pilot jet - why change them?

[sprocket]

I know that pilot jets have some influence over the entire acceleration range, but it seems that changing them causes idle problems in some engines.

Wouldn't just changing the main jet give better overall performance and the stock pilot jet better idle and OK low end?

[Bluefront]

It depends on what particular issue you have. Sometimes raising the needle is all you need for better low end performance. A different main jet does have some effect, but the pilot would be more likely for a low end problem. It seems like the mixture screw adjustment will have more of an effect with a bigger pilot than stock.

Different carbs also respond differently to jetting changes......it's a black art, this carb tuning.

[sprocket]

Thanks bluefront...

In the old days on motorcycles the air/mixture adjustment was called a 'low end' adjustment. It seemed that it had no effect on the mid to high ranges, which used different jets.

So, does adjusting the air/fuel mixture on these carbs effect the entire range?

[Bluefront]

Not really.....the mixture screw is mostly out of the picture over 3/4 throttle.

Poor me.....my dog bones are being taken away by somebody. I lost 13 bones in the last week or so. I'm so hurt.

[JR]

Easy explanation;

When you’re selecting a main jet remember not to blowup your engine. It’s better to go too rich at first then try smaller and smaller jets until it runs the best. If your engine really wanted to breathe, and you helped it do that, then a 20% larger main is a good place to start. Most likely the main you choose will be less than 20% larger what your scooter came with.

Something to remember when jetting your carb is that colder air requires more fuel and hotter requires less. What this means is that if you tune in the warm part of the day then go ride at night, when it’s 20 degrees colder, you’ll have a better idea of how close your jetting is. If your machine runs better that night then your jetting is too rich. If your scooter runs better in the day when it’s warm then it’s too lean. This is the ultimate test for choosing the correct main. The main you choose should have your scooter running about the same in warm or cold temperatures. The same holds true for altitude. The higher you go the thinner the air. At sea level your scooter will need a richer mixture for the dense air and in the mountains you’ll need leaner jets for that thin air. If your scooter is set up for sea level even going over 2000 feet can make you too rich. A rich main can make your engine slower and miss or sputter and hard to start. A lean main will lack power and the engine may feel like it’s running out of gas at full throttle. Another sign of a lean mixture is a loud hollow intake sound. If you open up the intake system so it breathes better without richening the mixture you may notice that the intake sound is louder. Once you find the main that gives you the best top speed you can go to the next step.

I like to skip around so the pilot is next. The reason the pilot is next is because the pilot or “slow jet” is effective to ¼ throttle then finishes tapering off close to ½ throttle opening. The main jet starts at ¼ throttle but really comes on from ¾ to full throttle. We’ll get to the needle but that’s after we get the pilot set. Don't forget we are talking about CV carbs here so it's possible your slide is riding lower than you think for any given throttle position.

Let me stress this again, "don't forget we are talking about CV carbs here so it's possible your slide is riding lower than you think for any given throttle position." For example the Ruckus or Met isn't really off the pilot circuit until about 20mph. Of course the main can effect lower engine speeds but if your slide is only at 1/4" opening and your throttle is wide open you are still on the pilot. This is a major source of headaches for most people new to CV carb jetting.

The pilot jet uses the same fuel circuit as the idle mixture screw. A lean pilot can make your machine feel like it’s out of gas, idle too fast or just be gutless from idle to just past ¼ throttle. The air/fuel screw on your carb can be adjusted to provide more or less fuel as need be. The mixture screw is not to be confused with the idle speed screw. The idle speed screw rotates the throttle wheel just like the throttle cable does.

Other help;


Some helpful tips and guidelines to use should you ever want to re-jet your carb;

Sizing at sea level:
With UNI filter 120-125
With UNI filter and low restriction exhaust 130-135
With UNI filter, low restriction exhaust, A12 cam, performance coil & CDI and NGK Iridium spark plug 135-140

Test procedure:
1. Run engine at less than half throttle a minimum of 15 minutes to close electric choke
2. On a 300' or 100 meter stretch or greater, run the engine at wide open throttle
3. While still holding the gas pedal to the floor, cut the ignition
4. Remove the spark plug and inspect end for color
a. White - Increase jet size
b. Tan/Brown - Correct jet size
c. Black - Decrease jet size
5. Clean spark plug with brass wire brush and check the gap prior to next test run

Hints:
1. Change jet size by 2 or 2.5 for fine adjustment by 5 for coarse adjustment
2. Reduce jet size by 2 or 2.5 for every 1500 ft or 500 meters of elevation
3. Stock fuel delivery may not be sufficient above a 130 main jet and may require larger diameter fuel lines and less restrictive fuel filter
4. Once main jet is selected, the air/fuel mixture screw can be adjusted for optimum idle


It all depends on a lot of factors, altitude, air ect. On my bali I just couldn't get it to go right without the mid range bog, changed from a 36 to a 38 pilot and she hit the sweet spot, purrs like a kitten. JRR

[sprocket]

Thanks for that in depth explanation...threw you a bone...

[JR]

Hey you're most welcome and thanks, hope it helped. JRR

[slowscootin07]

I have question for you jrryan and I'm not trying to thread jack since it's the same topic.

I have a SYM HD200 which is the same as a HD125 (125cc), but the motor has been bored and stroked to 171cc thus making a HD"200" .

I'm sure you're already aware of the motor spec, but to sum it up, it's a water-cooled motor with a 4 valve head and ceramic composite cylinder. I can only presume that the innerds are also modified so I'm going to gauge it as your last mod.

With UNI filter, low restriction exhaust, A12 cam, performance coil & CDI and NGK Iridium spark plug 135-140

I have a 30mm water-cooled OKO CVK carb with a 38 pilot and 132 main in it and the needle clip is in the center position

I know I don't need as big of jets as an air-cooled motor, but recently, I had to seriously richen the air/fuel due to lean popping out of the exhaust. I'm thinking of bumping it up to a 135 and seeing what happens. Last plug read was leaner until I adjusted the A/F screw. The reading afterward was normal but the A/F is near the max rich.

Any opinions? and thanks in advance--Jeff

[tomcas]

I wouldn't bother messing with idle jets. If it ain't broke, don't fix it.

[sprocket]

Ya, tomcas

That is my feeling...do the main and get it running well, then play around with the pilot and see if you can improve
things, if you must.

Doing both at the same time, will simply get you too far from perfect, and you have two variables to play with
instead of just one...double the trouble. Too hard to dial it in, IMHO

[JR]

Jun 24, 2010 23:22:43 GMT -5 slowscootin07 said:

I have question for you jrryan and I'm not trying to thread jack since it's the same topic.

I have a SYM HD200 which is the same as a HD125 (125cc), but the motor has been bored and stroked to 171cc thus making a HD"200" .

I'm sure you're already aware of the motor spec, but to sum it up, it's a water-cooled motor with a 4 valve head and ceramic composite cylinder. I can only presume that the innerds are also modified so I'm going to gauge it as your last mod.

With UNI filter, low restriction exhaust, A12 cam, performance coil & CDI and NGK Iridium spark plug 135-140

I have a 30mm water-cooled OKO CVK carb with a 38 pilot and 132 main in it and the needle clip is in the center position

I know I don't need as big of jets as an air-cooled motor, but recently, I had to seriously richen the air/fuel due to lean popping out of the exhaust. I'm thinking of bumping it up to a 135 and seeing what happens. Last plug read was leaner until I adjusted the A/F screw. The reading afterward was normal but the A/F is near the max rich.

Any opinions? and thanks in advance--Jeff

Just an opinion as if I understand you correctly you're running OK with your set up with the AF screw nearly maxed out? If so then my concern would be the screw coming out as it has done with others,

You got choices here, leave it alone and in that case just as a precaution I would take a small dab of silicone and place it on the A/F screw in a way it sticks to the carb body and screw so you could relax and know it might not vibrate out. If you ever need to adjust it again then the silicone is easy to break and not mess up your screw.

Or since you are maxxed out you could go up one or two sizes on the pilot jet and that should allow you to turn the screw back in and lean it back to where you want to be. When I did my carb I could richen it up all I could and I was just on the threshold of being right but if I adjusted it anymore my screw would come out for sure. I went from a 36 to a 38 and all was well.

Hope this helps. JRR