CRF100F Mods/Jetting

I have an '04 CRF100F and am interested in maximizing the breathing of the stock setup - sort of like the famous Gordon's Mods for the XR400. I plan to do a Uni filter, open up the air filter cover with a 1" breathing hole behind the number plate, maybe grind the header flange if it looks necessary and either add some 1/4" breathing holes in the exhaust silencer or remove it entirely.

Would you start off with 42/105 jets for this setup at sea level? Also, I am at 5,300 ft altitude. Do I use the correction factors as found for an XR400R carb? 0.95 correction for main and 0.90 correction for pilot? Thanks.

You get better throttle response using the stock airbox and filter, just drill some 1" holes in the box.

Just as a reference for everyone else: Current mods are ThrottleUp, 1" hole in stock air filter cover, Uni filter and Denso Iridium plug. Jetting is 38/102 (stock is 35/98, I believe) with the needle clip raised to the 2nd position (stock is 3rd). Seems to run much better. Dyno run said nearly 8 hp (not corrected for drive train losses) at 10,000+ rpm. Very nice, flat power curve. Runs like a champ :thumbsup:. Sea level equivalent jetting would be 42/108.

thanks for the info, what is the throttle up?

"Denso Iridium plug..."


That's new to me too.. where do you get one?

You can get the Denso Iridium plugs at your local Parts Unlimited dealer by using part number IUF22

You can get the Denso Iridium plugs at your local Parts Unlimited dealer by using part number IUF22



What makes this plug better than the stock Honda plug?

Oh, I'm working on all this info to get the kids' bike more power

without diggin into the motor, so please bare with all my questions! :thumbsup:


The ThrottleUp is a free-flowing spark arrestor exhaust tip that bolts onto the stock muffler of the XR/CRF/70/80/100. It sounds great and looks like it should flow much better than stock. Their site is here.

As for the Denso Iridium plug, several of the XR400 guys swear by them. They say it improves starting and is very resistant to fouling. Don't have enough experience with it to state this myself.

When you do any mods that affect air-flow, be sure to adjust your jetting. :thumbsup: If you do all of the mods I've listed above, you should notice a power difference. Post your riding altitude and I can ball-park your jetting requirements.

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