Question about 1979 Yamaha XS 650 F

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Elshaddai159: Thanks for your help so far with Yamaha 1979 650XS F. I've fixed the problem with the needle valves and it ran perfectly for a short time. Now, it is mis-firing on one cylinder. It does fire once in a while. I'm sure that gas is getting into the carb, so it must be electrical. The plugs are new and clean. What do you think?

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  • tdurrie Aug 28, 2010

    Thanks for this, so far. Do I have to remove the carb from the bike in order to do this? Actually, I have two sets of carbs: one set is 36 mm. and the set, now on the bike, is 34 mm. The 36 mm used to be on, last year, and it was misfiring in the same way. Now I have the 34s on and it's doing it again. It doesn't seem that the problem is in the fuel supply.



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It would pay to check your HT lead on that cylinder and also check that after it has been misfiring, remove the sparkplug and see if it is wet and not dry that way you will be positive that it is the ignition system.

Apart from these couple of things it could be the coil playing up.

Let me know, if you need more information.

Posted on Aug 28, 2010

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mixture or fuel regulating screw.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..
Check fig:---- Elshaddai159: Thanks for your help so far with - 314cab2.jpg
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..Check fig:---
Thanks. Keep updated for any more query. You can rate this solution and show your appreciation.

Posted on Aug 28, 2010


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