Illus 109

Effect of Crust Formation

These notes apply to carburetors which have been in service for some time and have become dirty, full of "crust" in the throttle barrel, and are found to be difficult to get adjusted properly. Usually the effect of excessive dirt or "crust" formation in the carburetor throttle barrel, around the throttle disc and in the fuel mixture passageways, is to cause the carburetor to have a lean spot off idle. This "crust" should be removed, particularly when a lean spot comes in at speeds off idle up to approximately 30 M.P.H. with the low speed (idle) adjustment set properly for idling. Idle adjustment should not be set to the very lean side when checking this point, but to a point about five to ten notches rich from the setting where the engine dies from leanness.

How to Remove Crust

1. Back off throttle lever stop screw (6) so throttle disc closes tightly. With a sharp pointed tool like a sharp pen knife or scriber, scratch a line deeply on closed throttle disc and also on wall of throttle barrel so lines on disc and on barrel meet. The lines scratched on disc and barrel must "line up" again when disc is replaced. Remove throttle lever (5), throttle disc and shaft, idle hole body plug next to idle holes in throttle barrel, body plugs in carburetor flange and carburetor body idle channels, and low speed (idle) lift lever (9) and needle valve assembly.

Also remove venturi and nozzle.

2. Scrape out caking or "crust" in throttle barrel with a scraper or knife, being sure not to cut into the metal. This "crust" can easily be wiped out with a rag dipped in acetone.

3. Clean up throttle disc by rubbing both sides on fine emery cloth on a flat plate and clean edge of disc all around, being careful not to round the corners or cut into the metal. Can be cleaned with acetone.

4. Clean out idle holes in throttle barrel next to the disc with correct size drills of Harley-Davidson clean-up tool kit Part No. 12012-38. Tool kit includes all drills and slot cleaners required for carburetors, but does not include tools for disassembling and assembling carburetors. Correct sizes for both holes are listed in "Carburetor Specifications", Page 125.

5. Clean the connecting slot between the large and small idle holes by inserting tool blade of correct thickness through slot. Tool with .009" blade (for M-51 and M-51L carburetors) has plain handle; tool with .0155" blade (for M-25 and M-35 carburetors) has two rings around its handle; tool with .020" blade (for M-75 carburetor) has three rings around its handle. Widths of slots are listed in "Carburetor Specifications," Page 125.

6. Clean out idle channels with the #42 drill. When cleaning vertical idle channel do not com pletely bottom drill as doing so may damage low speed needle seat.

7. Clean out low speed (idle) needle valve seat hole with correct size drill. The M-25, M-51 and M-51L carburetors are cleaned with the #53L drill. The M-35 and M-75 are cleaned with the #53L $2 drill which has a smaller handle. (This tool has two rings around its handle).

8. Blow out all channels and holes with compressed air and wash all parts in gasoline or solvent.

Attention to Carburetor Bowl

9. If carburetor bowl continually leaks, remove it from carburetor body, noting location of bowl fuel line nipple in relation to carburetor body. Remove all dirt with gasoline and compressed air. Hold bowl upsidedown so that float valve closes and suck on bottom of float valve seat. Valve and seat should hold this suction. If valve and seat leak after repeated testing, replace with new float valve and float valve seat.

10. If float is damaged or logged, replace with a new one. Remove old float after cutting cement seal around float screw which secures float to float lever. This seal can be cut with a pocket knife. Remove float screw and assemble new float to lever. This should be done with float valve, float valve lever, float hinge pin and screws, float valve seat and gasket assembled in bowl. Before tightening float screw securely, adjust as follows: Looking at bowl with gasoline inlet side away from you, pull float toward you to the limit of slot in float lever and about 1/16" to left of center line as shown in Illus. 110. This provides necessary body clearance.

Tighten float screw and cement top of float screw to float with Dupont Household Cement, with mixture of celluloid dissolved in acetone, or with thick shellac. When cement has dried thoroughly, check float height and adjust as explained in paragraph 11.

11. Check float level and, if necessary, reset to 1/4" (see Illus. 110). Measure directly opposite float lever with bowl held upsidedown (top of float to top of bowl). When readjusting carburetor float, do not attempt to do so by simply bending float lever upward in some manner, without disassembling from bowl. Adjusting in this manner bends and spreads fingers between which head of float needle fits, and thus develops lost motion between float and needle. Float and lever assembly should be removed from bowl, and lever then bent as required.

Before reassembling, see that needle head is a good free fit between float lever fingers with not more than approximately .003" play. This clearance can also be checked after lever is assembled in bowl, by carefully placing a small screw driver or a small rod against the valve head in such a position that it will hold the valve firmly against the seat and yet not bind the lever. Moving the lever up and down will then show the amount of actual clearance between the valve head and fingers. If this clearance is excessive, the float mechanism will not feed properly. After assembling note that float is approximately square with top of bowl.

12. Bowl drain plug can be removed for quick flushing of bowl. Before removing this plug, turn off gasoline at tank. Be sure to pull this screw up tight when replacing.

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