Re: does this make sense? drilling out something in the carb?
Both Elvin and Pete are quite correct.
The brass plug that prevents access to the idle "pilot" screw can be removed, but it should not be needed if the carb goes through a proper bath in real carb dip. The EPA required that plug so that we consumers couldn't easily tinker with the idle mixture settings, which could increase emissions. The screw is set at the factory, using very sophisticated instrumentation, and is 99% perfect 99% of the time. Attempting to set it in the field would require a very experienced ear, and a very accurate tachometer, as well as some testing time. Mal-adjustment in the field can cause a lot more problems that it remedies.
The 9.8A3 runs very lean, in order to keep emissions low. In order to accomplish that feat, and still keep the motor running acceptably, those carbs have long, narrow passages. That helps to keep the air/fuel mixture emulsified better. The trick does work, but those very narrow passages will varnish up easily, especially with today's poor fuels. If you religiously run the carb dry at the end of every day, you will minimize that buildup.
If the "clogged arteries" in the carb are marginal, it would be theoretically possible to enrichen the mixture with that pilot screw to regain some improvement in low speed operation. But that would not clean the carb. If the carb was then subsequently cleaned correctly, your low speed mixture would be too rich, causing premature plug fouling, and increased emissions.
OTOH, if the mechanic removes the plug, precisely counts the turns of the present position of the pilot screw, then removes the screw to gain better access to the brass seat that the screw meters against, completes a thorough cleaning, and resets the screw exactly where it was from the factory, no harm, no foul. In the rare event that a piece of metallic debris has wedged in that metering orfice, removal of the screw might allow it to be cleaned out more thoroughly.
