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  1. #1
    johnsonmtz's Avatar
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    '94 SLT750....Only 5700RPM @ WOT

    My project SLT750 is close.....very close. You can see here some of the troubles I had fitting the ski with a flame arrestor from a 780 http://www.greenhulk.net/forums/show...estor-on-a-750

    Originally with the 780 FA I was only getting 5500 RPM. I turned the HS needles out 3/4 from stock settings and got up to 5750. Today I put a stock 750 FA on the ski and tried it again. With the HS needles at stock + 1/8 turn out I was still getting just 5750 RPM.

    What gives?

    Piston wash looks great. It's perfect on MAG and CEN and a little rich on PTO.

    I've verified spark and compression (120PSI). The fuel pump is a new triple outlet. There are all new fuel lines, cleaned pickup screens in the tank, and there don't appear to be any air leaks anywhere.

    The only thing I can think of at this point is an electrical issue. Is there anything that would happen with the stator that would allow the ski to run just fine up to 5700RPM and then top out? There is never a misfire, stutter or anything that would indicate a specific problem with the stator.

    It's tough to do a leak down test with the motor in the hull, but I would think if the case were leaking I'd have lean looking pistons....which I don't. I have a nice smooth idle of about 1300RPM in the water and it runs around 3,000 out of the water. IF the case were leaking, wouldn't I get a runaway idle out of water due to the cylinder(s) going lean and no load on the pump?

    I'm at my wits end

    KJ


  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    I got to ask the obvious questions:

    Is the MFD hard to read? Maybe it reads 67, not 57

    What kind of speeds are you reading?

    If those are OK, you MIGHT have a water leak at the pipe/exh manifold gasket.

    Does the ski take off good?

    Does it sound like it "boggy" or "crisp"?

  3. #3
    johnsonmtz's Avatar
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    Quote Originally Posted by xlint89 View Post
    I got to ask the obvious questions:

    Is the MFD hard to read? Maybe it reads 67, not 57

    What kind of speeds are you reading?

    If those are OK, you MIGHT have a water leak at the pipe/exh manifold gasket.

    Does the ski take off good?

    Does it sound like it "boggy" or "crisp"?
    Yeah, the little round MFD is a little tough to read, but I'm pretty sure it's 57. I can tell the sound difference between the 780 and the 750. The 780 @ 6400RPM is far higher pitched than the 750.

    I've been paying attention mostly to the RPM, but I think today I saw about 38MPH. So, definitely down by at least 7MPH than what I would expect.

    I know I have a water leak at the thru-hull bearing, but I can't see a leak anywhere else.

    Acceleration is pretty crisp. It comes off idle without blubbering or falling on it's face. The ski seems to have pretty strong acceleration right up to 5500 and the it gradually climbs to 5700 (I think I saw 5750 once today).


    I'm so close I just can't solve this riddle.

    I'm thinking tomorrow I'm going to test the E-box and make sure the stator is in good shape. At least if that all reads good I can check one thing off the list.

    KJ

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    I don't think the stator would limit RPM, but by all means check it out.

    I was referring to a water leak at the exh manifold and pipe. It's an easy gasket to "f" up, and if water leaks into the exh stream, it can ruin the characteristics of the pipe. Mostly hurting the top end.

    The pipe likes to be "wet" down low for good accel (fools the gases into thinking it's longer than it is). It then wants to be a "dry" pipe for high RPM (acting like a shorter pipe for increased top end).

    That's why there's water injection systems. It can be programmed to inject water up to a cartain RPM, then it stops injecting it. Wet pipe for strong bottom end, and dry pipe for high RPM.

    Antwho, That's what I'm thinking.

  5. #5
    bowsniper's Avatar
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    Do you know exactly what prop is in there? on my slt, i had a different one from stock and never knew it..your slt750 wont go over 6250-6300 tops. it should go about 48 mph. stock grate in it? Lots of stuff in the forward compartment? Full tank of gas? although that wouldnt change speed that much, just make it bow heavy.

    Are the bearings in the stator ok? if there old, they could cause a little friction and slow ya down a little.. Base nuts tight? heads tight? if your prop clearence is too high, youll loose top speed. if the blades have changed pitch or are bent, that could do it also.

    Impellor leading edges perfect or chipped and dinged?. that will cause cavatation and slow ya down.

    How about the blades on the stator? are they perfect or chipped? that will slow you down too. that stator accelerates the water to the nozzle. Just a few ideas to kick around..

  6. #6
    johnsonmtz's Avatar
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    Quote Originally Posted by xlint89 View Post
    I don't think the stator would limit RPM, but by all means check it out.

    I was referring to a water leak at the exh manifold and pipe. It's an easy gasket to "f" up, and if water leaks into the exh stream, it can ruin the characteristics of the pipe. Mostly hurting the top end.

    The pipe likes to be "wet" down low for good accel (fools the gases into thinking it's longer than it is). It then wants to be a "dry" pipe for high RPM (acting like a shorter pipe for increased top end).

    That's why there's water injection systems. It can be programmed to inject water up to a cartain RPM, then it stops injecting it. Wet pipe for strong bottom end, and dry pipe for high RPM.

    Antwho, That's what I'm thinking.
    Thanks for the 411 on the exhaust manifold gasket. I haven't touched anything exhaust related. The ski was not running when I bought it last fall. The owner said it had a 'fuel problem'. AS soon as I heard that I was certain I could get this thing running. So, until now, the had no idea how this ski would do.

    I suppose I could get a new exhaust gasket and replace that just to eliminate another possibility.

    Today we're going to take out the boat and the 780. So, this evening I'll dive into the Ebox and check the stator leads. I'll post those back later.

    KJ

  7. #7
    johnsonmtz's Avatar
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    Quote Originally Posted by bowsniper View Post
    Do you know exactly what prop is in there? on my slt, i had a different one from stock and never knew it..your slt750 wont go over 6250-6300 tops. it should go about 48 mph. stock grate in it? Lots of stuff in the forward compartment? Full tank of gas? although that wouldnt change speed that much, just make it bow heavy.

    Are the bearings in the stator ok? if there old, they could cause a little friction and slow ya down a little.. Base nuts tight? heads tight? if your prop clearence is too high, youll loose top speed. if the blades have changed pitch or are bent, that could do it also.

    Impellor leading edges perfect or chipped and dinged?. that will cause cavatation and slow ya down.

    How about the blades on the stator? are they perfect or chipped? that will slow you down too. that stator accelerates the water to the nozzle. Just a few ideas to kick around..
    I haven't had the pump off to check the stator bearings. The engine turns easily by hand but I don't know for certain if they are good. I do know I need to replace the seals on the thru-hull bearing but I don't think that would effect RPM.

    I have the stock intake grate, but I don't know what impellor is on there. Looking through the intake the wear ring looks fantastic. In fact, everything inside the pump looks absolutely perfect. I haven't run a feeler gauge through there, so I don't know what clearances I have.

    So many things to check, but I'll look through them one by one until I solve this.

    The more I think about this, it does seem curious that a 1994 model ski, that doesn't appear to have been maintained all that well, would have a pristine looking wear ring and impeller. When I get the pump off tonight I'll have my answer but I'm starting to think that someone has worked on this pump at some point and possibly installed the wrong impeller. The OEM manual shows Part # 5131035 as the stock unit. I guess we'll see tonight what's really in there!

    KJ

  8. #8
    johnsonmtz's Avatar
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    ON a better note. While I was testing yesterday I played a bit with the ski. This '94 hull is a LOT more nimble than the '96SLT. I was able to do some spin outs and really slide through turns. It's also a lot wetter ride. The '96 really bites the turns and it much more a cruiser hull. This actually works out pretty well because the wife can ride the '96 and I can play hard on the '94......well as soon as I get this thing running.

    KJ

  9. #9
    johnsonmtz's Avatar
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    Open your ears and listen to the singing. The ski has the wrong impeller!!!!!!

    Normally I wouldn't celebrate finding the wrong parts, but I'm hoping this explains my problem. The manual says I should have Polaris impeller # 5131035 in my ski. The impeller that's on there is Part# 5131079 which the manual says is for a '94SL750.

    All of the Solas applications guides I'm finding show a 12/18 impeller for both the SL and the SLT. Is there any difference between the -035 and the -079 impellers? Are they the exact same, just different part#'s because they are on different ski's? I really, really hope the -079 is a higher pitched prop because that would explain my problem.

    KJ

  10. #10
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    The 035 was used on the 650, and would be a better impeller for pulling 3 people. The 079 was for the 2 seater, so it may contribute to the low RPM, but I don't think all that much. I may be wrong though....

    Since you don't know the history of the ski, might be worth popping the flywheel off and seeing if the elec stator was ever messed with???

    It's possible it may be slightly retarded, which would acount for not being able to turn high RPM.

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