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  1. #1

    How much RPM loss from heat? Chasing my last 200RPM... (GTX 260)

    Short version: My ski (GTX 260) will only pull about 7840RPM. Even extended top speed runs will not yield much more RPM. I saw 7900 once. Fresh plugs. Fresh oil change and level is just right (1/3 of the way up on the dip stick when warmed up). Literally was just at the dealer for a service and they said it checked out great. Was running great before I took it in too...

    Long version: So after scratching my head all last summer at what felt like a small loss of power, I found my spark plugs were probably 3/4 of a turn loose. They were just hand tight - they were not torqued enough to squeeze the crush washer down. In my trouble shooting I even verified my intercooler wasn't leaking.

    I found this out when I winterized. So, I winterized the ski and put the plugs back in hand tight. Start of the next season, I torqued them down properly (didn't change them), and took the ski out. I took it easy the first ride since it was the gas from storage (I did have fuel stabilizer in it). Ran it down to about 2 bars, poured in a bottle of fuel injector cleaner, and topped it off with fresh 91. Did some pulls the next run and it ran great. I was pulling 69mph pretty quickly and it would rev right to 8040PRM and hold it well.

    Fast forward a month and a half later, and I needed my oil changed. For the past few years, I've just been doing my oil change mid season. Not the greatest and I plan to change my oil end of this season when I winterize. This time, since I was close to 100 hours, I figured I'd take it to the dealer. I did so but I asked them to not change the plugs since they were relatively new. They did the oil change and I took the ski out and that's when I first notice it running about 200RPMs low. I should note that my ski had sat for about 6 weeks at this point (albeit with a full tank) and the air temp was up probably 15 degrees and water temp up 10 degrees. Otherwise it ran great. Good response and hole shot - just missing that top end.

    Well, 4 days later I took it back out and it was missing on one cylinder. It was surging under throttle and was shaking while idling. Throttle response was pretty poor. I noticed as soon as I got out of the no wake zone so turned right back around. Went home, swapped in fresh plugs (torqued them properly), and took it back out. Throttle response was back and it was not missing anymore but the last 200RPM up top still was not there.

    Now I'm stumped. Last year, when I got my plugs properly torqued I found my missing top end. Now, with fresh plugs, no improvement. Unless I have some other hidden issue, is it possible for a bit of heat and gas a few weeks old to cost me 200RPM up top? Water temp is 80* and air temp was about 115* but humidity is about 10% (AZ).

    Thanks in advance. This has been driving me nuts.

  2. #2
    tiderider's Avatar
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    Your chasing ghosts. Just ride the thing..


  3. #3

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    Your loss is 100% due to temperature. I also live in Arizona and mostly ride in Parker Needles Lake Havasu area. From the peak of summer like now until the weather cools down I noticed my peak loss is 3 to 400 RPMs. Just two months ago my 2019 RXT X300 would routinely hit 83 miles an hour and now I get 79 with the corresponding 300 RPM loss.

  4. #4
    JT jpt7779's Avatar
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    http://greenhulk.net/forums/showthread.php?t=249243

    Run the program in post 2 to see how weather conditions effect HP. Your high temps and RAD will kill 300-400 rpm easy.

    Just play with the numbers and watch the effect.

  5. #5
    Thanks for the replies so far. The calculator only found a 2% difference in power between the day it was cranking full RPM and where it was falling off, but it doesn't account for the 12*F difference in water temp which likely has a huge difference in intercooler performance and thus has a greater impact on engine performance.

    I'll keep riding it for now and will go for some speed runs when things cool down a bit.

  6. #6
    JT jpt7779's Avatar
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    What is the lake MSL?

  7. #7
    Quote Originally Posted by jpt7779 View Post
    What is the lake MSL?
    About 680ft.

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