27mm vs. 39mm carbs

[gorilla]

I have a 1978 GS 1000 and am looking at new carbs for it. I will be giving it a total rebuild with a mild performance web-cam cam with a 4-1 Vance and Hines exhaust. I am trying to figure out why I would go to a 39 mm over a stock 27 mm carb system. I already know that I will need to re-jet as I will also be putting pods on.

Looking for some insight into if this would be advantageous or not!

G

[chef1366]

You don't mean 29mm?

[gorilla]

Actually I think they are VM26ss Mikunis, not sure if they are original or not but they are in rough shape!

G

[Nessism]

Stock carbs are VM26's as noted. Classic upgrade is to go to 29's. 39's would be too large if you could get them to fit - assuming a road going bike.

[humhead]

Mikuni VM29 smoothbores are the biggest carbs that will fit on a small port (78-79) 1000 head.
This is a good setup for an engine with mild cams, light porting & an 1100 big bore kit. the downside is that VM 29s have been out of production for a long time. A decent set usually costs $400-500.
If you want larger, more modern carbs, get a head from an 80-81 1000G or 82-84 1100G/GK.
They came with Mikuni BS 34mm carbs. (Good for street use) If you're going racing, you can buy 34 or 36 mm Mikuni RS flatslides. 39mm is going to be too much carb for a 2 valve motor.
The V&H pro stock bike that ran in the 9's in the 1/4 mile used 35mm carbs on an 1176ccmotor.
The yoshimura superbike used 31mm carbs on a 1025cc motor. They both won championships.

Terry

[renobruce]

Mikuni VM29 smoothbores are the biggest carbs that will fit on a small port (78-79) 1000 head.

You can also use VM33 smoothbores on those heads, you just need to replace the intake boots with the proper ones.