mikuni pumpers for GS500?

[johnny ro]

Does anybody have an example where the CV carbs come off and flat slide pumpers go on?

The "Mikuni - TM33-8012" is the popular replacement for CV carb on the Super Sherpa, for example.

They go for around $225 on flea bay, so just under $500 on a GS.

Sherpas are worth slightly more, and take only one carb, so the economics may be a different thing.

The sherpas are famous for running badly with stock carb. Probably easier to fit one than a pair, with the linkages being what they are on the GS.

[SK Racing]

Flat slide carbs is a great idea - not that I would put it on my GS, because I think it's more work than I'd like to get into.

How about velocity stacks? It's easy to add and may have some benefits. Would it work on the stock CV carbs, though?

[Bluesmudge]

Nothing that I know of. Not sure its worth it either. The GS500 runs pretty good with stock carbs, especially with a little Dyno tuning.  Whereas the Super Sherpa is has issues no matter what you do to the carburetor.

I think its much easier to swap carbs on single cylinder bikes. I know DR650 and DRZ400 owners have also figured out pumper carbs that can be swapped for extra power and throttle response.

[mr72]

I know the carb Bonneville crowd rave about flat slide carbs, often getting more power than even stock fuel injection. But that's at the end of an exhaustive set of mods including increasing compression ratio, hot cams and porting. And they report pretty poor low rpm power, bad fuel economy. This is kind of the high revving high power race style mod.

If I were to spend money and effort trying to get rid of the hassles of the carbs I think I'd be putting efi from a Kawasaki ninja 400 or 650 on it. Maybe a combo rig of Kawasaki efi bits and a microsquirt ECU. You'd have to really be in love with your GS 500 to do any of this in favor of replacing the whole thing.

[chris900f]

If I were to spend money and effort trying to get rid of the hassles of the carbs I think I'd be putting efi from a Kawasaki ninja 400 or 650 on it. Maybe a combo rig of Kawasaki efi bits and a microsquirt ECU. You'd have to really be in love with your GS 500 tondo any of this in favor of replacing the whole thing.

I've done a megasquirt conversion before (different bike), but these days I would use one of these https://wtmtronics.com/product/no2c-v0-2-x/
because you can build one for under $100 and it runs TunerStudio. It's a do-able mod if you can find a good set of donor TBI's but the spacing (106mm) is rare.

[mr72]

That's a good call on that ECU. I might try putting one of those in my !vespa.

I would think throttle body spacing is one of those things you would have to be able to adapt and fab up. I know back when I had a Miata and before that my Jetta GLI 16V, there was a group of nuts who would put so-called "ITBs", individual throttle bodies, usually from a GSXR, on these car engines, not only with super hot cams for loads of top end power but also for the insane sound. Adapting the throttle linkages and fabricating up a custom manifold was all par for the course. On a car they would have to use tuned length runners to get enough low end torque, so they'd usually use plastic tubing bonded to a flat aluminum plate with epoxy. I'd think you could probably just use either stock GS500 rubber intake boots or DIY easily enough. So it'd be left to throttle linkage. What am I missing?

Not that I would really do this. But frankly, the amount of time and effort I have spent on carb tuning and the frustration I have with it all the time, I could have converted to EFI about three times over by now, at least in terms of time. A sub-$100 ECU makes this very attractive. I guess I'm going back to surfing for wrecked bike throttle bodies and injectors.

[chris900f]

Back in about 2004 I had been going nuts trying to get my Honda 1981 CB900F carbs tuned for pods and dual headers. I even tried out a set of Mikuni
VM33 smoothbores, I spent a ton of time and money on jets, but was never really happy with the results.

Megasquirt was quite new at the time, there were no bike examples to follow. Long story short, I came up with a way to take apart a set of TB's from a 01-03 GSXR600, and reconfigure
them so that the cable control was in the center of the four bodies, and the cables faced up, like a conventional set of carbs. I was extremely lucky that the GSXR 37MM tb's fit pretty well
with the Honda manifolds. With the help of an experienced welder/machinist/mechanic, I cut and extended the fuel rail to match the 80-120-80mm spacing of the old Honda, as well as extended the push tab on the TB with the cable drive.

Anyhow, the EFI works great on an old air-cooled engine and the fine control over fuel delivery gives you great throttle response and let's you take full advantage of intake and exhaust mods.

https://www.youtube.com/watch?v=mWVhpP7fRU8

So if you're in this far you have to start thinking about the fuel pump and how much power draw you can afford. You really should have an O2 sensor for tuning too...where can you fit everything?
How do you get it all wired up? Eg., If you replace the ECU, you need to add ignitors and relays...the pain goes on lol.

One thing I can tell you is that the Suzuki 01-03 throttle body fits perfectly into the GS500 rubber manifold, but each of the four TB's in a rack is a slightly different configuration
The two bodies that you need are the cable drive and the TPS, and I couldn't see how they would work as a pair without some serious surgery...if you are willing to do the fabrication
you could make it happen. At least the new ECU's are tiny, my 1st gen mega is the size of an old 8-track tape:)