Bs38 Carb Re Jet Question.??

[xscessivecompulsive]

Hey Guys, so Im sure this has been asked before but what is a good place to start when re jetting the Bs38's to accept pod filters and free flowing pipes?? It currently has the stock 135 main, and 27.5 pilots, and when I re jet do I need to move the e-clips to?

Thanks in Advance,.....

XsC

 

[littlebill31]

Try one up on the pilots (27.5), 4 or 5 up on the mains (132.5 or 135), and drop the needles one step, so move the the clip up to lower the needle.

 

[oldbiker]

hi a few years ago i purchased a kit from mikes,,, for my 38,s but at the tiny cost of jets i also went 2 sizes up and dropped the clip ,which raised the needle ...and i,m running mikes oval pods .. ,,also i,m running 45mm st/st pipes home made with alloy tracker mufflers ,,,and it does have somewhat of a bark when i pull 4000 in a hurry ,, but i,m a little more conservitive now,,, but it still has a nice groul to it regards oldbiker

 

[5twins]

Sounds like you have '78-'79 carbs? You really need to identify the year of your carb set if you want proper re-jetting info. Bill's info looks to be for '76-'77 carbs sets. Those settings would be good for those. For the '78-'79 carb set, start with one up on the pilots (27.5 to 30), one to three up on the mains (137.5, 140, or 142.5), needle clips raised one step to slot #2 (lowering the needle), and mix screws about 2.5 turns out.

It's very easy to over-jet these CV carbs because of their forgiving nature. They work on engine demand and the slide only lifts as much as the engine can use. That's why the bike may seem to run OK with a main jet several sizes too large. The slide isn't lifting all the way and that large main isn't flowing at it's full potential.

To properly test the main jet size in these CV carbs, you need to use absolute wide open, full throttle (just twist the grip open as much as it will go). In 2nd or 3rd gear and starting at about 3K, run the bike up to near redline. Watch for glitches (stumbles, flat spots, break-up) and note what RPM they occur at. This will tell you what needs tweaking (needle, main, or maybe both). Full details on this test and also one for the idle/lower midrange are in the carb guide. In fact, pretty much everything I just told you is in there, lol.

 

[Sundie]

or, as i always suggest, don't rejet. Unless you're going to be tracking. They'll deal with it fine.

 

[Tech7]

On my 79, I only raised the needle one notch, stock main jet and stock pilot. Pods and 1-3/4 pipes with small baffles. Plugs look great, runs sweet.

 

[xscessivecompulsive]

Hey guys, thanks alot for the help, its very apprciated! I purchased 140 mains and 30 pilots today and gonna put em in tomm! I also ordered 142.5 and 145 main jets from 650 Central and they should be here on Sat. so I will have a variety if need be. So this will be my first attempt at re jetting and tearing into the carbs! Are the needles easy to get to, and do you access them from the tops of the carbs??

Thanks guys,

XsC

 

[5twins]

Needles are in the slides which are accessed through the tops of the carbs. This can be done with the carbs in place, just remove the gas tank and maybe the breather hoses. Remove the carb tops and the slide/diaphragm assemblies can be withdrawn. Jets can also be swapped with the carbs in place, just drop the bowls for the pilots and change the mains through the drain plug (bowls still in place).

If your carbs are a '78-'79 set, the needle will be retained in the slide with a snap ring. You'll need some sort of snap ring pliers or needle nose with small enough tips.

 

[xscessivecompulsive]

Thanks 5twins, you the man!! Really appreciate the help guys!! I'll share my results when finished!!

XsC

 

[XSLeo]

www.amckayltd.com/carbguide.pdf

 

[alfredo]

On my 79, I only raised the needle one notch, stock main jet and stock pilot. Pods and 1-3/4 pipes with small baffles. Plugs look great, runs sweet.

How can I raise the needle on my 78 bs38's? Is it accessed from the top or bottom of the carbs? Can it be done with the carbs still on the bike? Complete carb idiot here looking to change my ways . Have been searching for a visual guide on this but haven't found one yet.

 

[azman857]

Carb parts diagram for a visual guide. Read the carb guide XSLeo points out.

 

[5twins]

Read post #8 but if you have larger than stock mains in there, raising the needle is probably not what you need or want to do.

 

[alfredo]

Read post #8 but if you have larger than stock mains in there, raising the needle is probably not what you need or want to do.

Just got them apart and the slide needle says 502. The bike tends to die out when I go to a stop light every once in a while. Also, can't do better than 65 without it sounding like it is pushing way too hard.

Thinking about moving the clip up one, but its such a small distance that I am not sure if it will help. The ignorance in me wants to move it all the way to the top clip position , but I will settle for the #2 spot for testing

 

[5twins]

I don't think you're understanding what you're reading here. There's raising the needle and raising the needle clip. The 2 do totally opposite things. Raising the needle is achieved by lowering the clip and makes things richer. Raising the clip lowers the needle and makes things leaner. You don't want to raise the clip to slot #2 unless you're running larger than stock mains. It will make the midrange leaner which is often needed if you install larger mains.

It sounds like you're running lean. First I would check the float settings (may be too low) then if they're OK, I would go up one on the pilots and one or 2 on the mains, and then, yes, you will want to lower the needle (raise the clip).

 

[alfredo]

I don't think you're understanding what you're reading here. There's raising the needle and raising the needle clip. The 2 do totally opposite things. Raising the needle is achieved by lowering the clip and makes things richer. Raising the clip lowers the needle and makes things leaner. You don't want to raise the clip to slot #2 unless you're running larger than stock mains. It will make the midrange leaner which is often needed if you install larger mains.

It sounds like you're running lean. First I would check the float settings (may be too low) then if they're OK, I would go up one on the pilots and one or 2 on the mains, and then, yes, you will want to lower the needle (raise the clip).

Gotcha now. Was confused a little. I spoke with another member and they suggested that since my throttle wasn't even halfway pulled when I am at max speed, he seemed to think the needle clip needed to be moved as the carbs were running lean. I am going to move it to position #4 in the picture below, but it seems like such a minuet difference. I am tempted to move it to #5. Am going to try the needle clip positions before re-jetting. Sound crazy?

 

[5twins]

It's just not the normal procedure for re-jetting carbs. It may help but it's really only a band-aid fix. If you never use over half throttle and never run the bike up to near redline, you may be OK. Riding it that way never puts full demand on the mains. Run it up to redline using full throttle and it will probably struggle above 5K. The reason Tech7 thinks his runs sweet is probably because he doesn't run it hard. I'm sure it would fall flat on it's face at about 6K under full throttle. You need bigger mains to get the full performance potential above 5K. Not radically bigger, usually just one to about 3 sizes, but you do need them.

 

[alfredo]

It's just not the normal procedure for re-jetting carbs. It may help but it's really only a band-aid fix. If you never use over half throttle and never run the bike up to near redline, you may be OK. Riding it that way never puts full demand on the mains. Run it up to redline using full throttle and it will probably struggle above 5K. The reason Tech7 thinks his runs sweet is probably because he doesn't run it hard. I'm sure it would fall flat on it's face at about 6K under full throttle. You need bigger mains to get the full performance potential above 5K. Not radically bigger, usually just one to about 3 sizes, but you do need them.

I have no intentions of doing anything higher than 70-75mph max. Could get it to 65 the way it currently is, but it was trying HARD. Might give the needle clip a shot to last me out the summer, then do a rebuild on the carbs later on.

 

[Grinder]

forgive the stupid question...just trying to learn. In an earlier post it was mentioned that when you put larger mains in you need to raise the clip(lower the needle). Doesn't that defeat the purpose of the larger main with the needle going further into the jet? Wouldn't just lowering the clip (raising the needle) do the same thing with a stock jet?

 

[5twins]

No, not exactly. Your carbs have 3 circuits. They overlap a bit and influence part of the one or ones next to them .....

A larger main gives you the needed extra fuel at high RPMs that mods may require but at the same time, it bleeds over into the upper midrange and can make that area too rich. That's why leaning the needle is sometimes called for when installing larger mains. The leaner needle setting cleans up your midrange and makes it right again but you still get the benefit of more fuel in the high RPMs.

Raising the needle only richens the midrange and a bit of the lower main (and upper idle) circuit. You're not getting as much extra fuel into the main circuit as a main jet size change would give you. As I said, you won't get the full performance potential out of the bike by trying to "jet" it with just a needle clip position change.

And when you're jetting carbs, you don't really go by or even consider speed that much. You look at RPMs and throttle opening or position. Those things give a more accurate indication of what carb circuit you're running in and what jetting components need changing.