The BS38 carbs are gone. These are SU type carbs, comprising a butterfly and a slide and needle operated by vacuum, a totally ridiculous design in my view. Anyway, the diaphragms being 46 years old, started developing tiny pinholes. I plugged a few with Sikaflex, but they just kept coming. Yo can't buy the diaphragms by themselves in OZ, you have to buy the diaphragm and slide at $300, damned if I was going to pay that to fix a couple of carbs I think are rubbish.
So I sent off for a couple of Mikuni VM34s. These are normal slide carburettors, simple and they work well, giving a slight performance boost. These come with cables and, manifolds and rubbers for $600, darned expensive, but heaps better than the BS38s. Carbs arrived and I bolted them on only to find the cable on top sits right where the fuel tap is. I contacted the seller and apparently, due to Yamaha's manufacturing processes back in the day, no two XS650s are the same, so the cables interfere with the fuel taps on some but not others. I'm just lucky, story of my life.
I did some checking and found that if the VMs are mounted in the original manifolds, the cables miss the fuel tap, trouble is the VM spigots are 40mm, where as the manifolds are for 48mm spigots. So I had three options: tilt the carb over at quite an angle to miss the tap, Fit the VM spigots with a 48mm sleeve or machine up some longer manifolds to place the carbs behind the taps. Options one and two, went out the window, option one for obvious reasons, option two because I'd have to turn the spigot down a little to make it smooth enough to take a sleeve. That's not something I fancied doing, so, I set about machining up a couple of longer manifolds.
The mounting flanges measures 90mm across, so I'd need a 90mm round piece of aluminium stock to turn down to 40mm for the spigot and bore through 40mm, that's a lot of wasted aluminium. So, better idea: turn up a couple of 70mm long 40mm spigots with a 34mm bore, machine a couple of 90mm wide flanges and press the spigot into place.
I made one and it came up excellent, so then I made another identical one, well, nearly identical, the second one came out 0.20mm longer, but I can live with that. Once finished I polished them, fitted them and fitted the carbs - success, The cable sits 10mm behind the tap and gives good access to the adjusting screw for syncing. Problem! The outlet spigot from the tap sits right up against the carb cable. Easily fixed, swap the taps from right to left and left to right and the outlet spigots now face forward, away from the carbs.
Next problem: the carbs are normally supported at the rear by the airbox, but as this custom doesn't utilise an airbox, the carbs would in all probability stretch the rubber boots and the carbs would fall off. So, now I needed some sort of a support at the rear. So, I made up two U shaped brackets to fit over the slot at the back of the carbs, fitted them with rubber grommets and positive stop T nuts, then I made a larger flat bracket which bolts to the rubber mounted battery carrier and mounted the U shaped brackets to that, then anodised them.
Worked a treat and not too obtrusive or ugly. Next step, shorten the cables, I machined up a 6mm ****** and soldered that in place, then routed some fuel hose from tap to carsb. Another problem; The taps have a 6mm spigot, whereas the carbs have an 8mm spigot. Back onto the lathe to turn up a brass adapter.
All done, so I wheeled it outside, started it up and tuned it. Runs like a beauty now. But geez, what a lot of work!
Some pics.
So I sent off for a couple of Mikuni VM34s. These are normal slide carburettors, simple and they work well, giving a slight performance boost. These come with cables and, manifolds and rubbers for $600, darned expensive, but heaps better than the BS38s. Carbs arrived and I bolted them on only to find the cable on top sits right where the fuel tap is. I contacted the seller and apparently, due to Yamaha's manufacturing processes back in the day, no two XS650s are the same, so the cables interfere with the fuel taps on some but not others. I'm just lucky, story of my life.
I did some checking and found that if the VMs are mounted in the original manifolds, the cables miss the fuel tap, trouble is the VM spigots are 40mm, where as the manifolds are for 48mm spigots. So I had three options: tilt the carb over at quite an angle to miss the tap, Fit the VM spigots with a 48mm sleeve or machine up some longer manifolds to place the carbs behind the taps. Options one and two, went out the window, option one for obvious reasons, option two because I'd have to turn the spigot down a little to make it smooth enough to take a sleeve. That's not something I fancied doing, so, I set about machining up a couple of longer manifolds.
The mounting flanges measures 90mm across, so I'd need a 90mm round piece of aluminium stock to turn down to 40mm for the spigot and bore through 40mm, that's a lot of wasted aluminium. So, better idea: turn up a couple of 70mm long 40mm spigots with a 34mm bore, machine a couple of 90mm wide flanges and press the spigot into place.
I made one and it came up excellent, so then I made another identical one, well, nearly identical, the second one came out 0.20mm longer, but I can live with that. Once finished I polished them, fitted them and fitted the carbs - success, The cable sits 10mm behind the tap and gives good access to the adjusting screw for syncing. Problem! The outlet spigot from the tap sits right up against the carb cable. Easily fixed, swap the taps from right to left and left to right and the outlet spigots now face forward, away from the carbs.
Next problem: the carbs are normally supported at the rear by the airbox, but as this custom doesn't utilise an airbox, the carbs would in all probability stretch the rubber boots and the carbs would fall off. So, now I needed some sort of a support at the rear. So, I made up two U shaped brackets to fit over the slot at the back of the carbs, fitted them with rubber grommets and positive stop T nuts, then I made a larger flat bracket which bolts to the rubber mounted battery carrier and mounted the U shaped brackets to that, then anodised them.
Worked a treat and not too obtrusive or ugly. Next step, shorten the cables, I machined up a 6mm ****** and soldered that in place, then routed some fuel hose from tap to carsb. Another problem; The taps have a 6mm spigot, whereas the carbs have an 8mm spigot. Back onto the lathe to turn up a brass adapter.
All done, so I wheeled it outside, started it up and tuned it. Runs like a beauty now. But geez, what a lot of work!
Some pics.
