SmedSpeed XS650


carburettor kits

When contemplating changing carburettors on an XS650, the most common change is to the venerable Mikuni round slide VM series of carburettors; they look "period" and suit the look of the engine, and are a very good carb.  However the design could have been improved, and in the early 80's Mikuni introduced the flat slide TM series of carbs which flow better still. The design of the jet block allows for an unobstructed path for the air to flow.

The design of the flat slide carb is so good that a 34mm item will pass the same CFM (cubic feet/minute) of air as the VM 36mm round slide carb. The picture below is an early XS650 CV type carburettor mouth, is it any wonder they flow so badly, the incoming air has to pass around the needle, then above or below the butterfly, at least in this position its a knife edge, when its at part throttle, the obstruction is worse still. 

Carburettor mouth

When you pass more air through a smaller hole, the velocity increases (Bernoulli's principle for you science buffs). This aids throttle response, fuel atomisation and fuel economy. Like many things in an internal combustion engine, the best performance is realised when the right sized item for the job is used, whether it be cam or carburettor.
All too often that overly large cam or carb seems just the ticket, when in reality less cam or carb would make the engine far easier to ride. Too much carburettor ruins the response, and loses more at the bottom end than it gives at the top end. The real downside for street use is the throttle can never be "snapped" open, as the engine just coughs and falters.

I use the TM series carburettors on all the XS engines I build, specifying 34mm for 840 and 750 engines, and 32mm for 650 and 707cc engines. There are many other far more exotic (read expensive) offerings available, but if they were any good then the Japanese would have used them in the first place. Items like Lecton are best left to millionaires searching for the last tenth of a HP, or for bragging rights.

Carburettor kit
The positioning of the carburettor relative to the engine has little to do with performance, but more to do with "packaging" the whole assembly. The inlet tract length is more suited to high rpm use, something an XS650 is not renowned for. In many non XS650 engine builds I have lengthened the inlet tract as an experiment by sometimes up to 125 mm; it always gives vastly better response, although sometimes accommodating the extra length can be problematic. However on XS650's with modified battery boxes, or none at all, the extra length of the runner and manifold is easily accommodated. The long manifold kits I make will not fit under stock side covers.

I manufacture long "Shell style" (Sheldon Thuett - Yamaha race tuner, deceased) inlet manifolds to suit TM and VM carburettors which aid bottom end power further.
Carburettor kit
The manifolds are fabricated from seamless steel tubing, then electroless nickel plated and include ethanol resistant coupling rubbers. Also included are stainless steel hose clips (4). All parts are freely available from UK suppliers or Smedspeed, in the event that spares are required in the future.

The manifolds fit 32 and 34 VM and TM Mikuni carburettors, as well as any other makes using a 40mm stub fitting inlet. They can be made any length to order. They will fit any XS650 from 1970 - 1984.

Contact me for more details

These are experimental parts intended for racing and as such no warranty is given or implied. If you decide to buy it is on the understanding that you assume complete responsibility for any possible injury, loss or damage that may occur as a result of using these parts and that you accept these conditions of sale prior to purchase.