XS500 carburetor problem.

[Joee]

So I have a 1976 xs500 and I am having too much trouble with the carbs. The exhaust right now is 2 manifolds down to one exhaust pipe which isn’t stock for that bike. Also I have air pod filters not the original air box the stock bike has. So right now the bike only runs with the choke on and that’s only at 3,000 RPM idle.

I have bought new pilot jet sizes because I felt like that need to be changed with the exhaust being more restrictive. Stock pilot jet is 30 and I tried 32.5 and nothing changed. Should I go bigger or smaller with the pilot jet and I also am having the hardest time trying to get the AF screw to the proper setting. Not sure if the two screws are different as far as setting goes on the BS38s I have. I am getting to the point where I may take it to the shop cause I have no where to turn.

Any idea how many turns I need on the AF screws and which pilot jet size I should use and whatnot.

Keep in mind the carbs are all clean and im pretty sure there is no vacuum leak. The bike simply starts only with the choke on then after 2 minutes or so I will turn the choke off and the bike will stay running for only another 2-4 minutes then will die unless the idle speed screw is high enough where it can idle at 2,500 ish. Anything lower than that without the choke on it’s not running.

 

[jpdevol]

The symptoms you describe (choke, high idle) indicate leaness, so I'd probably continue to larger size pilot jets. That would also be consistent with the modified pod filters that often require 2-3 sizes larger pilots in our 650 BS38 carbs. The 2into1 exhaust is an aftermarket part and may actually be less restrictive than the OEM. The Fuel screws on most Mikuni carbs set in the 1.5 to 3 turn range (more = richer)

 

[Joee]

Just tested size 40 pilot nets out and seemed to be a little better. Not sure it’s it’s too cool where I am at right now but the temp is about 73 degrees and the bike only started again with the choke on. I let the bike idle like that for about only 30 seconds then turned off the choke and it idled fine. I moved the idle down to 1,500 RPM which not sure if that’s too high but that’s where I set the throttle stop screw.

As for the AF screws they were both set to 2 turns out. But when I went to drive the bike down the street it would bog for a couple seconds whenever I would accelerate. This resulted in the bike dying on me. Any fix for this. Not sure if I need to turn the screws in more or maybe even go up another pilot jet size.

 

[jpdevol]

Sounds like that was the right direction; 30 sec on choke seems about right. 1500 idle is in the ball-park. The stumble on take-off might indicate the jet needle is a bit off, the cure is a little trickier to determine: too lean or rich? It's a coin flip, but I'd try raising the needles one notch (lowering the clip position) to richen it and try that first.

 

[Joee]

So I moved the clips down one and kept the AF screws at 2 turns each and didn’t touch the throttle stop screw. At first start, choke was on and the engine seemed very Rich it wasn’t bogging it was sputtering. (Whatever the opposite of bogging is called, that’s what it was doing). Then I waited about 45 seconds to turn the choke off and the RPM didn’t seem to die a little when it transitioned. (I believe that’s a good thing!?)



So now I figured maybe I’ll turn the AF screws out a little bit more and I tried 2.5 turns out and that stopped the sputtering (from being too rich). Although it is back to bogging a little only when I open the throttle very quickly. When I turn the throttle slowly it doesn’t bog.



I then tried 3 turns out and it still was bogging like it normally does. Not only that but it still likes to only hang around 1,500 RPM for idle still. I went and tested the ability of the bike and loosened the throttle stop screw and it went down to 1,350 or something like it. And it just seemed like it didn’t wanna be there but had no choice so it kept going. So I lowered it to about 1,200 and it shut off.



On a side note maybe I’m going crazy but when I had the needle at the original position (3rd clip) it seemed to run smoooooth. Now that it’s at the 2nd clip it seems to run and sound like a damn Harley! The vibrations and sound since I changed the needle clip down one just seemed harsh, rough, and violent like my bike was trying to be a badass. And I don’t want it to be that lol. Any diagnosis for this problem? Your guess is as good as mine!

 

[arcticXS]

What carbs? Stock CV type, or VMs? The idle mixture screw works opposite. CV screws normally leans out the mixture when screwing IN. VM carbs are opposite.

 

[Joee]

They’re Mikuni BS38 carbs so I think that means CV but not 100% sure. I also moved the needle pin back to the middle slot (where I had it before, you can see the symptoms of this 3rd position a couple reply’s up in this thread if your curious.) and I drove it down the street. It did seem to bog a lot at takeoff and then I eventually got up to about 45 mph with 4th gear or whatever it was and decided to slow down to a stop sign. When I put the clutch in and switched it to first gear while rolling to a stop the engine just slowly died on me. It seemed as if the engine was tired from getting me up to 45 mph and just randomly decided to die and leave me stranded. Any idea what this cause might be from?

 

[arcticXS]

It would definitely be helpful if you could take some pictures of your carbs and the "pod" filters. A picture of one of the filters off the carb, showing the inside of the neck that clamps onto the carbs will also give important info

 

[Joee]

Here are some pictures of the carbs and pod filters.

 

[arcticXS]

Those filters are very poor, and I doubt the bike will ever run right using them. That step is messing up the airflow in a major way, especially to the oval port at 12 o clock.
They are also very small, and not even genuine K&N, but some cheap knockoff. Pretty sure they cost you at least 5 hp compared to stock.
If you don't have the stock air box, filter and connectors, you should get UNI foam filters, like "2 many" uses on his bikes. I cannot remember the UNI part number right now.

 

[arcticXS]

There is a recommendation for a UNI filter in this thread:

https://www.xs650.com/threads/gas-leaking.64215/#post-795602

Just double check the dimensions on the UNI website against the available space on your 500.
I am pretty confident that 95% of your carb issues are caused by those crappy filters!

 

[5twins]

Yes, those air filters are junk .....

On the 650, we use the UNI UP-4200 but as Artic mentioned, you will need to check the available space on your 500 to see if this 4" long filter will fit in there .....

 

[Joee]

I will make sure they fit and then order them. In the meantime I’m just trying to understand this better. Kind of wanting the carburetor knowledge as well. If I understand what I’m looking at and what I’m changing it makes it easier to work and more fun to work on the bike.
So you’re saying that air filters would solve a problem of bogging. And doesn’t bogging mean it’s too lean meaning too much air ? Maybe I have all this backwards but ontop of the bogging at takeoff and just when I open the throttle quickly, the air filter could also be the problem for my bike randomly dieing on me after getting up to speed then letting go of the throttle? I was going 45 mph and it seemed fine, I don’t think there’s a problem with the main jet. But when I went to coast to a stop and let go of the throttle she went down to idle and then proceeded to go lower than idle and down to a complete engine shutoff.
Like I said earlier I just wanna gain the knowledge of carbs off you guys so I’m trying to know why an air filter would/could be the solution to all this rough running. Anything helps!

 

[arcticXS]

When you look at the carb bell from the filter side, you will see some holes or ports. One big oval at 12 o clock, and a smaller at 8 o clock. And possibly one more not showing in the picture ( not sure)
An uninterrupted, turbulence free supply to these ports is absolutely vital to the function of your carbs. The filters you have may either block these completely, or cause severe turbulence. No amount of jetting changes can compensate for this. Carburettors are finely tuned devices, and those shitty filters effectively ruin their function.
I would get stock filter setup, or UNI foam filters, and revert to totally stock jetting, and start from there.
For more comprehensive info, use the search function and type "BS38".

 

[Joee]

I understand now. I didn’t realize that the air needed to travel into those ports without having turbulence. That would make complete sense. I measured the area where the filters would go and the UNI UP-4200 ones would fit so I went and bought them and they should get here Friday. So crossing my fingers those along with me going back to the stock jets will bring my bike back from the dead.

Also not sure you read this complete thread yet or not but in the beginning I mentioned I have an aftermarket exhaust. The person before me must’ve bought a whole new system. The original for the xs500 (not sure if it’s the same for the 650) is separate exhaust manifolds going to their own separate exhaust pipe. The exhaust I have has two manifolds going into one single exhaust pipe on the right side of the bike. Curious if that would cause for a bigger pilot jet size even with the new air filters because I would love to go to the normal stock jets but I feel like that exhaust would cause for some change. AT LEAST. Maybe not tho?

 

[5twins]

Sounds like you have an aftermarket 2 into 1 exhaust. They can be difficult to tune for. Most are not made for best performance but rather just to fit on the bike they're made for. That means headpipe length and collector diameter are not usually ideal.

The long oval slot at the top of your carb's intake bell is for the slide and used to create the vacuum needed to lift it. It needs a smooth flow of air which your current pleated type filters don't deliver. Your filters are also physically too small and tapered. These CV carbs don't like a tapered filter either. The two small round holes down at 4 and 8 o'clock are the air jets. The one on the left labeled with the "M" is for the main circuit (main and needle jet). The one on the right with no label is for the idle circuit. The large rubber "stop" ring on your filters blocks these. Some guys notch out the rubber ring to expose these jets again but that still doesn't address the issue of the turbulent air flow these pleated style filters give. Take the filters off and take a short test run to see if it runs better. If there's no change, you may have other issues, like maybe the float levels being off. If they're too low, that could cause the stalling you're experiencing, maybe the bogging down when taking off too.

 

[arcticXS]

I still think the filters are the most obvious culprit. Once they are sorted, I suggest you check that the cam timing is correct, and ignition timing is correct. And verify you have decent compression. Then you can check carb float levels and fuel delivery as well, since sufficient fuel delivery is also important. Fuel delivery is best done by removing both plugs from bottom of float chambers, and making a small "gutter" to collect the fuel into a measuring cup. The absolute minimum fuel flow for a 50 hp fourstroke is 250 cc (0.25 L) per minute, but ideally it should be 400 cc. Debris in tank or petcock, or a blocked fuel cap vent can cause insufficient fuel flow.
Once all the items mentioned above are checked and found ok, you can proceed with jetting.

 

[arcticXS]

You may also have ANOTHER FILTER ISSUE! Those inline fuel filters may well restrict fuel flow enough to cause problems.

This thread is a good example of aftermarket items that can cause a lot of problems, that no amount of adjustments and tuning can fix....Now, I do like modifying motorcycles myself, so I am just as guilty as the next guy. But I am lucky to have a varied technical education and experience, so I mostly avoid big problems. And many of the most active forum members also have strong technical background. For those who haven't, it may be best to leave things stock, or at least do thorough research about what to buy, and what to stay away from.
Just my personal opinion though......

 

[arcticXS]

The 2-1 exhaust system may require some minor jetting changes from totally stock, but as a rule of thumb, intake side mods require larger jetting changes. 5Twins and others with experience in replacing stock air filter setup with UNI filters on the 650 will have an idea about what changes normally are required.

 

[5twins]

Yes, I missed those fuel filters. I never add them because of all the issues I've read about with some of them. An occasional carb cleaning is much easier to deal with, lol.