daves Mods altitude question???


8 replies to this topic
  • riding4life162

Posted March 31, 2008 - 04:23 PM

#1

I am in no way a carb expert, I am still learning how carbs work, by reading everything I can on them, but my questions is
1. Does Daves mods work in all altitudes??
The reason I am asking is because if you have to rejet depending on your altitude, how is it that this mod works for different places?

I would like to do it but not sure if it will work.

  • martinfan30

Posted March 31, 2008 - 04:25 PM

#2

I am in no way a carb expert, I am still learning how carbs work, by reading everything I can on them, but my questions is
1. Does Daves mods work in all altitudes??
The reason I am asking is because if you have to rejet depending on your altitude, how is it that this mod works for different places?

I would like to do it but not sure if it will work.


The recomended 158/55 is good all the way up and over 5k feet IMO. Just starts running a little rich. I would carry a 155 and a 52 if you go real high up.

  • RipperFox

Posted March 31, 2008 - 04:32 PM

#3

I gots a question too. What exactly does adding the washer under the needle as a shim do? What the theory?

  • martinfan30

Posted March 31, 2008 - 04:41 PM

#4

Since the stock needle has no adjustable clip positions, this is just raising the needle in the needle jet. Just like dropping the clip on a needle with multiple postions.

Just to richen the mixture a little at lower throttle openings.

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  • riding4life162

Posted March 31, 2008 - 04:47 PM

#5

The recomended 158/55 is good all the way up and over 5k feet IMO. Just starts running a little rich. I would carry a 155 and a 52 if you go real high up.


ok i see, so the reason everyone uses the 158/55 is because they are 5000ft AGL or below?

I live in utah now, at my house the elevation is 4800 feet, and it only gets higher, the mountains go up to 10,000ft. Should i use a 155/52 instead of the 158/55?

Also is there a link where it has the reccommended jets for different altitudes?

  • Purcell69

Posted March 31, 2008 - 04:52 PM

#6

I agree with Martinfan. Dave's mods will work pretty much at any altitude, as long as you adjust the jet sizes accordingly. RipperFox, the shim under the jet needle is to raise the stock needle up slightly, causing the narrower part of the stock needle to be in the main jet. The the needle in the carb works in conjunction with the main jet and the slide. As you open the slide when accelerating, the needle also goes up, allowing more fuel to pass through the main jet to mix with the extra air coming into the motor.

The factory needle has no provision to make adjustments to fine tune the mixture under acceleration, other than to shim the needle. Aftermarket needles, like those in the Dynojet kit, have a different taper and have a series of grooves and a clip that allow the needle to be adjusted heightwise without the use of a shim. Moving the clip up lowers the needle into the main jet, leaning the mixture. Moving the clip down raises the needle higher in the main jet, adding more fuel to the mixture.

Raising and lowering the needle is a further adjustment of the main jet size, ie. main jets come in say #155, #158, #160, etc. Raising the needle in a #158 main jet can make the motor run like it had a #159 size main jet, and lowering the needle can make it run like it had a #157 main jet.

-Joe

  • martinfan30

Posted March 31, 2008 - 05:43 PM

#7

ok i see, so the reason everyone uses the 158/55 is because they are 5000ft AGL or below?

I live in utah now, at my house the elevation is 4800 feet, and it only gets higher, the mountains go up to 10,000ft. Should i use a 155/52 instead of the 158/55?

Also is there a link where it has the reccommended jets for different altitudes?


Yes. Although I have ridden with 158/55 at 10k feet and still runs, but very rich. Thats why I keep a few jets on hand for a quick change out.

Just try experimenting and you'll get quick at changing the jets out.

4800 feet is where my house sits here in NV also. I run the 158/55 every day commuting. Just a hair rich, but helps keep operating temps lower.

  • Denn10

Posted March 31, 2008 - 07:58 PM

#8

no such thing as a quick change out for jets, this is not something you do on the side of the road. If your going somewhere that you can take 30 mins to change out it would be best to have them with you but I think your 155/52 for 5k and up will be pretty good. remember your air screw turning in gets leaner so start at the 2 1/2 and adjust to liking

RIPPER - the needle is tapered cuz when your about 1/3 throttle the taper starts to get smaller and as you pull more throttle your moving the needle up. This taper starts to get smaller and smaller as your going to 1/2 throttle and this starts to let fuel up through the main. The needle controls your 1/2 throttle range, little below and little more, the more you give throttle to 3/4 you get to the smallest part of the taper and this lets the main take over where you start getting alot more air so you need more fuel right? This is why a needle with about 5 clip positions lets you taylor your jetting for all areas of throttle, from 1/4 to 1/2 all the way wide open. Thats what your pilot, needle, and main control respectively. THE only kicker is your air screw work in hand with your pilot. Air flowing throught the throat of your carb causes suction thru the jets and as you open throttle and taper gets smaller the fuel is pulled through the main cuz its the least restrictive vs the pilot circuit. And say if your throttle is all the way closed the needle closes the main jet and your pilot circuit is the one doing the work.

Hope thats not a bad explination. LOL

  • martinfan30

Posted March 31, 2008 - 08:03 PM

#9

With the FCR, its pretty easy to just loosen the boots and rotate the carb enough to pull off the bowl and change the jets.

I dont see why the stock CV would be any harder?:prof:





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