82 758 yamaha seca

does anyone know what size main jets i should run at 6,000ft above sea
level?

 

There is a Mikuni tuning slide rule available in the Sudco
International catalog online. It helps the tuner set up the carbs for
temperature and altitude. I dunno what happened to my "Pocket Tuner".

If it was just a matter of the air pressure and density differences,
you would want to install a main jet with an area of 80% of the
standard size main jets,
which are #120.

So you would want #107.5 jets. They would be the right size for a
standard
day, when the temperature is 37.60 degrees F at 6000 feet.

But a hot summer day thins the air, and even #107.5's would be a bit
large.

The numbering system for round jets is based upon #100 having a
orifice hole which is exactly 1.00mm in diameter.

A #120 jet has an orifice of 1.20mm, so you can work out the
comparative areas of the orifices using the pi X radius squared
formula you learned in high school.

But, don't forget that the main jet controls the fuel air mixture when
the throttle butterflies are wide open.

You will also need to find your idle mixture screws and lean up the
idle mixture a bit. Your carbs have #40 idle jets, which are
relatively large idle jets, so you may need to turn the idle mixture
screws in until they are only 1/4 of a turn open.