Hard starting Mach 1

[Rick]

Jim's explanation works for me. The top of the float bowl needs to be vented to the atmosphere or drawing fluid out would create a negative pressure in the float bowl- atmospheric/hydrostatic pressure is 14.7PSI pushing down on the pool of fuel, but the taller the column of fuel in the jet passage, the more negative pressure in the carb throat would be needed- the 'inches of mercury' thing. So, if the tickler raises the level of fuel in the jet passage a given airflow should draw more fuel. I hope this still makes sense to me when I sober up.
So, I retract my attack on the ticklers of the world.
Rick

[Bevel bob]

For a long time my 250 would not rev out over 6,went through everything setting up to the specs,then i tried raising the float level till the carb flooded and then lowering the level to just below the point to just stop flooding, the bike now will rev to 10k goes like stink and will normally start with 2 kicks and a tickle, setting up by the book is a good place to start but old bikes and old parts can need a different approach .A surprising number of beautifull restorations dont get used because they just wont run well.Understanding the theory is good ,but your instincts are good too,do what works.

[wcorey]

A word or two about air adj screws...

The effect on the mixture that takes place through the volume of air adjusted by the air screw is by virtue of how much more fuel is drawn in by the venturi velocity that the air controlled by said screw allows. The volume of actual air it puts into the carb throat is so small as to have minimal effect on the overall mixture once off idle.

The idle circuit on the majority of carbs is like a little carb-within-a-carb, there is an air passage and a fuel passage. The fuel isn't drawn in just by the venturi in the main carb throat but by the air in the idle circuit air passage, opening the air screw is analogous to opening the throttle (to lift a slide or rotate a butterfly). The idle jet in the idle fuel passage has a similar function there as the main jet but operates at a much higher proportion of fuel to air.

Anyway, the point is that opening the air screw enriches the mixture, which is why even though it is literally an air screw, it should really be called an idle screw to avoid confusion.


Bill

[Bevel bob]

On most carbs the air screw operates in the opposite way including some Dellorto's ,another reason the SSi is missunderstood.