Throttle Shaft Seals - last ditch effort or ditch 'em?

[RPC3]

1978 Standard, stock BS38 carbs, stock air filters.

Looking for some input to see if there's anything else I'm missing before either living with my carbs with a higher than prescribed idle or trying again with another set of carbs. I've fully rebuilt my carbs twice; blown out ever port and passage with cleaner and compressed air, new jets, new carb boots, everything. Bike has a PAMCO with the e-advance and really runs great, except after two attempts, my throttle shaft seals still leak (OEM Yamaha, not mikes). Its mostly the shaft seal closest to the throttle cable, but spritzing some starter fluid bogs the engine on the other carb as well (either carb will bog when the shafts are sprayed). Even with the leak, I can get the bike to idle pretty well at around 1,500rpm and have it be reliable. I can get the bike to idle evenly at 1,000-1,2000 for a while, but if I keep it there on a ride it'll inevitably want to stall out on me a stop somewhere along my ride. Bike really does ride great off idle, no real flat spots, everything else is tuned well.

So I've got two choices - live with having the idle turned up a tick and a slight throttle shaft leak which I might be running a little rich to compensate for, or rebuild another set of bs38's and hope this set gets it done. I do have a second set of bs38's hanging around, I'm just balking at the $50 in parts, time and labor it'll take me to rebuild, balance, sync, tune, etc. I'm 99% sure I did the seals right (its why I took the carbs apart a second time to verify - the seals are facing the correct direction etc). Anything else I'm missing here? I'm pretty confident I don't have a leak anywhere else other than the throttle shafts. I have no clue what the origin of the carbs are, but I thought they were stock (they had the stock 78 jets in them when I opened them up the first time) and its a New England bike, so not subject to the dust and grit of the southwest, but who knows....the leak is there regardless of origin.

What would you do if these were your carbs? Ride it and forget about the leak? Rebuild the spare set of bs38s? Something else?

Thanks!

 

[5twins]

Are the butterfly shafts loose in their bores? Sometimes the bores wear out and then new seals don't help. But, these seals work on vacuum. They get sucked in tighter as the RPMs increase and seal more/better. They may not seal 100% at idle.

You should be able to get the bike to idle consistently around 1200 without stalling, it just may take some tinkering and fine tuning. Yes, they will up and stall for no apparent reason if you try to go lower than 1100 or 1200 on the idle speed. You need to get all the tune-up related items set correctly. That would include mix screws and sync on the carbs but also cam chain, valves, and timing.

 

[RPC3]

The shafts feel pretty sturdy to me without any extra play. Its very possible I just don't have it tuned right from an air mixture standpoint. I don't get any popping on decal or misfires when driving, so the problem really seems to be isolated to idling only. This is all with a big caveat that this is my only bike, I did all the work (cam chain tension, valve height, carb syncing, rebuilding, balancing, etc), so sure - I could have messed up on something along the way, but I'm typically pretty cautious and go slow after researching/consulting the forum quite a bit before delving into my work.

Based on the above, I'm inclined to ride and work on fine tuning before pumping money into more rebuild parts

 

[5twins]

I'd say yes, do that. It took me a couple seasons of learning about the bike and fine tuning to get my idle right. There's that illusive setting on the idle speed where it will idle right (not too fast) when hot but still idle pretty well (although at a lower RPM) when cold. I never trust it though when cold and at that lower RPM idle. I crack the throttle a hair or routinely give it little blips.

I tried running a lower idle speed, like around 1000, when I first got the bike. I thought the 1200 spec seemed a bit much. I couldn't do it without having random stalls. I've just gotten used to the 1100-1200 idle speed. It doesn't seem too fast to me anymore and besides, I know I can't set it lower because of the random stalling it will produce.

The mix screw spec for your '78 carbs is 2 1/4 turns out. That is just a starting point, an initial setting. From there you should fine tune them for best running. You want them set for fastest idle speed and smoothest running. Many times on the '78-'79 carb sets, that's a bit more than the 2 1/4 turns out initial setting. Many times they like somewhere in the 2 1/2 to 5/8 range.

You should be able to tune your mix screws by ear and without doing that "dead cylinder" method. Do it with the bike warm and the idle speed turned down to 1100 or 1200. Much faster and you won't notice the adjustment changes as easily. Kneel on the side of the bike next to the cylinder you're adjusting. Start with the mix screw at it's initial 2 1/4 turns out setting. Choose a direction, in or out, and start adjusting the screw in 1/4 turn increments. Wait a few seconds between each quarter turn to see what effect it has. Within 1/2 to 3/4 turns in either direction, the cylinder should start to slow and run rougher. Note that setting, go back to the 2 1/4 setting, and work in the other direction until the same thing happens. There is a setting range of maybe 1/2 to 3/4 turn where the idle will be fastest and smoothest. This is what we call the "sweet spot" and is what you're looking for. You'll want your final setting somewhere in that range. I set mine near the upper, richer edge because it gives less popping out the exhausts on decel.

 

[TwoManyXS1Bs]

If you have trouble finding that mix screw 'sweet spot', referenced by 5twins above, you may want to revisit centering of the butterfly plates. Many threads in here on "centering butterfly". For ideas, have a look at the butterfly postings in these:

http://www.xs650.com/posts/265633/
http://www.xs650.com/posts/265700/
http://www.xs650.com/posts/428155/
http://www.xs650.com/posts/445947/

And this album pic, and the 'previous' 10 pics:

http://www.xs650.com/media/71xs1b-1...mm-across-this-is-0-001-less-than-38mm.23254/