Measuring Fulcrum Arm Height

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Fulcrum arm height is something that is very critical, especially on the Walbro carb that is used in all the Yamaha/piston valve classes. There are many ways to check this, however I think you'll find that the following method will consistently give you the most accurate results.

As you can see from the illustration, I prefer to use a depth micrometer to measure fulcrum arm height. An inexpensive "depth mike" can typically be purchased for under $40, and is well worth the investment. The procedure is actually very simple, and with just a small bit of practice, you'll be consistently measuring the arm height to within .001. This can be fairly important since .005 can be a significant difference under certain conditions.

The Walbro carb seems to "like" an arm height in the range between .040" and .060" below the top of the gasket. This setting can be VERY critical under certain conditions (especially when running a bit on the low side). Generally speaking, the mixture will run richer throughout the range as you raise the arm... however the fuel "curve" will change as well (in other words: the fuel "distribution" through the rpm range will change slightly). As always, there is no substitute for experimentation and testing. Popoff pressures in the 7 psi to 10 psi range are most common.

Before checking the arm height, check the top of the inlet needle, the forked parts of the fulcrum arm (both ends), and the small button (on the diaphragm) that hooks onto the fulcrum arm ..... for wear. If any of these parts are noticeably worn, replace them first.

Remove the sheet-metal cover plate over the diaphragm, and remove the diaphragm. Leave the gasket in position on the body of the carb though -- it's extremely important that you make your arm height measurement with the diaphragm gasket in place. The pump side of the carb should remain completely assembled when checking the arm height as it's the only simple method to build pressure under the inlet needle.

Make sure the rubber tip of the inlet needle is "wet" (with gasoline or WD40, etc), and pressurize the carb by hooking your pressure tester to the fuel inlet (5psi or so is adequate). If your carb will not hold pressure, you may want to install a new inlet needle before proceeding (and/or check the condition of the "seat").

Hold the depth micrometer in position (on top of the gasket), and with the stem positioned directly above the end of the fulcrum arm. Slowly screw the depth micrometer down until you see the needle on your pressure tester drop. Read the setting on the depth micrometer.

Please note: when setting pop-off pressure, I recommend NOT cutting the spring under the fulcrum arm to lower the pop-off pressure. Cutting coils radically increases the spring rate of the fulcrum arm spring which is undesireable. Usually a pretty good squeeze on the spring (between your finger tips) should allow you to get the pop-off pressure as low as you might need.

Remember that when reassembling the carb, the small button in the center of the diaphragm must be "hooked into" the fulcrum arm. Use extreme care when assembling and disassembling the carb, as it is very easy to bend the fulcrum arm when removing the diaphragm.