Setting the needle clip

[STVR]

Hello,

So I have been tuning my bike (1975 XS650) for a few weeks now, I cleaned the carbs (original carbs) set the cam chain tensioner,valves,timing all of that. Had it running the best I have ever had it although setting the fuel screw never seemed to get it perfect. Then I was out riding it getting on it pretty good she stalls on me at a stop light. So I push her to a corner try to start and and sounded like she wanted to start but just wouldn't ( sounded like a carb problem for sure my fuel filters are from last year so that could have something to do with it). So today I recleaned the carbs took everything apart and decided to change the clip settings on my needle (the clip was second from the bottom and I moved it up one slot so now its 3 slots to the bottom), and she started right up jacked with the fuel screw for a bit and the idle and she sounds mean and strong now. Is my needle too short or should it be fine?

Steve

 

[CDNTX650]

needle only affects midrange. Idle is all screw /pilot.

 

[5twins]

I guess that would depend on what needle you're running and if you've done any rejetting for mods. If you've increased the main jet size then many times the needle must be leaned along with that. The #4 slot you found your needles in is the stock setting for the stock 4N8 needle in your '75 carbs. However, if you've replaced the needles with the generic ones included in rebuild kits, that setting will probably no longer apply.

 

[grizld1]

Needle position does not affect only the midrange. I've been trying for years to get across the point that there are strong secondary effects to consider in tuning carburetors, and this is especially true of older designs. In both BS and VM series carbs the interaction between the pilot circuit and needle position is especially strong: a change of a single step in needle position will force at least a change in mixture screw setting, and often forces a change of pilot jets. You can't read those component-and-range charts as though they define hard boundaries between component effects--there aren't any!

 

[jd750ace]

I keep this one and the carb guide (thanks guys!) and re-read them frequently while tuning. This one is for a 40MM pumper, but the theory, circuits, and behavioral effects in the overlap are described well. Just more info.
http://weislake.com/sig/mucker/carb/carb tech3.pdf

 

[grizld1]

The article on the V-Star carbs is a very well-written piece of work, jd--thanks for the link!

Last year I wrote a 26 page carb manual in serial form for the Yamaha 650 Society, covering OEM and Mikuni VM carbs in detail with a brief discussion of a few other aftermarket options. The manual is available from Don Lawson at the Yamaha 650 Society for $17; anyone who's interested can Google Yamaha 650 Society and click on the link for contact information.