BS38 Hanging idle

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veedub1955

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Ive just rebuilt my carbs with new keyster kits due to a mixture of hanging idle, and cutting out when pulling to a stop, and sticking floats. However the idle hangs still, cutting out is cured

Ive read the excellent carb guide over and over, all circuits are clean and there are no air leaks. There is a paragraph in the guide that states:-

"Too small of a pilot can be indicated by an idle that "hangs" after blipping the throttle or
by having the motor be slow to return to idle after running at higher speeds. Once again,
mix screw settings may help here"

So how do I tune out, richening or leaning? Any other advice would be great to if Ive missed somethig. My floats still stick now and again, although better mainly on the sidestand side, float levels are set at 24mm they have new valves

Thanks in advance, nearly problem solved all the new build issues
 
First see if your advance weights are sticking. Plenty of threads on this. Try a Google search that begins with "xs650" and then add search terms such as advance springs sticking weights service rod. PamcoPete has a vid showing how the weights are supposed to snap back, that should show up in your search. Also search this Forum's Tech section. This is a classic cause of hanging idle. After that, do you have stock exhaust and air boxes? If answer is no, you need bigger pilots, mains too, than come in the kits.
 
Cheers for the reply sorry should have stated ive done away with the advance and replaced with boyer kit. Ive non standard filters and exhaust any pointers on pilot and jet sizes? Cheers
 
Go up one size on pilots. If exhaust is conservative go up 3 sizes on mains. If exhaust is radical go up 4 sizes on mains. Raise the clip on the slide needle one position to second from top. Adjust mix screws for best idle while running.

Check for intake leaks, especially around the boots if they are old.
 
Follow their instructions - figure out what type of Mikuni main and pilot jets you have, then order on that basis. Not many places in the world are going to list "BS38" jets specifically. You need the N102.221 small round mains and either the VM22/210 or BS30/96 pilots. The type of pilot you need will depend on the year of your carb set. Since you failed to provide that, I can only guess.
 
Thats great info cheers i dont know what year the carbs are as they are not off the original bike, they have brass floats

20150912_133340_zpsmia0298o.jpg


20150912_133324_zpsoa7eipy3.jpg


20150912_133403_zpsnbr7skwd.jpg
 
That is a '76-'77 carb set. Some of the "tells" are the overflow drain nipples on the float bowls, the external bowl vents on the inner sides of the main bodies, and only 2 round holes (air jets) on the intake bells. They will use the VM22/210 type pilots. That carb set came with the smallest mains ever used in a 650 (122.5) so you may need to increase them more than on any of the other sets. You will probably need mains in the low 130s.
 
That carb set looks rather messed with. No telling what changes have been made inside from original specs. Here's a chart I drew up showing the stock jetting specs for the various 650 carb sets. Compare what you found to what is stock. Base your re-jetting on what was stock .....

CarbSpecsReducedSize.jpg


Of note for your particular carb set is the very small main. It gets quickly out-paced when you start modding things. Also of note is the very large Z-8 needle jet. That will be the limiting factor on how large the mains can be. While many of the other 650 carb sets can handle mains up into the low 140s, you'll never get that big on your set with that large needle jet. Like I said, low to mid 130s will probably be as big as you can go. Also note the float setting of 25mm. At 24mm, you're a bit too rich.
 
Brilliant thanks ive gone off that from the carb guide and ordered what you said for mains plus 27.5 and 30 pilots. Also noted the float settings cheers, going to pull whole carbs off and reset the floats instead of just pulling the bowls off for mains and pilots
 
Also of note the 4M1 was the needle i took out but have Y55 with the rebuild kit, which would you use? The pilots that came out were 25 without the small holes in the side, and the mains 122.5 just like your table. Ive ordered 27.5 and 30 pilots would you go for the 27.5 or the 30 first, thanks
 
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The Y55 is a generic needle. I would go with the original 4M1 first. If you can't get the carbs tuned to your liking, you could try the Y55 as a last resort. I would start with the 27.5 pilot. It's probably all you'll need with that rich Z-8 needle jet. You're also going to need to lean the needle settings one step to clip slot #2 with the larger mains.

The small holes on the sides of the pilot jet are air bleed holes. The way these pilots mount in the bowls, they aren't required, but won't hurt anything if they are there. The way your float bowl is designed, the pilots flow only fuel. The air is added on top or above the jet. Air bleed holes through the side of the jet aren't needed. The standard way of manufacturing the pilots is with the air bleed holes. That means your replacements will have them. All I can figure is Yamaha contracted with Mikuni to get production runs made without the holes. It probably saved them a few dollars because there was less machining involved to make the jet.
 
Great news ive put 132.5 mains in with 27.5 pilots it now pulls strongly right through the rev range
and idles miles better but now and again it will hang, so going to put 30 pilots in now, but i can ride
it properly now. Clip is on 2nd from the top as advised. Thanks for all the great advice !!

However my floats are sticking randomly all the time. I checked them out when stripping and put
new valves in. What can it be?? They are very free on the pins and the pins slip out easily, could
it be that, maybe cocking over and getting stuck. Is it worth turning up bushes for them so there is no
slop??
 
You can try the #30 pilots but I'm more inclined to think that the random hanging idle is associated with the random hanging floats. If the floats hang shut, a low fuel and lean condition can develop, and that could trigger the hanging idle. Maybe after nearly 40 years, the pivot area on those floats is worn out or worn funny, and they just need to be replaced.

I set up a pair of '76-'77 carbs for use on my '78. My final jetting was like you have now - 132.5 mains, 27.5 pilots, and needles leaned a step. I went larger on the mains while dialing them in but found that too big. I would experience break-up in the upper midrange under heavy throttle applications using 135s or larger.
 

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