Gurus,

I'm hoping your expertise can save me some time and keep me from pulling out my last two hairs.  In short, I acquired an AT1 with CT1 crank that I've installed CT1 top end on.  When I acquired it, it had a "new" mikuni carb on the AT top end but was running way too rich (black, fouling plugs, smell gas out pipe).   Inside of tank was roached, I've cleaned and re-coated it, then pulled and cleaned (sonic cleaner) the carb - quite a bit of tank metal made it past the filter and into the float bowl.

I'm trying to make the bike somewhat "stock" (although it's a 125 turned into a 175) and installed a '69 factory air box and filter (replacing the 'uni" type filter).  First, I adjusted the float to keep it from "pissing" when on side stand.  The bike had a 160 main jet and I've gone down to 110 or below to eliminate the "high/low" floating 3/4 to wide open throttle (still won't get beyond 6K RPM) - it seems to be happy with a 100 main jet.  The low end isn't crisp on the throttle and is bogging so I'm switching from a 20 to a 30 pilot jet.  Currently needle position is in the "middle", i've moved it from lean to rich just trying to find the sweet spot - right now it's in the middle - I think the needle jet is N8, not sure about needle.    My idle is stuck at about 2,500 rpm and doesn't respond to idle screw adjustments.   Air screw seems happy at about 2 - 2/12 turns.  All that said, I pulled the carb to clean it again just in case some passages where blocked, and noticed it was a 22 carb, was thinking it's a 24.

QUESTION:  Will Mikuni 22 perform well on CT1, or is it too small?  If so, what jetting might be recommended with stock air box/cleaner (I like this carb as I can pull the main jet straight out of side of the bowl:-) )?  Or, am I wasting my time and need to put a 24 or 26 on it?

Thanks!

How do you know it's a 22?

It's stamped on the cylinder / fuel side intake manifold:  22F 133 and is identical to the one at this link:  www.summitracing.com/parts/mku-vm22-133?...vY67HdhoCvDwQAvD_BwE

I'll cut to the chase & go read it all latter sorry. But sounds like the small Oring that seals the bowl onto the needle jet is leaking bad, if its there at all.
Ok either 'that' or your exhaust is blocked in the spark arrestor.

Thanks.  The O ring is there and looks to be pliable, and a clean seat to mate with in the bowl.  When I put it back together (going up to a 30 pilot) I'll pull spark arrester and see if it improves.

Not sure on yours but most spark arrestors on 250 & 360 are deep down in the tail & fixed in place with spot welds. Just have a good look when everything is removed--that will remove. Yours might be part of the tail section that comes out.

TY175s have 22mm carbs fitted. Putting a 26mm on, and a less restrictive airbox, makes them run far better.

A VM22 is not too small to the point that it can't be jetted to run properly.

From what I see that carb should have come with a N-8 needle jet and a 4D20 needle. Stock carb had a N-8 and and 4D3 needle.

The obvious big difference here are the needles... It took me a while to find the data since Mikuni slide needles have "some" consistency such as taper letter always means the same angle... but that's about it. And sometimes there is conflicting data... pretty sure what I found is accurate.

Basically, the 4D3 in the stock carb is 50.3mm long with the "D" (1 degree) taper starting 25.3mm from the top.

The 4D20 is 49.8mm long with the taper starting only 21.2 mm from the top.

Both needles have the same 2.515mm starting diameter.

Bottom line is 4D20 is a "richer" needle than the 4D3... A 4D3 with the clip in the bottom groove is roughly the same as a 4D20 with the clip in the top groove. Stock carb runs clip in center groove.

Lots of unknowns here... I have an excel sheet that calculates the open needle jet area by mm of slide travel. I did this years ago for the stock 4D3 needle... I have not done it for the 4D20 so I can't compare on a graph.

One thing I need to caution you on is to be very careful with main jets! You can make them smaller and it will lean out the 1/4 to 3/4 range but then it will be way too lean at 3/4 to full throttle.

(It sounds like someone already tried to lean the midrange with a small pilot... that carb should have come with a 30 pilot.)

You need to tune the carb based on throttle position. Mikuni Tuning manuals can be found in the Tech Library or on the Internet.

If you really want to replace the carb, an original one from a 69-71 AT1 or CT1 will work great. They are jetted the same. Or you could try putting a 4D3 slide needle in the new carb you have.

And before you start jetting please make sure it's not a plugged exhaust outlet (very common) or other issue like the o-ring. So many people (I'm one of them!) have chased their tail trying to "tune" to correct an issue when the problem all along is a restricted spark arrestor for example.

If you want a bit more power, and want to avoid spending hours messing around trying to get a 50 year old carb, to work reasonably well, get a 24 or 26mm Nibbi copy of a Keihin PE. JJH, 120, 42 set up should be pretty close.

Thanks y'all for the feedback.  I love the wealth of knowledge and experience you're willing to share.  I started wrenching on bikes with my first mini-enduro in 1971 and have always had "projects" and strays I bring home to keep running, with that, like so many subjects, the more I learn the more I realize how much more there is to learn.

In short, I'll dick with jetting in the 22 some more since it sounds like it can be made to work but not optimal, verify o ring seating, and check for blockage in spark arrester, and get it running the best I can for now.

My final project on this bike after tweaking carb is to re-install the autolube pump.  I'm sure I'll be back for more questions on that.  

Thanks again!
Marshall in NC, USA