carb needles
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wack 63
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- Main scooter: '71 GP TS1
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What would be the effect of using a needle of the same taper length and diameter but with a smaller tip? Obviously it would allow more fuel to pass as the taper would be steeper but where in the throttle opening range and would it also mean a smaller main jet would be used ?
Basically your talking about altering the taper angle by using a smaller diameter tip which i believe would have the biggest effect from 1/2 to 3/4 throttle but would also have a slight bearing on your main jet size as well.
I could be wrong but that is what i have been led to believe.
I could be wrong but that is what i have been led to believe.
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ForemanBob
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- Joined: Thu Sep 17, 2009 7:27 pm
- Main scooter: GP RT230
- Location: Isle of Naboombu
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You'd increase the taper on the needle, resulting in an increased rate of flow from just over 1/4 - 1/2 throttle to 3/4 throttle. The main jet only really kicks in fully beyond 3/4 when the needle has no influence, so should not affect the size of the main jet needed, but adjusting the taper allows you to richen/weaken the mixture at the required points.
length of taper and needle clip position effect 1/4 to 1/2 throttle mainly.
Taper angle (making the needle tip smaller) effects 1/2 to 3/4 throttle mainly but will bleed down below 1/2 throttle to an extent and will have some effect on the main jet.
There was a keihin diagram that i saw that explained this clearly.
With dellortos on scooters we are used to these needles x12, x7, x13, x2, x25 and these are the needles recommended by our suppliers to use and these all have the same diameter needle tip and we are told that raising the needle clip or using a richer needle (say change x7 to x13) will have an influence between 1/4 and 3/4 throttle. This is correct although the biggest area where you will notice the change is 1/4 to 1/2 throttle and because the needle tip is the same diameter the difference to fuel flow at full throttle is negligable.
Generally using just those needles keeps things simpler for set up and generally those needles are more than adequate for our demands, as it could be argued that if you need to richen up between 1/2 and 3/4 throttle, putting in a bigger main jet would probably surfice.
However changing the taper angle (altering the needle tip to a smaller diameter one) will mainly effect 1/2 to 3/4 throttle range and because the needle tip is thinner, this creates a slightly bigger bleed hole between the atomiser and needle tip at full throttle so may resullt in a main jet change.
Taper angle (making the needle tip smaller) effects 1/2 to 3/4 throttle mainly but will bleed down below 1/2 throttle to an extent and will have some effect on the main jet.
There was a keihin diagram that i saw that explained this clearly.
With dellortos on scooters we are used to these needles x12, x7, x13, x2, x25 and these are the needles recommended by our suppliers to use and these all have the same diameter needle tip and we are told that raising the needle clip or using a richer needle (say change x7 to x13) will have an influence between 1/4 and 3/4 throttle. This is correct although the biggest area where you will notice the change is 1/4 to 1/2 throttle and because the needle tip is the same diameter the difference to fuel flow at full throttle is negligable.
Generally using just those needles keeps things simpler for set up and generally those needles are more than adequate for our demands, as it could be argued that if you need to richen up between 1/2 and 3/4 throttle, putting in a bigger main jet would probably surfice.
However changing the taper angle (altering the needle tip to a smaller diameter one) will mainly effect 1/2 to 3/4 throttle range and because the needle tip is thinner, this creates a slightly bigger bleed hole between the atomiser and needle tip at full throttle so may resullt in a main jet change.