Theory and Operation? BS38 Carbs.

I'm looking for a good source of information and possibly a troubleshooting guide for bs38 carbs as well as just how to tune the carbs.

It appears as though I'm running a bit lean and I'm not entirely sure how to adjust the carbs. I'm running pods. And I have bought new manifolds from MikesXS.

I don't believe the carbs have been maintained much by the PO. Is there anything I should check or measure or just replace? The bike runs pretty great as of now but I'd like to just fine-tune it.

See link in post #1
http://www.xs650.com/threads/carbs-carburetors.43/

By going off the plugs, would this be considered lean? Or is this optimal?

Tiesco said:

By going off the plugs, would this be considered lean? Or is this optimal?View attachment 163187

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Looks a bit lean, but perhaps within normal parameters.
Make sure that you've followed the proper procedure for checking them: Use the choke minimally upon startup, then ride robustly for 10 minutes, then shut off engine before you pull into your driveway.

You are looking for a milk chocolate brown on the post ceramic.

I suggest riding it for awhile and concentrate on other restoration priorities.

I did up the needles a notch(made it richer) before riding around a very short distance but it probably wasn't enough. Definitely wasn't 10 minutes. I'll give it another go around the block tomorrow.

Your plugs don't look too bad, not extremely lean, but maybe a touch on the lean side. The outer edge, the part right above the threads, will always be dark or black, that's pretty much normal. What you look at when "reading" plugs is the side strap and the porcelain center. Those dark splotches on the sides of the porcelain may be remnants from starting with the choke. That will blacken the plugs right up almost immediately and takes a good run of several miles and/or 10 to 15 minutes to burn them clean again. What you're really looking at or for when checking the porcelain is what's called the "mixture" or "smoke" ring. This will be a ring of color around it and ideally, you want it down near the bottom of the porcelain .....



I shoot for trying to get it so it comes 1/4 to 1/3 of the way up from the bottom. The rest of the porcelain, the upper part, should remain relatively clean, staying white or off-white, maybe a light shade of gray or brown eventually after lots of miles are accumulated. The needle setting has a big effect on the size of the smoke ring. Had you checked it both before and after making your needles a step richer, you probably would have seen it raise up 1/4 to 1/3 more.

Your side strap looks good, all light brown or gray. As you get richer, it will begin to get black starting at the outside and reaching farther and farther in as you get richer and richer. Here's my plugs after I got the bike dialed in pretty good. The side straps are lightly colored but relatively clean and the upper part of the porcelain remains mostly white .....

Although it looks quite good, running CV's and pods needs rejetting and decent filters with possible air correction jets needed as well, there is a sacrifice in running pods in that you make a loss at one end of the rev range, this subject has been done to death on many forums and where i had pods many years ago on bikes the running was never perfect, these days i always run stock air-boxes, after all the manufacturers spent much more time and money getting them to run at optimum than i will ever have.

Besides learning how they operate and how to adjust them, you also have to take into account the year of your carb set. There were several different sets used over the years with different jetting specs. Re-jetting for mods is based upon what came stock in the year of the carb set you're using, not the year of the bike they happen to be on. Also, some sets require more "tweaking" than others. Here's a chart with the stock specs that I made up. I noticed from looking at some of your other posts that your bike is a '77, so take note of those specs and settings .....



As I mentioned, some sets need more tweaking than others and your '77 set is one of them. Notice that it's stock 122.5 main is the smallest used in any of the carb sets. While it's fine for a completely stock bike, it is easily and quickly out-paced when you start doing mods. From pics of your bike, I see you have aftermarket mufflers and they're probably a bit better flowing than the stock ones were. You also installed UNI pods but the model you chose, the short filter with the capped end, really doesn't flow much, if any more than stock. I know this because we put those same filters on a local guy's bike and didn't have to change the jetting at all. But, your freer flowing mufflers may require a jetting change. It shouldn't be much, maybe one or two up on the mains (to a 125 or 127.5). For the time being, I would leave the pilot jets stock and the needles set in their stock (#3) position.

My reason for switching to pods was because I was having trouble finding the rubber for the air box, otherwise I would keep it on. I prefer what the manufacturer has given me

I'd like it to run how it is supposed to run. Not how I want it to run. If you catch my drift.

I will try bigger mains and see how it works out and report back.

Where do you guys like to get your main jets and other various carb parts from if not a local dealer?

Tiesco said:

Where do you guys like to get your main jets and other various carb parts from if not a local dealer?

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niche cycle is good.

Tiesco said:

Where do you guys like to get your main jets and other various carb parts from if not a local dealer?

Click to expand...

....or make a "Wanted" request for a part in the Classified section.
One of us might have what you want and could toss it into an envelope for you.

If you need some other parts, like a tire for instance, enough to build your order to or above $89 and qualify for free shipping, then Dennis Kirk has a good deal on genuine Mikuni jets, less than $6 a pair .....

https://www.denniskirk.com/mikuni/1...-and-power-jet-carbs.p1763100.prd/1763125.sku

You're the man, 5twins.

The main jets are on the way. In the mean time I rode around a bit more with the needles raised a notch and it didn't seem to make much of a difference. If anything it ran worse. By worse I mean it was slower and popped every now and then when engine braking. I think I'm just going to lower the needles back to the original height and wait for the mains.

I'm not sure your carb set would stand for raising the needles a step because of the very large (rich) needle jets they come with. In fact, once you install the larger mains you may even need to lower (lean) the needles a step to clean up the midrange. What you have to understand about carbs is the three circuits in them overlap slightly. Changing one "bleeds over" and effects the one next to it slightly .....



Your new larger mains are also going to make the upper midrange richer. This may cause some stumbling under heavy throttle applications. The usual "fix" is to lean the needle setting a step (raise the clip, lower the needle). But, due to this circuit overlap, leaning the needle will also lean the upper idle circuit too. This can result in a hesitation or flat spot just off idle and will need a size up on the pilot jet to fix.

Very interesting information. That really helps to understand. I installed the 125 mains and lowered the needle back to the middle position and started it but I haven't taken it for a test ride. I will do so tomorrow.