confused and need some help


10 replies to this topic
  • byggd

Posted October 21, 2005 - 07:07 AM

#1

I’ve read dozens of threads on jetting and there seems to be conflicting information in one area.
1) Some threads recommend either a smaller bleed back jet or plugging it completely, which will increase the amount of fuel supplied by the accelerator pump.
2) Some threads (and the motoman393 site) recommend adding a screw to limit the movement of the accelerator pump arm, which will shorten the spray duration.

Which one is the better way to go for quick power at low RPMs? :banghead: I ride mostly tight/technical stuff so my main concern is being able to blip the throttle to get the front end up some to clear ruts etc.

I apologize if this is a dumb question but I’m just trying to clarify. Thanks in advance for your input. :banghead:

  • ARin

Posted October 21, 2005 - 08:58 AM

#2

I think that decreasing the squirt duration while maximizing the squirt volume is the goal. Need a strong shot of fuel, but not a LONG shot of fuel.

  • byggd

Posted October 21, 2005 - 12:11 PM

#3

I think that decreasing the squirt duration while maximizing the squirt volume is the goal. Need a strong shot of fuel, but not a LONG shot of fuel.

So I should do both? :banghead: Does anyone have any first experience doing or or the other vs both?

Anyone??

  • byggd

Posted October 22, 2005 - 06:32 AM

#4

Indy?

NCmtman?

Wheels?

Anyone??

  • SpeedBlitz

Posted October 24, 2005 - 05:16 AM

#5

I’ve read dozens of threads on jetting and there seems to be conflicting information in one area.
1) Some threads recommend either a smaller bleed back jet or plugging it completely, which will increase the amount of fuel supplied by the accelerator pump.
2) Some threads (and the motoman393 site) recommend adding a screw to limit the movement of the accelerator pump arm, which will shorten the spray duration.

Which one is the better way to go for quick power at low RPMs? :banghead: I ride mostly tight/technical stuff so my main concern is being able to blip the throttle to get the front end up some to clear ruts etc.

I apologize if this is a dumb question but I’m just trying to clarify. Thanks in advance for your input. :banghead:


I can't answer your question fully, but I recall the following:

Items 2) is for the FCR1 carbs that are used on the 400-426 bikes. Those bikes had too much squirt, so the screw was used to limit the squirt (I think this is in essence the BK mod). I believe I read a thread before (actually, I think it was the famous "Jetting Qs " thread, but not sure) that since the first generation FCR carbs used on dirt bikes were identical or very close to those develop for road racing and the squirt was close to what was used on a mutlicylinder road bike, so you ended with a squirt that was much too large for a single 400-426cc bike. Again, this would need to be verified as I'm just qoing of vague memory.

Item 1) is what is more commonly done on the FCR2 carbs on the 450s. For the 450s, you can tune your AP circuit with the leak jet (bleed back jet as you call it above) and timing screw. You can also change diaphragms, springs, pistons, but I think most get it almost perfect with only leak jet change (going to a 35-40 or blocking it off, some are happy with 50) and some minor timing adjustments.

Someone will most like be able to confirm, but check out Thumperfaq, I think all you need to know will be there, if not, you will find links to important threads.

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  • byggd

Posted October 24, 2005 - 10:34 AM

#6

I can't answer your question fully, but I recall the following:

Items 2) is for the FCR1 carbs that are used on the 400-426 bikes. Those bikes had too much squirt, so the screw was used to limit the squirt (I think this is in essence the BK mod). I believe I read a thread before (actually, I think it was the famous "Jetting Qs " thread, but not sure) that since the first generation FCR carbs used on dirt bikes were identical or very close to those develop for road racing and the squirt was close to what was used on a mutlicylinder road bike, so you ended with a squirt that was much too large for a single 400-426cc bike. Again, this would need to be verified as I'm just qoing of vague memory.

Item 1) is what is more commonly done on the FCR2 carbs on the 450s. For the 450s, you can tune your AP circuit with the leak jet (bleed back jet as you call it above) and timing screw. You can also change diaphragms, springs, pistons, but I think most get it almost perfect with only leak jet change (going to a 35-40 or blocking it off, some are happy with 50) and some minor timing adjustments.

Someone will most like be able to confirm, but check out Thumperfaq, I think all you need to know will be there, if not, you will find links to important threads.

Thank you for the information. Now that I see one applies to the 426 and the other to the 450 it makes more sense. I Have a 02 426 so now I just have to decide if I am brave enough to drill my carb for the screw :banghead:

  • ncmountainman

Posted October 25, 2005 - 04:55 AM

#7

not real sure about the 426's but the boyesen quickshot with a properly timed squirt sure helped on my wr and yz :banghead:

  • GCannon

Posted October 25, 2005 - 08:38 AM

#8

Here's what worked for me on my 02 wr426 (Canadian bikes have a different needle) . If you take your sub frame off and leave your carb on and your fuel tank you will see the fuel squirt when the throttle is opened. I was surprised to see such a big squirt for so little displacement. What you are trying to do here is reduce the amount of fuel being dumped by the AP (accelerator pump) and depend more on the precise fuel metering provided by the jetting. I highly recommend the BK mod as described by Motoman since it worked great for me. the timing of the squirt is also important the reason for the AP is to cover the lean condition during throttle opening (opening the throttle lets more air in) while the fuel metering system (and engine RPMs) catches up to the increase in air flow. You want the AP to happen as soon as possible when air velocity in the carb is fairly slow when air velocity in the carb is fast like half throttle and above the metering system works better to regulate mixture. After the BK mod you will need to increase the main jet since you will be reducing the total amount of fuel . this mod will make your bike accelerate more quickly and cleaner. Don't worry about getting the squirt timing to .3 second you cant be that precise since you are human anything under .5 seconds is fine. but the bes results comes when the AP squirt happens right when the slide opens without hitting the slide (very important). I ride California low desert and the 168 main jet works great my Canadian needle is different than a CA needle a JD jet kit is also a great way to go. Please carfully chose the fasteners you will be using on your carb. A dot of epoxy on the screw threads after it is all adjusted is also a good idea. Do the Bk mod up the main jet one from where you are now ride it and then make jetting adjustments. Good luck. :banghead:

  • byggd

Posted October 25, 2005 - 11:26 AM

#9

Great info gcannon! one question though

After the BK mod you will need to increase the main jet


Wouldn't it be the pilot jet I would want to increase? The power chart I have shows that the main jet doesn't even come on until 1/3 to 1/2 throttle :banghead:

  • ncmountainman

Posted October 26, 2005 - 06:53 AM

#10

Great info gcannon! one question though


Wouldn't it be the pilot jet I would want to increase? The power chart I have shows that the main jet doesn't even come on until 1/3 to 1/2 throttle :banghead:

it overlaps more than what they print in that graph,from my experience anyhow

  • byggd

Posted October 26, 2005 - 07:57 AM

#11

it overlaps more than what they print in that graph,from my experience anyhow

My bike has been running so well that I have been putting off ordering a JD jetting kit. Looks like if I do this mod I better put out the $ and order one up :banghead:




 
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