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air/fuel mixture screw

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Guest

Guest
I have 2 good screws and new orings on them. when you turn the one all the way in it makes the bike want to die , but when you turn the other one in all the way nothing happens at all. ? :confused::eek: What does this mean? Also the side that nothing is happening on is also the side that im getting the backfiring from.
 
hey... um... which way is rich and which way is lean? I can't remember right now and of course it isn't in the factory manual.

/\/\ac
 
Mac, I'm not sure if you're joshing me, but at least on VMs, the fuel screw on the bottom of the carb: clockwise is leaner (allowing less gas into carbs) and counterclockwise is richer. On the side air mixture screws, counterclockwise is leaner (allowing more air into the mix) and clockwise is richer. If I'm looking at this right, you set the bottom screws to get the proper mixture, and then the air screws for max RPM idle.
BTW, if one air screw doesn't seem to do anything, some PO might have bottomed it to hard and broken off the tip of the needle, which might be lodged in the passageway, thus not really adjusting at all. This can result in a rich condition that will cause backfiring. (But being lean can also cause backfiring! Fun, isn't it?). The tip can be carefully pushed out from the inside of the throat, and a new air screw installed. Do a search for broken air screw tips or something of that like, and you should find a more detailed explanation of how to do this
 
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Except that he does NOT have VM carbs, it's an '81 bike with BS carbs.

Kpark, my guess is that the tip of one is broken off in the orifice, blocking any mixture from getting through. That would be a possible source of your lean mixture (the cause of the backfiring) and would also explain why there is no change when you move the screw. Compare the tips of the two screws to see if they are the same. If they are, pull the carbs, remove the suspect screw, shine a light through the outlet of the carb, see if the hole is clear for the mixture screw.

By they way, Mac and Jknapp, carb screws will let more of whatever they are adjusting when you turn them LEFT and less when you turn them RIGHT. The trick is to know what they are adjusting. On the VM carbs, the screw on the bottom only controls FUEL to the pilot mixture. The screw on the side only controls AIR to the pilot mixture. On the BS carbs, the mixture is pre-set by the pilot air jet in the carb intake and the pilot fuel jet beside the main jet, under the rubber plug. How much of that MIXTURE is allowed into the engine at idle and low throttle openings is what the idle mixture adjustment screw does.

.
 
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ok thanks steve. going out to check on it now. i know they were the same when i put them in and i tightened them down lightly unlike the float bowl screw lol. ill pull them out and check the hole.
 
Are the carbs vacuum balanced? If not you can do it by setting up the idle and then alternately removing one of the plug caps - the idle should drop down to roughly the same level. Needless to say, the valves need to be adjusted to be in spec before any tuning work can be done.
 
Nope, wasn't kidding. I'm trying to finish up some painting in my old apartment, doing last minute packing, thinking about final goodbyes with my recent XGF tonight and wondering wtf else is going to go wrong before I get back to NY so I'm not exactly up to 100% on my troubleshooting.

/\/\ac
 
Yeah the carbs are in sync per my crappy gauge sync tool. the valves are all in spec. I just pulled the needle out and checked the opening and there is nothing blocking it. I even started the bike with the needle out and put my finger over it and there is suction? also changed idle when i would put my finger over the hole. so now im lost again as to why the screw isnt helping?
 
Are you sure that pilot jet is not plugged up?

I hate to ask you again, but did you get some additional pictures of the plugs.

The left one was white... did the left plug look like it was wet with fuel?

***edit*** does it smell like its running rich on one cylinder?
 
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every time i try they just come out blury lol. i'll try one more time. the plug on the left isnt the side im having trouble with its the right side carb. be back in a minute going to try the picture again.
 
Ok here you go rusty. the one on the top is from the right side which is the side im having trouble with. the one on the bottom is from the left side.
Picture532.jpg


Picture533.jpg
 
What is/are all the speckles around the circumference of the plug? oil, flakes, ect...

did/does the plug look/smell like fuel? (it appears to have coloration washed off by fuel)

is it running rich on the right hand cylinder?
 
its crap i got on it when i sat it on the ground. it has fuel on it when i took it out. but even if i turn the screw all the way in it doesnt change things. i even sprayed some carb cleaner through the hole and there doesnt appear to be any kind of blockage. i just cant figure out why when i turn the screw on the left hand side it changes the way the motor acts but doesnt do the same on the right side.
 
I'm also dealing with issues with my BS carbs (great name, I'm learning) on my '82 450T. Although, when I adjust my air screws all the way in on either side, the bike does run rough and want to die. So you def. have something else going on. Have you checked for any vacuum leaks?
 
Looks like the right cylinder is running rich so it doesn't need anything more from the pilot circuit. Maybe the float height is too high? Not sure.
 
My initial guess is that it may be a float (and/or) a float level issue and fuel has washed the coloration off the plug. it's either running rich, or the plug isn't firing because of no spark getting to the plug, or a bad plug.

can you check the float level(s) by making a fitting to go into the drain screw hole and check the fuel level(s) externally with a clear tube?
 
ok if this happens to be my problem just where did you find the new needle jets at?
 
rustybronco: i have an extra drain plug and have wanted to do that as well (make a measurement gauge). Can you recommend how to do that?

Thanks,
Matt
 
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