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Author Topic: The AT Mystery Screw  (Read 314 times)

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Offline Spike60

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The AT Mystery Screw
« on: November 22, 2021, 08:18:36 AM »
Thought it a good idea to contain the contamination introduced over in the 545 thread regarding the hidden screws, or threaded plugs, in the auto-tune carbs. And figured it deserved it's own thread.

Those things are not adjustment screws. They wouldn't put a welch plug over something that was meant to be adjusted. There are no secret procedures that dealers use on those screws that we aren't allowed to talk about; we never go near them. All of these carbs are put on a flow tester during the manufacturing process and that screw establishes optimum air flow for each carb and it is then sealed. That's all it is. The AT module is adjusting the fuel, not those screws/plugs. If you look at where it is, vs the fuel channels underneith the AT module, it starts to make a little more sense.

Turn that screw either way and the carb is never quite right after that. The saw will usually run, and in the case of the first gen 550's it seemed to improve acceleration in some early units that used the AT7 carb, and maybe even the AT7A. It kills the warranty on an expensive carb, and causes more problems than it could solve, so they should be left alone. They're not just on the 550's BTW. An even smaller welch plug hides them on 555/562's also. Now I know a lotta guys who mod saws and think they are smarter than the folks that designed them, will tinker with those screws, and that's cool within that context. That's what hot rodding is. But it's not a place to go to solve any ills in the fuel or ignition system. It will only compound whatever problem may exist.

Problem with so much of the info out on the web is that people who are otherwise smart, like the guy who started the other thread, struggle with the notion that they may not know everything. They are unable to fully understand something, and because they are clever, they feel they can fill in the blanks with their imagination in the absence of facts. Statements that begin with, "they must have been thinking", "the only reason they would have done this", "it has to be", "I gather", should all be taken with a grain of salt. Far better IMO to do some research and look things up than to theorize about something which you are unfamiliar with.  
Husqvarna-Jonsered
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