Idle and stalling

Dom

Dom

XS650 Junkie
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So I expected this to happen having replaced the tiny pleated cones for pod filters but it is worse than I had hoped, I'm struggling to get a stable and high enough idle and when approaching junctions it will stall when coming to a stop.

I think as the carbs look a pretty grubby fully stripping them down for a clean is in order but where can I learn what needles etc I need to build back up with?

Am I missing the point and should I be going right back to understand points ignition and setting that first? I've adjusted the cam chain which needed a very slight amount of tightening going by the Yamaha manual.
 
I'm unfamiliar with what bike you have and what you have done, thus far, to the points. General practice is to set ignition before messing with the carbs.
 
Yeah that's what I've been reading, so if I check the valve clearances, set/adjust the points then I can look to sorting the carbs which I might as well strip and clean while also checking for any worn seals and o rings.

I'm new to carburettors so this will be a big learning curve I think.
 
posting a pic here may help
 
Yes, you must I.D. the year of your carb set to properly re-jet it and tune it for mods. You base your jetting and re-jetting on the year of the carb set, not the year of the bike it's on. Yamaha used no fewer than 4 different BS38 carb sets and all are jetted differently .....

Carb Specs Reduced Size.jpg


'76 and later sets were linked together, bolted together in a "bank", and only used one throttle cable. '75 and older sets were separate carbs, each having it's own throttle cable, so it sounds like you have one of those sets. Yes, you'll want to strip them down, at the very least just to find out what jets and needles are in there now. These being 40+ year old bikes, there's no telling what you'll find, lol. Also, if an aftermarket rebuild kit was installed in the past, the jets and needles may be wrong. Many of those kits come with the wrong sizes.
 
Yeah you are correct @5twins I have a US import 1975 650b with dual throttle cables. I've just read through your very well detailed guide linked above, it somewhat baffled me but I think only because I need to re-read it as I do the processes. So as the only modification on my bike is the pod filters if I set it up to factory specs I have a good base to work from?
 
Yes, with only that one mod, and being that the early carbs were set up richer, you may not need to re-jet at all. But the only way to know for sure would be to try it, try one size larger main and see if the bike runs better like that.

Make sure you do have 45 pilots in there. I think the kit for your carbs comes with a 42.5, one size smaller than stock. That might help explain your idle issues if that's what's in there.
 
Don't count on it, lol. If I recall, the kit sold for your carbs comes with 42.5 pilots and 130 mains, the jet sizes for the '72-'73 carb set. It may also contain generic Y-22 needles which are way too short. Here's the BS38 needles compared .....

BS38Needles.jpg


The 4N8 needles for your carbs measure about 45mm long from the top clip slot to the tip. That generic Y-22 is only 40mm long, 5mm shorter, and that would make it way too rich. It might work in the '70-'73 carbs as a replacement for their 40mm long 4JN19, but no way will it work in your carbs.
 
Not sure if it was in your thread or someone else's .Mr 5T
adviced service of the advance unit
This sounds as such a situation
Stalling letting go of the throttle
Checking the functionality of the advance unit would be a first
If points are adjusted and ignition set.
Opening the cover run se if it retards fully .fex when it stalls try to check if it is fully retarded.
And moving in and out when giving throttle
 
Dom said:
The spark plugs were sooted up which in my Haynes manual suggest running rich
Click to expand...
I wouldn't conclude that's the cause until jet sizes are examined, timing verified, etc.. Plugs will appear sooted after starting on choke or idling in traffic with no high rpm running afterward to clear them off.
 
As I have a 650b I'm sure it has no barbs from each carb to balance them, if I'm correct how do do it?
 
Unfortunately for you, I don't think the vac ports appeared on these carbs until the '76-'77 set. Manifold mounted ones are probably your only option.
 
I tell you my experience, maybe it can help.
The first time I rode an xs650 the same thing happened, I couldn't get it to hold steady at idle. After finding this nice forum I realized that I had to check the advance unit and clean the carburettors. The unit had bad springs so I made a little cut and took a turn off and they came back to life, then I read the manual for the carbs and ordered some bigger pilots to try. At the end a little synchronization and that's it.
 
I've gone back to the beginning so I've checked and made a very minor adjustment the the valves. I've made up a 12v bulb to check the points but this is where I've come a little unstuck, the bulb is coming on well before the F mark and if I rotate it back it is staying on far past the point at which it lit. Is this most likely some of my running issues?
 

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