Motorcycle Repair: pilot screws and floats, vent hose, air vent

Question
Hi

I recently purchased a 97' GSXR1100.
It came with a D & D pipe and a K&N filter (not individual filters) installed.
I ran the bike for a couple minutes before buying it, it was running rough but not bad...the gas was old and the tank was full of rust.

First I used a product called KREEM on the tank, it work great.  Meanwhile I pulled off the carbs and cleaned them.

I noticed the floats where set to 12mm, I set them back to what the manual stated...14.7 and the pilot screws to 2 turns out, I didn't take notice of how they were set prior.

Then with fresh gas I fired her up and she ran like crap, sputtering and backfireing with extremely poor throttle responce below 5,000 RPMs or first 1/4 throttle (above that it's fine) and gas was pouring out the air vent hose!  I set the floats back to 12mm and that stopped the fuel from overflowing.  I then tried setting the pilot screw to 1.5 turns out, then 2.5 then 3 then 3.5 then 4 turns out.  3.5 turns out seemed to be the best setting giving me good throttle responce in low RPMs but during the test drive power was unstable in low RPMs...jerky at times.

I'm going to set the pilots back to 3.5 turns and try to set the floats to 13mm to see what that does, if anyone has a suggestions I would greatly appreciate it.

Thanks,
Bruce

Answer
Hi Bruce,

Here's some info:

Low end (full throttle / 1500 to 2000 rpm)
Step 1 (Best Main Jet) and Step 2 (needle height) must be selected before starting step 3!

Float height (AKA fuel level & how to)

o   To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling,  in 2nd gear from 1500 to 2000 rpm at minimum.

Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.
  
If the engine has a "wet" rhythmic, soggy area at full throttle / 1500 to 2000, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 1500 to 2000 leaner.

If the engine is "dry" and flat between 1500 to 2000 rpm, raise the fuel level. Example: change the float height from 15mm to 14mm to richen up that area.

REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.

Warning: If the engine is left with the fuel level too high, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.

Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.

Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all) and Kawasaki (if low speed problems occur). Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!

If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube") in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000 miles. Check them for "oblong" wear - the needle jet orifice starts out round! Factory Pro produces stock replacement needle jets / emulsion tubes for 36mm and 38mm Mikuni carbs. Click here

•   4. Idle and low rpm cruise
Fuel Screw setting (AKA mixture screws)

There is usually a machined brass or aluminum cap over the fuel screws on all but newer Honda. It's about the diameter of a pencil.  Cap removal details. Newer Honda carbs have no caps, but use a special "D" shaped driver, usually supplied in the carb recalibration kit. We do have them available separately, too. 800 869-0497 to order -

Set for smoothest idle and 2nd gear, 2000 rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear, 2000 rpm steady state cruise, and 1/8 throttle high rpm operation.  (pilot jet tuning information)  

Pilot fuel mixture screw settings, float level (but, you've "fixed" the fuel level in Step 3 - which you have already done!) AND pilot jet sizes are the primary sources of mixture delivery during 2000 rpm steady state cruise operation.

If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required.

Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.

NOTE: A rich problem gets worse as the engine heats up.

If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture.

NOTE: A lean problem gets better as the engine heats up.

If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!