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  1. #1
    1QwkSport2.5r's Avatar
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    ST S90K - mid 90's vintage question

    Regarding the (in)famous ST carb; How important is the seal between the barrel, rubber boot, and carb body in relation to air leaks and mixture fluctuations? On my example, it doesnt seem to have a "snug" fit as the barrel rotates so I question some of my mixture problems to a possible air leak here.

    My other question/issue is with the spring between the spraybar and barrel inside the carburetor. Somehow or another, I misplaced that spring. It did not have much tension when i took it apart, so I put a piece of fuel tubing in there to provide the same amount of pressure the spring did. Should I/can I replace the rubber boot and inner spring in my S90K with G-90 parts since they're still available?

    I feel like a dunce for misplacing that spring but since the engine ran no different before or after the loss, I figured it must not be too big of a deal.

    I use 5% nitro, 20% castor homebrew fuel.

    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  2. #2
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    RE: ST S90K - mid 90's vintage question

    The spring helps keep the carb barrel pressed out against the screw that guides the barrel in and out when it's rotated. It's not essential, but will help keep the idle mixture consistent. Otherwise, the carb barrel will float and the idle mixture can change a bit.

    The rubber boot is only to help keep dust out of the area between the carb barrel and the carb body. It's not essential, and provides no air seal. The metal parts must be loose enough to move smoothly, but not so loose as to rattle. There really isn't a need for complete air sealing between the moving parts of the carb.
    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  3. #3
    1QwkSport2.5r's Avatar
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    RE: ST S90K - mid 90's vintage question


    ORIGINAL: Bax

    The spring helps keep the carb barrel pressed out against the screw that guides the barrel in and out when it's rotated. It's not essential, but will help keep the idle mixture consistent. Otherwise, the carb barrel will float and the idle mixture can change a bit.

    The rubber boot is only to help keep dust out of the area between the carb barrel and the carb body. It's not essential, and provides no air seal. The metal parts must be loose enough to move smoothly, but not so loose as to rattle. There really isn't a need for complete air sealing between the moving parts of the carb.
    So does the g90 carb use the same size barrel spring as the S90K?
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  4. #4
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    RE: ST S90K - mid 90's vintage question

    Yes.
    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  5. #5
    1QwkSport2.5r's Avatar
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    RE: ST S90K - mid 90's vintage question

    Got a little kicker I'd like your opinion on, Mr. Bax. I ran my S90K today on a few different fuel blends. First is 5% nitro 20% castor +1oz acetone (homebrew). Turns up okay, running about 12,000rpm on a 13x6 APC but it will randomly stall. It stalls more often in the 1/2 throttle and below. Changed 4 different plugs, no change. No matter the needle settings, it would still quit.

    Second mix I ran was Byrons 15% nitro 18% oil (80/20) that I thinned down to 5% nitro and added castor to bring the oil up to 21% total. (just added methanol and castor) Different prop - APC 13x8 which turned 10,300rpm but the random quitting problem still persists. Way rich or Way lean didnt matter (idle mix). This time the quitting was at idle or advancing to 1/4 throttle.

    Still irritated that it wont stay running, I went to mix #3 - another homebrew of 80% methanol 20% castor with 1 drop of armorall added to defoam. Readjusted the needles for a nice idle and transition, WOT gave me 10,320rpm on the APC 13x8 with the lowest reliable idle of 1900rpm. It ran for 5-6 seconds at 1680rpm, but I dont call that reliable. Best part was it didnt quit on me at all. The plug that ran the best was a McCoy MC59.

    All of the parts in the engine are in top notch condition, and as it came from the factory. I got it having only ran 1 time so I finished bedding the ring and got it to hold a mixture setting without sagging. Break-in was painless and easy. No quitting or anything. Same fuel as my first mix except it had no acetone in it.

    Ever hear of one of these engines not tolerating any nitro whatsoever?
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  6. #6
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    RE: ST S90K - mid 90's vintage question

    SuperTigre engines are all Italian designs. Fuels with low nitro content are the rule there, so best performance is with 0%-5% nitro content in the fuel. Some engines may wind up with a bit more compression due to ganging of tolerances, so they are less tolerant of nitromethane when dead stock. Add an additional cylinder head gasket to lower the compression a bit and you'll find that the engine will tolerate more nitromethane in the fuel. Going to a cooler plug may help, too. We're not familiar with the McCoy plugs, we usually use an O.S. #8 plug for almost all of the 2-stroke engines we run.
    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  7. #7
    1QwkSport2.5r's Avatar
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    RE: ST S90K - mid 90's vintage question


    ORIGINAL: Bax

    SuperTigre engines are all Italian designs. Fuels with low nitro content are the rule there, so best performance is with 0%-5% nitro content in the fuel. Some engines may wind up with a bit more compression due to ganging of tolerances, so they are less tolerant of nitromethane when dead stock. Add an additional cylinder head gasket to lower the compression a bit and you'll find that the engine will tolerate more nitromethane in the fuel. Going to a cooler plug may help, too. We're not familiar with the McCoy plugs, we usually use an O.S. #8 plug for almost all of the 2-stroke engines we run.
    It doesnt bother me that the engine doesnt tolerate nitromethane. Its cheaper for me to run an 80/20 anyway.

    It seems everyone has OS as the "failsafe" whenever something goes wrong or whatever. I've never had good luck with OS glow plugs. I have used #8's and #6's in the past and at that time I had the best luck with traxxas car plugs (this was dealing with an RC Car). I've ran these MC59 plugs in every engine I own using anywhere from (now) 0% up to 25% nitro.

    The last time the S90K was having fits, I tried a K&B medium, K&B hot, Fox idle bar hot, Fox non Idle bar hot, traxxas medium, and a couple others that I dont know the brand but looked to be medium temp plugs. On the 5% nitro, the Fox car hot plug and the McCoy ran the best, some of the others wouldnt keep the engine running without glow connected. The reason I questioned the nitro tolerance, is there has been enough people on here talking about using 10-15% nitro and not having any problems and I couldnt get reliable performance from just 5%. Nonetheless, I am fine not using any nitro. The needle doesnt seem too sensative and it gives me decent RPM so I'm happy.

    What would be a realistic max rpm for this old girl? I know I've heard you and others say 11000-12000rpm on the G90's but this isn't a G90. Does it have the timing to turn 13,000rpm?
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  8. #8
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    RE: ST S90K - mid 90's vintage question

    The S-90 and S-90K engines will all run un the high 11,000's quite nicely. Prop them for there. A bit over 12,000 RPM will work. Trying for 13,000 RPM is a bit much. We'd not run them there.
    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  9. #9
    1QwkSport2.5r's Avatar
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    RE: ST S90K - mid 90's vintage question


    ORIGINAL: Bax

    The S-90 and S-90K engines will all run un the high 11,000's quite nicely. Prop them for there. A bit over 12,000 RPM will work. Trying for 13,000 RPM is a bit much. We'd not run them there.
    Sounds good. Thanks for your insight. I'll keep it around 12,000 or less.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  10. #10
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    RE: ST S90K - mid 90's vintage question

    Ok.. having more issues... Here's 2 videos of the engine just stalling. I've been messing with this engine all afternoon. 12:30 this afternoon it was running like a dream on my homebrew FAI 80/20 all castor fuel. APC 13x6 = 11,400rpm today. I tuned it by the book. Let it rest for a few hours so I could get some work done around the house and went to try out 5% nitro one more time. Ran and quit, ran and quit. Burned the 5% nitro fuel out of it and refilled with FAI. (I readjusted the carb each time I fueled it) I unstalled all new fuel lines inside and outside the fuel tank, and installed an OS Sintered bubblefree clunk. It will idle all afternoon until you hit the throttle. Transition is terrible. There is more on this here: Fuel Mixing Advice
    Just opening the throttle (engine warm) slowly:
    http://s153.photobucket.com/albums/s...MVI_0014-1.mp4
    Restart immediately, throttle up slowly to WOT, cleans out, back to idle and quits. This is with the idle set rich enough to not cut out off-idle but very close to the sweet spot. Adjusted spraybar all over the place, currently set 1/3 the distance between straight in and the engine mount lug. Seems to be the best spot so far. High speed needle is set 2 clicks richer than peak. No RPM change for 4 clicks once at peak. (going from rich to peak to 4 clicks lean, no RPM change, so I richen 2 clicks)
    http://s153.photobucket.com/albums/s...t=MVI_0015.mp4

    I probably sound like I'm crazy, but this thing just wont behave and I'm frustrated.. 3/4 gallon fuel through engine, broke in on 5% nitro 20% castor, 13x6 APC prop, MC59 plugs.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  11. #11
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    RE: ST S90K - mid 90's vintage question

    You didn't say what idle RPM you're running at. Try increasing the idle RPM a bit, and then re-setting the needles. If you're trying to idle too slowly, you'll never get the carb set.
    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  12. #12
    1QwkSport2.5r's Avatar
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    RE: ST S90K - mid 90's vintage question


    ORIGINAL: Bax

    You didn't say what idle RPM you're running at. Try increasing the idle RPM a bit, and then re-setting the needles. If you're trying to idle too slowly, you'll never get the carb set.
    The lowest idle I could get that day was 2200 but I raised it to about 3500 to keep it going. The throttle on the carb has so much travel from closed to wide open that my servo doesn't have enough throw so I idle it higher so I can get closer to WOT. I can adjust for a lower idle but then I'm that little bit farther from true WOT. The K&B that preceded this ST in my airboat didn't require as much throw. I will have to find a different servo, but that really has no bearing on the problem I'm having unless it's idling too fast to get the setting right. Though it hasn't mattered if it was idling slow or fast, It would quit anyway. I think the knurling on the needle valve is a bit coarse; I think if it was a bit finer, the retention clips might hold the needle between clicks. The idle needle is very sensitive, 1 full click can be too rich or too lean. Main needle has a 4-5 click window at WOT that rpm doesn't change.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  13. #13
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    RE: ST S90K - mid 90's vintage question

    If the needles are very sensitive, with little adjustment range, then your fuel flow is very limited. You should be able to have several turns on the high-speed needle where the engine can be adjusted from very rich to peak RPM. If your adjustment is limited to only a few clicks, then you have more problems than just a carburetor adjustment problem. Your fuel pressure may be much too low. See how things go on a test stand.

    If you cannot get the servo to move the throttle arm on the engine through its full range, you can either add a longer arm to the servo (or make one to bolt on), or drill a hole closer to the center of the arm on the engine. There's no reason you cannot get the servo to move the throttle through its full range.
    Bill Baxter, Manager Hobby Services/Futaba Service/North America
    3002 N. Apollo Dr. Ste. 1 Champaign, IL 61822 USA
    Service Phone: 217 398-0007
    Email: hobbyservices@hobbico.com

  14. #14
    1QwkSport2.5r's Avatar
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    RE: ST S90K - mid 90's vintage question


    ORIGINAL: Bax

    If the needles are very sensitive, with little adjustment range, then your fuel flow is very limited. You should be able to have several turns on the high-speed needle where the engine can be adjusted from very rich to peak RPM. If your adjustment is limited to only a few clicks, then you have more problems than just a carburetor adjustment problem. Your fuel pressure may be much too low. See how things go on a test stand.

    If you cannot get the servo to move the throttle arm on the engine through its full range, you can either add a longer arm to the servo (or make one to bolt on), or drill a hole closer to the center of the arm on the engine. There's no reason you cannot get the servo to move the throttle through its full range.
    Everyone seems to have a problem understanding what I say. If you read the last 2 sentences in my last post, I say the idle needle is sensitive to being rich or lean in only one click. This means if it's on the rich side of that click it loads up a little but will idle a long time. Just quits opening the throttle fast. The main needle is not sensitive - when peak rpm is reached, leaning 4 or 5 clicks doesn't change rpm at all. I use a tach to set wot mixture. I think the problem lies in the low speed mixture, not fuel pressure or draw. All fittings are clear of oil, debris, and machining burrs. I will pull the carb apart and run a piece of wire or drill bit (by hand) through the spraybar and see if that clears things up or sheds some light on the subject. I just had everything apart a few weeks ago and cleaned everything up. The only problem I noted was machining burrs on the edge of the spraybar orifice. I cleaned those off, rinsed all carb parts in alcohol and blew out.

    I will also change the location of the throttle linkage.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.


  15. #15
    1QwkSport2.5r's Avatar
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    RE: ST S90K - mid 90's vintage question

    I figured I'd post back and update. I learned that one cannot tune the main needle by tach but doing the pinch test at wot to set high speed mixture. Transition is a little rich with the spraybar adjusted towards the engine mount lug. The engine likes an idle of 2700rpm and I was adjusting idle mixture at this rpm. It needs more run time but right now it's turning 12,600rpm on a 13x6 APC prop. This is with the carburetor unmodified and using 5% nitro 20% castor homebrew fuel. It will turn 11,700rpm on a 13x8 APC prop. I'm going to make a mousse can muffler for it and see if it will run better yet. McCoy mc59 glow plugs work great.
    GlowHead Brotherhood #3
    Using Dynamite, Enya, Fox, Jett, K&B, SH, Super Tigre, Thunder Tiger, and Traxxas engines.



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