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  1. #11
    I dream skis Mr Noisy's Avatar
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    Re: Trying to setup dual 38 carbs for very hot 650 X2!

    ok guys, thankyou for your advice

    i'll look about for SBNs

    question is, IS it possible with dual BNs

    i have a feeling that with N&S correctly setup, as long as i can get them to run rich at WOT it'll be ok

    thing is, i dont know what happened to them, but now i know, i might be able to sort them out

    im good at engineering these things, but am only learning the carb tuning just now.

    dude i hear you on the weak signal with modified engines, is this something to do with crankcase pressure?

    i just checked my crankcase this morning and it holds air pretty good so im happy im getting a decent pulse and also not getting air leaks at WOT making it lean

    i dont wana annoy you guys telling me the way forward is SBN im happy to accept that its the truth, but im thinking that with twin fuel pumps, twin main jets, twin low jets i should be able to run the ski good.

    its kinda like saying will a 38BN run a modded 300 nicely and i bet it probably would, especially with a 1.0mm main jet

    but yeah i just need to know if this is ever gonna be possible, even if if takes me a lot of fiddling

    i mean, is a 1.0mm main jet capable of running one 310cc cylinder at WOT with high compression and hot timing.

    i guess thats the question.

    if you guys are running 1.5mm for two cylinders then it sounds ok to me?!

  2. #12
    PWCToday.com Is My Home Away From Home fastgtfairlane's Avatar
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    Re: Trying to setup dual 38 carbs for very hot 650 X2!

    yes of course its possible anythings possible, however how many engines could you potentionally go thru before you get it properly dialed in?? with the drill bits and such youre just guessing.. dual sbn 38s could run just as good if not beter than a single 44
    91 440; PJS Viper 7000, jetsport exhaust mani, factory full pipe, coffman waterbox,solas 17*, jetco worked 440 pump, ocean pro nozzles. shredmaster ocean rideplate and grate combo, wide tray, footholds, deep strake mod, driveshaft fill,650sx quick turn

    build thread part 1: http://www.pwctoday.com/showthread.php?t=331343
    build thread part 2: http://www.pwctoday.com/showthread.php?t=364872
    build thread part 3: http://www.pwctoday.com/showthread.php?t=396154

  3. #13
    I dream skis Mr Noisy's Avatar
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    Re: Trying to setup dual 38 carbs for very hot 650 X2!

    Well they're stamped 10 and a 1.0mm fits good so that's ok I think

    I'm gonna build them carefully, get the popoffs very good, and keep to the rich side, and this time double my high screw from 1.5 turns to 3.0 turns and see how she goes

    I do sympathise with people who say just stop wrenching and go skiing but I do kinda wana make the vintage option work

    Back in the day all the racers could use was dual 38s as a single 44BN couldn't deliver enough fuel I guess so I'm really hoping to get it right

    Gonna go see my machinist at his shop next week and ask him to diagnose the engine failure then repair it from there, also want him to check something on the head for me, and I'll face another try

    As long as 2x 1.0mm jets are essentially capable of delivering enough fuel to not seize a hot 650 I'm willing to give it my best shot

    I think

  4. #14
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: Trying to setup dual 38 carbs for very hot 650 X2!

    Quote Originally Posted by Mr Noisy View Post
    Then on day 2 I was WOT for about 20 seconds and I felt something go bad, so I slowed down and pulled in, thought I had maybe fuel starved so I filled up but then it wouldn't restart.
    after lots of messing about finally got it going but it wasn't right, sure enough, no compression on rear cylinder.
    being on dual carbs, I suspect a carb fault.
    Most likely there is more than one thing wrong. At this point the most likely
    cause is your impeller choice.




    Quote Originally Posted by Mr Noisy View Post
    both were basically the same with popoff at around 8 psi

    needle and seats are 2.0
    Not good, your fuel pumps produce a max pressure of 7 psi. In order for the
    pop-off to be that low with 2.0 seats, either the springs are cut, or the
    pop-off gauge is way out of calibration.


    Quote Originally Posted by fastgtfairlane View Post
    the fuel pump isn't very strong on them
    All fuel pumps produce the same pressure, but the newer pumps have significantly
    more volume.

    Quote Originally Posted by Mr Noisy View Post
    IS it possible with dual BNs
    Yes it is possible, and the 38sbn's are also a good option. Both types of carbs
    have their advantages. The low speed circuit of the BN is an advantage, but
    the SBN can emulate that advantage with reverse jetting. As fastgtfairlane
    mentioned the SBN is more adjustable to different set-ups.


    Quote Originally Posted by Mr Noisy View Post
    i have a feeling that with N&S correctly setup
    As mentioned there is a problem with the pop-off. It needs to be above 12 psi
    maybe near 15 psi

    Quote Originally Posted by Mr Noisy View Post
    dude i hear you on the weak signal with modified engines, is this something to do with crankcase pressure?
    That is "Klemm-n-ology" for basically the pressure differential created by manifold
    vacuum and air speed through the venturi.

    Quote Originally Posted by Mr Noisy View Post
    but im thinking that with twin fuel pumps, twin main jets, twin low jets i should be able to run the ski good.
    A single external pump will work fine.


    Quote Originally Posted by Mr Noisy View Post
    its kinda like saying will a 38BN run a modded 300 nicely and i bet it probably would, especially with a 1.0mm main jet
    Somewhere in the range of 1mm to 1.5mm main




    But like I said in the beginning: Start with your impeller

    I would recommend a very safe impeller like an 8/16 skat or
    or some other impeller that is smaller than stock. After you
    reach your goals and the engine is reliable, have the impeller
    modified appropriately.

    Purchase a good tach, like a PET from a Sea Doo or Polaris dealer.


    Bill M.
    Horsepower == Speed, RPM != Speed



  5. #15
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: Trying to setup dual 38 carbs for very hot 650 X2!

    Quote Originally Posted by Mr Noisy View Post
    keep to the rich side, and this time double my high screw from 1.5 turns to 3.0 turns and see how she goes
    The screws are only effective to 2 1/2 turns out.
    Horsepower == Speed, RPM != Speed



  6. #16
    I dream skis Mr Noisy's Avatar
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    Re: Trying to setup dual 38 carbs for very hot 650 X2!

    Thanks bill, just what I needed.

    As far as I can remember, the impeller is a skat 15, but I will check this.

  7. #17
    I dream skis Mr Noisy's Avatar
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    Re: Trying to setup dual 38 carbs for very hot 650 X2!

    Also, I really thought BNs were supposed to be 8-12 psi popoff?! :O

  8. #18
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: Trying to setup dual 38 carbs for very hot 650 X2!

    Quote Originally Posted by Mr Noisy View Post
    Also, I really thought BNs were supposed to be 8-12 psi popoff?! :O
    Like I said, all pumps produce 7 psi. Even a Walbro carb on a weed whacker or
    leaf blower produces 7 psi.

    You need a cushion beyond 7 psi so the fuel pump doesn't effect the needle
    and seat, and prevent it from doing its job.

    Maybe it's worth mentioning, we want atmospheric pressure to open the needle
    and seat, not fuel pressure.

    And yes, I can remember when people used to think that. Even at PJS, it was quite
    a shock when I found out they were selling 38mm sbn's with cut springs. Aghhh!!

    Quote Originally Posted by Mr Noisy View Post
    Thanks bill, just what I needed.

    As far as I can remember, the impeller is a skat 15, but I will check this.
    With a stock nozzle the impeller will limit the engines rpm to 5500rpm, and that
    is an engine killer on a 650. The ski should not be ridden until the impeller is
    replaced and you have a tach.

    This is one of those situations where if you plan to ride it, and rationalize it
    by telling yourself "I will just take it easy." It will bite you in the butt, and cost
    you more money.

    It would help if you knew the exhaust port timing, or take a picture with the
    pistons at 180 degrees to show how far the exhaust port is open.

    Do you have a pair of digital calipers? We can figure out the timing if you do.
    All you need to know is the deck height at tdc, and the distance from exhaust
    port open to the top of the cylinder. I can calculate the timing based on that
    information.

    You need an rpm goal in order to build a reliable engine. This is the first step.


    Bill M.
    Last edited by wmazz; 07-19-2014 at 06:24 PM.
    Horsepower == Speed, RPM != Speed



  9. #19
    I dream skis Mr Noisy's Avatar
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    Re: Trying to setup dual 38 carbs for very hot 650 X2!

    Hey bill

    Thanks again for your insight.

    I had figured that atmospheric has something to do with the needle valve but wasn't sure if it was affected by the fuel pressure aswell.

    Either way, I now see your point where by the valve is there to hold back all the fuel unless the pressure difference on either side of the diaphragm forces it to open.

    II will endeavour to find out exactly what impeller is fitted to the pump tomorrow

    I'm interested to find out more about the impeller pitches and how they affect rpms, clearly not as simple as I imagined!

    Exhaust ports I can get pictures of and also take measurements from no problem

    Personally I prefer a dial vernier but it reads to 0.01mm (with a suitable operator!) so I'm happy ebough with that

    Once again I'm immensely grateful for your help

    Otherwise I may never get this thing together!

    Regards a tach, and running total loss, I run a regular ignition coil, a dynacoil DC-1 as it happens

    Would it be suitable to pickup the tacho signal from coil negative as per a regular auto tach?

    Thanks, Sam

  10. #20
    I dream skis Mr Noisy's Avatar
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    Re: Trying to setup dual 38 carbs for very hot 650 X2!

    I can't check until tomorrow but I have a feeling the port height was 41mm last time I checked

    I remember I had a cylinder that was around 39 and I was told that was too high

    But I will confirm all this tomorrow as soon as I possibly can!

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