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  1. #1

    1999 GTX LTD MPEM questions (1998 MPEM swap?)

    Hi folks,
    I just bought a 1998 and 1999 GTX LTD last week. Can the MPEMs safely be swapped in order to troubleshoot RPM and missfire problems I'm having with the 1999?

    Stock 1999 GTX LTD, 120 psi rear cylinder, 137 psi front cylinder. Had been running smoothly all last weekend, started right up and accelarated cleanly, but max RPMs were 5940 in the water, max speed 50 mph. No sign of fuel starvation or rich conditions anywhere in that range. Anyone not familiar with what to expect from a 130 hp 951 might be happy with the performance, but I know it was not running at full potential. Not even as peppy as my former 96 GTX 787. Checked RAVE boots and they look brand new, springs intact, no spooge leaks. Bellows lift the Power valves when spring removed. Checked water box valve, spring intact. Checked the timing with strobe and degree wheel and find it's at 19~20 degrees BTC at 3000 RPM. Should be at 22 BTC. Seems a little odd. Maybe former owners had replaced the MPEM without getting it setup correctly at the dealer. So I was planning to go to dealer for MPEM programming to have the timing advanced. Didn't get over there yet.

    Before end of day pulling the kids on the tube, engine starts misfiring. Within 10 minutes of running it went from a mild case of misfire to only running on one cylinder. Found no spark on the rear. Replaced the spark plugs and got nice bright blue sparks. Again she fires right up. However, now there's an idle issue and bogging at the low end. Definitely sounds like an electrical to me. Idle speed had been 1500 in the water, but now without touching anything on the carbs, it had changed to 950 rpm and would not keep running by itself. Can keep it running if you squeeze the throttle a little. So I increased the idle speed with the carb adjuster, but it's certainly not running right on the low end. Once rpms get to around 3K, it accelerates fairly well, but again top speed of 50 mph and around 5900 rpm tops.

    Checked charging voltage and find it's only 12.8 volts at higher rpms on the trailer. It had been 13.5V or so when I checked over the weekend. Looks like it has developed a bad regulator/rectifier. Unfortunately Seadoo did not keep the rectifier modular on this series of craft. Don't know what Seadoo was thinking as it's going to be an expensive fix for Seadoo owners.

    Are there any work arounds for a bad voltage rectifier on this one? Pain to have to buy a whole new MPEM.

    To help isolate the problems to be sure the MPEM is the culprit of all these issues, I was looking whether I could put in the MPEM from the 1998 GTX LTD model. It is a different part number and although has the 3 connectors, they are in different locations and orientation. Please can someone give advice on this?

    Current 1999 GTX LTD MPEM: 278000906 (278001399) - however microfiche shows it should be 278001398
    1998 GTX LTD MPEM: Nippondenso 071000-1330

    Thank you,

  2. #2
    No, you cannot swap MPEM's. The 98 is single coil, 99 is dual.

    Don't assume the rectifier is bad if battery voltage is OK. 12.8 is a fully charged battery.

    You more than likely have a fuel problem; sounds more like a fuel starvation problem at high RPM. Have the carbs been cleaned? Do you still have the old gray lines? Has the fuel selector switch been cleaned? If you have the gray lines and haven't cleaned out the fuel system, rebuilt the carbs, start there first. Don't run it anymore until this is done. Starving a 2-stroke for fuel is the best way to blow it up.

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