BS38 Carb tuning

[tmkenney]

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...

View attachment 364624

You're amazing thank you! Just ordered a handful, should make the bike behave better. Will also be putting in some basic baffles

 

[grouchie77]

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:

I've been thinking of getting drag pipes for my Triumph...

 

[Jim]

You're amazing thank you! Just ordered a handful, should make the bike behave better. Will also be putting in some basic baffles

Look in Tech. There's good info in there on replacing those seals. Best to learn from our mistakes and not screw up the throttle/butterfly shaft.... which isn't hard to do.

 

[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.

Just wanted to update. I added some homemade lolipop baffles to the exhaust pipes, feels like it quieted down a tad and runs richer now, which was as expected. Got to go for a sketchy 3 mile ride the other day since it was 40⁰, ran great everywhere except low range, just sort of lazy and almost choked out, which was helped with almost bottomed out lean air screws. I downsized to the stock 27.5 pilot jets, and kept the 142 mains, and air screws are around 1 turn out each. Idles great and plugs look greyish at idle. Going to ride it again whenever it warms up, but should go well!