Carb trouble -- what am I missing?

[redzgrider]

I’ve been through my carbs a dozen times now, and still don’t have the bike right. Bodies are clean, all new O-rings and seals, floats set at 18mm dry/0 wet. Changed the air box boots to improve the carb out/in process. Now, the bike starts just like the old days -- maybe two revolutions on choke when cold, less than one when warm. Carbs kind of synch, but with only 14" of Mercury. Big problem is that while the motor revs well, it doesn't come back down for a couple of seconds -- which equates to no compression braking out on the road.
The low manifold vacuum and hanging revs (throttle blades snap closed) seem to indicate a vacuum leak – but where? The manifolds look good, with no signs of cracks or splits. The air box boots are brand new, with no signs of tears, cracks, or splits. There was a crack in the bottom of the air box, but it wasn’t nearly as big as SiSF has indicated, and I believe I sealed it well. What have I missed?

[Daytona_Mike]

This is a rough setting: 'floats set at 18mm dry/0 wet' 
When you do the clear tubing test is the level lined up with the bowl  split ?  ( the line where  the bowl sets against the carb)
That is how you check the fuel level in the carbs  when on the center stand.

Did you install over flow tubes?
Some folks will spray carb cleaner around the boots to see if the engine revs up indicating an air leak. Propane will do the same thing.
I cant think of anything else right now.

[redzgrider]

0 wet was intended to indicate that fuel went to the part line with clear tubing from the drain ports. Overflow tubes in place, at 5/16" (8MM) above the part line.
Tried finding a leak with start fluid, but didn't find a specific leak.

[MetrickMetal]

Are you sure the problem of the engine speed not dropping for a few seconds after the throttle is closed is not being caused by the throttle not closely properly due to possibly a kinked throttle cable(s), or some binding some place else within the throttle linkage.

The last time you did a carb sync, how much vacuum were the carbs pulling, and did you use the same gauges that you used before.

[Summit670]

My first thought was throttle cables too.

If checking fuel level with carbs on bike on centerstand, the carbs are not level.  I'm not sure if fuel hose placed in the middle of the seam (middle when looking from side view) will give same results as fuel hose against seam on a perfectly level carb.?  Hose on mounted carbs surely is not reflecting correct fuel level if held at either front or back of carb.

Also, my service manual for my 87 says fuel level .5mm above bottom of carb, +- 1mm.  So anywhere from .5mm below seam to 1.5mm above seam is the spec.

[George R. Young]

FYI, my Concours pulls about 10" Hg at 1100 RPM on my super accurate $10 vacuum gauge.

These guys
http://faq.ninja250.org/wiki/How_do_I_adjust_the_idle_mixture%3F
think that slow return to idle indicates that the idle mixture should be richened a touch.

[MetrickMetal]

I have always made the majority of my specialty tools for working on my bikes, and on the topic of adjusting air mixture screws, here is one of several custom made air mixture screw adjustment screw drivers that I have made over the years, and it will work nicely for adjusting the air mixture screws on the Connie.

It's a press on plastic thumb screw cap for a 5/16" SHCS, which I pressed in the cut off head off of a 5/16" SHCS into, then I epoxied in a 1/4" slotted hex bit into it. I have used these press on thumb screw caps in all sizes from #10 up to 3/8" to make all sorts of special low clearance tools, like epoxing in short sections of allen wrenches for turning hrad to get to SHCS.   

[jim snyder]

Hey Metric, the only thing you left out was when these would be available for sale to the general COG-dom.

[qman]

If the basic sync is out (ie if you disassembled the rack or messed with the screws at all) then it would be very easy for you to hear the throttle snap shut....for one or two but perhaps not all of the carbs. Then you could have a couple staying open. You say they "kind of sync" , they either sync or they don't. Watch SISFs video on carb sync to get a good run down. I think this may be where your problem lies. Make sure that your mixture screws are out 2 turns, that seems to be the magic #.
The other thing that can mess with your vacuum is the valve adjustment. If you haven't checked or adjusted them in a while then it's time. I think the interval is around 12,000 km (7500-8000 mi). I going on memory so check to be sure.
HTH
Rob
Edit
BTW Daytona_mike
 I LOVE your avatar, gives me shivers everytime I watch that guy drift!

[rwulf]

Hay "MetrickMetal" now all you have to do is come up with a
small hand that is burn proof and doesn't eat much.

[redzgrider]

Less than 1K since last valve adjustment. Isn't cables -- the blades visibly snap shut (I've looked at them far too many times now), and while doing a synch I'm twisting the grip forward. Push/pull cables insure the blades are closed when the grip is forward.
The problem is kicking my tail. I have two sets of throttle bodies, one the original from California and the other set from somewhere that had the restricted air domes.
About five or so attempts ago, I had a set assembled with the alternate throttle bodies, my original diaphragms and air domes, and clean jets and passages (I thought). That attempt ran well -- including throttle return -- but wouldn't start worth a dang. I switched back to the original throttle bodies on the next attempt, which restored easy starting, but introduced the hanging revs.
Today, I returned to the alternate throttle bodies, but with every port and passage absolutely clean -- I may not completely understand the reason the pilot jets feed so many different aspects, but I sure can flow carb cleaner out selective ports!
All back together, and it starts easy (yeah!) but runs exactly the same as yesterday!
I'm thinking the carbs themselves must be pretty correct, since I have such consistent results now. The only other difference is the new air box boots, but I have trouble believing they are the problem.