BS38 Carb tuning

[tmkenney]

Hey all, I've been building a 79 xs chopper for the last few months, and finally took it for a ride around the block tonight. I noticed that under acceleration more than about 5% throttle, it will sputter and slightly backfire. Not one big bang, just aggressive sputtering, makes me think its too lean. The bike will idle great and free rev while in neutral no problem. No hangups or lag. I will also say that putting the choke on will cause the bike to die within a few seconds once it's running. In trying to avoid pulling the carb bowls off 100 times again, what do you guys think? Currently has 27.5 pilots, and 142 mains. Screws are turned out 2 turns. I had 138 mains in, and after increasing to 142 I noticed no real difference, so I'm thinking its too lean on the pilot jets. Exhaust is straight pipes, and the intake is velocity stacks with mesh screens over them. I also made sure that the advance unit was free and not sticky, I removed and lubricated the assembly, as well as cleaned off the points.

 

[zrx1100]

@5twins is your friend. 142 main are too big assuming you have stock airbox and exhausts. but the main is not an issue at low throttle openings. what are your air intake and exhaust?

 

[tmkenney]

@5twins is your friend. 142 main are too big assuming you have stock airbox and exhausts. but the main is not an issue at low throttle openings. what are your air intake and exhaust?

Sorry I typed everything in one big paragraph so its hard to pick out, but exhaust is straight pipe 1.5in tubes, ending right around the rear wheel. The intake is just velocity stacks and mesh screens, so more or less completely open, no airbox or filters. It would make sense if I need a ton of extra jetting, but I can always drop back down to 138 mains if needed

 

[zrx1100]

usually a few seconds of choke is too much even for a properly tuned bike. since you don't have any air filtraition and straight pipes, you likely need larger jets. how do the plugs look?

 

[tmkenney]

usually a few seconds of choke is too much even for a properly tuned bike. since you don't have any air filtraition and straight pipes, you likely need larger jets. how do the plugs look?

Plugs looked pretty black, evenly on both sides. I just upped the pilot jets from 27.5 to 30 on both sides since its such a free flowing setup, and the air screws are around 2 turns out, they were 3 to 4 turns out before.
The bike starts great with no choke, seems to lightly cruise fine, and has plenty of power when you go past 70% throttle and is great there, its just that mid area that sputters and acts like its starved.
I will say I have not warmed the bike up at all for these tests, and maybe it would be a good idea to do so. If it gets better when warm, I imagine its just a bit lean. Currently have no brake light or plate on it so a bit tricky to put miles on lol.

 

[Its been a long time]

Black plugs is rich. But it's impossible to tell mixture by reading plugs until the bike is fully warmed up and run for a while off choke.

 

[tmkenney]

Black plugs is rich. But it's impossible to tell mixture by reading plugs until the bike is fully warmed up and run for a while off choke.

Exactly, I believe it runs good enough currently to warm it up for a good few miles then see how it reacts, the idle might just be rich and thats what im seeing, and maybe mid range is actually lean. For no filters and straight pipes, I find it believable id need big jets.
Not only has this bike never moved under its own power until yesterday, I've also never ridden an xs650 before this. We will see how it performs after a few more miles

 

[Its been a long time]

Exactly, I believe it runs good enough currently to warm it up for a good few miles then see how it reacts, the idle might just be rich and thats what im seeing, and maybe mid range is actually lean. For no filters and straight pipes, I find it believable id need big jets.
Not only has this bike never moved under its own power until yesterday, I've also never ridden an xs650 before this. We will see how it performs after a few more miles

If the starting point is black plugs, it's safe to ride as you're not going to ride the motor to holed pistons through a lean condition. I went through this fitting ER5 Keihin carburettors to my 650D and had to carefully check out the jetting. Ride a bit further each time, ride a bit faster each time until you're confident where jetting needs changing. Always better to start off rich and lean it out. Be prepared to use several sets of spark plugs. Only change one thing at time.

 

[tmkenney]

If the starting point is black plugs, it's safe to ride as you're not going to ride the motor to holed pistons through a lean condition. I went through this fitting ER5 Keihin carburettors to my 650D and had to carefully check out the jetting. Ride a bit further each time, ride a bit faster each time until you're confident where jetting needs changing. Always better to start off rich and lean it out. Be prepared to use several sets of spark plugs. Only change one thing at time.

Got it, thanks! As of right now I also am cleaning spark plugs off with QD electronic cleaner and a brass brush, works awesome. I fouled them bad enough to misfire a month back due to stuck floats and it cleaned up very well.

Side note, will the air screws affect midrange at all? Like if I think its too lean, and I reach down and richen them up a turn that might give me a direction to go too?

 

[zrx1100]

what diameter are your headers?

 

[tmkenney]

what diameter are your headers?

My exhaust is 1.5in as shown

 

[zrx1100]

Did you measure the float height?

 

[tmkenney]

Did you measure the float height?

Yes! I forgot thr exact measurements as I did it weeks ago, but I set them to the standard height against the gasket surface with the carbs off. If it makes any difference, the bike feels like it makes good power when it clears up wide open, so I want to say the mains are good?

 

[zrx1100]

i would try a smaller pilot. For mid range, what is your needle position?

 

[zrx1100]

check out this section. some good reading https://www.xs650.com/threads/carbs-carburetors.43/

 

[tmkenney]

i would try a smaller pilot. For mid range, what is your needle position?

I just went from 27.5 (stock) to 30 due to huge airflow increases, would you really go below the stock pilot size? Not trying to be an ass or anything of course.

And my needle position is 1 down from the middle from what I recall, maybe i should bring that back up to middle and see if midrange improves?

EDIT: needle height is one UP from the middle currently. Would up or down be more rich?

And I did check out that tech section for CV carbs, from what I can tell at least my main jets are good since it pulls hard when given lots of throttle.

 

[Jim]

EDIT: needle height is one UP from the middle currently. Would up or down be more rich?

Raising the needle (lowering the clip) makes you richer.

 

[zrx1100]

no problem. I have a built up motor and opened up my intake and exhaust. ended up going down to 25 pilot from 27.5. My bike is a 78.
The plugs are telling you that you are rich somewhere. one remote possibility could be that the reflection from the end of your straight pipe hits you right off the throttle. if it is a positive pressure wave that hits when valves overlap, the reversing exhaust wave will travel all the way to the carb and then will be sucked back. in the process, the oxygen-depleted gas travels through venturi twice, picking up even more gas. again, this is probably not what you are experienceing, but this happens often. check out this video:

 

[tmkenney]

no problem. I have a built up motor and opened up my intake and exhaust. ended up going down to 25 pilot from 27.5. My bike is a 78.
The plugs are telling you that you are rich somewhere. one remote possibility could be that the reflection from the end of your straight pipe hits you right off the throttle. if it is a positive pressure wave that hits when valves overlap, the reversing exhaust wave will travel all the way to the carb and then will be sucked back. in the process, the oxygen-depleted gas travels through venturi twice, picking up even more gas. again, this is probably not what you are experienceing, but this happens often. check out this video:

Oh nice! I will check that video out when I get home, thanks!
Just an update: richnened the needle by 1 spot, and adjusted air screws to be 1 turn out. 142 mains and 30 pilot combo seems to run great for me now, I just took it for a rip up and down the road.

HOWEVER, I noticed a weird idle randomly, and sprayed brakeclean on the throttle shaft. Surprise, idle drops consistently every time its sprayed. Throttle shafts have an air leak. I adjusted things to where it runs good and consistent right now, but I need to replace those before I think about tuning any further. Any recommendations on where or what to get for the bs38s? Everywhere i can find is out of stock.

 

[Jim]

Any recommendations on where or what to get for the bs38s?

From the Tech menu...

The great o-ring, seal, gasket, nut, bolt, sizes used on the XS650 thread.​

The first link in that thread is for Suzuki seals, which are a direct replacement, in stock at $2.20 each. Much cheaper than the Yamaha ones. But they're the same damn Mikuni seals. Go figure...