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JD, the jetting "ballpark" won't hold still. Main air/fuel jet issues.

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I've checked all the normal jetting sources you refer to & haven't really found the info I need so I need a fresh perspective on this project. The short story is I need to confirm that when I increase intake restriction I should need to make the main air jet smaller, & do I need to make main jet changes at the same time? When an engine is this far out of the ballpark with high RPM WOT issues should I begin with the main fuel or main air jet first & how can I tell which one is the jet I need to change?

The details are that this is a "hot" one-off 124cc engine based on a XR100 with Takegawa head, custom WebCam camshaft, custom Pro Circuit based pipe, custom JE piston, compression a few points higher than stock, race fuel blends, & a '03 CR85 based airbox & filter. The carb is custom & based on a Mikuni TM-24 flatslide that is oval bored to roughly 26mm worth of flow. This carb is a relative of your bread & butter FCR carb so your advice here will be of use to me with my CRF-450R too. I paid big $$ to get this "special set" carb for this application but the seller just sent me an as-is carb [see my other posts if you're interested in those details]. It had a 150 main, 10 pilot, & a roughly .010" main air bleed hole. The packaging makes it very hard to jet it with the airbox in place so I roughed the jetting in without the airbox then put it on once I get things close. That combo started one kick, no choke, the got real ugly from there with a nasty off idle bog, a way lean middle, then fell hard on its face popping/stumbling @ WOT with what seemed to be a way rich main jet. I got the mixture screw setting back in range with a 15 pilot & that got rid of enough of the bog to at least ride the thing & raised the needle to get rid of the lean spot. Swapping main jets within a large range made little difference on WOT so I enlarged the main air bleed hole until I got in a range that began working then converted the carb to screw-in air jets. I'm currently @ a 160 main, 15 pilot, & 1.3 bleed. With the 1.3 bleed & no airbox there is no WOT popping but it feels like it's down on RPM & power at WOT, & yes I have re-checked the ignition & cam timing. The weird part is the spark plug looks perfect. When I connect the airbox up it pops heavily WOT & falls on it's face but the plug color still looks good, & yes, I'm killing it immediately at the end of the WOT run without idling down & starting with a fresh plug each run. If I roll off the throttle, to 9/10ths, the popping goes away & the power leaps but RPM still seems limited. Yes, I have checked that the air jet channels are unrestricted & going up/down sizes on main air/fuel jets seems to make little difference other than plug color. Any fresh ideas on this would be appreciated.

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The airbox will richen your full throttle jetting ---except--- when the rpm's are too low and there isn't enough flow(vacuum) to pull fuel in.

If I roll off the throttle, to 9/10ths, the popping goes away & the power leaps but RPM still seems limited.

This is typical of a carburetor that's too big for the displacement.

--Is the smallest intake port area bigger than the venturi area? By how much?

You're faced with needing high rpm's to pull fuel, then adjust the main jet to find the best air/fuel mixture. Maybe 90% throttle will make better power, you will need to judge this.

The main air jet will lean the full throttle jetting if increased. Adding the airbox will richen jetting, so a larger air jet should help, except if there is not enough vacuum to run. I would lower the main air jet and then lower the main, just enough to pull through the mid-range. The upper (highest) revs will richen jetting, so test a range of jets for peak revs exclusively to get a feel for how it runs. Race another bike that's close in power on a straight to compare.

Hope this hwlps,

James

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JD, I appreciate the fresh perspective, thanks.

The part throttle range works good enough just roughed in that I know I can dial it in as the big jetting issue gets ironed out. It is strictly the 8/10-10/10 throttle ranges that I'm stalled on & can use input.

I'll reply to your post briefly by saying that the head/port/chamber work is pro flow bench stuff that worked well with a massaged "26mm" carb before now & I'm certain it is a jetting issue & not a port/carb size issue.

If I understand you correctly you are saying that to lean the top end mixture of a carb with main air jets [like the FCR] in response to increased intake restriction you go larger on the main air jet.

What is the riding test to dial in the main air jet separately from the main fuel jet?

If you need to get into more detail please PM me. I think we are both local so a phone call would be the hot ticket & I want to talk with you re: some other business anyway.

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