BS34 Air Screw

01ps

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On the BS34 carb, turning the air screw out is richer, in is leaner? Or am I wrong? I can never remember for some reason!
 
After some searching I found a very nice description of the mixture screw on XS650forum.com bu 5twins, as follows :

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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.

Read more: http://xs650forum.proboards.com/index.cgi?board=Fuelsystem&action=display&thread=1824#ixzz1JSYfNlqi

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Remember the xs400 used 2 different bs34 carbs 77-79 and 80-82. The 77-79 carbs use a non-o-ring needle and the 80-82 use a o-ring and washer with a stepped end on it. The 77-79 styles seem to break off in the bodies when over tightened.
 
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Remember the xs400 used 2 different bs34 carbs 77-79 and 80-82. The 77-79 the screw goes out for a leaner mix and the 80-82 (the capped off one that you have to drill out) out is richer. This only affects the pilot jet. This come from a 35 year yamaha tech.
Are the screw in the same position on both carbs, between the head and slide?
 
I am pretty sure my 78 gets richer with the screws turned out.

I have pods, free-flowing exhaust, and the bike wouldn't even start until I turned the screws out 4 times. By 5 or 6 turns I had a smooth idle at 1000 rpm. Another clue was that the bike was firing easy when I sprayed starter fluid in the air filters when the screws were closed; this isn't necessary when they are open. I went even further and once I got the bike idling I used a propane torch to spray gas in each air filter; When the screws are closed I get a surge in rpms, but when they are open the rpms do not move appreciably.

I can't be absolutely certain because I am flooding slightly at the moment, but when I turn them in--closing them more--while the bike is running my rpms shoot way up and the bike gets angry.

The problem is that running excessively rich or lean can sometimes produce the same symptoms. I won't know for sure what is going on until I adjust my float height.

I suggest starting at like 3 turns out and using the kick start so you don't kill the battery during the process. Won't start? Go another turn out and repeat. Once it starts running fine tune the screws. Once it has been running for a bit pull the plugs and take a gander. If it has black sooty carbon deposits or is wet with gasoline (not oil) it is running too rich. If it is running too lean they will probably be quite clean/white and may blister due to the overheating that tends to occur. Apparently they are supposed to look slightly yellow in color with no significant film or deposition.
 
I just put in smaller pilots and main jets and lowered my floats. Once I got it idling closed screws were causing back-firing and randomly it would die at idle. Several turns open and things seem smooth. I just need to sync the carbs now because I have noticeably less backpressure at idle from the left side, but pressure seems fine once I open the throttle.
 
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