Why do we use the highest idle method?

[JohnGS850L]

Thanks to the guru's on this forum, the bike is running better everyday. Almost everything in the fuel/exhaust system has been rebuilt/replaced/repaired, and I'm ready to set the fuel screws with the highest idle method and one last carb sync.

But why? Factory settings are for all fuel screws to be set out the same amount of turns (I think like 2.5 or something? Lean from the US factory cause of the EPA), but the highest idle method is done on each individual cylinder. Why do we tune each cylinder instead of turning them all 3 turns or so out? What would one screw finding it's highest idle at 2.5 turns and another cylinder finding it's highest idle at 3.5 turns indicate? How much variance is acceptable amongst the screws?

Thanks for your collective wisdom!

[chuck hahn]

because many factors such as bore wear, ring wear, valve and valve seat conditions vary across 4 holes...each is its own animal and likes what it likes...not what ALL like.

[Steve]

The "highest idle" method is used on many smaller engines, especially smaller two-strokes. The principle is that the engine runs best (fastest) when it has an ideal mixture. When you adjust the mixture screw (on the CV-type carbs) or the air screw (on the VM carbs), you will eventually find the mixture that makes the engine the happiest, regardless of how many turns out it is.

When the carbs were new, they were each connected to a flow bench that measured the actual mixture, adjusted and sealed. No doubt they were all within a narrow range, but I'll bet that they weren't just turned XX number of turns out and left there. When the carbs were new, none of the parts were worn, like they are now, so there probably wasn't much variance.

For what it's worth, when I tune a set of carbs, I go through them twice. I start with the screws three full turns out, then after I sync the carbs to make sure they are all breathing the same amount of air, I turn each mixture screw in, listening for highest idle and watching the vacuum gauges (still connected from the sync job) for highest vacuum, then move to the next carb. After I have gone through all of them, I go back, just to double-check the settings. They usually end up about 2 1/4 to 2 1/2 turns out, but there is no way I would just preset them and walk away.

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[JohnGS850L]

Thanks for the wisdom! Chuck, Steve: you two have consistently provided the knowledge allowing me to tackle repairs and maintenance that I've neglected on previous bikes or left to paid mechanics. My bike is running damn well thanks to you and the other Longtimer's and GS Whisperers of the forum. My continuing gratitude.

-John

[limeex2]

I believe that is called ''Idle drop method''. When used with a tach(inductive?) hooked to one spark plug lead works extremely well. It was used by Honda to tune problem bikes before fuel injection came out.