WZ507
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- Joined
- Dec 26, 2013
- Messages
- 553
needing said:
Background and Context
A drivability issue encountered with my recently purchased '74 850 caused me to remove the carburetors for a thorough review (never had an AMAL carb apart before). Problem turned out to be a loose jet holder in one of them (had almost completely unscrewed), but since they were off decided to rebuild and do some measuring. Thought my findings might be relevant in light of the recent comments here on annulus area of needle jet/needle, needle wear rates and other carb dimensions.
Carb Component Dimensions
The small orifice sizes (idle and main jet) were determined by probing with pin gauges sized in 0.0005" increments. The needle jet was sized with pin gauges of 0.001" increments. Jet needle dia at various locations along the length were assessed with an optical comparator.
Slide Fit
Depending on vertical position the slide could be displaced laterally in the carb body 0.012" - 0.015", so fairly loose.
Pilot Jets
Pilot jet dia is often quoted at 0.016", however the right and left fixed idle jets had diameters of 0.018" and 0.017" respectively.
Needle Jets and Jet Needles
Both carbs were fit with 106 needle jets and 4 ring needles. The jets had an ID of ~ 0.1065" dia. The 0.106" gauge pin dropped in easily and the 0.107" pin could not be inserted or even started into the jet bore. The needle clip was found in the top groove on both carbs (leanest condition) and the diameter of one needle was measured by mounting on an optical comparator and measuring at 10 mm increments along its length starting at the clip and moving toward the tip. A worn section was noted in the needle that corresponded to operation just off idle (position where needle is repeatedly subject to momentary high vacuum, thus side thrust, as engine transitions from idle to higher speed operating regimes), and where the needle would experience constant use (every time the twist grip moves from idle - every blip, every shift, every use) over its 10,000 mi of life to date. The needle was worn at this position from an original dia of ~ 2.535 mm to ~ 2.500 mm.
Main Jets
Ended up finding two different main jets in the carbs - R = 250 and L = 260. I realize the main jet IDs refer to the fuel they are capable of flowing (cc/min at a given pressure head), but nevertheless wanted to estimate the difference in main jet orifice size. I was a bit surprised how close the 2 jets were in size, where a 0.0545" pin would pass through each jet, albeit slightly easier in the 260 than the 250, and a 0.0550" would not pass through either jet. So perhaps the jets differ in orifice size by a maximum of 0.0005", which seems a very small difference (~ 1.9% difference in orifice areas) given that the flow difference between the 2 jets is 4% (260/250 = 1.04).
Needle Annulus Area vs Main Jet Area
With the above data in hand, the annular area between the needle jet and throttle needle at various positions along the needle's length were calculated and compared to the area of the main jet. The rationale for this undertaking was simply to create a plot showing how needle taper affects mixture strength at different throttle openings with this "special 4 ring needle" intended for use with a 260 main jet. With the needle clip in the highest groove and the throttle totally closed, the point at which the needle enters the 0.1065" diameter section of the needle jet orifice was assigned the position "0 throttle opening" and the remaining throttle opening positions were assigned as fractions of the 32 mm of needle that exist below this point, which in total cover throttle openings from 0% to 100%.
The worn section of the needle referred to above is readily observed in the plot at ~ 1/8 throttle, where the worn out needle section creates an undesirable rich spot. Disregarding the worn needle section for the moment, the plot suggests that from 0 to 3/8 throttle, a virgin needle jet/needle combination would not change mixture strength significantly, since there is minimal, if any, change in needle dia occurring over this range of throttle position. From 3/8 to 3/4 throttle the needle jet/needle provide a very significant change in mixture strength, and it appears that by 3/4 throttle the needle is withdrawn sufficiently from the needle jet that it no longer regulates/limits fuel flow since the annular area has now exceeded the main jet area, thus the main jet is now limiting fuel flow in the system. It should also be noted that the needle clip provides a total adjustment of ~ 3.4 mm, i.e., by dropping the clip to the bottom needle groove the red "throttle opening scale" shown on the plot shifts to the right ~ 3.4 mm (the blue vertical bar becomes the new 0 throttle starting point for the throttle opening scale), thus providing increased mixture strength relative to the scenario where the clip is positioned in the top needle groove.
Bottom line, with new needles/needle jets and clean, freshly synchronized carbs, bike ran great. Even with loose slides the idle is very stable at 1000 rpm.
Examining the new vs old needle jets, no difference in size could be ascertained with the pin gauges on hand (0.106" and 0.107"), albeit a coarse means of measuring a precision hole.
