Monday, 22 February 2016

Carb boring 101

A while back I wanted to fit flatslide carbs on my old Kawa Z1000J. Bearing the prices for a nice set of Mikuni RS34s in mind, I was looking for cheaper alternatives. The first generationof 750cc GSX-R's actually sported a set of very, very proper OEM Mikuni flatslides, which go by the name of VM29SS. Now putting 29mm flatslides on a bike, which was originally delivered with 34mm CV carbs yielded some nice performance gains, but not quite what I had hoped for.

After going through a lot of literature I can to the conclusion that all this carb boring isn't all that difficult, if you have access to a lathe and bear a few things in mind. Interestingly enough, the throats are already at 33 (and a bit) mm, but the backplates are choked to 29mm.

The thing that makes this whole job a lot easier is that you may take the backplates off the carbs and as such the bodies can stay where they are and you only have to remove the slides.

Next step is to put the backplates in the lathe for boring them out to the same diameter as the front. Now the only thing you have to bear in mind is to bore the carb eccentrically, as you can see, I did this with a few strips of copper sheet wrapped around one of the jaws on my lathe. If you bore it centrically (which is of course possible and would yield larger gains in terms of maximum flow) you have to lower the needle jet and recut the grooves in the carb body that make the slide seal at the bottom. Furthermore this would really upset the idle mixture, so basically it's not really worth it...

And this is the finished product. Yes, it did work but the gains weren't as drastic as I had hoped. I should have offset the carb less and bored it in a wider circle as I ended up with an oval carb.
(By the way something Mikuni did in the early eighties on a round-slide carb intended for the Suzuki DR500!)