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  1. #1
    IDoSeaDoo's Avatar
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    Building my first modified 787, where to start?

    Hey guys. I'm finally to the point of engine building on my dream XP project. It's been 8 years coming. Here is the list of goodies going in there:
    42mm Novi carbs (look stock, but are bored with NOVI bombsights and no choke plates)
    port-matched OE rotary valve cover
    Spec1 Factory Pipe
    PSI Adjustable dome head (as well as a good stock head)
    MSD Water Injection
    97 SPX water box
    MSD ignition control
    1mm bored cylinders running weight-matched ProX (ART) pistons
    R&D Power Plenum Flame Arrestor

    I'm not sure how to go about breaking in and tuning this thing. I am afraid of running this thing lean and burning up the pistons on break-in. I am not too sure about this PSI head, so I was thinking of experimenting with it later, after I have the motor broken in. Initially, I was going to just install the stock head and stock ignition, and maybe even leave off the water injection. Break it in, then install the other goodies. The carbs come with 162.5 mains and 77 pilots with a silver spring and 1.5 seats. What should my pop-off be? I'm not sure if this spring is OE (probably is). How many turns out should my screws be? I plan on not running the water regulator, but instead hard-jetting it with a 150 main jet. Should I use the front or rear of my Factory Pipe?

    I'll definitely be using premium gas and Klots PWC oil for assembly and break-in. Once I break in, I'll switch to Amsoil Interceptor. I clearanced the RAVE valves and chamfered all the ports on the cylinders. Some were downright NASTY. I did not raise the exhaust ports as I was recommended. I was a bit nervous to do it.

    This is my first time delving into such big mods. All hints greatly appreciated.


  2. #2

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    Apr 2018
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    Pearl River, LA
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    How is it coming ??? I acquired a 1996 XP that has some mods, factory pipe, buckshot carbs, some kinda fancy nozzle, LOL. The engine is on its way to the shop for rebuild. I'm just gonna put it back together as is. Should be interesting but I suspect this ski is set up pretty well. Maybe some tweakin will be necessary. Good Luck with yours !!

  3. #3
    IDoSeaDoo's Avatar
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    Not good. I assembled it, but it seemed to run lean. I seem to have set it up according to a consensus between many tuners, but I didn't like the way it ran. It even seized once on me. I immediately restarted it and idled around to cool it down. I opened the high needles up after that and got it ashore and put in 165 mains and changed to lighter springs for even less pop-off. Still, the ski had a huge hesitation from idle, enough to kill it at times. When I get a chance, I will attempt another leak-down test. I'd be very surprised, as I already did one before installing the motor...

  4. #4

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    Apr 2018
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    What about putting EGT probes in the exhaust. That will certainly tell the tale. I'm sure you talked to Matt B.

  5. #5

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    Apr 2018
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    Pearl River, LA
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    You have a lot of variables to figure out with so many modifications. Did you fatten up the low speed jet ?? I'll be a cheap laser thermometer would be a help. Check the exhaust temps for each cylinder. Somebody spent some money on this ski so I'm gonna go with it as is.



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