Suzuki SP400 tuning help

Discussion in 'Thumpers' started by trailer Rails, Apr 13, 2013.

  1. trailer Rails

    trailer Rails Long timer

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    I just picked up a 1980 SP400 the other day and need a little direction in making it run correctly. It idles great but just off idle it boggs real bad and is tough to get it to build revs above that point. It lasts from 2500 to about 4000 rpm. I was guessing plugged up pilot jet. It took the carb apart and inspected and cleaned everything. I started playing with the pilot jet adjustment, I unscrewed the screw a fair bit. It runs better but it still boggs and smells like it is running rich.
    #1
  2. Booze&Meat

    Booze&Meat Bar Crawler

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    What yinz need ta do is get six a them there bottles of sea foam and pour them in the gas tank and drive all the way out sheetz's and get somma that fuel injecter cleaner. They gots good hot dogs n'at.
    #2
  3. trailer Rails

    trailer Rails Long timer

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    I done did that, I also put a whole can of PBR in the tank.
    #3
  4. Andyvh1959

    Andyvh1959 Cheesehead Klompen

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    Da frozen tundra eh? 1.5 mile west of Lambeau
    Since it idles ok by your report, but stumbles through the mid-range, then it is likely the midrange needle jet and needle position. What is the clip location on the needle? Go to a lower clip position to make it richer, higher clip position to make it more lean.

    You don't say if the air intake and exhaust are stock or modified. If the exhaust is modified, like more open, then you need to make the mid-range more rich. Same for if you opened up the airbox.

    Also, the carb is a CV style with two diaphragms, one for the slide and another for the lean burn valve on the side of the carb. Any tears or holes in the diaphragm will cause running problems.

    That engine is the same as in the GN400. Go to Yahoo Groups and search for the GN400 forum. Lots of good info there.
    #4
  5. NJ-Brett

    NJ-Brett Brett

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    Time to take the carb apart and clean it, and check that the jets are correct.
    Replace the spark plug also.

    People confuse mid rpm's for mid throttle.
    The carb uses different things at different throttle openings, which is not the same thing as different rpm's.

    If the motor breaks up at full throttle and 2500 rpm's, its the main jet/float level, not the pilot circuit or needle.

    If the motor always breaks up at half throttle and any rpm, its likely a needle problem.

    But the first step with any old bike is to take the carb apart and see what you have.
    #5
  6. SkiBumBrian

    SkiBumBrian DualSport Crazy !!

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    My money is on the diaphragm. Like was said, inspect closely for rips and/or pinholes... Make sure it is seated correctly. But, while you have it apart you might as well soak the carb overnight in PineSol, blow cleaner through all passages and follow with compressed air. Nice clean carb!!
    #6
  7. trailer Rails

    trailer Rails Long timer

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    I had the carb apart and blew all the passages out. I should have inspected and double checked the jetting. I don't know if anyone changed it. I recently had another issue, the tank rusted through and someone put epoxy on the hole, the epoxy melted and the tank stated leaking again. I have a new-used tank coming. I'll be able to continue to diagnose after that.

    I was also leaning toward a diaphragm problem. I inspected it when I had the carb apart and it looked decent. Any suggestions on where to find a new diaphragm? My google search turned up a few but they were asking around $200. :1drink
    #7
  8. Andyvh1959

    Andyvh1959 Cheesehead Klompen

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    "Blowing" out the passages and jets doesn't insure they are clean. Any carb on an old bike that I have cleaned always included probing all ports, passages and jet holes with strands of copper wire pulled from 16ga stranded wire. This has always been successful for me.

    After the wire probe, I follow with carb cleaner, and then air blasts. This always works and I know the parts are clear and clean. Do this before spending $$$$ on carb parts that may not solve the issue. If the tank was that rusty, could be some got through into the carb, assuming the petcock valve strainer didn't stop it. Have you checked/cleaned the petcock also? Also confirm the carb float adjustment as a too low float can cause runnign problems above idle also. Once everything is clean the tuning adjustments start.

    Check Sudco and Nichecycle for parts. I have seen some slide/diaphragm parts go for up to$169 but never $200. Most I have seen are in the $70 range.
    #8
  9. letank

    letank Adventurer

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    try to find a carb for a DR400 of the same period and do not to deal with the diaphragm.
    #9
  10. trailer Rails

    trailer Rails Long timer

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    Good idea, thank you. I have been pretty busy lately and the bike has been pushed to the back of the garage.
    #10