Considering VM30s or similar

[Yamaguy55]

Hello all. I'm new to the forum, but am not new to motorcycles or working on them.

I brought home the daughter's beginner bike (XS400SG) that has sat unloved and unused for six plus years. At this point, I'm exploring my options. I'll probably scrub up the stock junk carbs to get it functioning, then go for the long-term fix.

Since the carbs are now full of yellow jello, and the BS34s aren't performers anyway, I'm considering replacing them with a set of new carbs from Mikuni or Keihin. Pushing me in this direction is that both diaphragms are about to go. By the time I throw enough new parts at these to make them run, I could have bought new several times over.

Dime City Cycles sells VM32s set up (jetted) for 400-500 twins. Around $130 each.
Jetting 185 main, 35 pilot, 6DH7 needle, 2.0 air jet, 159-P5 needle jet.
(Generic is 200 main, 35 pilot, 6DP17 needle, 2.0 air, 159 Q6 needle jet.)

Sudco sells several sets ready to go for Honda CB350s and the Kawasaki EX250. I'd guess that similar types would work very well for the XS400.

Anyone have experience with these? I ran a set of VM36s on my 1980 XS650, and it ran very very well. Drastic improvement in throttle response and seat-of-the-pants power output. It never ran well with the emission controlled 1980 BS34s. The older 1977 vintage BS38s were good carbs, but the later ones were crap.

Comments?

 

[16VGTIDave]

I'm curious, where are you buying your OEM parts from to rebuild your carbs? I can fully rebuild a pair of carbs for well below $260 (plus taxes, shipping, etc), and I'm in Canada where we pay more taxes, more for shipping, etc.

If replacing diaphragms is the big expense, have you looked at the offerings from JBM Industries? A number of our members have installed them with great results.

From what I've seen here, carb swaps are not something that people do very often. I've considered it, and came to the conclusion that I'd be better served installing an aftermarket fuel injection system like MicroSquirt. The initial build would be slightly more involved (a "winter project"), but the tuning afterwards would be a 5 minute job. And one could also have custom mapped ignition as well.

My point is, does your daughter's beginner bike really need a carb swap? Or do you just need to source your parts better?

 

[Yamaguy55]

Thanks for the reply. What I was looking for is field feedback on slide/needle carbs. What worked, what didn't. Often, on small engines, they either need to be quite small compared to CV types, or have an accelerator pump to prevent the "sudden-open bog" many complain about when they fit slide needle carbs on something that originally came with CV carbs. Which is why the factory fits them to begin with. At this point, I'm merely gathering info. Once I get the original carbs cleaned and back together well enough to start it up, I'm going to assess where I'm at and how/if to proceed.

This bike was used quite a while ago by my daughter to learn to ride. It is far from a looker, and after she got married, it sat in a non climate controlled area for a long time until I rescued it last weekend. It is in very tough shape. A full strip and clean, plus most likely a top end job are in order. All of the bare alloy on the engine is nice and chalky. I'd call it a $250 parts bike with title at this point. However, it is paid for.

The carbs need diaphragms, floats, plus all of the usual suspects. That assumes I can remove all the internal corrosion as well. (really, these are in tough shape) By the time I put that much into these BS34s, I can apply it to carbs worth keeping. I went through this exact thing years ago on an XS650G. I pitched the BS34s and didn't/don't regret it, although if I had them now, I could at least cannibalize them. The BS34s work well on completely stock machines, but aren't much good for anything else, IMO. The BS38s from 77-79 are probably the most streetable set available, but I was wondering about the success rate for the various slide needles.

If it is worth fixing, I'm looking to make a project bike out of it. If that happens, I'm interested in upgrading anything I can. The stock fueling and exhaust are prime areas of improvement. Next would be modern suspension and brakes. I wouldn't have a problem fitting USD forks and good shocks. I bring that up as I'm not looking to keep it stock.

I currently have a call in to Sudco, but I like second opinions. Both their CR29s and VM30s sound like expensive, but really good setups. However, If I sold carbs, every carb I sold would be ideal for every situation.
I'm concerned about going too big, and don't want to reduce power by going too small.

Again, which is why they put the CVs on to begin with. I'm one of those types that does the legwork up front, then does the mods.
Caveat Emptor.

Sorry for the saga/book.

 

[16VGTIDave]

You may want to take a look at this thread, in particular this post. The carbs in question are VM30's. And on page 11 there is more discussion regarding jetting.

Hope this helps.

 

[Yamaguy55]

Yes, it does help. Since he lives not all that far from me, perhaps I can go see it if he gets it sorted.
That's quite a project. I hope that XT350 frame can take the extra engine weight. When (and I doubt it ever will) my WR250R engine has enough, I may shoehorn a two stroke into the frame.

I'm thinking that small engines like the XS400 might need smallish slide/needle carbs. The old Honda CB/CL 350 series used either 26 or 28mm Keihins if I remember correctly. The 450 used the same Keihins that the DOHC Yamaha 8 valve 500 twin first used. Actually, that is a thought: they would no doubt work very well. No diaphragms: machined AL slide/piston that I believe didn't even have a return spring, but relied on gravity.

You can run much larger slide/needles with an accel pump and other advanced circuits like the Keihin FCR has, or use a CV like the factory did. If do I stick with the BS series CV carbs, I'm switching to the earlier alloy capped diaphragm chamber version. They were much more tunable.

Once again, thanks for the assist.

 

[16VGTIDave]

Glad to be able to help a fellow inmate!