Mikuni VM carb conversion.

The 74 TX650 I’ve been working on originally came with BS38 carbs, these are vacuum carbs, being 50 years old they were well beyond their use by date and simply not worth spending the money on them to refurbish. So, I decide to upgrade to simple slide type carbs and bought a couple of Mikuni VM34s. These carbs also provide a slight performance increase.

The carbs came with manifolds, cables and a few jets. The manifolds were well made but the mounting holes were elongated and for 8mm bolts, where as the studs were M6. The manifolds also sited the carbs directly under the fuel taps necessitating tilting the carbs over quite a bit for the cables to miss the taps - so, into the bin they went.

So, to fix the problem I made a couple of new manifolds to fit the VMs to the bike. I first spun up a couple of runners 65mm long to site the carbs behind the taps. These have a diameter of 46mm for 5mm, the next 20mm was turned to 40.10mm and the remaining 40mm to 40mm.

Some 25mm 6061 was shaped for flanges to fit the inlet ports, bored to 40mm to take the runners and a recess turned at the base to accept the runner lip. The runners were then pressed into the flanges. The extra 40.10mm at the base of the runners allowed for a nice press fit inside the flange bores.

The manifolds were then mounted on the lathe and the bottom of the flanges/runners faced. After that it was just a matter of drilling a 6mm hole through the flanges, turning up a couple of 6mm brass barbs and pressing them into place for syncing.

The taps have 6mm barbs, whereas the carbs have 8mm barbs, so I also spun up a couple of 6-8mm adapters and fitted them in the fuel line.

 

Nice work!!