need XS2 carb advise

[tmikeyb]

I cleaned the carbs on my XS2 prior to starting after years of rest. My right side carb had an air leak at the intake, so I replaced the carb holders on both carbs. Now when starting from cold, it still only runs on the left cylinder while it's cold. Spraying fluid around near the joint sort of indicates a leak, but not like before. If I rev the engine up, the right cylinder kicks in at about 1500 RPM, but drops back out at idle. After the bike gets warmed up, the problem seem to go away, and both cylinders are OK. When I cleaned the carbs, I re used the old gasket on the cover on the side of the carb, over the enrichener passage I think, because the kit did not come with a new one. I realize that nobody can troubleshoot my issue from your house, I'm just hoping to get an idea where to start, or a hint or two. THANKS

 

[5twins]

Are the carbs synced and is the idle speed turned up enough on the right carb? If it's set too low, that carb will be effectively shut off at idle.

 

[tmikeyb]

Good points, I feel pretty stupid right now, because I remember reading in the manual that it was important to sync the carbs. But honestly, I completely forgot to do that. I will check both of those things first thing tomorrow. THANKS

 

[gggGary]

A cross over tube supplies air/fuel mix for the RH choke on the 72 carbs, make sure it's in place and sealed, the spigots it fits on depend on a taper fit into the carb bodies.

 

[Niels B]

if it has been sitting for a long while the seals on the throttle shaft may have hardened, thus may leak a bit of air there - i would replace them - it's a cheap fix

 

[tmikeyb]

if it has been sitting for a long while the seals on the throttle shaft may have hardened, thus may leak a bit of air there - i would replace them - it's a cheap fix

I did replace the throttle shaft seals when I cleaned the carbs.

 

[tmikeyb]

I just went out and checked the idle settings, and syncronization. Both seem fine. I started the bike and it again ran only on the left cylinder. I started spraying carb cleaner at various spots on the carb. It appears to be leaking air at the throttle shaft seal on the out side, i.e. the side where the spring and linkage are located. I replaced all of these seals on both carbs. But 2 of the old seals were stuck hard in the recesses and I had to dig them out. This could be one of those spots. Maybe I missed digging out a bit of the old seal, or maybe I gouged the aluminum seat. I'll remove the carbs and see if I can get to the bottom of it.

 

[tmikeyb]

pulled the carb, popped the cover cap off of the seal. It's obvious that there is a bit of slop in that end of the throttle shaft. The inner lip of the seal is not quite touching the shaft. From eyeballing it, I'd guess that the gap might be .003 to .005 inch. Could it be that the throttle shaft is worn that much. Since it rides in the seal, it seems odd to me if it's worn that much. I think that I may have read that the vacuum pulls the seal in toward the shaft, so could this be normal?? These carbs are new to me, so please forgive me if I seem ignorant. It's only because I am.

 

[Its been a long time]

pulled the carb, popped the cover cap off of the seal. It's obvious that there is a bit of slop in that end of the throttle shaft. The inner lip of the seal is not quite touching the shaft. From eyeballing it, I'd guess that the gap might be .003 to .005 inch. Could it be that the throttle shaft is worn that much. Since it rides in the seal, it seems odd to me if it's worn that much. I think that I may have read that the vacuum pulls the seal in toward the shaft, so could this be normal?? These carbs are new to me, so please forgive me if I seem ignorant. It's only because I am.

Put a dial gauge on the end of the throttle shaft and move it up and down. Then you'll know how much clearance there is between the throttle shaft and carburettor body. Should be very little clearance on the shaft.

 

[tmikeyb]

Yes, I thought about doing that. I will set it up after lunch. Just for curiosity, what would be an acceptable amount of clearance?

 

[Its been a long time]

Yes, I thought about doing that. I will set it up after lunch. Just for curiosity, what would be an acceptable amount of clearance?

Well, they're 50 year old carburettors. I would hazard a guess that on such a small diameter shaft, anything more than 0.05mm or 0.002" would be too much.

I am just going by general experience there. Someone here no doubt knows better than I, but I think I'm not far off there.

 

[tmikeyb]

OK, I set up a dial indicator and measured it. I put the indicator on the shaft, as close as I could get it to the carb body. I'm thinking that would give me the most accurate measurement. The total movement back and forth is .005 inch. That would mean a clearance of .0025 or thereabouts. I agree with you that it seems like a lot. In the meantime, I looked for a new throttle shaft online and did not find one. It seems that a lot of stuff for BS38 carbs is no longer available. When I search for BS38, most of my results are changed to BN38 or other numbers.

 

[Its been a long time]

OK, I set up a dial indicator and measured it. I put the indicator on the shaft, as close as I could get it to the carb body. I'm thinking that would give me the most accurate measurement. The total movement back and forth is .005 inch. That would mean a clearance of .0025 or thereabouts. I agree with you that it seems like a lot. In the meantime, I looked for a new throttle shaft online and did not find one. It seems that a lot of stuff for BS38 carbs is no longer available. When I search for BS38, most of my results are changed to BN38 or other numbers.

I would say from experience that 5 thou clearance is too much. It's a very small shaft. I am not surprised that the rubber seals don't work.

Others with more experience may know more than I. Anyone else out there done this check?

 

[Mailman]

Random stuff re: chasing air leaks,
I have spent more than my fair share trying to chase down air leaks that caused poor idling. Here are a couple of odd ones that gave me grief.
I had a BS38 that had wear inside the carburetor body from years of the throttle plate rubbing. It created too much gap and the idle was always erratic. The cure for this one was a replacement carb body.


Another weird one ( and I found this on more than one set of carbs ) casting residue that wasn’t letting the cover seal properly. The fix for this was a fine file to smooth out the bumps and a new gasket.


And finally my super high tech leak detector. Start your bike, insert an ear plug into one ear to block out ambient noise, stick a piece of tubing in your other ear and with the open end, start probing around your carburetors and their mounts.
Believe it or not, I had great success with this, as soon as I got that tubing anywhere near a leak that sound would be very apparent through that tube.

 

[Its been a long time]

@Mailman did you ever measure shaft to bore clearance as above? Gut feel says to me that the 0.005" measured is too much?

 

[Mailman]

@Mailman did you ever measure shaft to bore clearance as above? Gut feel says to me that the 0.005" measured is too much?

No I never did. I really am a shade tree mechanic.

 

[Its been a long time]

No I never did. I really am a shade tree mechanic.

I guess there's no official numbers for this. But 5 thou on a small shaft feels far too much to me. When I changed the BS38 shaft seals recently I was a little surprised how crude it was. I would say from experience those seals have no chance of sealing properly with that much shaft clearance. But for me that's just based on experience. For sure, on a machine that I'm used to (turbines, compressors, pumps etc) it would be intolerable.

Someone out there might know better than me with actual experience?

 

[Jim]

In the meantime, I looked for a new throttle shaft online....

Almost all my recent experience is with BS34's. They're very similar, but not exactly the same, so take this with a grain of salt...

The throttle shaft is made from brass and then chrome plated... very durable against wear in other words.
The carb body on the other hand, is made from zamak... which is a zinc alloy. If there's excessive wear (and I suspect 5 thou is excessive), you would expect it to be on the softer carb body and not the shaft.

 

[Its been a long time]

Almost all my recent experience is with BS34's. They're very similar, but not exactly the same, so take this with a grain of salt...

The throttle shaft is made from brass and then chrome plated... very durable against wear in other words.
The carb body on the other hand, is made from zamak... which is a zinc alloy. If there's excessive wear (and I suspect 5 thou is excessive), you would expect it to be on the softer carb body and not the shaft.

I agree. On my 77 BS38 carbs the shafts were perfect. I was surprised how close fitting the shafts were in the bodies. I didn't measure it but probably not much more than a thou or so clearance.

I was surprised to see the carburettor bodies actually have bronze/brass bushes that the throttle shafts rotate in. In other words, a competent machine shop could actually rebush the carburettor bodies if required. After rebushing, line reaming to final size would be essential.

 

[Niels B]

Agree on all the various comments above - one thing though - you did put the seal in the correct way right ? The lib facing out should (in theory ) help the seal to expand a tiny bit, if air is drawn in at the shaft.