Upsizing Main Jets to Compensate for Pods

Another thread caused me to wonder about why upsizing main jets is recommended when going to pods.

My understanding is that up until about 3/4 throttle it's the difference in diameter between needles and needle jets that controls fuel flow. Beyond 3/4 throttle the area between needles and needle jets becomes larger than the main jet area, and fuel flow is then controlled by the size of the main jet. If my understanding is correct then what is gained by going to larger main jets if one never approaches 3/4 throttle?

That's pretty much like saying what's the point of brakes on a 150mph machine if you never go above 5 mph..
I'd say go buy a bicycle.

HRM... This backs up your thoughts....

zed1015 wrote: That's pretty much like saying what's the point of brakes on a 150mph machine if you never go above 5 mph..
I'd say go buy a bicycle.

I was hoping you would reply because of your knowledge. Few riders go above 3/4 throttle except for short bursts. I was asking a serious question to help with my understanding of how a carburetor functions, particularly on Z1/KZ1000 bikes. If I read between the lines I think you are agreeing that upsizing the main jets is only useful at above 3/4 throttle?

Now. I believe your theory might be correct as long as the main is large enough to NOT restrict the needle and jet flow...

Zed1015... I wouldn’t even ride a bicycle without brakes

Mikaw wrote: Now. I believe your theory might be correct as long as the main is large enough to NOT restrict the needle and jet flow...

This is from the Kawasaki training manuals for carburetors. It's what caused me to wonder about this.

Overall jetting is a stack up of all fuel circuits and not just the main.
The main jet does not just affect 3/4 opening and beyond but overlaps the needle and needle jet well before half throttle so the larger main also richens up the fueling before 3/4 throttle.
Try running the bike with a massive main jet or no main jet and you will find you have an unrideable machine due to over fueling problems much lower down.

OK...as the slide moves upwards more air flows through the throat, which means the air velocity increases, which means the pressure in the carb throat decreases. This causes a greater pressure differential between the bowl (basically at atmospheric pressure) and the throat, which drives more fuel up the main jet and past the needle. If the main jet is larger this means more fuel passing by the needle at any given throttle setting. Is this correct?

Yes! An overly large main will flood the needle jet tube with fuel causing rich running at any opening.
Blocking the needle jet tube air bleed will have a similar effect.

zed1015 wrote: Yes! An overly large main will flood the needle jet tube with fuel causing rich running at any opening.
Blocking the needle jet tube air bleed will have a similar effect.

OK...I think I understand and thanks by the way.

Going back to adding pods...it allows more air into the engine, which leans out the mixture if carbs are not adjusted. Going larger on main jets compensates at mid to higher rpm's. Adding restrictors reduces air flow to the pilot circuit for lower rpm's.

Back to the main circuit for a moment. Lifting needles enables more fuel. When would you lift needles instead of going with larger mains?

Zed1015.... Make sense when you consider the fuel being PUSHED up the bleed tube. I can see now how a “to large” main causing rich conditions... I always thought fuel was sucked by pressure drop causing a vacuum.... thank you for enlightening me...

Now as HRM ask... better to change main or raise needle?