Main jet Size By Doug in San Diego (Read 1,109 times)

[Doug in San Diego]

Hi guys

been awhile since I posted here. Here is my question.

With a UNI filter and a free flow exhaust what size jets do you suggest

I read this but it seemed really high

Properly Selecting And Installing Carburetor Jets
[12,251 views - 6 comments - Rating 5/5 ***** ]
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

[Bashan]

Hey Doug, I read that too somewhere. I think that's off a buggy site (gas pedal?) so maybe that affects the jet size. I thought that was way too high on the jets myself. I had installed a Uni and exhaust some time ago and ordered the upgrade set of jets from Scrappy. The biggest one was the " guzzler" at 135. At what I'd poured into the bike I figured this would work with a little tweaking. It instantly turned the plug oily black and it ran like crap. I think I ended up using a 115 and it ran really well. I have jets from 104 to 125 in close increments to experiment with. I think each bike is different and those formulas just don't work. I'm a firm believer in that no two Chinese engines are alike because no two Chinese engines are made alike. There's some exceptions like CF but I think that's generally true. Rich

[sprocket]

Go up two sizes for the UNI and 2 sized for the exhaust.. total 4 sizes... rule of thumb

On most GY6s a #110 will do the job well...if your stock jet is a #106 or #104

Leave the idle jet alone...

Then re-tune the air/ fuel mixture and throttle