EFI to carburetor conversion?
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Thread: EFI to carburetor conversion?

  1. #1
    Senior Member Trail Woman's Avatar
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    EFI to carburetor conversion?

    So I googled the very question and found no responses except the opposite conversion.

    I'm considering a second bike.A 2 stroke 125cc. And with them slowly being converted to EFI wonder how difficult, if possible, would it be to convert to carb?.

    If I could fix them myself I'd have less of an issue with it but the fact that I have no chance of doing it myself seems criminal.

    If I had the money right now I'd snag a yz125....Even the used ones are not cheap. I'll have to do some fund raising.
    Last edited by Trail Woman; 03-14-2018 at 04:27 PM.
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  2. #2
    Senior Member Peterb's Avatar
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    I don't understand why you would want to do that. Japanese fuel injection is very reliable and works beautifully. Also on a small bike it would be a very simple system. I'm pretty sure you could fix it if you had a problem.

  3. #3
    Senior Member Trail Woman's Avatar
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    Fuel injection systems requires computer diagnostics software to find and fix issues. EFI is both water and additive sensitive and for a dirt bike in dusty conditions sounds like a bad idea.

    I've had my bike 3 years and the only time I got it serviced was for them selling me the wrong oil filter. I've done everything else either by myself or with help from a friend and I prefer to keep it that way. The local shop leaves bikes outside with keys in them in the rain....not an option for me. Plus it's more money and time away from my bike then I can afford.
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  5. #4
    Senior Member SportsterDoc's Avatar
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    A YZ125 liquid cooled 2 stroke has a Mikuni TMX 38 flat side carb.
    No conversion necessary.

    https://www.yamahamotorsports.com/mo...s/models/yz125

    BTW: Most EFI issues can be deduced with a simple multi-meter...not that I do not generally prefer carbs.

    The EFI on the XT250 is superb.
    Last edited by SportsterDoc; 03-14-2018 at 05:22 PM.
    Trail Woman, Ken, jbfla and 4 others like this.
    2014 Honda CB1100
    2018 Yamaha XT250
    2016 Moto Guzzi V7 II
    2017 Yamaha TW200
    2012 Triumph Bonneville
    2002 H-D Sportster 1200 Sport
    2003 H-D Sportster 883
    1976 Honda CB750F
    1975 Honda CB360
    1970 Yamaha CT1
    1972 Yamaha CT2
    1972 Yamaha AT2/CT2
    1970 Honda SL350
    1970 Honda CL350
    1967 Honda CL160
    1967 Honda CB160
    1962 Honda CA110

  6. #5
    Senior Member WECSOG's Avatar
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    My biggest problem with EFI is that it requires a fuel pump; the one component that is most likely to fail on any given vehicle that has one (or more).
    Any motorcycle that uses EFI should be no major problem to convert to carb. I wouldn't put it past Yamaha though, to spend a lot of money figuring out how to make it as difficult as possible. It's just how they roll.
    '87 TW200
    '94 Sportster 1200
    '74 XL70 Frankenbike
    '76 CT90
    '72 CT70
    '72 XL250

  7. #6
    Senior Member SportsterDoc's Avatar
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    Quote Originally Posted by WECSOG View Post
    My biggest problem with EFI is that it requires a fuel pump; the one component that is most likely to fail on any given vehicle that has one
    I understand the concern, as gravity is much more certain, however, I have owned 3 M/C and a couple dozen 4 wheel vehicles with electric pumps, without failure. 2003 Wrangler did have a check valve issue, which required replacing the pump, but it did not leave me stranded (had to turn on key and let pump operate a few seconds before cranking...otherwise it cranked longer than normal)

    To me, the bigger negative is the requirement for a higher capacity electrical system and not being able to use all the fuel in the tank.
    Last edited by SportsterDoc; 03-15-2018 at 10:04 AM.
    WECSOG likes this.
    2014 Honda CB1100
    2018 Yamaha XT250
    2016 Moto Guzzi V7 II
    2017 Yamaha TW200
    2012 Triumph Bonneville
    2002 H-D Sportster 1200 Sport
    2003 H-D Sportster 883
    1976 Honda CB750F
    1975 Honda CB360
    1970 Yamaha CT1
    1972 Yamaha CT2
    1972 Yamaha AT2/CT2
    1970 Honda SL350
    1970 Honda CL350
    1967 Honda CL160
    1967 Honda CB160
    1962 Honda CA110

  8. #7
    Senior Member SportsterDoc's Avatar
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    Quote Originally Posted by WECSOG View Post
    Any motorcycle that uses EFI should be no major problem to convert to carb.
    The issue would be
    a. finding a carb to match the intake manifold
    b. adapting a petcock to the tank
    c. probably remove the fuel pump for more fuel capacity
    d. blocking injector port
    e. dealing with CEL for the absence of injectors, etc. This may be the most pain, unless the ECM is replaced.

    The positive would be a higher capacity charging system to power accessories. However, with LED lighting, that is less of an issue than formerly.

    For a Yamaha comparison, the TW200 schematic is much simpler that the XT250...although the XT250 has a very basic injection system...which works very well.

    I am more familiar with Sportsters. 2003 was the last year of the ignition module. 2004-2006 was the last years of the carb, with a crank position sensor providing fire signals to the ICM. 2007-2013 went to an ECM. 2014 went to CANBus.

    These links show the increased complexity:

    2004-2006 wiring
    Rubbermount_EVO 2004-2006 wiring info - The Sportster and Buell Motorcycle Forum - The XLFORUM®

    2007-2013 wiring
    Rubbermount_EFI 2007-2013 wiring info - The Sportster and Buell Motorcycle Forum - The XLFORUM®

    CANBus (2014 up)
    CANBus major connections - The Sportster and Buell Motorcycle Forum - The XLFORUM®
    Last edited by SportsterDoc; 03-15-2018 at 09:48 AM.
    jbfla, Trail Woman and WECSOG like this.
    2014 Honda CB1100
    2018 Yamaha XT250
    2016 Moto Guzzi V7 II
    2017 Yamaha TW200
    2012 Triumph Bonneville
    2002 H-D Sportster 1200 Sport
    2003 H-D Sportster 883
    1976 Honda CB750F
    1975 Honda CB360
    1970 Yamaha CT1
    1972 Yamaha CT2
    1972 Yamaha AT2/CT2
    1970 Honda SL350
    1970 Honda CL350
    1967 Honda CL160
    1967 Honda CB160
    1962 Honda CA110

  9. #8
    Senior Member WECSOG's Avatar
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    If I had one I was trying to convert, I would probably choose the carb I want and either modify the intake manifold or build a new one. Get rid of the pump and the ECM, adapt a petcock or use a different tank altogether, then look at what it would take to make it work with an existing CDI. That last part might be the most tricky, depending on how the factory setup works. But either way, it could be done.
    If necessary a DC powered CDI could be used; then all it would take is a trigger signal (which any ignition system is gonna have in one form or another). There are stand-alone, programmable digital CDIs currently available that can be made to work on any bike.
    I'm not crazy about DC powered CDIs either (their built-in inverter is a failure point), but a bike like that would certainly have enough reserve electrical power to supply one.

    My TW btw is an '87 with '01-present ignition and electrical and a simplified, hybrid wiring harness. My Sportster is a '94 so it still has the carb and ignition module.
    Trail Woman likes this.
    '87 TW200
    '94 Sportster 1200
    '74 XL70 Frankenbike
    '76 CT90
    '72 CT70
    '72 XL250

  10. #9
    Senior Member Trail Woman's Avatar
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    MY best guess is to start with retrofitting parts off of previous models. I'm mostly unclear as to how integrated the EFI is. For instance if the manifold port is smaller for injecting fuel then a carb needs to adequately feed the engine. Or it the EFI tank parts add any weight.

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