CR Special Carbs / '83 650 / occasional single carb fuel leak ?

hello all ....so, I have had the CR Special smoothbores rack-of-4 on my 650 since the mid 80s (see pic), and thru regular maintenance, cleaning & adjustments, they have served my Kaw very well, never any major issues or problems, except this one: on occasion, on long backroad rides with lots of high Rs and cruising 65 to 70 MPH, one of the 4 carbs will begin leaking fuel "at idle only", never more than one carb at a time and never the entire rack of 4 carbs (it did it on my ride yesterday, first #2 carb, then stopped, then # 3 carb, then stopped) ....I usually take back off opening the throttle way up, shifting thru the gears at high rpms and "something" clears or resets and the leak stops when I come back down to idle at stop after a little cruising ....2 years or so ago I completely disassembled not only the entire rack, but each carb individually, did an ultrasound cleaning, cleaned and/or replaced all stages of jets, checked for float level & stop, etc, etc ....bike starts, runs, idles and pulls great ....has for years (64,585 original miles) but highly modified .....I mean I know it could be a speck of dirt or whatever I did not get helping the float for the leaking carb to stick ....but man oh man the internals on these carbs are clean ....to note: interior fuel tank filter, the tall one just above petcock inside, there and clean but I am not running a filter on the fuel feed line from petcock to carbs ....lastly, I run high octane in the bike, could it be something in the brand of gas I'm using ? primarily Shell #1 and 76 #2.....all insight & assist appreciated, thanks 

Beyond a piece of dirt sticking in the float valve, is at all possible the float itself could be shifting over and rubbing on the bowl causing it to hang. I’ve seen this on the stock VM26SS carbs. 

If there is a fuel level spec, I'd check that.  Float height is not a definitive test for how much fuel there is in the bowl.

I could drop the 4 bowl drain plugs and blast up in there with carb cleaner ....or just drop all 4 bowls and blast some carb cleaner up into the lower bowl chambers & floats....I wonder if anything can be getting past my in-tank filter that could cause this ? ....it would have to be really tiny, as that in-tank filter is super fine mesh ....

If there is any "pitting" the valves can stick even without debris in them, also the floats can catch on the sides of the bowl if they are slightly bent

I have had CR's on my bikes since back in '82. Sounds like your problem is the little o-ring that goes around the removable seat for the needle and seat. The standard factory Keihin o-ring around the seats were only good for five or six months before they shrink enough to let extra fuel by to cause the carb or carbs to start leaking fuel. What normally happens first is one or two cylinders would start to run rich. Then after that the carbs would start to leak fuel. I never ran anything but Unocal 76 race gas or VP110 fuel in my bikes with CR's. So that might speed up the o-rings going bad. I try to drain the fuel out of my carbs if the bike is going to sit more than a week before I would ride it again. I always keep a box full of Keihin o-rings on hand because I know I will need them sometime in the near future!!!

I would start out with new o-rings around each needle and seat and set the float to 14mm in the unloaded position. In all my years of building bikes I have NEVER used inline fuel filters when I used a factory Kawasaki petcock or a Pingle petcock with the filter in good shape on the petcock. I kinda chuckle when I see those funky looking inline filters on a bike!!!!!!

Collection of Keihin parts I keep on hand.
 

A set of CR33's on my Kz1000.
 

excellent intel, thank you Dr G ...see attached, assuming the o-ring I'd need is #35 ? ...should probably replace the needle valves (#31) and seats (#34) while I am at it ...those have been in the carbs since the mid-80s ....highly appreciate your informative reply ....I also see the seats come in 9 diffident sizes but the o-rings & needle valves are universal ....guess I'd have to install the size seats I pull out of my CRs .....

Are the needles Viton tipped?  If so, the seat won't wear enough to matter, just the needle.

And I'm surprised to read about an O-ring shrinking.  I wonder what's going on there?  Typically, O-rings swell in gasoline.  This is what creates a nice tight seal.  Buna-N/Nitrile O-rings are the most common.  You can use Viton if necessary too.  That's more gasoline resistant.

yep, understood, but if these o-rings have been in place since the mid 80s when I installed the CR Specials, they could use a swap out for sure ....not sure on the needles and seats but they are cheap enough that my 650 would enjoy the refresh thru all 4 carbs ....

#35 is the o-ring you want. Keihin part number is 16075-MBO-0030. Sudco number is 018-694. Sudco is the Master Distributor for Keihin parts in the U.S. They always had my parts in stock over the years. Here is how you are suppose to check float height right out of the factory Keihin CR manual. Make sure you spray the new o-ring and where it fits into the carb body with like WD-40 so you don't hurt the o-ring as you push in the seat.
 

 

thank you Dr G, great intel ....if the seats & needle valves have never been replaced since new / mid-80s, do you think I should swap them out too ? ....maybe just the needle valves, as it's those that make the contact to seal off the bowl from continuous running fuel - correct ? their tapered sides could be worn .....

A google search shows that Jets R Us sells spare parts.  The needles look like common Keihin parts with a Viton tip.  With needles like these the seat won't wear much if at all.  That said, the parts are all pretty cheap so...

www.jetsrus.com/individual_parts/018-676_su.html