authenticnovelty
XS650 Enthusiast
Forgive me if this has been covered somewhere else. i’ve read threads on throttle shaft issues, but none of them discuss such a consistent reoccurrence. Figured I may as well throw this scenario out for opinions....
I have an air leak on my BS38 carbs that I cannot get rid of. I have a 1978E (standard) with factory air boxes and a joined OEM exhaust from an xs650 special (previous owner’s doing). The carbs are normal BS38 with standard jetting, floats, needles, etc and setup going to dual petcocks from fuel tank. Mix screws are usually 2.5-3.25 turns out. Cylinder compression is consistently matching on both when tested, just under 130 (just shy of 150 when 1tsp of oil is put in cylinders).
In less than 9 months i have stripped and cleaned the carbs four times, removed and reinstalled them countless times, installed three different sets of carb boots with and without vacuum barbs using three different sets of boot gaskets, most recently yamaha boot gaskets. I’ve replaced/reinstalled all four throttle shaft seals at least 3 times cleaned and used fresh dielectric grease. At this point i’ve stripped one of the screw holes on a throttle shaft and find it hard to believe a fourth try will help anything.
It is always the inner shaft seal on either carb individually or simultaneously, but intermittently. Never the outer seals. It comes and goes.
Some mornings it starts right up, some days i have to spend 10 minutes turning the choke on and off strategically as i rev the engine in small blips and it still will die. Sometimes I can be sitting at a light and i’ll watch the idle shift 500RPM one direction or the other and stick. Typically i’ll just adjust my idle screw and keep going. On most good days it will idle at 1200 RPM.
To add insult to injury i finally gave in and bought newly restored carbs from Spydercycleworks, when i saw my exact set online recently and these also leak air in a similar way. I’ve resync’d the new carbs three times in the last week and each time requires different between idle mix and sync screw settings as the air leaks shift.
One guess is that the engine will occasionally backfire/miss subtly when idling (especially when starting on some days) sometimes just on left carb, sometimes on both. Always through the carb into the airbox, almost never through the exhaust pipes. Is it possible the backfire/miss is reseating the shaft seals from the abrupt pressure change? The throttle shaft air leaks often begin after revving hard or a miss, but not in a consistent pattern. Sometimes restarting the bike can fix/cause it.
Does it may sense to try and double up on seals?
A bit long winded, but want to cover all my bases. Am i missing something?
Here is a pretty boiler plate video of carb spray on my inner shaft seals if of use to anyone. about 5 minutes before i filmed this, the bike was idling comfortable at about 1200RPM and i could not get any response from any carb spray anywhere. Then i pulled each spark plug off to check sync and the bike went to about 1700RPM and stayed.
I have an air leak on my BS38 carbs that I cannot get rid of. I have a 1978E (standard) with factory air boxes and a joined OEM exhaust from an xs650 special (previous owner’s doing). The carbs are normal BS38 with standard jetting, floats, needles, etc and setup going to dual petcocks from fuel tank. Mix screws are usually 2.5-3.25 turns out. Cylinder compression is consistently matching on both when tested, just under 130 (just shy of 150 when 1tsp of oil is put in cylinders).
In less than 9 months i have stripped and cleaned the carbs four times, removed and reinstalled them countless times, installed three different sets of carb boots with and without vacuum barbs using three different sets of boot gaskets, most recently yamaha boot gaskets. I’ve replaced/reinstalled all four throttle shaft seals at least 3 times cleaned and used fresh dielectric grease. At this point i’ve stripped one of the screw holes on a throttle shaft and find it hard to believe a fourth try will help anything.
It is always the inner shaft seal on either carb individually or simultaneously, but intermittently. Never the outer seals. It comes and goes.
Some mornings it starts right up, some days i have to spend 10 minutes turning the choke on and off strategically as i rev the engine in small blips and it still will die. Sometimes I can be sitting at a light and i’ll watch the idle shift 500RPM one direction or the other and stick. Typically i’ll just adjust my idle screw and keep going. On most good days it will idle at 1200 RPM.
To add insult to injury i finally gave in and bought newly restored carbs from Spydercycleworks, when i saw my exact set online recently and these also leak air in a similar way. I’ve resync’d the new carbs three times in the last week and each time requires different between idle mix and sync screw settings as the air leaks shift.
One guess is that the engine will occasionally backfire/miss subtly when idling (especially when starting on some days) sometimes just on left carb, sometimes on both. Always through the carb into the airbox, almost never through the exhaust pipes. Is it possible the backfire/miss is reseating the shaft seals from the abrupt pressure change? The throttle shaft air leaks often begin after revving hard or a miss, but not in a consistent pattern. Sometimes restarting the bike can fix/cause it.
Does it may sense to try and double up on seals?
A bit long winded, but want to cover all my bases. Am i missing something?
Here is a pretty boiler plate video of carb spray on my inner shaft seals if of use to anyone. about 5 minutes before i filmed this, the bike was idling comfortable at about 1200RPM and i could not get any response from any carb spray anywhere. Then i pulled each spark plug off to check sync and the bike went to about 1700RPM and stayed.
Did 50/50 gasoline and 10w40 motor oil on the filters and brake check valve on the dual breather hoses with the reducers in the breathers removed for science. 
