TM27-D1 dumb questions: what is this jet?

I've got a pretty good understanding of how stock CV carbs work, and I can usually troubleshoot and set up stock carbs on stock bikes easily enough. I've never worked on any flatslide type carbs, so I'm hoping some of you smart people will help out. This is mostly for me to learn on a project, so going back to stock is my last-resort.

There's some discussion here: www.kzrider.com/forum/forum-index/10-new...om-srsupertrap-in-co

I'm at the point where giving the bike any gas at all will rev it up high, and it won't come down.

I leaned out the air screws 1.5 turns and it'll idle more poorly, and die with throttle. However, if I'm real slow with the throttle, it'll rev up high, and then slowly come back down. Any more than about 1.75 turns, and the bike won't start or idle anymore.

Even with the leaned out air screws, the spark plugs look a little rich after a few minutes, so I'm interpreting all this as needing a leaner pilot jet. Then I can actually fine-tune the air screw.

However, there's a removable jet on the intake side, and I can't figure out where it comes into play. Edit: Matches the "Air jet" on jetsrus.com. Can someone explain which circuits it affects and how I can select a correct one? Current size says 0.5.

That is a air correction jet or Mikuni calls it a Main Air jet, if you spray some carb cleaner thru it you will see it come out at the needle jet.

It comes into play at mid throttle levels. I really don't think you need to be concerned about those, sizing would be good.

If the motor won't idle down it sounds like you have a vacuum issue, check for vacuum leaks or the valve clearance.

It's an air corrector jet for use when removing the airbox.
Read the attached thread for explanation etc.
www.kzrider.com/forum/23-for-sale/611992...lide-mikunis?start=0

<quote>If the motor won't idle down it sounds like you have a vacuum issue, check for vacuum leaks or the valve clearance.</quote>
First thing I checked. Can't find any leaks, and carb boots are brand new. I found a couple mikuni tuning guides that said this behavior would also happen on over-rich pilots.
The whole motor was just reassembled, and valves were checked as part of that.

@zed1015: Thanks, reading now.

Forgive me for sounding too simplistic, and you haven't mentioned it, but are you absolutely sure the throttle mechanism is returning to the idle stop and isn't hanging up somehow? It wouldn't be the first time something like this happened to someone.
Rick H.

Rick H. wrote: but are you absolutely sure the throttle mechanism is returning to the idle stop and isn't hanging up somehow?
Rick H.

Yup, absolutely. These are flatslides with a push/pull throttle, and I physically see all four slides completely returning to closed position.

I agree, a hanging high idle is indicating you have a vacuum leak.

A couple of things to check & verify. You have new Z-1 Carb boots installed. The boots do not have o-rings so did you apply Black RTV between the carb boots and cylinder head and let it sit for full cure? Second, did you mill out the carb boot ID for a slight interference fit with the Carbs? I ask because the first time I forced the Flatslides into the stock Z-1 carb boots mine leaked and experienced the same exact high idle issues. I verified my valve lash and nothing had changed since engine reassembly.

Your pilot jet should be a 30 or 32.5 if you're at sea level. I am running a Wiseco 615, Yosh, K&N. My air screws are turned out about 3/4 turn out. The vacuum port between Carb 1 & 2 goes to the fuel petcock vacuum port. These Flatslides were only made for GPz/KZ550 so the air corrector jet has not likely been changed and is N/A unless its clogged.

These Flatslides are simpler than RS Flatslides so keep at it.