bs38 questions

RacerRex

RacerRex

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Since I'm a rookie to rebuilding carbs I'm in need of some guidance.
I've been reading some of the threads on here and found some great info but I can't figure out a few things to get it running correctly.

I have a 1973 650 with bs38 carbs and points. I've cleaned the carbs, changed diaphragms, adjusted chain, adjusted valve lash, adjusted the points and also started to mess with the timing. I did the banshee swap so I don't know exactly where the range to set the timing is at. At the moment it idles ok but I can tell it's a bit off. When you give it gas, the left carb pops (sometimes shoots out the boot).

I've read how to check if a diaphragm is bad you should put your finger over the air passage way at the mouth of the carb and lift up on the slide. It should then come down slowly. I tried this to my set and figured out the left side (choke control side) comes down quick and the right is slow. I figured, the newly replaced diaphragm was not seated correctly so for shits and giggles I swapped the diaphragms and came up with the same problem. The left still drops fast and the right is slow. I would have figured the right would drop fast now and the left slow.....:banghead: wtf? am I missing something?

I think I need to rebuild/ clean the carbs better. Also I haven't synced the carbs yet. What's the best way to do it on the bs38s since there isn't a vacuum nipple on the boots?

If this question has been answered before please point me in the correct thread. In my search I may have missed the answer.

Thanks!
 
The dead cylinder routine is for setting your mix screws, syncing is different. Syncing your 2 cable carbs without gauges of some sort is a 2 step process. First you must match the idle speeds between the 2 carbs, then match when the butterfly plates begin opening by setting equal freeplay in each cable.

With the motor off, back both idle speed screws out until they're not touching the cable arm at all and you can see a little space between the two. Check to make sure there is still some freeplay in the cables. That will insure the butterfly plates have closed completely. Now turn the screws back in until they just touch and give them one more full turn in. This will open each butterfly plate the same amount. Start the bike and adjust idle speed to 1200 RPM by turning BOTH adjustment screws the same amount. This will maintain the matched plate openings. Now you can do the dead cylinder thing to get the mix screws dialed in. When done, re-set idle speed to 1200 if need be, again turning each screw the same amount.

With idle speeds set and synced, now you can sync the butterfly plate opening points. Set your desired amount of freeplay (usually about 3/16") in the left cable. Now set the right one to match. Lift the left cable sheath out of the holder on the carb just enough to show the freeplay, not enough to move the cable arm at all. Roll the throttle open a little to pull the cable sheath back down and take up the freeplay. While holding it there, check the right cable for play. I intentionally loosen the right cable way up. While holding the throttle open slightly to take up the left cable freeplay, I then adjust all the play out of the right one.
 
You make have a choke problem, as in the plunger is not sealing properly. That would cause even a good diaphragm/slide to drop fast.
 
5twins said:
You make have a choke problem, as in the plunger is not sealing properly. That would cause even a good diaphragm/slide to drop fast.
Click to expand...

Would this allow extra fuel into the cylinders causing a rich condition and possibly cylinder wash? Also, how can you fix the sealing problem?
 
The usual fix is to replace the choke plunger. It has a rubber tip on the bottom that seals the hole. They can harden up, get deformed, break apart, whatever, and then don't seal right. I would think that cylinder would run richer as a result. Not as rich as having the choke full on but richer than having a good plunger that seals properly. As far as washing the cylinder walls goes, I guess that would depend on just how much it's leaking.
 
Ahhh! Still cant get the timing correct. Its either spitting out the carb or out the pipes.
just have to tweak till its correct i guess...
Thanks, I'll check the choke.
 
Hey Rex, I've got a 73 w/ bs 38s that sounds similar to the problem you were having. Did you get a new plunger assy or "seal" from Mikes or have you replaced anything yet? How does it run now? The knowledge on this site is fantastic1!!
 
I didnt replace it yet, but I did order the plunger from mikes. I also ghettoed it to see if that was my problem and it was. She sounds much better now! Still needs some tweaking.
 
If you go to Hugh's Handbuilt web site and look under his installation instructions, He tells how to install a sticker that has timing marks on it. As long as you run points you will need this. Even with an aftermarket ignition it will help.
I think he might sell the stickers seperate from the kits.
There is also a way to mark the timing marks you need with out Hugh's sticker, search through all the Banshee swap threads. It's in there.
Leo
 

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