my pilot circuit... - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 1 (permalink) Old 06-15-2013, 02:04 AM Thread Starter
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my pilot circuit...

I am done with my pilot circuit. It is a hair lean throughout the full range, but does not warrant the next size jet. The pilot air adjustment screws are at 2 turns, and the bike 'burbles' when decelerating. The pilot jets are, I believe stock #38.

Probably 95% of the riding I do is using the pilot circuit. This circuit controls the first 25-30% of the throttle. The intermediate circuit (needle) is inactive until the throttle is at the last pilot orifice. Rpm does not dictate the limits of the pilot circuit, only throttle opening does. A M/C is almost always in this circuit or wide open. Few riders will ride with a sustained 1/2 or 3/4 throttle and a CV carb will not function well when this is done.

The test. The goal is to run the same constant speed with less throttle opening. If you are in fact lean at cruise in a higher gear and you add gas the bike will accelerate. Or you can run the same cruise with less throttle. The amount of available gas you can easily add is quite small and would produce a very small acceleration. However, when you have to decrease the throttle to maintain a constant speed you have feedback, though small. I chose a calm day, level straight road, 4000 rpm and 5th gear. At steady RPM/MPH, I eased the choke on slowly to my mark. The mark is obtained by using the choke (enrichment circuit) to obtain a 3000-3500 RPM idle with a warm motor. The goal is not to tell how much rich/lean, but to determine if it is rich/lean. There are reasons for this. If the M/C does not require more or less throttle it is likely a hair rich. If it requires more throttle it is severely rich. Mine took such a small amount less throttle, I had to repeat the test a number of times. Given the extraordinarily large jump in jet size from 38 to 40, I will leave mine alone.

CV carbs have no mechanism to produce an instant 'jump' on any amount of throttle opening. If the bike appears to leap when you whack the throttle, You are adding air to an already overly rich condition. Mine pulls smoothly from any RPM/amount of throttle.

Keihin jets. Genuine Keihin jet sizes are 0, 2, 5, 8. Pilot jets will be available in 35, 38, 40, 42, 45, 48, etc. Main jets are available as 135, 138, 140, 142, etc. These numbers are not random, they are the jet diameter expressed as a percentage of 1mm. A 38 pilot jet is 38% (.38) of 1mm (.039) in diameter. In order to understand jetting, it becomes necessary to calculate the area of the jet orifice. Using the standard formula, the area as follows.

#38=.0001723 sq in (OEM)
#40=.000191 sq in (about 11% larger than #38)
#42=.0002103 sq in (22% larger than #38)

#132=.002080 sq in (OEM)
#135=.002176 sq in (4.6% larger)
#138=.002273 sq in (9.2% larger)
#140=.002339 sq in (12.5% larger)
#142=.002407 sq in (15.7% larger)

The amount of gas increase from 1100 idle to 3500 idle is very small when cruising at 4000, nothing on the order of the increase to a #40 pilot jet.

Question, given the huge increases in each pilot jet increase, why aren't some of these bikes pig rich? I will attempt that next.

Last edited by wmsonta; 06-23-2013 at 08:48 PM. Reason: spelling
wmsonta is offline  
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