‘76 xs500 Carburetor tuning (BS38’s)


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Hey I’m new here and crossing my fingers you guys’ wisdom on this topic can get my bike finally running!! So basically I haven’t been able to work on my bike that much lately since it’s been too humid outside. With that being said I worked on it a little yesterday and finally got to balance the carbs. My neighbor happened to have a carb balancing tool and he helped me work on doing that. We got the bike running and found that the one carb was about 2 psi (or whatever unit it is) off from being balanced. (Didn’t know 2 psi could cause that much of a difference). The carbs were then balanced and then we looked at the spark plugs to see how they looked and they were very black.

Since they’re fouled we knew the mixture was too rich. So we adjusted the air/fuel screw a little to see where we are at. So to lay some groundwork down the bike seemed to run very well and it just sounded like it should. The throttle response was very good (when opening the throttle at a normal rate) only thing was, when we opened the throttle quickly it would bog a little. A day later I tried to ride the bike around and it started up fine and felt as I said. After about 5-10 minutes of idling it was bogging HORRIBLY. I changed nothing from the previous day but it just maybe got too hot and that’s the reason for this. I DONT KNOW. Do these bikes like to die after warming up for 7 minutes or am I just frustrated?

My question is, since the bike runs well at a normal throttle turn (on a good day lol) what would be the fix for getting it to respond just as good when I open the throttle quickly? My neighbor and I were thinking of changing the jet sizes a little bit but we’re not sure. The reason we think this is because of the bogging of course but also because my bike has a 2 into 1 exhaust which is aftermarket. The exhaust is supposed to be 2 into 2. On top of that I have the uni red foam air filters which were recommended by some big xs650 guys.

My neighbor kept saying those air filters are letting in a lot of air (implying it’s not restricted) so we should make the jets bigger. Finally we also thought maybe the jets should be bigger because the exhaust is a little more restrictive than the stock one. Does this exhaust/air issue call for bigger jets or no?
Do these bikes like to die after warming up for 7 minutes or am I just frustrated?
No to dying and Yes to frustrated (that is pretty much par for the course when working on carb mixture).

Before you go to buying a bunch of stuff, you should gather some information first. A bike can bog when both rich or lean. It sounds from you plug color and descriptions that your bike is running rich. I gather that you have not had the carbs apart and looked at what size jets you have and the position of the jet needle clip. That info would be good to know as well as how many turns out your pilot mix screws on each carb are. Don't assume everything in the carbs is stock or setup properly, check it yourself before you get in too deep.

Just an aside - as the bike heats up after starting it, it will start running richer. That is why most bikes need application of the choke or enrichment circuit to start. Those are reduced and finally turned off once the bike is up to temperature and the fuel mixture gets richer.
Thank you for the info! I opened the carbs up last summer and cleaned them all out and if I remember correctly the clip for the needle is in the middle (I could be wrong though). And the jet sizes that are in there now are the stock ones which are 122.5 for main and 30 for pilot.

With that being said I typically don’t need the choke on to start the bike when it’s warm out like it has been. The scenario I was talking about I started it without choke and it stayed running like that. And what would happen was it would run for about 10 minutes just idling then die on me. Then I would sit there and “let it cool” so to speak for about 3 minutes and start it very easily and it would run normal again with that good throttle response I was talking about but not good response when opening it up to quickly. After the bike kept idling for another like 5 minutes it would do the same thing and die randomly. It’s really weird and I’ve never seen anything more bipolar lol.
Can we assume it ran fine since last summer and this is just a recent issue with no changes done? Have you checked valve lash adjustment? Maybe when it heats up the lash gets too tight and drops compression?
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Can we assume it ran fine since last summer and this is just a recent issue with no changes done? Have you checked valve lash adjustment? Maybe when it heats up the lash gets too tight and drops compression?
Last summer it still didn’t run fine. It’s been this way for a while now and I have a strong feeling it’s something to do with the carbs. Like I said I just now synced them and it definitely seems better. But it’s bogging like I described. As far as the valve lash adjusting goes I have never done that and to be honest don’t really know how. The last owner would have driven/tuned this bike back in 2003 ish so it hasn’t been messed with since then.

What I will say is about a week before balancing the carbs and messing with the air/fuel mixture screws it ran normal it seemed. For instance I was able to ride it around my town for about a good 30 minutes. Stopping at stop signs and getting up to speed during that time. After I was done riding it didn’t die on me. It never bogged that whole time and was completely fine. The next day I tried to do the same thing for about 30 minutes and it started to die/bog badly whenever I would stop and slow down to a stop sign. Then I would have to wait about 2 minutes for the throttle to respond normal again just to go drive to another stop sign and have the same problem.

Like I said earlier. This is the most frustrating/bipolar thing. I feel like it’s everything to do with the carbs but I just don’t know how to tune them correctly.

To add on to the valve lash adjustment topic (I don’t know if this will help answer) but back when I first picked up the bike I checked the point gap for the timing and there was nothing wrong with that. So I feel like if that didn’t change in the 20 years the bike sat then maybe the valve lash adjustment didn’t. I don’t know.

I also found out about a jet calculator website and I typed in my stock jets and chose the options for aftermarket exhaust and pod filters and the calculator gave me main jet size of 130 and pilot size of 33. Should I try that?