Mikuni RS Flat Slide Pilot Adjustment Screws

Will someone please help me understand this :pinch:

Is the Mikuni RS Flat Slide carburetor spec setting of 1/8 to 1/2 turns maximum out accurate for the Pilot Screws, or should it really be 1/4 to 3 1/2 turns out?

I just don't get how just 3/8 turn of that fine-thread screw can provide adequate adjustment, if any at all...:S

Pic of a screw? Are they in front of the slides or behind them?
Steve

SWest wrote: Pic of a screw? Are they in front of the slides or behind them?
Steve

The RS36 set I had before the 34's:


Screws removed:

Hey, I've got a set of RS38's in my shop right now, as - is take off from what was a professionally dyno tuned ex racing GSX-R750 Slingshot. I'll check how those are set after dinner and get back to you. I think you are correct, though - 1.5 to 3 turns out sounds right-er than 1/4 to 1/2!

-Scott

Well, I don't think this is going to be the final word you are seeking...I just checked those racing RS38's, outside carbs pilots are about 6.5 turns out, insides are 5.75 and 6 turns out (+/- 1/8 turn on those #'s). It was a serious road race bike, though, and I don't know if the carbs have been modified in any other ways. But I think that those numbers can't be how they were run on the engine. Probable that someone took the screws out and just stuck them back in "loose"... The carbs have been on my buddy's shelf for about 20 years before I got them recently.

I'm thinking you should try 2 - 1/4 turns and go for long ride cruising at constant slow speeds / low throttle and check your plugs...or send an e-mail to Sudco or APE...

The Pilot Fuel Screw adjustment range according to the Mikuni HS40 Tuning Manual is...1/4 to 3 1/2 turns out.

See page 1, idle circuit, 3) Pilot Screw:
www.mikuni.com/pdf/hs40_manual.pdf

But...

The Pilot Fuel Screw adjustment range according to the Mikuni RS Series Radial Flat Slide carburetors is...1/8 to 1/2 turn out maximum

See page 5, Fuel Metering Circuits, Closed Throttle to 1/8 Throttle
www.mikuni.com/pdf/rs_app_parts_lists.pdf


Help

I'm working on a similar problem. At 3 ish turns out, bike starts and runs, but pops through carbs, plugs are wet and fouled. At 1/2 to 1/8, it won't start, just backfires through the exhaust. Battery is flat now from cranking, so that's all the experimenting I can do tonight.

Do the screws have a hole in them lie my 33's?
Steve

jberger635 had RS36's on his GPz1170 prior to his 125 mph racetrack crash. When he gets back from his long weekend vacation, I'll ask what his idle screws were set at. He tuned his with an air/fuel meter. He always said they ran rich at low to mid RPM, even with the smallest pilot jets and the idle screws turned in as far as he could before the engine wouldn't reliably start. In part because of the difficulty he had tuning his Mikunis, he is now working on his own design for micro-squirt F.I. for his new 1400 cc engine. He is using my machine tools to build the hardware...

Just to add - every Japanese carb I've ever used (mostly Keihins) worked best with idle screws 2 - 3 turns out. I haven't ran any Mikuni RS flat slides yet myself, although I have tried 32 mm TMR's and will be trying out those RS38's and a new, never out of the box, set of RS40's, I bought the 40's because they are 1/2 the money of the FCR's (I consider the FCR to be the best perf. carbs). I want to get a base tune with the RS38's on my GPz1170, then I'll change them over to break in my 1400 cc engine with as little carb fiddling as possible, Once the new engine is run in, I'm switching it to the RS40's to see if the extra $$$ for the FCR's is worth it over the RS (probably, as these things usually go).

-Scott

It is certainly dependent upon which type of adjustment screw you are talking about.

An air screw(usually on the air side of the carb) has a direct effect on the ratio of air/fuel in the pilot circuit, as it controls how much air is allowed to mix with the fuel pulled up by the pilot jet (slow jet in some carbs). But does not have any effect on how much of this mix is allowed to enter the carb usually below the slide but near the engine side.

A pilot screw usually on the engine side of the carb (sometimes called a fuel screw) does not have a direct effect on the ratio of air/fuel in the pilot circuit. Instead the ratio is controlled by the size of the pilot jet and the pilot air jet. This screw controls how much of that mix is allowed to enter the carb throat on the engine side of the slide.

So if you change the size of the pilot jet and/or pilot air jet the setting on a pilot screw (fuel screw) may be in different positions than stock setting would suggest.

So the combination of Pilot Jet and Pilot Air Jet size must be correct to fall within the Pilot Fuel Screw adjustment range of 1/8 to 1/2 turn out (on all four carbs), where it produces the best idle and low-end throttle response.

Ok, but then the accelerator pump injects a squirt or two of complexity to the situation...:S

SWest wrote: Do the screws have a hole in them lie my 33's?
Steve

No sir. The Pilot Fuel Screw tip position relative to the orifice allows precisely metered fuel to pass.

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