early GT250 carb upgrade to 28mm???

[jabcb]

Seems to be a somewhat popular GT380 carb performance upgrade to replace the stock 24mm carbs with GT550 28mm carbs.

The early GT250 has 26mm carbs. Is there any corresponding benefit to upgrade to 28mm carbs?

The early GT250 (73-75) has flange-mounted 26mm carbs. The late GT250 (76-77) has spigot-mounted 28mm carbs.
Per this topic, the late GT250 carb holders will mount in place of the carb insulators on the early GT250: viewtopic.php?f=2&t=13987

This possible paths to 28mm carbs:
1) late GT250 carb holder & carbs.
2) TBD??? late GT250 carb holders + GT550 L&R carbs. Will the carbs fit the holders?
3) TBD??? the 71-75 TS250 has flange mounted carbs & uses the same insulator gaskets as the early GT250. Two TS250 carbs might work. Float bowl height might be an issue.
4) TBD??? GT250 X7 carbs. The X7 was not sold in the US, so I have no insight on this option.

[markush]

Hi!

1.) yes, but you have to widen the inlet port of GT250K/L/M-RamAir-Cylinder to fit the 28 mm.
done here: http://suzuki-gt250.de/bildernavi/Bilder.php?b=1&m=MR1

2.) may be, not. The later, bridged 550-carbs have 35mm flange diameter, (don't know early single 550-carbs), the GT250A/B/C- carbs have 37mm.
Possibly, if you do not use the original GT250A rubber flanges, but some from the accessory trade

3.) don't know

4.) X7 carbs are 26mm. No advantage.

[jabcb]

Thanks.

Did a Google search based on your answer to #2, and found these carb holders with 35mm spigot OD. Holders have slots for the studs. But they have the same 28mm ID that you mentioned & the same inlet issue you mentioned for #1.

The holder description mentions use with modern Mikuni VM24, VM26, VM28 Roundslide carbs, TM24 and TM28 Flatslide carbs.
So option #5, modern Mikuni carbs also has the same inlet issue you mentioned for #1.

https://www.mikunioz.com/shop/mikuni-fl ... 16fd43adaa


Markush, how did the mods on your GT250 work out? Do the extender tubes between the carbs & air filters make much of a difference?

[markush]

Hi!
The 28mm inner diameter of the rubber flanges in your link are OK.
Original GT250-Flanges are the same there.
The only issue is the inlet opening of the GT250KLM-cylinders, there are exactly 26mm to fit the smaller, bolt on carbs.

Do the extender tubes between the carbs & air filters make much of a difference?

Not very much, but a bit.
The engine is set to provide more "power" in the lower RPM range, for use in "Mickey Mouse - Tracks" etc., with mainly curves. An attempt to reinforce the already existing properties of RamAir cylinders.
: Raised compression & a bit longer resonance length of exhaust and inlet.
According the maths the inlet length of only the carburetors was to short.
The engine reluctantly turns above 7000 rpm, but until then it pulls pretty well.

[dollydog]

hi jab, you've got a few easy choices for carbs. back in the day everybody fitted 112.5 main jets. i still fit them as standard when i rebuild carbs. also fit the shorter slides if you can find some. slides fitted as normal were the long ones, which restricted how far you could whack the throttle open. you could always fit the carbs from the gt250a from 1976, as they had 28mm carbs the rubber mounts fit exactly onto the studs - which you will have to shorten, or use shorter m8 studs. you can do alsorts with these engines without going mad and blowing them up. use good 2 stroke oil up top, and check the gearbox for leaks - usually from behind the neutral light switch. obviously if the oil level in the gearbox drops low enough it will starve some of the gears and the centre main bearing. apart from that, these engines/gearboxes are very strong and reliable. i love 'em
cheers, dd.

gt350 ramair barrel with gt250a inlet:

[jabcb]

Turns out there are 3 versions of GT250 carbs:
1) K/L: flange-mounted VM26 with long slides & idle speed adjuster at top of carb
2) M: flange-mounted VM26 with short slides & idle speed adjuster at side of carb (slide part # same as A/B version)
3) A/B: spigot-mounted VM28 with short slides & idle speed adjuster at side of carb

Bought some incomplete M carbs sets from eBay & now have a good set of M carbs with the short slides.
We need to pull the current K/L carbs because of cold start problems, so I figure its a good time to switch to the M carbs.

Its been my experience that the gear shift lever oil seal is another spot that can leak because its below the tranny oil level.

[markush]

...2) M: flange-mounted VM26 with short slides & idle speed adjuster at side of carb (slide part # same as A/B version)...

'same slider' is correct, but it isn't "short" (overall length 47.5mm). In the M-model carburettor, this slider bumps against the mixing chamber lid just as with the previous models before the carburetor cross section is completely free.
You can either shorten the slider (by about 4.5 mm) or use the higher mixing chamber lids from the A-model carburettors.

[jabcb]

...2) M: flange-mounted VM26 with short slides & idle speed adjuster at side of carb (slide part # same as A/B version)...

'same slider' is correct, but it isn't "short" (overall length 47.5mm). In the M-model carburettor, this slider bumps against the mixing chamber lid just as with the previous models before the carburetor cross section is completely free.
You can either shorten the slider (by about 4.5 mm) or use the higher mixing chamber lids from the A-model carburettors.

The old carbs are still on the bike so we have not yet done a comparison between the K/L & M carbs.

Do you know of any other model that used higher mixing chamber lids that would fit? The 72 GT550 I’m working on has that type of lid but they are smaller OD & won’t fit the GT250.

[markush]

I only know the GT250A carburettor lids, which fit on the older carburetors.
But it's not a problem to cut the sliders, so why not take that path when there are no suitable lids.

[jabcb]

When the slide is fully inserted, the guide that orients the slide is close to the top of the slide. Will need to measure it to make sure the shorted slide still fully engages the guide.
Any suggestions on what to cut the slide with?

[markush]

Between the guide and the top of the slider are 10 mm. More than enough.
I cut mine with a small saw and then sanded the cut surface by hand.

IMG_0009.jpg

IMG_0005.jpg

[jabcb]

The GT250 is a friend’s. He has ridden my 69 T350 and complains about how much hotter the motor is. Its a bigger difference that I would have expected given the modest displacement increase from 247cc to 315cc. But it has those ginormous VM32 carbs. So I’ve been a bit curious about what bigger carbs would do to the GT250.

Thanks. Your slide mod looks easy enough. My friend’s GT250 has a new top end with very few miles on it. I’m going to hold off on telling him about the mod until he’s got some more break-in miles on it.

[dollydog]

hi jab, this is the difference between the long and short slides - both off an early gt250 ramair with the pull type tickover adjustment. left is the normal slide @ 47.5mm overall height, and right is the shorter one @ 40mm. unfortunately the short ones have become almost impossible to find nowadays, especially over here
cheers, dd.

long and short gt250k slides: