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Author Topic: Carb Conversion - 75 Yamaha GPX 433G - from Keihin Carbs to Mikuni Round Slides  (Read 419 times)

MACK

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Looking for some guidance here.

I have a '75 Yamaha GPX 433G where we'd like to convert it from it's stock, OEM Keihin Carbs to the Mikuni Round Slide Carbs.

The more I talk with people regarding this sled, it seems to be the popular consensus to make this carb conversion.

I will not be racing this sled. This sled will simply be a fun, enjoyable, trail riding sled.

Knowing many people have already traveled down this carb conversion path already, so that we're not wasting a ton of time and money trying to figure things out on our own, (we're hoping to not have to recreate the wheel here), we're looking for a starting point for either the 38mm or 36mm carbs in regards to recommended Needle, Needle Jet, Slide Cut Aways, etc...just to get us close. A place to start. Then we'll take the fine tuning from there.

The places that this sled will be ridden will be in the Midwest, meaning the state of Indiana and the state of Michigan.

It will be ridden at elevations from the 750 foot above sea level to possibly around the 1,750 feet above sea level. The state of Michigan has some smallish, hilly areas that might get upwards to those elevations.

We'll take care of the pilot jetting and main jetting in regards to air temps, etc. Knowing our climate is indeed changing and the Winters are indeed warming...I'll want to make sure that we're jetting these carbs a bit "fat"...intentionally...to enure the cylinders are getting enough fuel to be happy and run slightly cooler on those warmer days, but yet have enough fuel for those colder nights as well, to prevent lean condition and prevent any burn-downs. Nights up in Michigan can easily hit 0 degrees or below. Yet, the days can always be in the mid to upper 20 degrees with some low 30 degree days mixed in. If there would be days where I would deem it to be too warm for the GPX free-air sled, I'd let that sled sit for the day and simply ride my fan-cooled Exciters. Then wait for the cooler days and cooler nights to enjoy the GPX.

A couple of additional questions:

- Are there any of these Mikuni Round Slide carbs out there on the market that have the provisions to them that would allow us to hook up the oil feed lines from the oil pump, in effort to retain and continue using the oil injection pump?

- Would like to continue using the Tach as well, knowing it runs off of the oil pump


Any help on this would be greatly appreciated. Thanks in advance.

« Last Edit: October 26, 2021, 06:59:22 pm by MACK »
- 1975 Yamaha GPX 433G

- 1978 Yamaha Exciter 440

- 1978 Yamaha Exciter 440

Vintagesleds.com

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    big danno

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    If you can find a sno jet sst440 free air manual from 1975 then you will have your info. Jetting may change a bit due to the twin pipes on the gpx but otherwise the same animal. You will lose oil injection though.

    jschaffer

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    It's going to be hard to make the oil injection work.  The stock carbs have a pump rod that runs off the carbs to the oil pump to pull the pump open and it's variable. 
    1975 Yamaha GPX 433G
    1977 Yamaha SRX 440A
    1979 Yamaha SRX 440C

    MACK

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    If I'm forced to have to lose the oil injection and remove the oil injection pump....I'm fine with that. I'll run the pre-mix gas. If this sled will run better (more reliably) with the Mikuni Round Slide carbs and less fuss with those carbs, than with the OEM Keihin carbs, then that is what I'm after.

    Just need a starting point to know what size Mikunis to run, and a starting point to know what setup with regards to the Needle, Needle Jet, Slide Cut Aways and so forth...just to get us close.

    I'm digging around to see if I can find some documentation on the carb set ups for the '75 Sno Jet SST 440 Free Air sled, but no luck as of yet. Maybe someone here has one of those '75 Sno Jet SST 440 Free Air Service Manuals that they can scan and email those carb set up pages to me??
    - 1975 Yamaha GPX 433G

    - 1978 Yamaha Exciter 440

    - 1978 Yamaha Exciter 440

    thefoolsbro

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    • Hale, MI
      • Hale Area Trailblazers
    I put a set of rack carbs off a '95 v max 500? could have been a 600 can't remember for sure. they slid right into the mount boots.
    was a bit doggy, but I never fine tuned it, and it could have been worn clutch or belt related also. worst part was fitting the cable
    because of the way those rack carbs pull open, if I remember I welded a tab to the metal that connected the carbs together to attach
    the cable to. with round slides you will have to change the throttle assy also as they have a longer pull than those keihins. sorry no
    pics, this was about ten years ago. the vmax carbs are usually cheap and plentiful.
    1970 moto-ski 375 (372 jlo) most original vintage winner 1st  Hale show
    1971 chap firebird with an executive hood
    1974 El Tigre 340
    1974 polaris colt 175

    1974 chap. ssx 340

    jschaffer

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    You don't remove the oil pump (leave it on so your tach works.)
    1975 Yamaha GPX 433G
    1977 Yamaha SRX 440A
    1979 Yamaha SRX 440C

    big danno

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    Vintagesnow.com has the manual you are after

    MACK

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    Thanks Big Danno! I will look into that!!
    « Last Edit: October 27, 2021, 02:40:01 pm by MACK »
    - 1975 Yamaha GPX 433G

    - 1978 Yamaha Exciter 440

    - 1978 Yamaha Exciter 440

    MACK

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    • *****
    • Posts: 1116

    You don't remove the oil pump (leave it on so your tach works.)


    Got it. Thank you.
    - 1975 Yamaha GPX 433G

    - 1978 Yamaha Exciter 440

    - 1978 Yamaha Exciter 440