Air Jet/ Pilot relationship

[dardoonk]

On a BS34 carb what is the relationship between these? I know the mixture becomes more rich by increasing the pilot jet or turning the air screw out, or reducing the size of the air jet. I don't know the effects of changing the air jet though.

Let's say (EXAMPLE ONLY) you have a 40 pilot jet, but your mixture screws take 4+ turns to get your idle mixture right. The obvious option is put in 42 pilot, turn the screws back in and recheck, but another option would be to reduce the size of the air jet for say a 170 to a 160.

Is reducing the air jet on size equal:
A] going up one size on you pilot?
B] some number of turns in on the mixture screws
C] a "half-step" up in the pilot jet?
D] some other value?

 

[hampshirehog]

It's D. It's not a straight line correlation as far as I can tell - no idea why.

 

[trippivot]

On the carburetor info I use it states that it is best to have the biggest air jet in there to increase the total volume / total vacuum that the carb is capable of moving. Adjust with the fuel jets. More volume of air makes more vacuum which means more pressure differences to pull fluid and raise the slide.

All the fuel systems - inside these BS style mikuni CV carbs stack up and flow all at once as the carb opens up in a progressive manor. Decreasing the pilot air bleeder will make the pilot circuit richer by using less air. using less air will make less vacuum to pull the fuel up into the carb body. (float level?). And by flowing less total emulsified A/F/mix you will change the total amount the bypass circuit is able to flow which is a second circuit controled by the pilot jet as it reaches 100% flow -- which is before the needle/needle jet emuls. tube takes over.

I have to say [ D ] some other value

The air flowing thru the carb is what makes the magic of a CV carb happen. adjust these carbs with fluid/fuel jets - you'll get better results.

 

[hampshirehog]

One other thing I have just remembered. For a few of the bikes for a few of the years different markets had different air jets but the same pilot jets. And by quite a bit if I can remember correctly - eg GSX1100 1980 or 81 - air jets were 120,140 and 160 in the US, UK and Europe (non UK) models. It does affect running so you have to know what market your bike was built for. The EZ and ED/ESD models had larger air jets (170 / 180 I think) and these were the same the world over....unless you know differently.

The 1981 workshop manual is right but the late 80s version lists the specs incorrectly.

And the parts list shows the right part number as 'jet oil gallery bal shaft'.