Off idle response fine tuning??

9 replies to this topic
  • Jason_in_KC

Posted October 15, 2001 - 09:34 PM


I have done some searches and read info from JD and Taffy, ie, the guru's. I am in KC MO, not much above sea level..maybe 200ft. Temp ranges now are 40-60 degrees. Current jetting, 45pilot/stock needle/1clip leaner than stock/168main..BK mod timed to .5 and the spray doesn't hit the slide, 2 turns out on the fuel screw.
I have my idle set to where it is purring fast and not thump,thumping.
Wondering how to get more snappy off idle response?? The midrange is strong and top end seems fine, but cruising in 2nd/3rd on flat ground at idle it seems sluggish for a moment before leaping into the midrange pull. It will bog(slightly) if I whack it too fast.I haven't changed any of the air jets..don't know where they are but have a manual for that! I also have not messed w/the fuel screw it has been set at 2 turns out for a while? Anyway, the plug is pretty clean at the electrode and black out towards the threads? I have enjoyed tinkering ever since doing the BK mod, just looking for a litte more snap down low to transition into the midrange...any guru's chime in and set me a harescramble race this weekend that is full of GP style open areas and lots of 90/180 degree turns, need to come out of those turns hard and not use the clutch, can't have those 2smokers in front of me! Thanks for any info, Jason

  • Boit

Posted October 15, 2001 - 11:19 AM


Are you keeping your air filter clean and not riding it right away after installing a fresh one? And one that is not over-oiled? I would try turning your fuel in(leaner) at quarter turn intervals and see if you get a bit more response off idle. Since you already know that it is set at 2 turns out, you can always go back if you don't like how the engine responds running a bit leaner down low. I have mine set to where I get a little muffled exhaust pop when the throttle is chopped and this gives me instantaneous response.

  • kc_husky

Posted October 15, 2001 - 02:58 PM


Boit, what jetting are you running? and what are you riding conditions there, ie, temp, elevation?

  • Boit

Posted October 15, 2001 - 05:31 PM


My jetting probably won't apply to many other 426 owners since I use VP MR2 and a different exhaust system. But, if you want to know, here it bee: Main jet is a #160, Pilot #42 with fuel screw at 1 1/2 turns out, stock needle at stock clip position. As the ambient temperature drops, I will go at one step richer on the main jet and will probably have to enrichen the fuel screw a little.

  • Jason_in_KC

Posted October 16, 2001 - 04:48 AM


The air filter is ready for a change, but I know the routine involved w/those..learned the hard way! I've had this bike a year and followed the info from this board.
How does the race fuel work for you, do you still get the black carbon on the inside of your silencer, or is that just typical? I have been thinking about trying race fuel. I will probably be asking the great KCHusky for assistance and check/adjust the valves in my beast. Also, I have heard of some dirty area of the carb..something w/the accelerator pump area??? Couldn't find anything in a search but recall folks finding this area a bit dirty? I will try the fuel screw and see how that helps, also drop the main from the 168 to the 165 for testing? Thanks, Jason

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  • Boit

Posted October 16, 2001 - 04:31 PM


I used C-12 for over a year and the 426 responds very well to it. Again, jetting is critical with C-12 since it requires a leaner main jet. MR2 is the opposite. I'm back to a #162 now and may need to go richer when the outside temperature gets down below about 45F. MR2 is a bit more responsive, especially right off idle. I see no carbon build up at all the exhaust port has a light ashy grey color. Very clean.

  • KTM250rm

Posted October 17, 2001 - 03:29 AM


I am posting this for Jason as we are going to look at his bike tonight.

My question, on a 4 stroke, what should a properly jetted YZ426 sparkplug look like. I know what one should look like on a 2 stroke, but never really considered what it should look like on a 4 stroke.


'01 DRZ400S
'00 KTM250exc

  • Hick

Posted October 17, 2001 - 01:25 PM


Originally posted by KTM250rm:
My question, on a 4 stroke, what should a properly jetted YZ426 sparkplug look like. I know what one should look like on a 2 stroke, but never really considered what it should look like on a 4 stroke.


Here is my two Rupees:

Looking at the plug on a four stroke is at best an unreliable and subjective way to determine proper jetting. Even if properly done the only area that is tell tale is that at the base of the insulator, which is hard to see with the naked eye. My bike is jetted very nicely and the plug is black all over. I’ve never bothered to do a “plug chop” where you kill the motor at WFO and then yank the plug, but I have a feeling if I did I would find my plug is black all over.

Having said that, jetting by the “seat of your pants” is also subjective to a degree, but can be reliable as well as educational.

Finally, there is no way to jet for all the different carb circuits by reading the plug. With the “plug chop” you may get the right main, but on these bikes that helps you very little. The right pilot, needle and clip position (and for that matter the right air jets) make more difference to your every day riding impression.

  • Jason_in_KC

Posted October 18, 2001 - 04:16 AM


I found some good info from Rich at DRN about plug readings and what a 4stroke plug should look like. He had no problems w/a plug being white w/the black ring around the edge near the threads.
I did richen my clip though just in case I was too lean...I am at 1clip richer than stock and adjusted the fuel screw to 1.5 out per Boit's info. Wow, now I think I have the fullest the BK mod has to offer. It will snap into the mid range much quicker and I even have a little back rumble when I let off of the gas. I will go to the stock clip and leave the rest alone just for seat of the pants comparison..right now though..its loop out city. Would I benefit from a larger main than the 168? It seems to jump into and pull really hard there, but seems to want to keep pulling but goes kind of flat..granted I don't take this bike to the rev-limiter..thought about a 170-172main??? Anyone?

  • Boit

Posted October 18, 2001 - 11:20 AM


It wouldn't hurt to try one step richer on the main jet. If nothing else, you will get a sense of how your engine will run with rich jetting. If this jet makes it too rich, the engine will blubber at some point high in the rev range and the power falls off sharply. It's very easy to change the main jet on a 426. Just drain the bowl and remove the plug at the bottom of the bowl with a 17mm off-set or boxed end wrench. Using a 6mm socket, you can get the jet out in a snap. Reinstalling is a bit more tricky as the jet is offset to the rear(air box side) of the hole. Make SURE you don't strip it when going back in. Just snug and then a tiny bit more is all that is needed. I can change a main jet in about 5 minutes now.

[This message has been edited by Boit (edited October 18, 2001).]

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