75 GT750 with VM carbs from 72 GT750

[jabcb]

Bought the bike this year & am looking to finish sorting it out.
What I have:
75 GT750 with 72 GT750 VM carbs & K&N pod filters
3-into-3 chambers (I think the are J&L)
Newtronics ignition

The 72 GT750 VM carb specs per service bulletin Specification-6:
main jet: R&L 102.5 / C 100
needle jet: R&L P-4 / C P-3
jet needle: 5F16-3
cutaway: 2.5
pilot jet: 30
air screw: 1 1/2
float level 27.25

Currently starts quite easily when cold & runs nicely on choke.
Warm & off choke it idles nicely but doesn't have the pep it should.

Am planning on checking the timing & then going through the carbs.

With the 3-into-3 chambers, the center carb should be setup the same as the other two.

Looking for advice on how the early model VM carbs should be setup.
Or should I switch the bike back to CV carbs?

[tz375]

Pity it didn't come with real VM carbs but that's OK.

Alan Pulsifier had his K set up on the dyno and ended up with 125 mains, #30 pilot jets and I think everything else was stock. His bike had a mild port job and a set of Jemco pipes.

Some old chambers lost most of the power below 6k and signed off at 7. The pipes may be part of the issue. I'd try cleaning the carbs and getting timing spot on with new plugs and add larger jets and see how that works.

[jabcb]

Thanks for the info.

Forgot to mention one other item. PO had converted the stock vacuum petcock to manual. I installed a NOS vacuum petcock. Also have inline fuel filters.

Bike has a quite noticeable power increase at around 3,500 rpm. Haven't used the bike much yet. Am looking to get this sorted out before doing something nasty like holing a piston.

tz375, you mentioned #30 pilot jets. Per Specification-6, they are stock, so then the only change would be to up the main jets from 102.5/100 to 125. (Just making sure.)

Looked at sudco.com & motorcyclecarbs.com & don't know which jets to get.

For now, the bike is in storage for winter and am looking to get the parts for spring.

[tz375]

According to my notes, 72 used a #188 needle jet and that takes Large Round jets. Later ones used #159 series needle jets and they use Large Hex jets.

You will need to pull one carb to either get the main jet out or even better, confirm which NJ series is fitted. Threads are different to make it impossible (!) to fit the wrong jet, but people do it anyway.

AP kept stock slow jets according to my notes. Start with that, but I'd buy the next two sizes up jurst to be sure. They are normal VM22/210 (IIRC) slow jets and not BS30/96 jets fitted to later carbs. http://pinkpossum.com/GT750/carb/BS40carbs.htm" onclick="window.open(this.href);return false; has a picture of both. Do not mix them up.

[egor]

The Loyal Order of Water Buffalo are give to installing slide type/ '72 carbs on later engines: weld and grind and drill and tap. One of our CMA members also added a connecting tubework between (and above) the rubber carb-to-cylinder spigots. It looked like a drill-and-silicone assembly using 1/2 copper tube. Elbows at each end and a tee in the middle. Same principle as the exhaust couplers on early models.
'nuff sed,
Lee in Alberta

[jabcb]

FYI... Fred Flintstone & Barney Rubble belonged to the Loyal Order of Water Buffalos.

Thanks for the additional info.
Was hoping to get parts over the winter but it looks like the carbs will have to wait till spring.

[5twins]

You can get parts over the winter, nothing wrong with that. You just need to nail down exactly which ones you need, IE which jet type. Mikuni now makes a #32.5 VM22/210 pilot and it's about time. 30s and 35s were many times too big or too small. This new 32.5 is many times just the ticket. When jetting for mods, the big thing you have to realize is that the 3 circuits in your carb overlap. Change one and you influence the one next to it .....



No one seems to mention it here but if you do go up 9 or 10 main jet sizes (from 100 to 120 or 125), you're going to have to adjust the midrange to compensate. This would mean changing the needle clip position, the needle jet size, or maybe a combination of both.

[tz375]

Good point. The overlap between circuits is considerable (see the VM manual for details).

We also don't know if the carbs have 159 or 188 series needle jets. A #100 large round in a 188 series needle jet is approximately equal to a 115 hex jet in a #159 series NJ, and 125 hex= 105 in large round ( 2 sizes up from stock).

That's why it was suggested that the owner pulls the carbs to work out which type of jets were installed in those carbs.

[Buffalo-guy]

My former hotrodded buffalo was a bit of a mish mash of parts, but, the combo I came up with, worked well. The bike and bottom end were both 72, J, model. At one point it required a re-bore, and a later M (?) cylinder was used for the more advanced porting. J&R Powerpipes were the chambers, and the early VM carbs worked much better. The later J and K spigot mount manifolds go right on the later cylinders. It was a bit lethargic at first, but started getting results when I got jetting to work. The final results were: # 47.5 pilot, stock slide and needle with clip in middle groove, and # 115/112.5/115 mains. I stayed with the differential jetting, and it worked well. I believe that the combustion chamber shape governs that jetting variance, not the exhaust, but I could be wrong. Regardless, the last time I had it at the track, that old girl took my 210 LB carcass down the quarter mile in 13.3 seconds, with a 1.8 sec 60 foot time. Made a few kids on their sport bikes quite disgruntled when they saw my tail lite right away. We're at 3450 feet elevation here, and with my 56 year old (at the time) reaction times, I was pleased with the results. One other thing, I found a large, single K&N filter that I grafted onto a stock J intake manifold rubber, and that worked well also. I suspect even better results can be gained with bigger carbs, and more experimenting. Always an adventure. We of the "Loyal Order of Water Buffalo" salute all such endevours. Cheers, and good luck experimenting. Happy New Year to all here on Sundial.
Fred (the Grand Poobah)

[jabcb]

Thanks for the info.

Sorry for the comment referring to the Flintones cartoon having the "Loyal Order of Water Buffalos". Wasn't sure what to make of egor's comments since he just joined the forum.

[Buffalo-guy]

The Loyal Order has been centered in my garage for close to 20 years, and it amazes me the we appear to be the only ones using that monaker. I grew up with the Flintstones, and it seemed like a natural when the Water Buffalo nickname was attached to the bike I bought, way back when. These great old bikes have huge potential in them, and Suzuki only scratched the surface with their TR 750s. The "Hacker" is really tapping in with his fabulous drag bike, and Shannon's friend in Australia going 240+ mph, is really cooking. I watch with great interest as all these great bikes on this forum evolve. Suzuki Rules!! Cheers.
Fred

[two-stroke-brit]

i have an early 73 k model with jemcos and k&ns stock porting and one up on the stock pilot and 120 mains (large round).
its still a little rich (safe side woos)
needle in the middle and 2 degree retarded timming.
just offering for info really
cheers mark

[oldjapanesebikes]

The Loyal Order has been centered in my garage for close to 20 years, and it amazes me the we appear to be the only ones using that monaker. I grew up with the Flintstones, and it seemed like a natural when the Water Buffalo nickname was attached to the bike I bought, way back when.

Right on Fred !