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  1. #1

    Stock '98+ Pro 785 Jetting and other questions

    What is everyone running for jetting near sea level on their '98+ Pro 785s? I just bought and rebuilt a '98 stocker and set the carbs to the factory settings. I pulled the head after a 4th of July weekend and it is definitely running rich. Run quality off-idle is very poor and the thing doesnt really seem to want to idle at all. It also needs choke on most starts.

    Here is the run-down of what I did during rebuild:
    -New pistons, rings, upper rod bearings, honed cylinders
    -Cleaned and inspected power valves
    -Rerouted pipe cooling routing to Watcon routing with Jetworks valve
    -Replaced all 4 motor mounts (all 4 were shot)
    -Crank main bearings and lower rods inspected (good) with new crank seals
    -Replaced cavitation-damaged 5-vane aluminum stator with a new SS 6-vane
    -Drained and cleaned fuel tank, replaced all tempo lines
    -Cleaned and inspected carbs/needles/seats

    The problem that I have seen is that I am only turning 6800-7100 rpm. My powervalves are opening when running out of the water (need to check in water). I am still running the 4.5 Mod Slim overlay prop (5131949)...does this prop load the engine harder than the 12/17 prop on the 99+'s? I have a 12/17 on the way. I'm hoping that this, combined with what may be fat jetting is hurting my RPMs. Has anyone else fought this? I cant see any exhaust leaks.


  2. #2
    I'D RATHER SWEAT THAN SHIVER hyoctane23's Avatar
    Join Date
    May 2007
    What is your compression? You need to check that especially if you just rebuilt the motor. Should be around 120 PSI.

    Did you check the crank phase to be sure it hasn't been forced out of phase?

    Does it run better with the seat off? If so, that could mean exhaust leak. Those can be hard to find...

    Did you double check that the fuel restrictor was still in place on the center carb fuel return line? Mine was gone and caused a horrible low end bog.

    Could you confirm what jets you had on both the highs and lows? Stock screw settings for a '98 are 1 turn out on the lows and 0 turns out on the highs.

    Did you check the pop-off pressure to ensure the needle and seats are in good working order? When you say you cleaned the carbs, did you replace all the diaphragms, check valves, and other parts?

    What exactly does your wash pattern look like? Do you have a picture?

    Low RPMs usually points towards the power valves not opening. You need to confirm that they are opening all the way up. Also, double check that one or two of them were not accidently installed upside down. This is very easy to do and can cause a loss of RPM. I'll have to find a picture to show how they should be installed.

    A lot of people prefer the Skat Trak 12/17 over the NuJet 4.5 for stock boats so you'll probably see a little difference there, but that prop shouldn't be limiting your RPMs at all.

  3. #3
    I am getting a little bit closer...took a ton of slack out of the powervalve cables (adjusted per service manual) and now I am turning 7400-7500 on a longer pull. Everyone that owns a pro should go do this NOW!

    Still a terrible low-end bog, to the point that it is dropping cylinders. As soon as its up on plane there are no issues.

    Hyoctane...thanks for the tips...Compression at 120 all across, phase is good, forgot to run with the seat off (will try this week), fuel restrictor is there, pilots and mains are stock 98+ with stock screw settings, need to check pop-off pressure.

    Ski runs alot better now, but still want another 300-400rpm outta her! Thanks!
    Last edited by bigred70; 09-01-2010 at 12:23 AM.

  4. #4
    thester71's Avatar
    Join Date
    Jun 2009
    central ma.
    Are you running the oil injection still? If so did you stil premix the fuel at 50/1 for the break in? If no oil injection what ratio are you mixing? How many hours on the top end? I would be hesitant to reach max rpm till 10 hr break in is complete. You can get 300 rpm from opening the highs up a bit, but I would let it be till break in is complete.
    The low end bog could be caused by the excess oil loading the plugs, if not try opening the lows 1/4 turn out. My pro is going in the water this weekend after a complete rebuild, so I have been watching the boards closely, it seems a few people are struggling with a low end bog, does it clear out as the rpm's increase? Good luck

  5. #5
    Running stock oil injection and amsoil dominator. I mixed the 1st tank at 40/1 in addition to the oil pump...that fuel is all long gone now. The top end now has ~5 hours on it. The bog cleans out completely once the boat is on plane, and throttle response is awesome. I'm beginning to suspect faulty needle/seats. I took a look at them when cleaning the carbs and didnt think they needed replacing...maybe I was wrong. Where can I buy or build a pop-off tester on the cheap? Thanks!

  6. #6
    Getting even closer...swapped out my '98 prop (Nujet 4.5 I believe) and put in a stock '99+ 12/17 swirl...this prop seems to work alot better with the 6-vane stator. RPMs now flash to 7500 and climb to 7700 on a long pull in flat water. Top speed is right at ~60mph. I turned the low-speeds out 1/4 turn and it seems to idle alot better on the stand and hesitate alot less in the water, although still bogging a bit on takeoff from a dead stop.

    Overall pretty happy, but will be trying hard to find the last 150rpm and 3mph if they are out there...any suggestions?

  7. #7
    Well I ripped out my exhaust last night, and found something that may be contributing to my low-rpm woes. My center-cylinder headpipe had a broken o-ring seal (between the headpipe and expansion chamber), and the rubber coupler was filled with oil. I also noticed that there was only one o-ring on each headpipe, but there are grooves cut for two. I am going to get rings on their way from mcmaster and try it out next week.

  8. #8
    I've been trying to diagnose for a little while why my '98 pro only runs 7600-7700rpm. The boat is bone stock with stock jetting. The boat has 67.7 hrs on it.

    I ran a compression test tonight and came up with the following:


    Some background...I bought this ski this spring at 58.1hr on the MFD and it had a thrashed PTO piston and cylinder (60psi comp). I pulled the engine, split the cases to replace all the oil seals, and installed OEM polaris 785 pistons, rings, and upper rod bearings. I had the PTO cylinder sent out to Millenium for re-nicking and honed the other 2. I assembled everything myself.

    Back to tonight...I pulled the head and saw what appeared to be minor scuffing on the intake side of the mag and PTO cylinder walls...nothing near enough to snag a fingernail on, but a decent rub mark on both cylinders. Tore the cylinders off and the mag and center pistons look beautiful, but the PTO is scuffed bad enough that I wouldnt put it back in (again, only intake side). Piston wash on all 3 looks very safe/rich with zero sign of detonation. Keep in mind these pistons have LESS than 10 hours on them! The port chamfers on the millenium cylinder look good, there is no sign of debris on the piston top or head. I am running the stock oil injection with Amsoil Dominator and all lines are pumping oil.

    What else could cause this? Only thing I could think of was water intrusion (Ive never rolled or sank the machine, but some water does work its way into the hull...the pro is a wet ride). I am running the Watcon pipe routing with a single jetworks valve with a delrin ball.

    I can snap some pics tomorrow if that would help.

    I am also now in need of one single 785 piston and ring set (now both discontinued). Anyone have a spare or two collecting dust that they'd be willing to sell? I'd consider used as long as its perfect shape and low hours.


  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
    Join Date
    Oct 2006
    Cleveland OH
    What's the ring gap set at?

    Do you warm the engine up properly before going WOT?

    goBENGALS was selling used pistons for a good price if you want to PM him.

  10. #10
    Well...I was able to run the freshly-rebuilt 785 this past weekend. I used Polaris 785 pistons and 750 rings with a fresh hone. I saw 7750rpm on a decent pull and it flashes to 7700 almost instantly out of the hole. The entire package just seems to work much better. Now I get to start applying the go-fast goodies from Lugs!!!

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