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How to determine correct Pilot Jet size?
Posted: 04:35 pm Jul 01 2018
by ericr
I have a '02 KDX 200 on which I just installed Ron's Carb and Head mods. I installed a DEK-4 and the carb came with a 40 pj, and a 152 mj. I want to tune it for 3000-7000' and 80 degrees. I'm running 40:1 premix.
My question is, how do you determine the proper pilot jet size? I've read the jetting guide, but Ron's air screw circuit mods don't work in the same manner to allow you to determine if your pilot is too large or small. I've searched this sub-forum, but have not found an answer. How have others done this?
How to determine correct Pilot Jet size?
Posted: 05:52 pm Jul 01 2018
by SS109
The jetting guide works perfectly even on an RB modified carb. From the jetting guide...
As already stated, start with the pilot circuit. Turn the airscrew all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the airscrew for the best response. Now is the time to determine if you have the correct pilot installed in your carb. The airscrew position determines this for you, making it very simple. If your airscrew is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet.
Re: How to determine correct Pilot Jet size?
Posted: 06:06 pm Jul 01 2018
by ericr
I'll give that a try. I was completely thrown off by the instructions Ron included with the carb. I interpreted it to mean that you could not adjust for highest idle with the carb mod. Thanks for clearing that up!
One thing though, Ron ships with the air screw at 7/8 turn out. According to the jetting guide, that calls for a larger pilot doesn't it?
How to determine correct Pilot Jet size?
Posted: 06:01 am Jul 02 2018
by SS109
What Ron is saying, that once you have the proper pilot jet is that now, with his idle circuit mod, you can really fine tune your off idle performance. Follow his directions after you have found the proper pilot jet.