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  1. #1

    MSX 150 Loss of RPM

    Just had a customers 2004 MSX 150 out for a test run after head gasket replacement. (previous gasket replacement was done badly by a dealer). Also overhauled the cylinder head while it was of, new valve seals, valves lapped and lash readjusted.

    Apart from a bit of cavitiation it ran perfect for the first hour, 7700 rpm. JUst as I was about to head back and load it on the trailer I noticed the rpm at WOT had dropped to approx 7350. NO alarms on display, (FUEL LEVEL WAS VERY LOW ALRGHT). Gave it a few more runs up the river but it would no rev past 7350-7400.
    So back at home I checked the usual suspects, compression, oil level, coolant level, plugs, MAP sensors, all 100% perfect. Connected up the DIgital Wrench and all checked out ok.

    Anyone have any idea as to what might have caused this loss of RPM. Maybe the fuel was so low it may have let air get to pump. Will try with more fuel today. Pump does need a new Impeller as there is a good bit of cavitation low down.

  2. #2
    should have mentioned that this mSX is totally standard. How accurate is the REV counter on this ski? What max RPM should I be seeing on a standard MSX 150?
    Hitting 7700 on the factory fitted rev counter before the RPM dropped off. From reading this forum I think it should be lower.

  3. #3
    ripcuda's Avatar
    Join Date
    May 2010
    Newman Lake, WA
    Max RPM is targeted at 7500 on the stock ECU. Here is some details posted in a long ago post by Desperado (the Weber ECU guru) regarding 7700 RPM on an MSX150.

    your PWC ecu controls the throttle opening based on many inputs. It is normal to briefly see higher rpm figures when you press the throttle to the bars.... but the ecu will always pull back the throttle plate to the target 7,500 maximum rpm. Also, please note that above 7,500rpm, the ecu will gradually retard timing and detune the engine.

    Your rpm fluctuations to 7,700 are likely related to boost spikes from the turbo. A ''sticky'' wastegate mechanism leads to momentary boost spikes..... leaking boost reference lines (those small 3/16'' id. rubber hoses), also lead to momentary boost spikes. Boost spikes will raise rpm levels beyond normal for short periods. Sometimes, these boost spikes don't last long enough to trigger an overboost condition from the ECU. I would suggest you give a good look at all your turbo reference lines to ensure they are in good shape and you have no leaks..... and as you know, your wastegate mechanism should be moving freely. I would not be concerned if your RPM jumps to 7,700 for durations less than 2 seconds and then settles to the usual 7,500.... I would be concerned something is wrong if all the sudden your rpm is 7,700 all the time when it used to be 7,500 in the past.

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