VM carb conversion.

toglhot

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The BS38 carbs are gone. The diaphragms being 46 years old, started developing tiny pinholes. I plugged a few with Sikaflex, but they just kept coming. I wasn't going to pay good money to fix a couple of carbs I think are rubbish, so I sent off for a couple of Mikuni VM34s. These come with cables and, manifolds and rubbers. I wasn't impressed with the manifolds though: they were well made but the mounting holes were for 8mm bolts and elongated, amateurish to say the least. Carbs arrived and I bolted them on only to find the cables on top sit right where the fuel taps are. 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 did some googling and found a number of like problems.

I did some checking and found that if the VMs are mounted in the original manifolds, the cables miss the fuel taps, trouble is the VM spigots are 40mm, where as the manifolds are for 48mm spigots. So I had three options: tilt the carbs over at quite an angle to miss the taps, 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 spigots down a little to make them 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 measure 90mm across, so I'd need a 90mm round piece of aluminium stock to turn down to 40mm for the spigots and bore through 40mm, that's a lot of wasted aluminium. So, I had a 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 spigots into place.

I machined up one, pressed the parts together 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 cables sit 10mm behind the taps and give good access to the adjusting screws for tuning. Another problem! The outlet spigots from the taps sit right up against the carb cables. 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 could possibly stretch and the rubber boots deteriorate. 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 nipple and soldered that in place, then routed some fuel hose from taps to carbs. Another problem; The taps have a 6mm spigots, whereas the carbs have 8mm spigots. Back onto the lathe to turn up a couple of brass adapters.

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!

So, here's something else for some of you to make silly comments about.

Some pics.
 

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Welcome back Toglhot!
I personally have missed seeing and following the amazing work you have been doing.
You skills as a machinist and builder are astounding. I appreciate how you think out of the box and just create.
I understand and realize that you have had some bad interaction previously with a few members, one in particular;
but I truly doubt that anyone has made fun of your talent or craftsmanship. I believe that there are others like myself that
have missed following your amazing build. I don't think there ever was malice involved in your prior scrapes.
I know I'm an outsider and everything comes down to point of view.
I usually take into account where and who any perceived insult may come from.
And yes it can and does occasionally feel insulting when you know your skills.. know your plan and vision;
yet someone questions your plan. Especially when you have spent so much time developing and executing it.
When that happens it really doesn't seem to matter whether that perceived insult was not meant as such. It just feels that way.
I think mostly the members here are genuinely trying to help their fellow members.
I also think that it sometimes may seem like the 6th grader is trying to school the PHD, even if they don't realize it.
I just realize they don't really know better and it isn't an attempt to degrade me, you or anybody's work. Even if it may seem that way to myself.
There are many neophytes on this forum that come here for every piece of advice because they do not have the knowledge required,
There are also many very talented individuals such as yourself that really don't need nor do you seek any advise.
Your experience and skills dictate your decisions and that is awesome. Better yet that your talents are shared to open our eyes
to new ideas and ways of doing the same old thing in a different way. Personally I am quite confident in my abilities to create my vision.
I am also kind of a little bit out of the box and as such I am certain I have a few around here scratching their heads about me.
It is ok. I am still very open to suggestions and ideas as my plan is like a rough outline and subject to changes.
I can just as easily disregard any suggestions I do not feel forward my vision. But I might just get 1 or 2 or 3 that I choose to adopt.
This forum is loaded with some amazing talent. Some are talented machinist, and builders like yourself.
Others are incredible teachers filled with untold amounts of knowledge regarding these motorcycles.
I am hopeful that you can let the past incidents roll off your shoulders and down your back.
I would very much love to see more about your build. Especially now that it is running.
Thanks for sharing
 
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Very pretty work indeed. VM carbs are light enough that you don't need rear support. The rubber hose will last for years even with heavier carbs like the 33 mm. 4-stroke pumpers that are hanging on my alloy mounts from 650 Central. In any case, you might consider working on some vibration damping.
 
Very pretty work indeed. VM carbs are light enough that you don't need rear support. The rubber hose will last for years even with heavier carbs like the 33 mm. 4-stroke pumpers that are hanging on my alloy mounts from 650 Central. In any case, you might consider working on some vibration damping.
Everything is mounted in rubber, at the rear, twice. I think they call that vibration damping.
 
Welcome back Toglhot!
I personally have missed seeing and following the amazing work you have been doing.
You skills as a machinist and builder are astounding. I appreciate how you think out of the box and just create.
I understand and realize that you have had some bad interaction previously with a few members, one in particular;
but I truly doubt that anyone has made fun of your talent or craftsmanship. I believe that there are others like myself that
have missed following your amazing build. I don't think there ever was malice involved in your prior scrapes.
I know I'm an outsider and everything comes down to point of view.
I usually take into account where and who any perceived insult may come from.
And yes it can and does occasionally feel insulting when you know your skills.. know your plan and vision;
yet someone questions your plan. Especially when you have spent so much time developing and executing it.
When that happens it really doesn't seem to matter whether that perceived insult was not meant as such. It just feels that way.
I think mostly the members here are genuinely trying to help their fellow members.
I also think that it sometimes may seem like the 6th grader is trying to school the PHD, even if they don't realize it.
I just realize they don't really know better and it isn't an attempt to degrade me, you or anybody's work. Even if it may seem that way to myself.
There are many neophytes on this forum that come here for every piece of advice because they do not have the knowledge required,
There are also many very talented individuals such as yourself that really don't need or do you seek any advise.
Your experience and skills dictate your decisions and that is awesome. Better yet that your talents are shared to open our eyes
to new ideas and ways of doing the same old thing in a different way. Personally I am quite confident in my abilities to create my vision.
I am also kind of a little bit out of the box and as such I am certain I have a few around here scratching their heads about me.
It is ok. I am still very open to suggestions and ideas as my plan is like a rough outline and subject to changes.
I can just as easily disregard any suggestions I do not feel forward my vision. But I might just get 1 or 2 or 3 that I choose to adopt.
This forum is loaded with some amazing talent. Some are talented machinist, and builders like yourself.
Others are incredible teachers filled with untold amounts of knowledge regarding these motorcycles.
I am hopeful that you can let the past incidents roll off your shoulders and down your back.
I would very much love to see more about your build. Especially now that it is running.
Thanks for sharing
I started work at 15 as a factory labourer with grade school education. I spent every lunch hour teaching myself to weld with no help from the resident welders, despite asking. I ended up on the bench as a welder. Along the way I taught myself to fabricate, spraypaint and polish metal and ended up as leading hand by the time I was 20. Another few years and I quit, never working as a welder, fabricator again, instead continued it as a hobby. My brother was a mechanic but had nothing but disdain for me as I showed no aptitude for mechanical or electrical things, so again taught myself, he now seeks my assistance. Everything I know I taught myself, with no outside assistance. That led to many jobs and job offers over the years using skills I taught myself. As a result of my earlier experience, I neither seek assistance nor appreciate it, in particular from credentialled tradesmen. Call me bitter, call me anything you like, but I do not need assistance, I prefer to use my brain and that's the way I like it.
 
Really nice work! But I still have a silly comment.....
Aren't those air filters rather small? Or is there no room for larger filters?
They work fine, despite the size. The filter are made from thin stainless steel with very fine holes, similar to oil filters. They provide plenty of air. Paper element filters are a different matter.
 
Lovely job on the intakes.
My bike came courtesy of the PO with a pair of VM34s and I was mildly surprised to see no securing bracket at the waggly end - that's something I've always meant to remedy, but many years later the front rubber mounts have shown no signs of giving up the game.
I've never been too sure if the PO enlarged the bores, or whether the power output of the engine is purely down the better breathing of the VMs, but I've had no cause to complain about them. They even stay in sync for far longer than the twin-cable arrangement that was on the BMW R80 I had.
I've thought about the angled adjuster tops for the cables and they will take the cable away from the carbs at a better angle, missing the tank and taps, so I'll probably get a pair from Niche Cycle if they are where I saw them last.
 
I've polished the float bowls and wanted to paint the carb bodies to match the engine. I'd painted the old BS38s with engine enamel, but the paint tended to peel off at the bottom where the float bowl attaches. So I did a little. Experiment: Painted an old bowl and place it in the oven at 100c, waited for it to cool, then placed it upside down in a saucer of petrol. The damn paint peeled. I could use two pack, but the colour will be different. I might give it another try in a hotter oven. The wife loves the smell. Petrol flavoured roast beef - yummmm!
 
Love your work toglhot, and your perseverance, with the carbs and life/work. I and a few others over here went for option one -tilt the carbs. Never had an issue and you wouldn't notice .
 
Oldfart - can you tell me a bit more about tilting the carbs please. Which way ?. Did you have to do this even though you had the right angled cable adjusters?.
Can you show a photo please.
Regards Ray.
 
I got the vm 34s from mikuni oz and the carby tops touched the fuel petcocks so I asked Geoff at Geoff's xs what he does as I know he uses the same carbs and it's common to just tilt the carbs slightly inwards to clear the fuel taps. Yeah I'm lazy but they work fine.
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Very pretty work indeed. VM carbs are light enough that you don't need rear support. The rubber hose will last for years even with heavier carbs like the 33 mm. 4-stroke pumpers that are hanging on my alloy mounts from 650 Central. In any case, you might consider working on some vibration damping.
With "rubber hose", do you mean some generic armored and fuel resistant hose, or actual molded rubber carb connectors?
 
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