Air Screw

Gpaws1968

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Ok, in the picture is the carb on my 1981 XS 650, can anyone tell me where the air mixer screw is.
 

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...... and technically, it's not an air screw, it's a true fuel mixture screw. It regulates a portion of the amount of fuel air mix coming from the pilot jet .....

mixtureadjustmentplug.jpg
 
So I can't adjust it?
You can and probably should adjust it. They were set very lean and capped at the factory for emissions. If the caps have not been removed already, not too difficult to drill/pop them out. You will want to remove the screws plus spring washer and O-rings in order to clean this circuit. First, before removing, gently turn the screws clockwise by half turns, counting as you go. Stop as you feel resistance as tightening them completely will damage the tips and their ability to meter mixture accurately. This will give you a baseline setting. When reassembling, return screws to baseline and then adjust. Clockwise to lean. Counterclockwise to richen. Have fun.
 
See red circles
View attachment 260698
From the factory they are capped with a pushed in brass plug.

You can and probably should adjust it. They were set very lean and capped at the factory for emissions. If the caps have not been removed already, not too difficult to drill/pop them out. You will want to remove the screws plus spring washer and O-rings in order to clean this circuit. First, before removing, gently turn the screws clockwise by half turns, counting as you go. Stop as you feel resistance as tightening them completely will damage the tips and their ability to meter mixture accurately. This will give you a baseline setting. When reassembling, return screws to baseline and then adjust. Clockwise to lean. Counterclockwise to richen. Have fun.
I thought I was going crazy yesterday when I couldn't find the screw, thank you
 
The BS34s were pretty E.P.A. strangled so like more. On a stock bike, somewhere in the 3 to 3.5 turns out range is usually needed. If you've modded the bike and changed the jetting, a bit less will work. On my '83 with one size larger pilots and one size smaller air jets, the screws are set at about 2.5 turns out.
 
It is hard to keep running when cold, the slightest move of the throttle and it dies and when you do get it to rev it backfires, when warmed up it runs great.
 
You need a richer mix when the engine is cold, that's what the choke is for. If it's doing this with the choke on then maybe the chokes aren't working, or working fully. Maybe the choke feed jets in the float bowls are plugged or partially plugged. Here's how to check them .....


Choke Jet.jpg


ChokeJet2.jpg
 
You need a richer mix when the engine is cold, that's what the choke is for. If it's doing this with the choke on then maybe the chokes aren't working, or working fully. Maybe the choke feed jets in the float bowls are plugged or partially plugged. Here's how to check them .....


View attachment 262957

View attachment 262958
Yeah, I have the chock fully out, then when I can rev it I go halfway with the chock then when I start riding it, I push the choke all the way in
 
So, I'm back :p

I turned the screws out 4 turns and it starts better when cold but has a dead spot at low revs and intermittent backfire, I'm getting a 90-degree screwdriver so will start tuning.
 
I found this to be useful for adjusting bs34 idle screws. Craftsman 41380. It will fit 1/4" ratchet but I've always used it on its own. Drop the bit down the hole and turn slowly until the blade falls into the screw slot. You can maintain downward pressure with a finger. A line across the top and side represent 0 or 12 o' clock. Easy to read and make precise adjustments.
 

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