Mikuni Starter Jets

After owning these RS carbs for five years, I just learned that multiple starter jet sizes are available, and have no idea what size was factory installed.
In order to establish a starting point for replacement, such information would be helpful.

See near bottom of last page:
www.dynoman.net/carb/pdf/mikRS-access.pdf

The starter jets are bolt/press fit into the bowl. I wouldn't waste a thought on it!!! NEVER CHANGE A RUNING SYSTEM!!!

Looks like #58 on the exploded parts view. They range from #15 to #100... sort of weird sizing steps.

I bought my RS34's used from a friend who bought them new, so those jets were never changed, they are #42.5 in the RS34.

Are you having cold start issues? I have had no issues with starting those on my KZ650 with 831cc motor.

PS - Disabled the accelerator pump system and removed the fuel pipes between the bowls. #120 main jet with #20 pilots.

wrote:

Looks like #58 on the exploded parts view. They range from #15 to #100... sort of weird sizing steps.

I bought my RS34's used from a friend who bought them new, so those jets were never changed, they are #42.5 in the RS34.

Are you having cold start issues? I have had no issues with starting those on my KZ650 with 831cc motor.

PS - Disabled the accelerator pump system and removed the fuel pipes between the bowls. #120 main jet with #20 pilots.





 

Do the carbs function well for you with the accelerator pump system disabled and blocked off?

While disabled it has worked great for me, but I just gotta try that system again with the upgraded cylinder head...
My current carb jetting seems ideal for cool 40-55F temps: #25 Pilots, .08/.09 Pilot Air, 125 Mains, Jet Needle lowered (clip in 2nd slot).

I will likely try the Starter Jet #60.  The engine acts like it wants more fuel with the 42.5 size.
After struggling to start, the engine stumbles badly with the plunger knob fully out.  With the knob held halfway the rpms increase and the stumbling ceases, but the plungers close and the engine dies when I let it go.

RS series Starter Jet




Size relationship:  Starter Jet Left, Pilot Air Jet Right




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My bike runs really well with the accelerator system blocked off.
The motor seems to be getting enough fuel and I should have done this on the GSXR 33 flat slides I was running on it previously. I have a slight bog at 3K that could be eliminated with some needle or pilot screw adjustments but it got too late in the season to address.
I have not found a issue starting the smaller motor with the stock starter jet, having a bigger jet in there could not hurt anything as far as I can see.

daveo post=859100After owning these RS carbs for five years, I just learned that multiple starter jet sizes are available, and have no idea what size was factory installed.
In order to establish a starting point for replacement, such information would be helpful.

See near bottom of last page:
www.dynoman.net/carb/pdf/mikRS-access.pdf

The starting circuits are designed for a wide range of situations, hot, cold and so on. My 28's were the same way. Full on the bike would stutter and I'd have to find that "sweet spot" and use it every time in normal situations. When extremely cold more would be better just like none in hot weather. As I began to lean it out I found I'd have to use the choke more and more. I wouldn't worry about it unless it's flooding the engine at the lowest settings. 
BTW the VM 28/33's don't have starter jets just a fixed jet in the tip of the emulsion tube and on the 33's there's one pressed in at the bottom of the venture in the float bowl. That makes two for them.
Steve

As I understand, the RS Flat Slide carbs utilize a fuel enrichment system that differs from a choke. 
The system is very similar to BS34 CV carbs, though the RS series provides the option to meter a tunable stream of fuel for an engine to start running more smoothly. 

I believe for all the years Mikuni RS carbs have been produced and as popular as they are, there should be a ton of useful user information on them readily available.

Unfortunately there is NOT...




 

I adapted a set of CVK34 carbs from a 1990 Ninja 750 water cooled bike to mount on a 750 Spectre motor. I enlarged the pressed in starter jets on those by drilling them with a pin vice to approximately .60 MM because I found it slightly hard to start. No issues after enlarging those orifices.

I think if your looking to enlarge pressed in jet orifices you need to be extremely careful as those micro drill bits are usually cheap and can break if your not very careful. I believe any starter jet can be enlarged by drilling but that would be a last resort, look for other ways to reduce hard starting first.