Main Jet vs Jet Needle vs Throttle Valve vs Pilot Jet - Which Tuning Part Does What?

[pete]

I came across this diagram on the Mikuni Australia website (www.mikunioz.com):



Reading through these carb threads reveal that a lot of people seem to get confused about what to adjust based on throttle openings (and I'm no different), so I thought I'd pop it up here for all to see in case it helps.

http://www.mikunioz.com/jets.htm

Hope it helps someone...

[Big T]

This should be a sticky

We see it every day - my bike runs poorly off idle, my main jets are ***

[pete]

Yep, it seems every second carb thread starts with the same question... but in saying that it is easy to get confused if you don't have something like this to refer to...

[Dave8338]

Cliff will snap this up...

[pete]

I'm surprised to not see the "this has been posted x number of times already" response, but maybe that's still coming... figured somebody had to have put this up before?

[DanTheMan]

Actually, this site http://www.iwt.com.au/mikunicarb.htm has a shortcut to it somewhere in the GSResources, I just don't remember where. I saved it to my favorites. It has a good explanation (came from the upside down side of the world as well)

[BassCliff]

Hi,

That same diagram is in the Mikuni VM/Smooth Bore Manual on my website, page 9. I suppose I could post that diagram separately on one of the carb pages. That would make it easier to find.

EDIT: OK, it's up on this page:

Carb Notes:
Synchronizing/Troubleshooting/Theory

Thank you for your indulgence,

BassCliff

[pete]

Cheers BassCliff! I think it's something overlooked by many and maybe being buried in a manual is making it a little obscure perhaps...

[KEITH KRAUSE]

That chart is approximate. Look around and you'll find others that differ slightly. But it certainly is close enough to help you understand.
I've found that the jet needle has effect a bit earlier (lesser throttle openings) than shown, especially the VM carbs. Biggest issue many of us has is dealing with the overlap effect from one jetting circuit to another. Makes it more difficult at times.
There are still many out there that think you can just increase the main jet to allow for certain mods and you're good. This will help them understand.