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  1. #1

    I Need 787 Tuning Help

    97 XP with Group K Sleeper Kit. Top end rebuilt with all OEM parts, bottom end untouched.
    Ran great for about 10 hours, now has two issues that I have not been able to solve and I need help.

    The ski has a terrible off idle bog that LS screws do not seem to affect at all. In water idle is 1430 on my PET 2000 Tach. When you hit the throttle, the ski bogs for about 2-3 seconds and then takes off upto 6900 rpm and runs very clean on the top. When you chop the throttle, the rpm will drop and hang at 2400 rpm for about 15-20 seconds before returning to idle rpm. If you hit the throttle while it is at 2400, it will take off instantly with no bog. If you bump the throttle a few times with it out of the water while at idle, it will try to take off like a "lean runaway".

    Remember that after the build, the ski ran fine for several hours. I don't want to start chasing jetting and other variables. It acts like a LS circuit issue so I took the carbs off and removed the LS screws, blew through the circuit with compressed air, no debris. The acc. pump appears to be working. You can see a small stream of gas shoot into the throat of each carb. I had GK rebuild the carbs with new Mikuni parts at the time the other mods where done.

    I have not pressure tested the tank other than blowing by mouth into the return line. I did not hear any leaks and it held pressure for at least a minute. I know that is far from a leak down test, but if there is a leak, it is not severe. I did not do a pressure test on the engine at the time of the build up, but again, ran fine for several hours.

    The second issue is RAVE related. Both RAVE units were completely rebuilt with new o-rings, bellows, etc. For some reason, one of the valves will not open fully about 20% of the time. You know instantly by sound and the rpms will only reach 6500. Many times, you can close the throttle and hit WOT again and it will pop open. The sliders are clean and move freely.

    I don't know if the two issues are related, but I wanted to put them both out there. Please hit me with some hints or suggestions and maybe we can track down what is going on.

  2. #2
    Compression: Mag 162, PTO 165 plugs out RAVE open
    Spark plug boot resistance Mag 4.52, PTO 5.41 (would like less than 5.15)
    Cut off 1/4 of wire and replaced boots
    Rectifier 14.18 (my understanding is good if less than 15v) Most rectifier problems will not allow engine to rev. My revs to 6900 all day, just bogs badly for 2-3 seconds off idle and then off we go.

    Any ideas appreciated.

  3. #3
    I dont know if im exactly right but I thought if it takes a few seconds to get back to idle after stamping it that means its lean. How far did you mess with the LS screws? try richening the low speeds like a full turn and see what happens

  4. #4
    LOL, I turned them till one of them fell out! That is what convinced me that I had some debris in one of the pilots, "LS jets". I did not and I was like oh crap, now what? I just knew that was the problem.

    I talked to Harry Klemm and the spec for the sleeper is 1.25 out on the LS screw with 70 pilot jets. He told me that I may need to go larger, but remember the ski ran fine for about 10 hours before this problem developed. Did I develop an air leak? My plugs look good and my tach shows no sign of deto or dropping rpm on top. It runs 6900 and Harry thinks that it may be over propped just a small amount. It should be about 7100 on top, it never has been.

  5. #5
    I think that I can rule out an airleak in the fuel system. I pressurized the tank by blowing into the return line and then put the line back on the carb. I came back 4 hours later and when I took the cap off, it still had pressure. I guess that means the check valves are holding and no lines are loose.

  6. #6
    Group K says that sometimes an engine will need more fuel after the rings seat. It certainly acts lean, but does not improve with a more open LS screw. I will move from a 70 pilot to a 75 and see where that takes me. Anybody have any suggestions?

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