I figured there were enough of us using the Mikuni RS34's on our 750's now, that we should compile some tuning and general information. I, for one, had a difficult time finding info on these carbs. Through some help from DoctorD and MRieck, I gleaned some info., but also sniffed a little out on my own. I am, by no means, an expert on these carbs. I just wanted to share what I have learned and how I learned it.
Here we go:
For basic tuning of the carbs, this is what you want to do. There really isn't a lot of info on these carbs. Searching for rs36's, 38's, or hs40's will give you more info. You need NEW early style intake spigots. The carbs will be hard to put on. You can put them in the oven at 200 degrees for 15-20 mins, as well as remove some material from the lip of the spigot with a dremel. The accelerator pump adjustment is a PITA. It's best done on a dyno (I used a Wideband O2 sensor), but the backyard way to set it is to have the plunger start moving just before 1/4 throttle and stop before 1/2-3/4 throttle. I like it earlier (like just off idle), but haven't figured out the later yet. Bench synch those @&$%ers, or you'll have a hell of a time. Then, you absolutely must use a manometer to synch them while running. These carbs make a rattling noise at idle (some refer to them as rattle slides), but if out of synch, they're even louder.
Excerpt from one of the online forums referring to RS36's (I've edited some of this for RS34 content):
"If your mixture screws are okay put them back in and tighten them just until they stop (carefully) and back them out 2-3 turns each. Then take your float bowl covers off and check that your floats are each at 17mm. The way I do it is to flip the carbs upside down and using a vernier gauge or thin ruler put it inside the float bowl area and measure from the uncapped float bowl edge to the highest point of the plastic float. For the jetting I usually like to leave my needle clips in the middle setting and start with the main jet and work from there. Another thing, this "hanging up" of the throttle you wrote about could be many things, vacuum leak, sticking throttle cable, or it could be the accelerator pump is spraying too much fuel when you whack the throttle. If you look between carb 2 and 3 you will see this skinny rod that gets pushed down by a white plastic piece when the throttle is opened. This rod actuates a piston which squirts fuel through a tiny brass pipe in the carb throats. There are small screws on the top white plastic piece that can be adjusted to control how much gas is squirted in and for how long. This is primarily for those times when you just whack it open, it is squirting fuel in to compensate for the sudden rush of air coming in. One last secret I'd like to share....I think this may help a lot of people who shy away from buying flatslides because of the "heavy throttle". On the Mikuni RS flatslides there are two heavy coil-wound springs that force the throttle shut. I unhooked one entirely and on the other one I unwound it a couple of times and reattached it. The throttle felt like a STOCK bike (This doesn't work, as there is no return spring tension whatsoever. Develop big wrists). You MUST be running a push/pull cable (This is very true. The RS carbs create too much vacuum for them to close all the way by just the return spring)."
Stock push/pull cables will work, but you need to solder new barrels on the carb side and trim them down some. It is a very tight fit and you need to pay very close attention to cable routing.
You can access the needle through through the top of the carbs, and the mains through the large hex head on the bottom of the float bowl. If you need to change pilots (you shouldn't have to), you'll have to pull the float bowls in their entirety.
HS40 Tuning Guide (Single carb application of the RS carbs, but the info is awesome. Disregard anything that starts with an H and ends in a Y).
RS Series Parts List and Throttle position to Jet relationships.
There's an installation manual that comes with the RS34's that, for the most part, is kinda worthless. It has info on accelerator pump adjustment and carb synch on the second to last page. I'll try and post it when I have it in my hands next.
For those of you with RS34's please post any relevant info you have including, but not limited to:
Engine Modifications
Jetting
Dyno Graphs
Accel Pump Settings
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