View Full Version : xs650 Carb problem

02-13-2010, 7:05 PM

I had recently bought a 1980 xs650 for a bobber build.

Upon getting it home yesterday, I first ran into badly leaking carbs. I tore them apart and cleaned them and put them back into place only to have another issue arise.. It seems like now I'm not getting any fuel to the plugs. If I shoot a tad of gas into the cylinders, it runs for a few seconds. So, I know it fires and runs but it just seems like it's not getting fuel?

FYI: I have the air boxes and filters totally off right now and I'm not using the gas tank. I've got a funnel full of gas connected to the fuel line.

Thanks for any ideas!

02-13-2010, 8:22 PM
1. Check all of your jets to make sure they are clean and not plugged, specifically your idle jets.
2. Those carbs need some sort of vacuum to operate properly. Running with no restriction on the inlet makes those buggers tricky to tune.

02-13-2010, 8:44 PM
Thanks for the reply.
I just cleaned the carbs again, concentrating on the jets & I'm going to try to start it again. They were full of gunk.

02-13-2010, 10:30 PM
After putting it all back together, it still is having problems starting. The only time it fires is when the choke is fully pulled out and it seems to only run for a second or two. I still have the air boxes/filters off.

I'm not sure if this is correct, but I have also plugged both nipples at the intakes.


02-14-2010, 12:06 AM
Run a vacuum line from one nipple to the other, and put the air box/filter back on and try it again.

02-14-2010, 1:06 AM

I'll give that a shot. I'll have to dig around for the air boxes. I hope I didn't toss them out already. I had planned on using some K & N pods.

Thanks for the input!


02-14-2010, 6:07 PM
Once again (4th time), I cleaned the carbs, checked the brass and put the whole thing back together to find the same carb spewing gas everywhere. After a moment it had stopped and I turned the motor over and the opposite side ran very weakly for about 2 seconds. The side with the leaky carb didn't seem to be firing at all.

With a line connecting them, I had the two vacuum lines at the intakes running to one another.

I did not hook the airboxes up due to the fact that the filters had some corroding foam that I did not not want to get sucked into the carbs.

I cannot help but feel that the poor running of the motor has to do with the vacuum factor of the carb. What does everyone do with the intake nipples when your run the K&N pod filters? Do you connect the two, leave them both open or plug one and leave the other open (the last is the way I bought it)?


02-14-2010, 10:45 PM
This bike should have the BS34 mikuni cv carbs. If you pull the carb bowl off. You'll see a hole in the mating surface of bowl. There's a brass tube that is attached to the carb body that fits into it. Down inside the hole in the float bowl is a small non-removable jet. This jet is the starter (or choke) jet. If it's clogged, your choke will not work properly. There is a hole in the wall of the bowl that connects with the bottom side of this jet. These bikes actually use fuel enrichment instead of choking the air. Check to make sure those jets are not clogged, but do not use anything that will enlarge them.

Here's an example (NOT an xs650 carb but same procedure)

You may also pull the slides with rubber diaphragms out. Holding the rubber diaphragms towards a light, pull the edges taught to check for any pinholes or cracks in the rubber. If there are cracks or pinholes the diaphragms or slides need to be replaced ( you can buy just the diaphragms seperately here http://www.jbmindustries.com/Yamaha650.html ). Also blow thru any passages to make sure there are no blockages.

I've also seen instances that the bowl vent is clogged and fuel can not be "siphoned" thru the jets.

Let us know how it works out.

02-14-2010, 11:41 PM
Oh yeah, the vacuum nipples serve two purposes
1) they provide a place to hook vacuum gauges to when syncronizing the carbs
2) they provide a vacuum source to operate vacuum controlled fuel valves.

If you are using a non-vacuum fuel valve, you can just cap both of these. If they are not capped, it will be a major vacuum leak resulting in poor running and a high idle. They are not hooked together from the factory and I don't see any point in hooking them together. Just cap them both individually.

02-15-2010, 8:56 AM
Thanks a ton for the help, Earz!

I'll give that a shot!


02-15-2010, 11:59 PM
Holy SHIT, earz!

Low and behold in one of the float bowls, the orafice was completely constipated. Upon some rotor rooter and Pepto Bismol, she gave up the ghost!

I found a couple other minor problems with the adjustment that operates both accelerators at once.

The diaphrams seemed OK.

After putting it back together I covered both nipples (on the carb) and fired it up.......Instant runner!

Now to tear the bike apart and get to work fabricating!

Thanks a lot for the help!!!!