* . . . Finding and fixing the cause of the degradation that leads to intake manifold leaks

• On January 15, 1968, the MoCo issued a service checklist to aid in testing and repairing (instead of returning for warranty) Tillotson carburetors due to some dealers having difficulty diagnosing problems on 1966 → Sportsters & 1967 → Electra Glides. ¹⁾
• Before disassembling the carb to correct any (non)performance, it should first be established that external adjustments have been made and are correct. In many cases, the intermediate and idle speed needles are incorrectly adjusted cause poor performance. ²⁾

• Correct adjustment can be determined in the shop and verified by a road test:
• Adjustments (a tachometer is recommended): ³⁾

1. Seat idle and intermediate needles, gently, and open both 7/8 turn.
2. Start the engine and warm up for at least 3 minutes at fast idle.
3. Set the engine throttle so that the engine runs at app. 2000 RPM.
4. Without changing the throttle setting, turn the intermediate needle slowly in the direction which produces the highest RPM. The engine should not miss or surge at this position.
5. Back off the intermediate needle 1/8 turn to slightly richen the mixture. This is the correct intermediate needle adjustment.
6. Readjust the idle needle & the idle speed stop screw to produce a smooth idle at desired speed (900-1100 RPM).

• All these tests should be performed (in sequence) before further disassembly or repairs are made.
• Testing (prior to removal from engine): ⁴⁾

1. The accelerator pump should be inspected for proper operation. Remove the air cleaner and prime the carburetor by inserting a toothpick through the small hole in the bottom of the plastic pump cover and gently work the diaphragm several times. Operate the throttle lever (both rapidly and then slowly several times with the fuel valve turned on. The pump should deliver a strong & constant jet of fuel with each stroke. Failure to do so indicates defective diaphragm valves or pump plunger.
2. Clean the high, intermediate & low speed channels to (most likely) dislodge any loose dirt in the passages. Remove the high speed screw plug located on the rear side of carburetor opposite intermediate adjustment needle. Lightly seat the intermediate needle and apply air pressure (90 psi max.) to the screw plug hole. Next, open the intermediate and idle needles three or four turns and again apply air pressure (90 psi max.). Reset both adjustment screws (see adjusting measures above). Ride the bike to evaluate the carburetor's performance.
3. Check the inlet needle and seat for leakage. Make sure that all of the plastic cover screws are tight. Remove the fuel and vent lines, install a bulb tester (94750-68) to the fuel inlet fitting, plug the vent fitting with your finger and pressurize tester while noting any leakage. A moistened needle and seat should hold app. 1 to 1-1/2 psi. Release at app. 3-5 psi. A dry needle and seat will not hold as well as a moist one.
4. Remove the carb but do not disassemble it yet. Inspect the intermediate adjustment needle and spring to see if the spring coils are binding before the needle seats. If the needle does not seat grind a small amount of material from each end of the spring. You can check to see if the needle is seating by applying blue dye to the needle taper and screwing it down lightly into the seat and noting the mark on the needle taper.
5. Check the main nozzle ball check valve for leakage. Seal one side of the venturi with your finger and apply alter­nate pressure and vacuum by mouth using the grommet end of tool (96960-68) seated in the venturi. Vacuum should release ball & pressure should seat it in the nozzle assembly.
6. Removing the main nozzle welch plug. If leakage is evident, the carburetor must be disassem­bled and the main nozzle check valve assembly replaced. To replace the main nozzle, puncture welch plug with pointed tool (avoiding center). Remove the nozzle welch plug and use the stepped end of a punch (96962-68) on the nozzle, tapping it through and into the venturi using a plastic hammer. Use the larger end of the tool to install the new check valve in the same manner.
7. Inspect the idle needle and seat in the carburetor bore for any distortion or a cracked casting.
8. Inspect the choke relief disc (upper half of the choke) for distortion or stress cracks at the area rotating on the choke shaft.

• Disassembly, inspection and replacement of parts (replace damaged parts only after completing all of the above tests). ⁵⁾

1. Remove the plastic diaphragm cover and inspect the accelerator pump leather for fold-over or the coil spring out of the correct position. Check the accelerator pump outlet ball check valve to see that the ball is free (Late 1968). Inspect the gasket and diaphragm for distortion or misplacement on the carb body. The diaphragm must not be stretched or have a rippled appearance particularly within the valley portion which should be uni­form in shape (gasket should be assembled next to the body).
   Lightly, attempt to rotate the metal diaphragm washer (riveted to the upper side of the diaphragm). If the diaphragm plate rotates freely with no drag, replace the diaphragm assembly. The diaphragm plate should not be loose. Prior to removal of the inlet lever, the initial needle seat leakage test should be performed 10-12 times with the bulb tester. Close the bulb valve. Apply pressure to the inlet to seal the vent fitting. Open the bulb valve and again apply pressure. This repetition checks the sealing of the needle in the seat insuring that it is not sticking open at the lever pin or at the groove in the needle.
2. Inspect the inlet needle lever for correct adjustment. It should be flush with the surrounding floor of the carb body. If not equipped with a shackled needle, replace with kit (27588-66). Torque seat to 45 in-lbs.
3. Test the economizer ball check for leakage and correct operation. Using the hose end of tool (96960-68), place it over the economizer welch plug hole so that it seals off the sur­rounding area. With alternate pressure and vacuum applied by mouth, ball check should release and seal. Replace any defective parts.
   

• During the mid 1979 model season, HD makes a recommendation to dealers for corrective measures regarding the carburetion and ignition system on (1100, 1200 and 1340cc) models with emissions control design changes. They operate with leaner air/fuel mixtures and are more sensitive to carburetor mixture settings / ignition spark. ⁶⁾
• For proper carburetion it is recommended to check the following items as corrective measures for irregular running and spit-back which are symptoms of an excessively lean mixture. Also check for proper ignition.
  • Check to see that the intake manifold seals and band clamps are installed properly. Using a squirt can with stanisol or kerosene, squirt some on these connections with the engine idling and see if there is any change (increase or decrease) in RPM which would indicate an air leak.
  • The air cleaner backing plate must be flat against the car­buretor flange gasket surface and the air cleaner mounting brackets must be adjusted and tightened properly per the FSM with no air leakage at the carburetor mounting flange.

• Carburetor modification kits (including jet(s) and an accelerator pump stop screw) were made available for 1200cc, 1340cc and 1000cc models to provide leaner fuel mixtures when operating at elevations above 4000 feet sea level. ⁷⁾
  • Normally, leaner fuel/air mixtures are required for proper engine operation as the elevation above sea level increases. This high altitude kit (27096-79 for 1979 XLH (XL), XLCH, XLCR-1000) should be installed where there is evidence of a rich condition causing loss of smooth combustion, stumbling on acceleration or such other carburetor issues at high altitudes.
  • The kit for Sportsters includes a #160 main jet, accelerator pump stop screw and the front down tube on the frame.
  • Install and adjust the acc pump stop screw to extend 1/8“ past hole in lever.

• Note, any motorcycle modified for high altitude operation must be converted back to standard if operated at altitudes below 4000 feet. An overly lean condition can cause engine damage.

• Inspection:
  • A close inspection around the walls of a welch plug can reveal possible leaks.
• Removal: ⁸⁾
  • Whenever a welsh plug is removed, a new one should be installed in it's place, especially since a large part of the time, removal of one means destroying or deforming it.⁹⁾
  • Drill an 1/8” hole thru it (just deep enough to break thru to the other side) off center and pry it out with a small punch. Be careful not to drill too deep which could destroy the nozzle assembly or casting. While prying it out, be careful not to damage the casting counter-bore edges around the plug.
• Installation: ¹⁰⁾¹¹⁾.
  • The plug should be seated with a flat end punch that is slightly smaller than the diameter of the plug. The plug should be flat and not concaved to assure a tight fit. If leakage is suspected due to a rough or damaged welch plug seat in the casting, apply a small amount of epoxy or suitable sealant to the edges after installing it.
  • Installation Recommendations from Hubbard Spring http://www.hubbardspring.com/install_reco.php?cid=45
• Some Welch Plug dims: http://www.hubbardspring.com/index.php/category/43/cupped-plugs-and-cupped-plugs-pierced-catalog

Here are some excellent videos of servicing the pre-CV Keihin Butterfly carbs:¹²⁾

• #1 - Tips and tricks.
  
• #2 - Disassembly and cleaning.
  
• #3 - Assembly and testing.
  
• #4 - Testing and start-up.