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I have a 1981 Yamaha XS650. I recently put in K&N stock replacement filter (ie, they are in the airbox and conform to the size of the original), however they may be leading to a leaner mixture because they purportedly let through more air.

I want to tune my carbs (yes, I know that the B34s are notorious for being lean in the first place). I have attempted adjusting the pilot screw, which did alleviate the popping symptoms and gave me a much smoother ride at 25 mph.

In order to truly tune my carb, I want a better understanding of the symptoms of lean vs the symptoms of rich. Terms often used are: Stuttering Hesitating Running Rough etc etc

And worse, many claims of lean conditions and rich conditions are contradictory.

Please advise me on what the real symptoms of lean vs rich system is. Thanks.

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3 Answers 3

the easiest thing to do is to raise the needle a notch (or more) if it is too lean. Just don't loose the tiny "E" clip that locates the needle in the throttle slide. Sometimes they spring over a little so be ready for that.

Some carburetors do not have adjustable needles. You can enrichen them with a little washer under the E clip. I have used pop rivet washers for that.

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A shop manual (particularly one with colour photos) will offer some spark plug diagrams / photos, to compare against your plugs, but they're not particularly useful.

As you said, claims are contradictory. The problem is, that a symptom of too lean/too rich is that it doesn't run as well as it should. That's both ways. So a too-lean symptom for one person is the same as a too-rich was for someone else. The best bet is to do one direction or the other and see if it gets better. Sometimes this can be accomplished by tricks (blowing in the vent tube to lower the gas level and lean it out (but just try to do that at full throttle!)) but more often it's going to be trial and error with part changes. This is why people pay money to dyno operators to tune the bike and do all the jet parts x throttle position x engine speed matrix testing at once. You can take the long approach and tune it yourself with ears, feel, butt-dyno and notes - lots of those! - over some time.

In order to tune your carbs you also need a better understanding of how the various circuits affect the mixture at different throttle openings.

I'd suggest starting with a read through the Mikuni VM Manual. The VM models are older, direct-action slides rather than the vacuum-operated BS34, but the same principles apply. In particular the last diagram "Functional Range Effectiveness of Tuning Parts in relation to the Throttle Valve Opening (Approximation)" illustrates which parts (mostly) affect which throttle positions. It's good that they put "(approximation)" in the title - because the diagram lies - everything seems to affect everything else, but at least it gives you an understanding to start from.

To tune your carbs, you need to either go through everything systematically, starting at the main jet and working down, or you can choose to just identify some problematic parts - particular throttle openings x engine speeds - and address those. Factory Pro has a CV carb tuning procedure for lower rpm engines that's an appropriate systematic procedure for your XS650.

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A lean mixture in a motorcycle will symptomatically be hard to start - and you will see less/little fuel in the carburetor and if you remove the plug(s) immediatly after starting attempts, the plugs will be dry rather than an overly rich mixture where they might be "wet" with fuel.

If your bike runs rather bad/worse when cold, but better when warm it is likely running lean.

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