BS38 Tuning Issue

So I decided to re-tune and sync the carbs on my 79’ today because, since I finished this build and initially tuned and synced the carbs, the left cylinder seems to run smooth and the right cylinder just seems to not be running as smooth. It runs, but just not with the same steady cadence as the left cylinder.

Before I initially tuned the carbs (only like 40 miles ago) I adjusted everything else (timing, cam chain, etc.) first like you’re supposed to. Now just so you know, this bike is running open headers and pod filters. So I’m running #130 pilot jets (one size up from stock #127.5) and a larger main. When I actually got to the carbs, I set the mix screws at the stock turn out, and then adjusted the by sound from there. I DID NOT use the carb guide method at that time......yet.

So with the right cylinder starting to annoy me, I decided to start over and use the carb guide method to see if it would enlighten me as to what may be going on.

So I do the the left cylinder/left carb first. Everything goes well and I’m able to find the sweet spot as I use the carb guide method which was probably at about a half turn in from the stock mix screw setting with the larger pilot jet. Then I move to the right cylinder/carb. I start to go through the carb guide method and I start to find that the more I lean/screw-in the mix screw, the faster my idle goes. So I keep adjusting the the throttle stop screw accordingly and then go back to the mix screw. To make a long story short, I literally was able to adjust the mix screw ALL THE WAY IN and still run a fast idle on only the right cylinder! I’m no genius, but at this time I’m thinking I may be onto whatever my issue is with the right cylinder.

So obviously the right was running like shit before because it was running rich when I had the mix screw out about 2.25 turns on my initial tuning job because something’s not right with the right side mix screw. So what could the issue be? I’m thinking either my right side mix screw needs to be replaced or, and hopefully this isn’t the case, the carb body is damaged around the mix screw somewhere. Has anyone else had this? Is there something else that could be causing this?

I've got a 79 also. Your issue sounds exactly like mine which I have not resolved yet. Left side idles nice and steady, right side likes to pop and stutter a bit. When I get on the throttle they both run fine.

I replaced the mix screw on both carbs. What I noticed as being odd is when I turn them both in till they seat, then unscrew them till they come out, the right side will come out in about 4 turns, the left side about 7 turns. I was thinking the carb body was screwed up, and it may be.

But now after reading your post maybe it came that way from the factory? Is yours the same way? The right side screw will come out in less turns then the left?

Mickster said:

What I noticed as being odd is when I turn them both in till they seat, then unscrew them till they come out, the right side will come out in about 4 turns, the left side about 7 turns. Is yours the same way? The right side screw will come out in less turns then the left?

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I'm not sure. When I get a chance to pull them I'll have to see. Even if that's the case, it still wouldn't explain why I can idle the right side the best with the mix screw set all the way in. It sounds like you may have a different problem entirely that may not even be related to your mix screws.

Well before I went Real deep I'd be pulling and checking everything about the choke circuit on that side. Note that we call it a choke but it is really an enricher circuit. See why it's suspect? Bottom of the plunger has a rubber seal that sits against a seat when the plunger is closed it typically has a visible groove where the seat touches, it should be an even smooth groove. Have the butterflies been synched?

A fast check of the choke circuit is the slide drop test. Raise slide cover top port on intake, If the choke isn't closed that slide will quickly drop.

gggGary said:

Have the butterflies been synched?

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Yeah I re-synced before I started messing with re-adjusting the mix screws. I suppose if there’s an issue with the choke on that side, that could also be causing running rich and rough on the right. I’ll also have a look at that for sure. Other than the choke plunger and mix screw, any other things within the carb body that could cause the symptoms I’m having?

Just to confirm, I also pulled the plugs after the bike cooled just now and the right plug was noticeably darker than the left. Not really dark or anything, but definitely running richer than the left.

gggGary said:

A fast check of the choke circuit is the slide drop test. Raise slide cover top port on intake, If the choke isn't closed that slide will quickly drop.

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I did do the slide drop test before I reinstalled the carbs last time. Leaving the holes unplugged let the slides drop freely and when I plugged the holes they both lowered slowly at what seemed like the same rate. So I think all is good there.

confirm float level with the clear tube method.

Mix screw.............Start to think of it as a Pilot screw, (correct name). It is a part of the pilot circuit that also has a pilot jet...........Thinking about one part and using the correct term helps the mind to associate the whole

How does fuel level in the float bowl affect richness in the pilot circuit? I understand some of the ways these carbs work but I’m unfamiliar with the relation of the float bowl level and how it affects the pilot circuit running rich.

not 100%, but it does, high fuel level tends to richen up everything. AFAIK The slide needle doesn't seat completely so fuel can up there.

I’ll have to check when I have a chance and measure the floats again. Everything was fine when I put them on last but who knows.

How low the float would have to go to affect the pilot circuit i dont know..........the fuel is drawn from the bowl by a venture system. air flowing through the pilot circuit sucks up the fuel and the pilot screw acts to stop or regulate the flow of fuel to the inlet............there are other holes on the pilot circuit before the pilot screw as well, they deposit fuel into the inlet on the carb side of the butterfly, this is the idle, so if the pilot jet is blocked it affects idle

Not the best pic for the 38's but the 34's give a good indication on how the circuit works
BS34.............................BS 38.......................................Pilot circuit holes and Pilot screw hole


Simplified version of a CV carb fuel delivery

gggGary said:

confirm float level with the clear tube method.

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I’m not sure I have a way to do it on the 38’s with what I have laying around. On the 34’s I can just slide tubing over the drain nipple.

650Skull said:

How low the float would have to go to affect the pilot circuit i dont know..........the fuel is drawn from the bowl by a venture system. air flowing through the pilot circuit sucks up the fuel and the pilot screw acts to stop or regulate the flow of fuel to the inlet............there are other holes on the pilot circuit before the pilot screw as well, they deposit fuel into the inlet on the carb side of the butterfly, this is the idle, so if the pilot jet is blocked it affects idle
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I’m assuming the float levels could potentially be too high causing a rich condition and too low could cause a lean condition?

i suspect the orings around the emulsion tubes need replaceing. if float levels are correct.

Could be, what about spark? poor spark can make a cylinder act rich. Were floats leak tested?
lotta carb info here. keep reading. Mailman’s XS2 a full on restoration

Assume???

79josh81 said:

I’m assuming the float levels could potentially be too high causing a rich condition and too low could cause a lean condition?

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The pilot circuit relies on a vacuum ...........as long as the circuit is in the fuel then it will draw the fuel into the idle and pilot circuit........don't think this will cause to much of a lean/rich running.................lean or rich circuit is more controlled through the mains and needle setting............Yes float height is critical for this........

At the end of the day it is a combination of getting all of the sums to match

angus67 said:

i suspect the orings around the emulsion tubes need replaceing. if float levels are correct.

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Are you talking about the needle jet o-rings?