120XR Sport Jet Stalling, W/Alarm

[AutoTechman]

I recently acquired a 1996 Sugar Sand Mirage with a 120XR Sport Jet. My first impression of this engine was good, It started right up, ran fairly smooth and accelerated well....After a 3/4 throttle run across the lake, My son jumped on the tube and I was pulling him around the lake..still at 3/4 throttle...After a while he wanted to go faster so I went full throttle after about 3-5mins the engine simply shut-off...And when I tried to restart it the low oil/overheat alarm turned on, and the engine cranked very hard (kind of like a high compression engine/ or if the timing is too far advanced)...I left the engine sit a minute or two and it fired right up and I noticed the upper cyl's felt obnormally warm, so I baby'd it back to the dock and loaded it up...It ran fine all the way back (at 1/4 to 1/2 throttle)...Just shy of the dock I just went full throttle momentarily and it pulled very well..But I trailer'd it anyway...Being that this boat sat for three years, The engine is in spectacular EXTERNAL condition and I seen the bolts were never out of the T-Stat housing, so I removed the t-stat, this where my curiousity was killing me...The t-stat was placed in the engine with the pointed (rad side) of the stat pointing outward...towards the cover and spring...Is this how these are supposed to be positioned? Does anyone have any Idea why this thing just simply shut-down...then sounded the alarm? The oil bottle is full and I have tested the switch with a DVOM (Digital Volt/Ohm Meter) and it does switch properly, I also tested the thermostat in 130* water and it does open at 130*. The Engine is a 1996 Mercury Sport Jet 120 XR , Ser: OE189241, JMP Ser:OE150356, I have included as much detail as I have at this time...ANY help is greatly appreciated. I like to work on my own toys.

 

[AutoTechman]

Re: 120XR Sport Jet Stalling, W/Alarm

I've replaced the t-stat just to be safe, But one other thing that's bothering me, after researching some posts....I wonder if it is possible for the rev limiter to be the cause...The hard re-start has me baffled.

 

[wilde1j]

Re: 120XR Sport Jet Stalling, W/Alarm

You indicated that the motor was abnormally warm, horn came on, and cranked hard after it shut down. When was the water pump last serviced? Did you touch the engine w/ cover off right after shutdown? Was there a good stream from the tell tale? Is or has motor been run in saltwater? I'm not that familiar w/ recent Merc's, but usually the hot horn has different sound for "hot" vs "low oil" ... check your manual.

 

[AutoTechman]

Re: 120XR Sport Jet Stalling, W/Alarm

Wildeone, Thanks for the reply.....<br />I am under the impression that a Jet Sport does not use a waterpump, they use the actual pressure created by the jet nozzle to circulate the water...I did feel the engine after it shut down, it was warm but nothing excessive, I could always keep my hand on it, I did notice the upper two cyls. were a little warmer than the lower two...I have not had the engine in saltwater, neither did the previous owner, and by the looks of what I've seen in the cooling system (clean) I temd to believe him. The alarm is the same for both the low oil and the overheat...I verified this by the wiring diagram.

 

[ODDD1]

Re: 120XR Sport Jet Stalling, W/Alarm

Tech,most likely the alarm was an overheat...oil in the tank is oil in the tank, no mystery there...the stalling I would suspect the rev limiter, have a history of simply killing the ignition and not allowing a restart..[you ground the black/yell wire to kill ignition on these units]...check the manufacturing number on the rev limiter [2x2 black box] if it has a relatively low suffix number it is highly suspect and should be changed [like a7, a11, a12.] new ones show up with high numbers a28 a30 etc...<br />back to the overheat...the spring on therm allows water to dump through head at higher rpms...if the therm is packed full of sand/gravel this cant happen, and an overheat shows up...

 

[wakataka]

Re: 120XR Sport Jet Stalling, W/Alarm

Techman - <br />Your story sounds a lot like mine, but I hope you have a different problem. I purchased a 1993 Bayliner Jazz with a Sport Jet 90. The boat had been sitting for about a year, with gas in the carburetors. I drained the tank and filled with fresh gas, and thought I would see how it ran first before I cleaned the carbs. <br /><br />It was a little hard to start, but seemed to run OK. I ran it below 3/4 throttle for about 45 minutes, then went full throttle for about 1 minute before the engine siezed. It didn't stop all at once, but kind of wound down quickly, like it was pulling against a load. I tried the starter and found it slow to turn over. Let it cool for about 30 seconds and it started up and seemed to run OK at lower throttle settings. I didn't try full throttle again that day.<br /><br />I took it home and cleaned the carbs. There was a little varnish slime and some flakes in the bowls. <br /><br />The next weekend, I was pulling my son on a tube and the engine started running on two cylinders. Pulled the plugs and found that the center one had been hit by something inside the engine. We got a tow back to the dock. I took apart the engine and found a broken piston ring and a badly scored piston in the middle cylinder. A small piece of the broken ring had worked it's way past the piston and was beating up the cylinder head and piston crown.<br /><br />I think what happened is that the middle carb got partly plugged, causing the middle cylinder to run lean and overheat. I'm looking at a top end overhaul.<br /><br />Run a compression check. If you've done any serious damage like broken rings, it should show as low compression. If you haven't done it by now, clean out the carbs.<br /><br />As I said, I hope you're problem is different (and cheaper!!) than mine, but the symptoms sure sound similar.