Hi Pete,
I've been working on this part of my bike recently so I'll tell you what I know. Ixion or Motu will be around later to correct any of my mistakes...
The spring is a critical part. If you have a look at the carb again you'll see that it holds a rubber diaphragm down into the carb. This rubber diaphragm supports the jet needle, which is shaped like a javelin ie it tapers to a point. The pointed end sits in the needle jet.
The Venturi Principle dictates that when a fast moving fluid (air in this case) is accelerated past the top of an otherwise enclosed volume (in this case the float bowl, by means of the needle jet, emulsion tube and main jet), the pressure above that chamber drops (the “venturi effect”). The area of the carb throat controlled by the rising and falling slide is called a “venturi”. The venturi effect creates a low pressure above the needle jet (essentially, a weak vacuum), which draws fuel up from the float bowl (which is kept at atmospheric pressure by means of a vent channel above the carb intake). At small throttle settings, airflow going under the slide and above the needle jet is minimal so the venturi effect is minimal and little fuel can be drawn up from the float bowl. But, as the throttle opens, the volume and velocity of the air passing over the needle jet rises, so the venturi effect creates a lower pressure above the needle jet (therefore a stronger vacuum pulling fuel up from the float bowl). The slide, which is controlled by mass-airflow and differential pressures between the volume under it and the volume above it’s diaphragm (which gets it’s “pressure-signal” from a connecting channel above and upstream of the throttle plate), rises and carries the needle with it. The rising needle, with it’s tapered shape, exposes an ever-greater amount of the needle jet orifice to the venturi, allowing an increasing amount of the fuel to rise from the main jet.
However, I don't think the spring is your main issue. My guess is that when you put the tops back on the carbs you haven't quite sealed the diaphragms correctly. This would cause a weakening of the vacuum which will stop the slide moving properly. Take the carb tops off, and check the diaphragms are sitting properly on the top of the carb. Also ensure that when you put the carb tops back on that the springs sit properly straight - the ones in my carbs have a propensity to come a bit loose when you put them in.
Failing that, pull the tops off again, and check inside the carb venturi to see if all of the needles are still in there. Or that they aren't bent.
Good luck. I hope my suggestions help.
Last edited by riffer; 27th November 2006 at 19:59.
Reason: found some good info and did a "cut and paste" to make it read better
And I to my motorcycle parked like the soul of the junkyard. Restored, a bicycle fleshed with power, and tore off. Up Highway 106 continually drunk on the wind in my mouth. Wringing the handlebar for speed, wild to be wreckage forever.
- James Dickey, Cherrylog Road.
Reply With Quote
Originally Posted by riffer
Bookmarks