sportjet120 WOT problem

[industriallift]

I have a 1995 mercury sportjet. I can only obtain 4500 rpm in forward. I am able to obtain 5100 rpm in reverse which is what the book says is supposed to be WOT speed. I have check the throttle cable adjustment. It is correct.I have checked the the tower shaft adjustment.It is moving and stopping at exactly the same place in both forward and reverse. Timing for WOT is correct. Any suggestions would be appreciated. Also no diffarence with the Rev limiter unplugged.

 

[rodbolt]

Re: sportjet120 WOT problem

in reverse your most likly cavitating/ventilating the impeller. sounds like your either missing a cylinder or have a fuel delivery issue. do a compression and spark test first. if it passes we will go to the next staep.<br /> does it idle correctly still or is it ragged now ?<br />when was the last time it ran correctly and what has been done since then?

 

[industriallift]

Re: sportjet120 WOT problem

Just rebuilt Compression is 145,150,150,145 cold. Rings barely seated. Spark test is good. Idle 1050 and smooth. WOT 30 degress btc. New carbs. Starts right up. Runs great. No miss. Tach smooth. Only problem is I can only attain 4500 rpms. Like I said the rpms in reverse are 5100. Specs for the motor are 5100 rpm WOT. Its only costing me about 5 miles per hour. Boat is running great other than that. In line fuel gauge shows 6 psi at WOT. Book says 6.5 max. Stator and switch box both test. No misfire. Not sure what the rpms were before rebuild. Installed tach during rebuild.

 

[industriallift]

Re: sportjet120 WOT problem

Stiil waiting for any suggestions that might help with this problem.

 

[kenimpzoom]

Re: sportjet120 WOT problem

Are you saying you lost 5 mph after the rebuild?<br /><br />Is the boat weighted down differently. Did the foam get waer logged somehow?<br /><br />It really sounds like one cylinder is not working properly.<br /><br />Maybe pull one spark plug wire off at a time, ground it properly, and then run WOT. This could narrow it down as to which cylinder is not working.<br /><br />Ken

 

[funpilot]

Re: sportjet120 WOT problem

Does the engine rev freely until the jet pump loads up and then it drops to the 4500 RPM range? On my boat, I never see more than 4900 under load, WOT going forward. On start up with WOT bevore the pump loads with water I sometimes see as much as 6000 RPM before the rev limiter kicks in. It is impossible to over rev these engines once the jet pump loads up, the rev limiter is only for protection on hole shot and jumping a wake.<br /><br />Since there is no difference in the action of the motor in reverse or forward, except for the position of the thrust reverser bucket and the speed and direction of the boat, I would surmise that the higher RPM you see in reverse is due to the jet pump not being fully loaded with water which is not impossible. How long do you motor about in reverse at WOT when you are performing that test?<br /><br />Have you checked your impeller clearances? What I'm hearing here is that your engine going in forward is unable to over come the resistance of the jet pump with enough horsepower to allow the engine to make better revs (since it is a direct drive system, always pumping water, always on). I think Ken is onto something with the spark plug test...<br /><br />fp

 

[Kevin Turner]

Re: sportjet120 WOT problem

Did you verify the before rebuild speed? <br />Are you sure it is 5 mph slower? Does it feel slower? <br />Is the tach set to the correct setting? <br />I see you checked cable adj, but are throttle plates wide open? <br />Reverse RPM has little to do with "forward loaded RPM". It should "easily over rev" in reverse. <br />Just some of my thoughts, Kevin

 

[funpilot]

Re: sportjet120 WOT problem

I disagree Kevin. Reverse in this boat is simply slipping a clamshell diverter over the jet nozzle on the jet pump. The impeller never reverses direction. Short of it not fully loading the jet pump with water (air is compressible, water isn't) the motor/L-drive jet pump combination works pretty much the same forward or reverse.<br /><br />regards,<br />fp

 

[Kevin Turner]

Re: sportjet120 WOT problem

Fun Pilot, I should clarify.<br />As the boats starts moving backward, the pump will start to unload. The intake is designed to feed the pump going forward, not backward. Add aerated water (from the reverse gate) to the mix and pump pressure drops, making it easier to spin or over rev the powerhead.<br /><br />Also, the hull design was not posted. Shallow deadrise hulls, can easily over rev while backing.<br /><br />I would like to point out, the rev limiter protects the engine in other ways than hole shot & jumping wakes. I:E: Clogged intake, damaged impeller, worn impeller and/or wear ring. Damaged hull/intake (feeding aerated water to impeller) etc... <br /><br />Regards, Kevin

 

[funpilot]

Re: sportjet120 WOT problem

Industriallift has the same hull as I do. I generally find it very difficult to unload the pump in reverse once it is loaded.<br /><br />Your comment about the rev limiter protecting the engine is true. It was an oversimplification on my part to offer only two areas the rev limiter protects. If you want to be very specific, the rev limiter also protects the engine when you have a perfectly shaped piece of granite lodged in the impeller hidden by the blade overlap. Don't ask me how I know that!<br /><br />fp