naliahs]

I looked st the reed valves the best I could and seemed ok. I really didn’t want to take it all apart. Can reed valves cause this issue?

 

[Redbarron%%]

If the pump is cavitating then the RPM will increase perhaps as high as the RPM limit.
The first thing I would do is to check that all of the cylinders are firing.
An inductive timing light will give a little clue if it is flashing steadily on all plug wires.
Second a good compression test, or perhaps before the spark test.
Many Sport jets need more than 3800 / 4000 rpm to get on a plane,
You can test with the boat tied to a dock as the rpm will be just about as good tied or running free on the water.

 

[Naliahs]

I did do a compression check and all were 135-150 range. Which seems good. I removed the ride plate and silicones the crap out of it to ensure a tight seal. I have done a spark check while running and all are firing. With the boat tied it is still limited to 3800 rpm.

 

[Naliahs]

Can a carb issue cause any of these symptoms?

 

[Naliahs]

Bump

 

[jerryjerry05]

They made a bunch of Sport jets.
Always include a year and model if possible.

It's possible the rev limiter(if it has one)(model/year) is cutting the motor out at 3800??

Do what barron suggested, a spark test while running wide open.

Is the throttle working right? Have someone push it forward and then see
if you can make it move more by hand.

 

[Naliahs]

I did get a spark tester that flashes and tested all cylinders definitely lights up. It’s a 1997 sea rayder with a 120 sportjet ce from force/mercury. It does have a rev limiter. Throttle is opening all the way and by hand can’t open further when wot.

 

[jerryjerry05]

Take it for a run.
Wide open for 4-5 min.
Then turn the key off, don't throttle back just kill the motor.
Needs to be done in the water under load.
Then remove the plugs and mark them indicating which hole they come from and compare(pics) the burn.
Post the pics.

 

[Naliahs]

Will do

 

[Scott Danforth]

if you are only getting 3800 RPM, something is wrong. that motor should hit well over 5000 RPM

check compression
check quality of fuel
check (read) plugs

 

[Redbarron%%]

Sounds like a dead cylinder.
Tie the boat to the dock and one at a time remove a spark plug wire and run the motor WOT.
If one disconnected makes no difference in RPM that one is dead.
It may be more than one dead.
On my 90 hp Sport Jet one cylinder will do that that is 33% less power call it 60 hp instead of 90
The 120 hp 4 cylinder would be 60 hp as well (more or less)
Remember that the power absorbed by the pump increases on the square with rpm. The difference in the 90 and 120 hp Sport Jet is 1:1 on the 90 and 1:1.4 if IRRC.

 

[Naliahs]

What is the repair protocol for a dead cylinder?

 

[Naliahs]

Sorry for the delayed response work duty calls. Thanks for the help

 

[Scott Danforth]

you have water in cylinder #2 that is steam cleaning your plug

 

[Jiggz]

Nbr 2 plug doesn't look like it's firing or it does but as mentioned could have water in it and being steam cleaned. That doesn't necessarily mean it doesn't have spark but could be lack of fuel oil mix due to broken reed or water intrusion. Nbr 3 is also questionable. But for now troubleshoot nbr 2 first by doing the following:

Disconnect plug wire on nbr 2 and ground it.
Start motor and see if you'll hit 3800 rpm. If you do and you know for sure that there's spark on nbr 2 plug stop motor and dismount the upper carb and intake adapter to check the reed. Or if you have a mini-borescope (~$20) you can just use it to inspect the reed through the carb but making sure the throttle is wide open to have access.

Another good indication of broken reed is blow back of fuel-oil mix coming out of the carb's mouth while running. This is more prominent while idling. To check for this make sure you dismount the air cover off the carb.