Anyone have a late style carb sitting around to check something? (Mid-Main jets)

[herennow]

Hi Folks,

Does anyone have a late style carb sitting around? My curiosity is piqued by the "mid-main" jet and, like others here, I have had no success finding a manual for our specific carbs or even ANY Mikuni info regarding the 3 circuit carbs with mid-mains. Actually, it seems that the gs500 is almost the only bike had these carbs with mid-mains as I can't find any other info on them...

What I am interested in is where the mid-main jet delivers fuel into the throat of the carb. If anyone has a carb could you blow some air up the mid-main to see where it comes out in the carb throat? I suspect that it comes out of its own hole close to the butterfly, logic tells me before the butterfly but the little I have read makes me think it might be after the butterfly. It might also deliver through the needle jet but I doubt that.  A photo of that orifice would be fantastic!

I should have of course done this when I rejetted but my hindsight is so much better than my foresight.

Cheers in advance.

HnN

[user11235813]

I do have one lying around as it happens! You mean the 60 jet? I took it out and there's what looks like another little jet hole in there!

I blew some air in but I couldn't tell where it's going. There is a small hold near the butterfly but I don't know if that's where it exits. Let me know if you want some other shots.

[TGTwin]

I had my carb apart for cleaning today and was thinking the exact same thing do I did some investigating. I can confirm that the choke pulls fuel from the mid jet orifice to some degree, but the only other possible connection for the mid just fuel path that I could see was to the tiny hole directly under the butterfly, and slightly "upstream" of where the pilot fuel path comes out.
What's stranger is that near the top of the mid "tube" inside the carb bowl there is a hole that would seemingly pull in fuel vapour from above the float fuel level. I sprayed cleaner through all these so it is definitely connected but I couldn't figure out exactly why.
I'd have taken photos if I known someone else was thinking about this.

[The Buddha]

Yea that mid main is an extra "pilot". An unmetered orifice that is supposed to add fuel at a constant flow rate regardless of throttle opening. These tend to make it richer @ idle and leaner as throttle openings get larger. AKA reverse taper.
The mains and needle OTOH add in more at larger throttle openings than at smaller. AKA proper taper.
I've found the 01+ carbs rather unimpressive. The tiny 20 pilot clogs easily cos its a tiny hole in a jet that sits deeper in the bowl sucking up more crap from the gas, the 60 mids nearly useless for tuning and no one seems to have 62.5 around me in both cities I have tried looking, and of course the needle wont take any washer we can buy, but there is one on top of the e-clip that will go under it ... yyyyeaayyyyy.
Whatever problem it was designed to solve, It hasn't, and its made our solving of the problem that much more frustrating.

However To be perfectly honest, on the 89-00 carbs I made over a 100 runs with throttle position marking and an O2 sensor screwed into my exhaust and made pages and pages of notes in the 98-2000 time frame to come up with the +1 formula (which was available free in the manual). Although to be honest, most of my runs where with K&N and pipe. That was an eye opener cos it was lean till the high 140's, landing on 150 to be correct.

Cool.
Buddha.

[herennow]

Hi folks,

I never replied and thanked the folks for looking into this and replying before. Got sidetracked and wanted some more info before replying. This took some time ;-(.

So, while working on my 6k rpm flat spot I've gotten a better understanding of the so called  "mid main" jet. I've been playing with my A/F sensor and was going crazy as changing the "mid-main" jet size had no measurable effect on the A/F ratio in normal riding scenarios. I went up to a 67.5 size just to be ridiculous and force something but Nada.

However, one clue I had:  going from 60 to 62.5 to 65 etc was that I could use less and less choke each time on cold startup. Too much choke and engine would die with a very low AF ratio (too Rich).

This led me hanging out on some other sites where our carbs are used (ATV and snowmobile) and interestingly those folks call this jet a "starter jet" or "enrichener jet". And according to them it is only used for the choke/enrichener circuit, not for any other running.

This is supported by my AF sensor testing. TGTwin, you also were on the money it seems with  your test showing a link between choke and "mid-main" jet.

Just my 2c in this ongoing saga. But, if true , this means that like the older carbs it's primarily main jet/needle position that are the tuning parameters we need to play with...