Cracked Carb Diaphragm Ring--(How Much) Should I Worry?

1980 XJ650G Maxim with those lovely Hitachi carbs. The picture pretty much says everything:



I noticed the crack in the plastic ring atop the carb piston. The carb seems to work just fine, the bike runs pretty well, idles, winds out without breaking up, and spark plugs do not seem to show any signs of mixture problems. I don't know the function of the ring, beyond to mechanically secure the diaphragm to the piston. I do know that replacement parts don't appear to be readily available and I have no particular desire to purchase a whole new carb rack just over this.

So... something to worry about? Anything I should do to fix it?

 

The ring being compromised might allow air to bleed past possibly reducing the diaphragm lift and leaning the mixture some, there is a small bleed at the bottom of the piston itself so honestly I don't know how much the cracked ring will affect this but I'd consider finding a replacement. I did do a video last year of the needles in action on my XJ750 placing a small camera in the airbox and taking the motor to the 9500 redline several times. You can see the actual movement here and I' going to rework the videos this year using two GoPro cameras, one in the airbox and the other on the tach.

 

Not really.....that crack isn't that wide compared to others we've seen. Brand new Hitachi vac piston units can have split rings (oddly, this situation almost never happens on Mikuni models, only the Hitachi carbs, so Hitachi either used a poor formula of plastic, or didn't have good control of their swaging procedure or design).

There's no real "fix" to the problem, superglue won't bond it back together, and you don't want to disrupt or damage the rubber diaphram while attempting a repair. Remember......that split may have been there from the very beginning, over 40 years ago, and if they bike runs well then leave it alone!

 

Good vid. Be interesting to see the throttle position vs piston height as well. Then another vid running on pods or no filter, this would show the effect on piston height..

 

Not sure I can do anything with pods being a GoPro is far larger and I'm a bit leery running no air filter being I'm rural with lots of gravel roads but the plan is to get a better focused cap of the sliders in action with a better view of the main floor jet and how it feeds this fuel thru both the RPM and throttle range. My purpose of this was to gather what I could to possibly solve if not answer some of the reasons why the pods don't work well on the slider carbs. There was a time before climate change when I had a 1980 Honda CBX sport and the PO had installed pods more for accessibility, the curved layout of the carburetor rack made things far difficult and these were also diaphragm metered but didn't have the same midrange bog as the XJ... or maybe it did and I never noticed being more interested in that howling 6-cylinder sound. It had a 6-into-1 and sounded absolutely wicked above 8000. Regardless I'm determined to figure out why the HSC32 doesn't play well with pods and in addition to the GoPro in the airbox a wideband O2 is also in the plans as time allows.

 

What I do with those on a rebuild is simply pack the little gap with black RTV.

 

It runs well enough, which is to say it would probably benefit from a Colortune but is otherwise producing good power, not breaking up, not fouling plugs or doing other bad things. So I think I will take your advice!