Swap Bs32 with VM26?

[Kyler]

Will the carbs off my '77 GS750 (VM26 carbs) work on a '80 motor (BS32 carbs)?

The VM carbs work (the motor doesn't - severe piston slap). The carbs I got with the '80 motor need rebuilding and I'm feeling lazy.

 

[Guest]

As far as I know the quick answer is no. The long answer is yes. I don't know much about the 750's but you would need the right sized carb boots / rubbers, install the push pull throttle with the VM's and probably rejet the carbs due to differences in the size of the intake ports. Way easier to rebuild the carbs IMHO

 

[John Kat]

No, because not only is the diameter of the pipe intake different but the spacing of the spigots is also different

 

[Nessism]

No, because not only is the diameter of the pipe intake different but the spacing of the spigots is also different

You sure? The same set of VM29's will fit both VM26 and BS34 equipped GS1000's. Spacing must be the same for that model.

 

[John Kat]

Ed, I'm not talking about the spacing between the carbs but the spacing between the bolts that hold the rubber intake boots

 

[Nessism]

Ed, I'm not talking about the spacing between the carbs but the spacing between the bolts that hold the rubber intake boots

Ah, got it. And I agree. The spacing is not the same.

 

[Steve]

The carbs I got with the '80 motor need rebuilding and I'm feeling lazy.

If you are that lazy and want the EASIEST route, rebuild your present carbs.

It's not hard, certainly a lot easier than coming up with adapters for the other carbs, chaning the throttle grip to accommodate the dual-cable arrangement, etc., etc., etc.

.

 

[1_v8_merc]

I don't blame him, I would have Attempted the same thing, lol.

VM's always run good, BS clog too easily.

 

[Steve]

VM's always run good, BS clog too easily.

Then I must be doing something wrong. :-k

I have five GS bikes. After the intial cleaning, which happened right after the bike was acquired, NONE of them has needed another cleaning/rebuild.

Short story long: Last year, I had to do some work on my wife's bike. It started with replacing a missing exhaust bolt. That 'missing' bolt was actually broken in the head. In the process of removing the exhaust to be able to get it out, I broke twp other bolts. Time to remove the head and take it to a shop. In the process of removing the head, the little bolt at the front also broke. When I took the head to the shop, I also took the cylinders, just for inspection. Found a very questionable area in one of the cylinders, so ended up changing one of the liners.

After all that work, I decided to go ahead and "do the carbs" because it had been six years since I rebuilt them. They looked nearly perfect, all the o-rings looked good. In fact, if I hadn't had a new set of o-rings on-hand, I would have been tempted to re-use them.

As I said, I must be doing something wrong, all my BS carbs seem to be doing quite well.

 

[Kyler]

thanks folks. I'll suck it up and rebuild. I have two sets of BS32 carbs (one for each motor). Although I didn't mention it, these carbs expect a push/pull throttle cable. My racing hack uses a single cable with a lever (like a brake lever). I'll try using a spring in lieu of one cable.

 

[koolaid_kid]

Just a note, the VM carbs are the reason I sold my 78 GS750E. A 2 stroke rider of over a decade, I was used to an immediate snap and star pointing position of the bike when I whacked open the throttle. The GS just bogged. It was soon replaced by another RD350.

 

[Kyler]

well, the VM carbs worked fine for me. When racing sidecars "smooth" is critical, "snap" is bad--can result in a lost passenger and real bad yaw.

 

[koolaid_kid]

LOL. No sidecars for an RD.
The VMs are good if you have to go 4 strokes.

 

[Chuck78]

You sure? The same set of VM29's will fit both VM26 and BS34 equipped GS1000's. Spacing must be the same for that model.

This has got me wondering... I realized that the good set of rebuild rejetted VM22SS carbs we have on my wife's GS550 will be too much work and slightly different spacing to fit onto the BS32 equipped GS650G head we're swapping on. I was wondering if the GS750 VM26's would be adaptable to the BS32 intake boots. The ports on the 650 head are MUCH MUCH LARGER than the older GS550 engine.

The GS750/VM26 "bogging" issue you mentioned has me wondering if it was just a tuning issue. The BS carbs I was told are more prone to a slight lag when cranking the throttle, because the needles are operated based on engine vacuum on the diaphragm, and when you gun it really quickly, the engine vacuum drops a lot momentarily, resulting in the needles dropping (leaning out) momentarily before they raise up. The vacuum operated slide needle seems better for streetability, smoothness, and economy, but with a manual/mechanical slide carb, it is garaunteed that you are not going to momentarily lessen the amount of fuel restriction through the mains when you crank the throttle rapidly.

 

[Chuck78]

Reda's re-jetted, podded, glasspack running, VM22SS fed 1977 GS550B actually responds immediately and very crisply to rapid twists on the throttle, and my VM26 GS750 which barely idles until warmed up (very dirty carbs and suspected vacuum leaks) hesitates a bit at first, but when warmed up, is also a fire breathing monster that absolutely screams. I'm keeping those for sure and rebuilding when I upgrade to 850 jugs and 72mm MTC pistons.


The big thing I am getting at here is that I'd like to keep the simpler and more reliable VM carb style on her bike when we do a 549cc 8.6:1 to 673cc 9.4:1 top end upgrade, and I don't want to drop $150 on a "nice looking" set of BS32 carbs that could potentially need $400 worth of four vacuum diaphragm slides replaced. If I got them at BikeBandit, they'd be $600!!!!!! They ain't gettin any cheaper or easier to come by. the slides on a VM will take forever to wear in the bores, metal to metal with no rubber (neoprene?) to repeatedly flex during normal operation and eventually crack...

I will be comparing those GS650 BS32's and hopefully some GS750 VM26's or VM29's at the junkyard tomorrow if they have any VM26 racks. I think there is actually a junk set in the carb parts crate sitting outside to compare at the very least. VM26 on a "GS550" sleeper would be killer, the older GS650 heads will take the GS550 top cover and cam covers, and if you're not paying attention to the slightly larger carbs with added ribs cast into them, you may not notice that the VM22/550 head has been swapped for a VM26 and 673cc head!

 

[koolaid_kid]

This has got me wondering... I realized that the good set of rebuild rejetted VM22SS carbs we have on my wife's GS550 will be too much work and slightly different spacing to fit onto the BS32 equipped GS650G head we're swapping on. I was wondering if the GS750 VM26's would be adaptable to the BS32 intake boots. The ports on the 650 head are MUCH MUCH LARGER than the older GS550 engine.

The GS750/VM26 "bogging" issue you mentioned has me wondering if it was just a tuning issue. The BS carbs I was told are more prone to a slight lag when cranking the throttle, because the needles are operated based on engine vacuum on the diaphragm, and when you gun it really quickly, the engine vacuum drops a lot momentarily, resulting in the needles dropping (leaning out) momentarily before they raise up. The vacuum operated slide needle seems better for streetability, smoothness, and economy, but with a manual/mechanical slide carb, it is garaunteed that you are not going to momentarily lessen the amount of fuel restriction through the mains when you crank the throttle rapidly.

The bike was right off the showroom floor. I don't pretend to be an expert on the VMs, but Steve explained it was the immediate drop in vacuum that is the issue, which is why the CVs do not do that, the slides raise slower, allowing the slides to rise gradually.

 

[Steve]

The bike was right off the showroom floor. I don't pretend to be an expert on the VMs, but Steve explained it was the immediate drop in vacuum that is the issue, which is why the CVs do not do that, the slides raise slower, allowing the slides to rise gradually.

ALL carburetors depend on having air moving across the jets at sufficient velocity to lower the pressure in the venturi to draw fuel up through the jets.

If you are running at a relatively low throttle opening (VM or BS carbs), your velocity will be high. If you suddenly whack the throttle open on the VM carbs, the engine, which has been creating a vacuum against the closed throttles, will suddenly have that vaccum filled with a rush through the carbs. Once that vacuum is gone, there is no longer sufficient air velocity through the venturis to draw the gas up through the jets.

If you try the same trick with the BS carbs, the slides will likely be in a lowered position because there is not much air moving below them (their position has NOTHING to do with engine vacuum). When you suddenly open the throttle butterflies, the lowered slides will ensure that the air velocity remains high (that's why these are "CV" carbs, that stands for "Constant Velocity"), allowing the jets to still do their job.

When there is not enough air velocity, there will be no fuel to burn. That can be alleviated by adding an accelerator pump, but I know of only ONE GS bike that has one.

.

 

[koolaid_kid]

Thanks, Steve. Irritated the crud out of me, since I was young at the time and pointing the wheel at the sky was a big thing. These days, not so much.
There were other brands (Kaws?) that had a few models with the accelerator pumps, but CVs fixed all the issues.
My XS650E had CVs, which confused the heck out of me. I pulled the top off one of them, expected to see a slide just like all my 2 strokes and early Hondas had. Instead, there was this rubber diaphragm thing. ??? I immediately put the top back on and left it alone.

 

[Bookmon]

first glance- I tought it was swapping the B52s. I was in for that deal. I will swap them for Justin Beiber any day. Poor kids of today- their music choices are rather poor.