TOPIC:

This is what the throttle shaft seal looks like. It is in fact a seal as opposed to an "o-ring". I'd call it a "quad-ring" due to it's square shape (4 sides). In this case (my bench-carb) the seal fit snugly on the shaft but it had shrunk away from the bore. The throttle shafts can traverse .022" (measured) when the throttle plates are opened. The dust-cap came off easily with a flat-bladed screw driver placed under the cap edge with a prying up motion. Like a bottle-cap.
Replacing this seal is very easy on the "capped" end but the throttle plates would require dis-assembly and the shaft removed to replace the seal on the "linkage' end.

I'm of the personal opinion that the latter would be the last procedure on any list of carb work. The need to remove the throttle-shaft to replace the seal on the "linkage" end would be best determined if serious problems arose from the carbs not holding a sync., not holding a consistent idle or other unexplained anomaly's. But again: my opinion.

I found this on the topic: "Keihin carb end bushing leak" and there are a few more articles on the subject ! This may be an area of carb maintenance which deserves more attention and respect.

I'm going to look into this further !

I have the cap off of mine but haven't pulled the circlip, washer and seal yet.
Did you find wear on the shaft or the bushing? Quad seal or a U-cup seal would probably live with .005" clearance before being unable to seal on that shaft diameter. also, Partszilla only shows a requirement of 3 seals for the three carburetors. Doesn't show a seal behind the throttle shaft spring end (No breakdown in the pic either)

I just got a reply to my inquiry regarding oring material for gas with ethanol (typical automotive pump gas) and Parker Hanifan is recommending V1163-75 which is Viton for the orings and Ucup seals. (If your interested)

Once I finish measuring and specifying the orings, I'll post the sizes. I also just picked up an oring /U cup catalog for reference.

Auto makers went to Viton seals in carbs decades ago to replace the old neoprene because of some of the new additives being used to replace the lead additives. But, almost nothing will stand up to lacquer thinner. FWIW, I have had excellent luck(?) using Seafoam to soak the injector bodies of sets I have rebuilt. it takes a while to loosen the really stubborn crud, but I'm usually not in any hurry. And, the stuff doesn't affect the seals, except to clean them.

Kawboy, if you talk to those guys again, ask them if/how such materials as "Rubber Renue" affects their Viton seals.

If new seals are available, then I don't see the problem with removing the throttle plates (butterflies) and sliding the shaft out to replace the seal trapped by the lever. If you are worried about the screws backing out after reassembly, then red-Loctite them back in and they will NEVER come back out! The current seals have lasted more than 30 years, so some of us will probably not still be here in another 30, and the bike will surely be completely wore out by then!

I found no noticeable wear between the shafts and bores on either my '80 or '79 carbs.

It could be assumed the reason "Partszilla" only shows 3 seals is due to the fact that only the cir-clipped end can be readily accessed for replacement.

On both the '79 (currently on the bike) and the '80 carbs I removed the caps which cover the cir-clipped end of the throttle shaft (only on #1 and #3 carbs - no room for cap on #2 carb) and placed a drop of oil in the bore, then replaced the seal/washer &clip. Then I used a small amount of gasket seal on the inside mating edge of each cap and installed the caps.
The theory: This will seal the cap to the shaft-boss regardless of the rubber seal. Whereas the "linkage" end seal can not be accessed, I placed a drop of machine oil on the inside and outside of each shaft. The lubrication factor is obvious and the oil may migrate and "rejuvenate" the inaccessible seal.

In my opinion: Unless there is discernible/measurable lateral movement of the throttle shaft, we shouldn't "over-think" the concern as I believe the concern is minimal.

This isn't to say the discussion is dead..........

scotch wrote: In my opinion: Unless there is discernible/measurable lateral movement of the throttle shaft, we shouldn't "over-think" the concern as I believe the concern is minimal.

Here, here! I second that notion!

I thought about that inaccessible seal last night and tried to figure a way to inject some silicon plumber's grease in the space on the outside. I figured that the vacuum would pull it into the seal. But then it dawned on me, just how much vacuum is there at that point? In theory half of the shaft has vacuum applied to it while the other half does not. In time most of the grease would be sucked in and mostly gone. Another possible issue is that plumber's grease is not very thin and I'm just not sure if it would affect throttle-closing response time. IF it did work well, it would surely stop any air leaks at that point. Gasoline also does not wash away silicon grease.

Just my mind wandering through the possibility library!

For those that don't know me I can't leave well enough alone. I've always torn things apart to understand how it was designed/built. I had to pull the throttle shaft to see if there was a seal underneath the throttle return spring and there is. On the carb I just took apart, the throttle shaft on the outboard end had the anodizing worn off on the engine side and the seal was in good shape. The spring end of the shaft was like new but the seal was shot. It had no elasticity and was shrunk as if the entire seal was about 2/3's the original width. It was spinning on the shaft, and loose in the bore. This may have happened from some kind of cleaning solvent somewhere along the life span of this motorcycle. I suspect the outboard seal was saved by the steel cap over the end of the shaft. These seals are definitely U-cup seals.
I've got all the orings and seals out now and I plan on sizing them up and going to Parker Hannifan and getting replacements made of Viton 75 durometer.