Basic Carb tuning (Covers most 4 strokes, but R6 based)

[R1stu]

Carb Tuning, mixture screw/pilot jet explained
------------------------------------------------------------
For all of you guys puzzles about tuning your carbs. Many of you with slip-on pipes and the common "lean" pop pop. Here is a article from factory pro explaining how and why the adjustment does what it does

I hope this helps some of you with your tuning problems It might put some things in perspective to why you are doing what you are doing with the screws.

The affect of pj(pilot jet) size change on idle and cruise operation


Question:

> I have installed one of your 1.0 jet kits on my R6 and I am coming to the final parts of tuning it.

> It runs well and passes most of the tests as provided on your tuning guide. It does, however, appear to be lean low down, as there is a "hole" when cracking the throttle wide open at below 2500rpm and at high rpm/part throttle, it shows some surging. I have though set the fuel screws, so that it idles correctly and returns to the correct rpm, when the idle is set below 1000rpm and the throttle is blipped.

> To fix the lean patch, is it better to go the next step up in pilot jets, rather than adjust the fuel screw?
> Will upsizing the pilot jet, require re-adjustment of the fuel screw and thus negating some of or all of the gains made by the larger pilot jet?


Answer:

As far as the pilot... Look at it this way...

There are 4 outlet holes for the pilot mixture.

1 hole is controlled by the mixture screw and with the other 3, max flow is limited by the size of the pilot jet.

At idle, 1 hole (metered by the fuel screw) and 1 un metered hole are open.

At cruise, when the "butterfly" is just "cracked", all 3 unmetered holes + the 1 metered one are uncovered -

So.....

At cruise, you get those 3 unmetered holes + the metered hole... So, at least 75% of the fuel delivered is limited by the size of the pilot jet.

At idle, you get 100% of the trimmed 1 hole + a butterfly valve-trimmed amount of the other 3 metered holes.

Rule of thumb....
If you go 1 size larger or smaller on the size of the pilot jet, you will change the fuel screw ~1.5x richer or leaner to retain the original idle mixture -

Example:
You have a #40 pilot jet installed (with the proper main, needle height and fuel level already done) and to get best idle, you are 4.0 turns out (from lightly bottomed out).
(Using another "rule of thumb", if you have the correct pilot jet, the fuel screw will end up at between 1.5x and 3.0x when set for best idle.)
Cruise seems lean....

So, I'd expect that I could richen the cruise with 1 size larger (42) pilot jet (size of pj is 75% of cruise mixture) and the "trim" the fuel screw "in" for best mixture for best idle (size of pilot is 25% of idle mixture).

To go back to ~ the same idle mixture, after going from 40 pj/4.0x to the 42........ try 2.5x

40 pj / 4.0x = same idle mixture
42 pj / 2.5x = ~ same idle mixture
38 pj /5.5x = ~ same idle mixture (this is a "stretch - after 4.5x, not much changes)
____________________
Carbon closet tart! https://www.bikepics.com/members/r6stuk/02r6/
Stolen bike. 2000 R1: https://www.bikepics.com/members/r6stuk/00r1/
Current bike 1991 Honda ST1100 Pan European, 1986 Honda cb350sg (Finished)

[R1stu]

EASY TEST to set up your mixture screws.

First off go with the suggested settings in the kit and adjust accordingly.

If at 4,000rpm; (steady state riding)
lean surging happens you need to richen the mixture (going outward 1/4 turn at a time.

A rich problem gets worse as the bike heats, lean gets better as the bike heats up.

Now to do the test: Heres how it works (and it will get you dead spot on!)

Set your idle below 1,000 (6-700rpm works good)
1) Blip the throttle quick, lightly. If the idles rises up then drops below the set point you selected (6-700), you are too RICH, you need to go IN with your mixture screws 1/2 turn at a time.

2) If you blip the throttle and the needle rises, but then hangs for a second before dropping to the perfect set point of 6-700rpm you are too LEAN, you need more fuel, go OUT 1/2 turn at at a time with your mixture scews.

Now huse your head. If you turn the mixture screws 1/2 turn oneway or the other and the problem goes back and forth from lean to rich, you need to turn less.......try 1/4 turn.

Once done, you should be able to set your idle back to normal, give a blip and have it rise fast, and drop fast right to the perfect spot. Now your mixture screws are set

If you run into problems and it just won't work. Make sure you have not intake leaks, bad exhaust leaks. Intake leaks (carbs to motor) cause the most frustration.
____________________
Carbon closet tart! https://www.bikepics.com/members/r6stuk/02r6/
Stolen bike. 2000 R1: https://www.bikepics.com/members/r6stuk/00r1/
Current bike 1991 Honda ST1100 Pan European, 1986 Honda cb350sg (Finished)