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Post by jimmuck on May 18, 2010 13:47:55 GMT -5
I have a 1979 xs650sp with what appears to be bs34 carbs, the lh cylinder misfires even though I think that I have cleaned the carbs out, it is definitely the carb as I have moved the rh carb over and the lh cylinder then runs ok.
The air screws are free but will not unscrew out, is there a reason for this as I cannot see what is stopping them screwing out, the screwdriver slots are mutilated but I can get a screwdriver to grip.
Any help would be appreciated
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Post by 5twins on May 18, 2010 15:51:52 GMT -5
The mix screws should come out. There's nothing holding them in once they're unscrewed all the way. They have a spring, washer, and o-ring on them that may be causing them to stick in the hole. If you cleaned the carbs using carb cleaner with the screws in place, that cleaner may have melted the o-rings. That could be what's holding the screws in. Unscrew them until you're sure the threads are disengaged then try grabbing them with some pliers and tugging them out. You may not have gotten the passageway clear that these screws fit into and regulate fuel/air mix through. You really can't properly clean it with the screw in the way. That may be the cause of your misfire. Also, that little o-ring needs to be in there and in good shape for the screw to regulate flow properly.  |
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Post by XS Rod on May 19, 2010 12:01:15 GMT -5
I know alot of you guys know this and some don't... As I recall ...
The mixture adjustment bleed is like a hole in the side of a straw, it bleeds air into the gas which is flowing through the port and creates a fuel mixture. The size of the bleed hole is metered by the screw and the amount of air allowed to bleed in is controlled by the screw adjustment... Fuel to Air ratio or mixture.
Hopefully if you didn't know you now have at least a visual on what's going on when you turn that screw.
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Post by 5twins on May 19, 2010 13:28:50 GMT -5
What you've just described is an air screw. The CV carbs on the 650 don't use an air screw, they use a true mixture or fuel regulating screw. Opening the screw more makes things richer. Opening an air screw more produces a leaner mix. How do you tell the difference? On 99% of the carbs out there, if the mix screw is located between the slide and the engine, it is a fuel regulating screw. If it is located between the slide and the air filter, it is an air regulating screw. The 650 mix screw regulates the all ready made air/fuel mix that comes from the pilot jet. The strength of that mix can be altered by changing the pilot jet size on the BS38s. On the BS34s, it can be altered by changing the pilot jet size and/or the size of the air jet that feeds it. The air/fuel mix from the pilot jet is delivered into the main bore through 4 tiny holes. 3 are clustered together about where the butterfly plate closes. These flow directly from the pilot jet and are unregulated. Whatever mix the jet sizes are producing will come out here. The 4th hole is off to the side (BS38) or ahead (BS34) of the 3 others and is the regulated feed from the mix screw. The mix screw gives you control and adjustment over approx. 25% of the feed from the pilot jet.  |
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Post by XS Rod on May 20, 2010 8:49:33 GMT -5
Got that bro, I stand corrected, thought I recalled an air bleed in the circuit from the carb diagrams I've looked at.
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Post by jimmuck on May 20, 2010 17:51:14 GMT -5
After much working backwards and forwards I have managed to get the mixture screws out but nothing was left of the slot on top. I intend to try to use a junior hacksaw to cut a slot for a screwdriver, o rings seem good and the passage way seem clear (I hope)
thanks for the help it really seemed like there was a stop as the screws would only turn easily so far.
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Post by XS Rod on May 21, 2010 8:43:57 GMT -5
I'd replace them ... but that's me.
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Post by jimmuck on May 21, 2010 17:09:07 GMT -5
I intend to replace eventually them but where do I get them, mikes xs does not sell to the uk and the ones on the xs650shop.de site look different
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Post by 5twins on May 21, 2010 18:29:45 GMT -5
Different how? Like this?  If so then you may want the different screw with the nice taper (left above). Is your bike a USA import? The stepped screw tip is an E.P.A. mandated thing. The tapered style would probably run better, richer, and be more adjustable. I know I switched to European mixture screws in the Delortoes on my Moto Guzzi LeMans. Same deal - better taper and more adjustable. |
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Post by jimmuck on May 22, 2010 5:18:07 GMT -5
Yes exactly!.
My bike is an import from Japan 1979 xs650sp it has the screw on the right as standard, the screws on the site xs659shop.de have the taper as the screw on the left and are listed as 3L1
Thank for the info I will order the taper screws
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Post by 5twins on May 22, 2010 13:34:39 GMT -5
Be aware that the original screw ran best at 3 to 3 1/2 turns out but it was E.P.A. strangled. Your new screw probably won't need that many turns out. I would start it at 2 or 2 1/2 turns out and fine tune from there. Who knows, it may even need less. What I'm saying is you're going to need to tinker with the setting and find what works best.
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Post by jimmuck on May 24, 2010 11:37:38 GMT -5
I assume that the original/epa screw does not allow much adjustment if any with that long parallel bit at the end. Now lets hope when I get the new screws all will be well, there was indeed dirt on the misfiring cylinder's screw. I just hope everything thing else is ok.
I have learned a lot guys thanks
PS I had only previously heard of the e.p.a. in the simpsons movie
doh!
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