78 BS38 Carbs - General Rebuild Questions

[Oddjob]

So I am knee deep in rebuilding of my 78 BS38 carbs.. So far with the help of the carb guide and numerous posts on the subject in this forum things have gone reasonably well...

The only bump on the road has been the condition of the right side throttle shaft... it would appear to have a groove cut into where the lip of the seal made contact with the shaft... it has cut through the chrome and appears to be wearing into the brass... the amount is small but you can easily feel it with your fingernail... I've also shown the left hand throttle shaft, with its minimal wear, below as a comparison.

So my question is, would the use of a quad seal or o-ring seal be recommended over the use of an OEM seal on this shaft? My thoughts being that the quad or o-ring seal would most likely seal on slightly different spot on the shaft hence avoiding the groove shown on the shaft.

Is there any drawbacks to using a quad or o-ring seal over the OEM lip seal?

 

[5twins]

That is odd. I've never run across that before. Was that seal really hardened up from age? They're very, very soft when new and it's hard to imagine they could do something like that. But yes, you can use a quad style o-ring for a seal. I tried them out several years back before I discovered originals could still be had quite cheap right from Yamaha. The ones from Mike's were like $5 each back then so I was looking for a cheaper alternative. I found this American size to fit perfectly and for slightly more than 2 seals from Mike's, I could get a whole bag of 50 .....

I can't attest to the longevity of these compared to an original because I put them in a spare set of carbs that only sees occasional use. I've probably only run them for a total of a few months but they are still working. But, you're in luck that your problem is a left side seal. That can be changed out without having to pull the butterfly plate and shaft. You might even consider swapping shafts between the carbs so that grooved one is in the left carb. Then you could change that seal out with the carbs even still on the bike. You could easily experiment with different seal types this way. Try an original, try the quad. If the quad type works but wears quicker, it would be no problem to swap a new one in every couple years. They only cost about a quarter after all.

As I said, I don't really use these much anymore since originals can be had cheap enough. I'm sure I still have some laying around. Send me a P.M. with your contact info and I'll send you a couple to try. If they work out then you could look into getting yourself a bagful. I would suggest using original seals in the other 3 spots.

 

[Oddjob]

Hi 5twins thx for your reply, since you mentioned that you've never seen something like this I decided to have a closer look at this and reassembled the throttle shaft. I would appear that the groove is slightly inboard of the plastic bushing... the plastic bushing is somewhat of a loose fit on the shaft, so perhaps something got wedged in between the bushing and the shaft and caused the grooving...

Based on this I'll probably re-use the shaft and run OEM seals...

Having said that, if anyone has a right side shaft for a linked 78 carb that they are willing to part with then please let me know...

 

[5twins]

Or maybe it was even made that way? Is that another groove I see at the bottom of the machined flat on the end?