ProCircuit Platinum Pipe / Silencer -Jetting 91KDX

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May 30, 2001
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#1
OK, I sprung for a ProCircuit Platinum Pipe and a ProCircuit Silencer. Now that being the case, does anyone have any info in regards to what the jetting should be changed to when I install the new pipe/silencer on a 1991 KDX 250? I have a stock carb and no other mods have been made as of yet. Any info. would be apprectiated. Thanks!!:confused:
 
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Dec 10, 2000
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#2
Change the main to a 162, the needle to one step leaner (can't remember the number off the top of my head:confused: ) in the second from the top groove, the pilot to a 38 or a 40, and either get a leaner slide or grind 1mm off your existing slide. This combo, combined with 32:1 ratio, will rip!

I think the FMF Fatty pulls harder from low to mid, but the PC will rev a little higher, and is smoother. Any exhaust system is light years better than the boat anchor that came on the bike, so you will notice a huge improvement in the power.
 
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#3
Thanks

Thanks for the Info, I'll grap some jets this weekend and see how it all goes Any idea where you can get a 1mm smaller slide? I have yet to try and get one of those....:)
 
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#4
The stock slide is a #6, you want a #5. It is pricey, though, about $90. You can make yours into a #5 by grinding 1mm from the airbox side of the slide, but it is permanent once done, so be careful. You won't regret this mod, it transforms the running manners and power of the engine completely.

There have been several threads on the proper way to cut a slide, just do a search. I believe there was also an article in MXA on this as well, either this month or last month.

Also, Those are the jets I run in 90+ degree weather here in Georgia. I jet slightly richer in the winter. I don't know the temps or altitude where you live, you may want to start one size richer than those sizes and see how they work for you. The slide mod is a must for that bike, though. You will never get the most out of that engine without it.
 
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dirt bike dave

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#5
Spanky, I think you have the numbers reversed. I thought the slide #'s referred to mm of cutaway. Grinding 1 mm off would increase the #.