View Full Version : BS38 carb help needed

06-30-2010, 4:15 PM
I just replaced my butterfly shaft seals and this damn thing will not run right on the pilot circuit. Cruising is fine, but it will not idle right. It will be fine sometimes, then it will just start accelerating all on its own. Other times it will pop and backfire like it wants to die. Sometimes it does. Also taking off it likes to pop and sputter instead of running like normal. Now I realize that new shaft seals will decrease the amount of air entering the motor, and its been getting hotter and hotter here in Phoenix, so the logical thing would be to drop the pilot a size or so and play with the air screws, but I have always heard that a high or hanging idle is usually caused by a lean mixture and bumping the pilot UP is the fix. The bike is a 79 XS650 with open exhaust and the XS K&N filters. Anyone got any advice?

06-30-2010, 4:34 PM
check for air leaks yet? spray soome flammable stuff around the boots and see if it revs up. heard propane works too...

06-30-2010, 4:47 PM
Not yet, but would an air leak cause it to accelerate on its own?

06-30-2010, 5:40 PM
i guess if its intermitten. the mix would lean out and rev. kinda like when you run out of gas itll rev up just be fore it dies.

06-30-2010, 6:06 PM
I just checked for leaks with propane then carb cleaner and I got nothing. It was still idling pretty high even with the idle screw backed all the way out. Could it be a problem with the butterflys? When I put them back in I made sure to get the bevel right, but could they be slightly crooked? It looked like there is no room for adjustment on them.

06-30-2010, 7:58 PM
So does anyone have a pair of VM34 round slides that wants to trade me carbs? I'm tired of tinkering with these CVs.

06-30-2010, 9:05 PM
Alright, I just swapped pilots from a 35 to a 30 with no change. I also tried testing for leaks with carb cleaner when it was trying to auto accel with no change as well. One thing I noticed is when it is idling high or running away if I manually put a bit of pressure on the throttle bellcrank on the carb it will come back down, but as soon as I release it goes right back up. Sometimes it will stay down, but as soon as I blip the throttle and release it back to the stop it revs right back up. When I put the butterflys back in, I put it so the bevel is flat with the throat when the throttle is closed. Is this backwards? I honestly didn't look at it when I took them out, I was just using my own judgement upon re-installation. Also the black carbon stains were on the slide side of it, not the motor side.

07-01-2010, 11:32 AM
heres a good read on stock carbs. youll have to register to read it.

read it twice..

07-01-2010, 12:56 PM
my 38s on my 72' do the same thing. the throttle shaft seals are beat. makes it impossible to tune the bike to where id like it to idle. pressure on the shafts drops idle. i have a second set of 72' carbs im gonna try when i get the bike back together but im savin to go to the vm carbs.

let us know if you figure it out.

07-01-2010, 2:36 PM
you cant beat a set of vm 34's from 650 central.....im just saying...

07-03-2010, 5:37 PM
CV or VM's...gona be hell gettin it right w/ those short pipes

07-06-2010, 9:57 PM
Alright I fixed it. Turns out the butterflys have roughly 0.002" that they can be adjusted. I simply loosened the screws and opened then released the throttle a few times to let it center itself, now its running like normal again.