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  1. #1
    IDoSeaDoo's Avatar
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    787 Challenger EGT issue. PTO runs 100degF cooler?!

    Hey guys, I recently put together a 787 carb engine for my 97 Challenger and the PTO cylinder is running 100degF cooler than the MAG. The EGT probes are routed to the same place on both sides of the manifold. I shaved off and drilled a hole right in the middle of that square nub on the side of the Y-pipe. Anyway, prior to reassembly, I cleaned up the casting marks inside the motor and smoothed out many of the transitions in the ports, nothing big, just fractions of a mm here and there. I assembled the motor after making sure everything was even and within spec. I pressure tested the motor, and that checked out fine. The carbs are tuned to stock spec, 1.5Low needle 0High needle, 30psi pop-off, 70 low jet and 147 high jet. Compression is 150 on both hoes. My temps differ by 100degF across the entire throttle range. My WOT rpm is roughly 6580. I feel that my water regulator may be tuned too far out, running my lower pipe too cool, but I can't see that affecting EGTs like this... Swapping plugs makes no difference. Swapping thermocouples makes no difference. I originally had my high needles turned out a bit, but screwing them back in didn't make a diff. My PTO is running around 930, and my MAG is 1030. 1030 is where I want to be roughly, but I feel that having this gradient is bad news. How do I proceed? What do I look for?


  2. #2

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    double check mixture and sync carbs together. you have to do it with a multiple carb setup to balance the cylinders

  3. #3
    IDoSeaDoo's Avatar
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    Are you saying that one carb has to be tuned substantially different from the other? Right now they're identical.

  4. #4
    SURFnTURF's Avatar
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    787 cylinders run differently due to the exhaust runners being different lengths and shapes.951s are eaual but the 787 have a tight bend that changes the scavenging effect and temps

  5. #5
    IDoSeaDoo's Avatar
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    So since the PTO cylinder has more of straight path in the exhaust system, I'm guessing it gets more of the scavenging effect, right? Which means that maybe I should put a slightly smaller high-speed jet in there to match it with the MAG cyl, right? If I'm not mistaken, I believe these Challengers should be able to hit 7K rpm's with the single 787. I'm only reaching 6500 or so. I'm not sure what prop I'm running: it had ABSOLUTELY no markings, but was really seized on the shaft. I don't think any of the POs have messed with it, as the wear ring appeared to be oem as well. I put a new wear ring on it and reconditioned the prop, fixed my dirveshaft seal to get rid of any cavitation. Now she won't rev all the way and my cylinders run 100degF different. I don't think SeaDoo would've allowed their engines to come engineered to where they run 100degF apart from each other, WOULD THEY? I've never heard of anyone having such a large difference in EGT temps...

  6. #6
    Myself's Avatar
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    If that were mine I would richen the hotter one a bit until the temp comes down to the lower one.

  7. #7
    IDoSeaDoo's Avatar
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    I suppose I can try it, but the plug readings look really good. As the matter of fact, I broke in a pair of plugs with the Hotter Cyl rynning 1/2 turn out on the high screw, and it's plug looked slightly more dark. I experimented closing the High needle on the hot cyl (mag) to see what it would get to (the book calls for the highs to be closed btw) and it reached 1140 or so. Knowing that Aluminum melts at 1218, I wanted to play it safe and be below 1100. Back to 1/2 turn it went. Only turning 6700 rpm with just me and about 100lbs of gear, planing at 5k rpm an doing probably 38 or so. Going to clean raves, I think they're getting sticky or something. Could a worn rave cause an egt difference?

  8. #8
    Myself's Avatar
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    I don't believa a worn RAVE would raise exhaust temp but I'm sure one that's not opening fully could. Didn't you clean them up when the motor was all apart?

  9. #9
    IDoSeaDoo's Avatar
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    Well, I guess I don't have a reference for what a truly dirty rave is. They were black and sort of wet with what I can guess is partially burned gas/oil. They didn't have any solid deposits on them, just a "coat" of black. They moved in and out of the hole smoothly with not much resistance, so I figured they were ok. I will def. remove them and do a thorough cleaning before returning to the water though. If one is dirty or not moving properly, would that cause it to weep that black goo down the side of the motor? I noticed a tear trail on my PTO, but I know both of these RAVEs and housings are less than a year old. The boots were replaced not long ago as well, they're the green ones.

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