BS38 Question?

Kevin Werner

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So many denizens have offered advice and knowledge. I am grateful beyond words. I have looked at the Tech section and did not find my answer there. An air mixture screw is referenced but pics show it at the carb bellmouth, What is this screw/adjustment (screw driver shaft in screw)?My BS38s have nothing at the bellmouth. I did scan the Mikuni BS38 site and I did not see an answer.
 

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There is no air screw in the intake bell of any Mikuni carb. Air jets are fixed in XS650 BS38 carbs. Aftermarket carbs and some OE carburetors have jets that can be changed for tuning purposes. Pilot air jets enable fine tuning of the pilot circuit mixture. BS 34s on late XS650s have changeable PAJs. Main air jets control air flow to the needle jet; an MAJ that's too small will result in poor emulsion, one that's too large will result in the mixture leaning out at high rpm. Our motors don't turn that fast in stock or streetable form, which is why the MAJ that comes with VM carbs is usually removed for our application. The pilot fuel screw in BS series carbs does not add air to the mixture; it controls vacuum over the pilot jet. Turning the screw out admits more vacuum=more fuel=richer mixture. Vacuum control type mixture screws are located on the engine side of the slide tower (there may be an exception to this somewhere, but I don't know of one). Screws that add air to the pilot circuit mix are located on the intake side of the slide tower. Again, there may be an exception somewhere, but I haven't seen it. Turning an air screw out leans the mixture.
 
Got some potential confusion re. the pic. The big steel slotted screw covers the synch port in 76-79 carbs. The little brass slotted screw at the end of the screwdriver is the mixture screw.
 
My mixture screws were at 2 1/2 turns. Yesterday I ran the bike at 2 turns. It seems idle and part throttle was better. The chart 5Twins (?) posted indicates my year/configuration should be 1 1/2, +/- 1/2. Maybe I am working backwards 🤔. I have played mainjet and needle height to get to a pretty civilized hard running motor.
Thanks all.
 
My mixture screws were at 2 1/2 turns. Yesterday I ran the bike at 2 turns. It seems...
Mix screws are for fine tuning. It's not a matter of setting to X amount of turns and going for a test ride. They have to be tuned by ear and feel.
Whatever the book calls for in turns is just a starting point... it's not set in stone.
The idea is to set the screws at the book value, warm the bike up and then adjust them "individually" for best idle. Some use the "dead cylinder" method, some don't. Do a search here on dead cylinder tuning.
With the bike warm and at idle, start turning the screw out. If idle drops or it starts running rough, go the other way. continue turning until rpm increases and then starts to drop again..... keep going back and forth... play with it. This will give you a better feel and "ear" for what makes a carb happy. Turning out too far and idle will drop and/or get rough... turning in too far will do the same. Once it's where the engine sounds the best, blip the throttle several times.... you're also looking for best throttle response "off idle." Me personally, I like to find highest/smoothest idle, then continue opening the screw up slightly until the revs "just start" to drop. That usually give me the smoothest idle with the crispest throttle response. I should add, once you're close, just turn a little at a time... like an 1/8th of a turn or so.
Also... keep blipping the throttle during this procedure. That helps prevent the plugs from loading up and give you a better idea of where idle will fall to on decel.
As far as the number of turns, I don't really get concerned unless I get way outside what the book calls for. For example, if it say a max of 3 turns and I get my best idle at 4, I'm OK with that. Anything past 4 though would give me reason to think about re-jetting the pilot circuit... or not, depending on my mood.
One last thing, this is an individual carb thing. If one's happy at 2 turns and the other at 3, so be it. These screws are for "fine tuning" after all... and no two carbs are the same.

Idle mix tuning is fast becoming a lost art. It's something our dads (or mentors) taught all of us old gearheads at a young age, as it was a necessary skill back in the day before fuel injection. Might take a while to get the hang of it, but it's very rewarding to be able to listen to and adjust an engine and know when it's happy.
 
As Jim says, adjustment from carb to carb can vary. The mixture screws are steel and the carb bodies are an aluminum alloy. Not 100% sure if this is the case with the BS38, but if someone overtightens the mixture screw on other carb models, it’s could potentially damage the needle seat and will throw standard settings off. When adjusting, lightly turn the mixture screw in until it stops then back it out the number of turns for the starting point of adjustment.
 
With your older carb set, you should be able to set the mix screws by ear. You're seeking the point where the cylinder you're adjusting idles fastest and smoothest, what we call the "sweet spot". It will be easier to hear the effects of your adjustments if the idle speed is lower, so turn it down to maybe 1000 to 1100 RPMs. Get the bike nice and warmed up so it will idle OK at this lower speed setting. Do one cylinder at a time. Kneel next to it and start adjusting it's mix screw 1/4 turn at a time. Pick a direction, in or out, and note what it's set at when you start. Within 1/2 to 1 turn from your initial setting, the idle speed should start falling off and the cylinder should start running rough or stumbling. Note how many turns that happens at, set the screw back at your starting point, and "work" it in the other direction until the same thing happens. You should find a small range of adjustment, maybe 1/2 turn or so, where that cylinder runs fastest and smoothest. I set my mix screws at the rich end of this range (more turns out). I find this lessens popping on decel.

Usually, if you increase the pilot jet size, your "turns out" setting gets a little smaller, not larger like you found your screws set. And certainly not a full turn above the recommended spec. Have you checked your pilot jet size and has it been increased? If not and if you find your mix screw setting is best at 2.5 or more turns out, this would indicate to me that you need a larger pilot jet. When jetting these 650 carbs for mods, it's common to need to change the mains AND pilots.
 
So many denizens have offered advice and knowledge. I am grateful beyond words.

This thread is nearly as good as being given a one-one tutorial. Last year set Miss November's carbs by following the advice given here. Idle tuning de-mystified, settle down to the job, learn to listen and feel, get the bike running better, immense feeling of satisfaction.
 
Yep, lots of ways to get 'er done. A few symptoms to watch for in pilot mix tuning: In BS38s, if you're seeing the idle still rising with the mix screw backed out 3 turns (3-1/2 turns in BS34s), don't back it out more. Past that point the screw won't meter well and the spring will be too loose to hold the screw in place. Install a PJ 1 step richer, and start over. If idle speed rises by more than ~300 rpm from first steady idle to full warm, the cause could be slightly lean idle mix; try opening the mix screws 1/8 turn (if there's mechanical advance in place, that could also be responsible). Exhaust popping under engine braking is another lean pilot circuit symptom.
 
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