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Discussion Starter · #1 ·
I work in a machine shop, and I run a 40mm Mikuni on my KX5, but I know its not the best for drag racing, could I just bore it here at work? What is there to look for when boring a carb? Anything else that needs to be done? Bigger jets are obvious, but is there anything else? Just cant go bigger than the slide, correct? I was thinking only going up to 41-42mm.

Any insight on this would be much appreciated.
 

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OK, check the carb for taper. A lot of carbs taper in to the middle, then back out to the front side to increase velocity. I dont think the mikunis and keihins are as bad as the lectrons in that aspect, but wouldnt hurt to check it. If it is a straight bore then pay attention to leave enough meat for the slide to seal on each side, and that you dont get the front nose too thin and have it break when tighteneing down. Of course you want to gut it out first before doing anything, then make sure the pilot hole feed doesnt have any roughness around it when done. Smooth it and blend it out the back, then your ready to reassemble. Step up one pilot and main jet size to start with and check it from there. Hope that helps, Bubba
 

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Discussion Starter · #3 ·
Well, I did it. My 40mm Mikuni was a straight bore, so I bored it out to 42mm. I still have to smooth out around the jet holes, and out the back. But my main question is, how much bigger do I go on jets for a starting point for 2mm bigger? I have heard 1 jet size per 0.5mm bigger on the carb. True?

I am at 390 main, and 40 on the pilot. 2 turns out on the air screw.
 
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