Why can't I understand jetting theories?

9 replies to this topic
  • Dougie

Posted April 09, 2002 - 04:16 AM


Please tell me I am not the only one that does not get the theories of jetting. I'm college educated from a respected Big Ten university. I hold a technically demanding job in digital broadcast television. I read regularly.

But damn it...if you held a gun to my head and demanded I jet a bike or die, it would be curtains for me. I have never been so frustrated by anything. You would think eventually it would come to me. But I just can't get it. Oh I am adept at pulling the carb and tearing it down. That is a good thing I guess. I've read the sites that try to explain all aspects of the jetting. But I might as well be reading Chinese.

I do appreciate all your help and patience. You guys that get it command me respect. So please bere with me as I request more info and help in the coming days, weeks, months, years. Thank you, Doug

  • LarryCO

Posted April 09, 2002 - 06:51 AM


After this weekend, I'll hopefully have us Colorado boys extremely close to an excellent setup. I'll post my results with the 38PJ and 35PJ on that "High Altitude Jetting" thread I started months ago.


  • jim-in-so-ore

Posted April 09, 2002 - 08:26 AM


If it is any consolation, that jetting stuff and the deoctopussing are both Greek to me

  • Mark_Cantrell

Posted April 09, 2002 - 08:35 AM



I rationalize it by imagining this is a complex carb. It is, but some people manage to understand it.

OK, there is no venturi, it has a pilot jet that may or may not be the emulsifier tube, the ET may or may not feed the main, it has a low speed fuel screw where most have low speed air screws, it has separate pilot and main air jets yet Sudco's pilot air screw is a popular aftermarket item, it has an accelerator pump with a leak jet that may let fuel into AP or leak squirt back into bowl or both, there is an adjustment screw for AP timing and a popular aftermarket replacement for less volume and an addon screw to adjust duration, there are more combinations of needles and jets than I can keep straight, it has a cutaway that can be changed for low speed jetting, and if it is a WR it has a deceleration antibackfire circuit AKA air cut valve that can be plugged to give lean or the spring/diaphram removed to give rich but left alone it masks pilot jetting. Finally after being drilled about the various circuits pertaining to throttle position, not RPM, I have a bog on the very low end that is independent of throttle position and AP. Oh, and the CDI/neutral switch/TPS/blue wire/gray wire, the pipe and baffle, and the cam timing all give off jetting like symptoms. Oh, and one thread, Taffy and JD were guiding two aspiring jetters to use the FCN(?) needle and one ended up with a 175 MJ and the other a 155 MJ and both swore by their jetting.

And we don't understand it clearly?


  • Guy

Posted April 09, 2002 - 10:24 AM


:D Clear as MUD :D :) :D

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • KerryT

Posted April 09, 2002 - 10:32 AM


Hey Dougie, I put in a EKQ needle clip #2 with a 165 Main this weekend. Jake and I went to Pueblo, which I think is the same altitude as Denver, and the bike seemed to run good except:

1.) It seemed a little hard to start. (maybe It was the 1st ride of the season and I was a little lazy)

2.) It would sputter and/or backfire upon deceleration. I turned the fuel screw in about 1/2 turn and that seemed to get rid of it.

3.) It still will stall if I crank the throttle off of idle. This is the problem I would really like to get rid of. I've never checked or cleaned my accelerator pump so maybe this could be the problem. Can I take the pump apart while the carb is still installed or do I have to remove the carb from the bike? Am I going to lose a bunch of springs and washers? Anyone? Bueler?

[ April 11, 2002: Message edited by: KerryT ]

  • Chaindrive

Posted April 09, 2002 - 12:06 PM


Thanks a lot, Mark... :)

I ALMOST had the courage worked-up to try some of these mods...

Kinda like coming around the corner, WFO, committed to conquering that monster jump, past the point of no return, and seeing the ambulance loading up the best rider on the track... :D :D

  • Dougie

Posted April 09, 2002 - 04:22 PM


Kerry, I'm going to try a 42 pilot and 162 / 160 main. Then I will try to drop the needle a clip to #2.

I prefer to take the carb off to get to the pump. There is a spring in there but more importantly, there is that very tiny washer that will fall straight out and you will never find it. Then your bike won't run and you will drive your self nuts becuase you didn't realize there was that little 10 cent washer nor that it would not allow your bike to run. Take it from someone who went through that until Clark brain stromed the washer idea. And sure enough, it was gone and sure enough, once I replaced it (Vickery had it in stock bless them) the bike ran fine.

Plus it is just easier to work with because of those tiny screws and the hose clamp holder thingy that goes on it.

May go to Rampart Friday or Saturday to test the jetting. Care to join me and we can compare horror stories?

  • Taffy

Posted April 09, 2002 - 09:13 PM


i've NEVER suggested or recommended an 'F' needle. the nearest i got was to try and find the time to test one on "jetting Qs". i never made it!


  • Magnum

Posted April 10, 2002 - 05:42 AM


Hey Dougie,

I'll be at Rampart on Mon. and Tue. I'm going to Milliken for the Nat. HS. We have some friends that live in Greeley. The only thing is, I ride a 400 EXC.



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