BS34 vs BS38 ?

Grewth

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Has anyone ever bothered to dyno test a stock XS650 motor running on BS34 carbs, and then tested the same engine again with BS38 carbs fitted ?
I'm sorting out parts for my '78 650SE, which was imported into the UK from the US.
Factory carbs were BS38's which I've still got, but need some work.
I've got a better set of chrome top BS34's, along with the rubber manifolds and airboxes.
I'm just wondering if I'll lose much in terms of performance if I fit the smaller carbs ?
 
No, I don't think you will. I have a '78 with it's original BS38 carbs and a later model with BS34s. There's little, if any, difference in the performance between the two bikes. Both bikes have the usual minor mods (different exhaust and pods) and the carbs have been re-jetted for them.

I think the BS34s have gotten a bad rap. It's true they do come set up rather lean from the factory, and the U.S. versions have that fixed needle so you can't "tune" the midrange, but they can be remedied with some minor jetting changes. Normally, on an older carb set with adjustable needles, as you increase the main and pilot jet sizes, eventually the bleed-over from them makes the midrange too rich. You fix that by adjusting the needle, leaning it a step. On the BS34s, the midrange comes set up pretty lean so the bleed-over from larger jets just makes it right. There's no need to adjust it. The BS34s have several factors going for them. They are a newer design and lighter than the BS38s. They also have a removable air jet that feeds the idle circuit and tinkering with it is a way to fine tune that circuit. Mark my words, in the coming years I believe the BS34s are going to become the most desired 650 carb set.
 
Thanks for that.
I've had a personal preference for the 34's for a while now, based on my own experiences.
I've also found a cheap supply of replica diaphragms from China for the 34"s, while I can't find any for the 38's so far.
The last 650 Special I ran had 34's fitted to a US spec engine which originally had 38's fitted.
It pulled like a train with excellent acceleration and fuel economy, but I think the Halco Tuning exhaust played a big part.
 
Yes, Cruzinimage sells complete BS34 slide/diaphragm assemblies for about $20. I've seen just the rubber diaphragms from China cheap but haven't figured out how to get the old ones off and the new ones on. The plastic rings they fit between hold them on very tightly.
 
Yes, Cruzinimage sells complete BS34 slide/diaphragm assemblies for about $20. I've seen just the rubber diaphragms from China cheap but haven't figured out how to get the old ones off and the new ones on. The plastic rings they fit between hold them on very tightly.
You can't... at least I don't see how. the two rings and diaphragm are set into a cutout, and the top of the slide is crimped over (outward) to lock it all in place. There's no way short of grinding the crimp off to remove the rings. And if you did grind the crimp off, there's nothing there to re-crimp it back in place.


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I suppose you could use a Dremmel and cut the top ring like a split washer then epoxy it all back in place.... but I'm not sure I'd trust it after that.
 
I've already done a pair with the loose Chinese diaphragms.
It's not easy but it can be done.
Basically you have to successfully pull the old worn out diaphragm from between the two plastic rings, then force the new diaphragm into the same gap with a blunted plastic knife or whatever tool you can devise.
Plenty of lubrication is required, and I reckon that I was very lucky not to tear or split the new rubber.
Hardest bit is keeping the locating bulge on the diaphragm correctly aligned to the carb piston - get it wrong and you can start again
 
Color me impressed. :)
The rings on the one I destroyed are pretty brittle... and the diaphragm has a pretty hefty rib on the inside. I wouldn't have though it possible to force it in there. Well done. :cheers:


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Do you think you could substitute a zip tie or two to secure the new diaphragm in place?
 
I don't think so. Sealing would be an issue where the zip ties joined together... and the diaphragm opening is slightly larger than the slide. You'd have to bunch it up quiet a bit to get a zip tie on it.
 
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