BS34 jetting with modified exhaust?

Cooltouch

Okay, I guess it's my turn to ask you carb guru guys what kind of jetting you'd run for my application.

The bike is an '81 SH, stock except for a Mac 2-1 exhaust system, which I have modified.

I removed the baffle from the mufler, cut it leaving about a 2" stub at the outlet, then tack welded about an 8" piece of same diameter tubing to the stub, so I would have about a 10" piece of straight-through tubing. Then I drilled a couple hundred 5/32" holes in the tube and wrapped it with fiberglass I had left over from a previous glass repair. Stuffed the new "resonator" back into the muffler and screwed it in place.

The result is a muffler that is quite a bit louder, but now has a deep warm tone. I like it a lot. But I know that my jetting is probably way off. I'll bet the PO did not change the jetting when he installed the Mac 2-1 system. So I know it needs it, especially after this latest mod.

I'm figuring that, as long as I'm changing the jetting, I should go ahead and get a set of John's needles.

Question about jets -- where's the best place to buy them? How about the ones Mikes sells? If they work okay, I can add them to an order I'll be placing in the next day or two.

Best,

ichael

xsjohn

With my kit you can tune that thing any way you want....now and in the future.....and the instructions will cover what you are doing now....and what you might want to do later...I know I sound like a broken record but this particular thing is what I do......mikes jets are fine......xsjohn

nj1639

Hey CT,
I've got an '83 Heritage Special, bs34's with 2 into 1 pipes. Not sure if they're mac's. No change in baffles that I can tell. Stock jetting with Johns' needles. Runs fine, runs great actually, but i'm a novice. What do I know? Don't see a reason to change.

Cooltouch

Well, nj, I get quite a lot of back-cackle on trailing throttle -- more now than before, which I've always interpreted as a lean condition, so I think the carbs could use a bit of tweaking.

John, I plan on leaving the motor stock except for the exhaust mod. I will probably be selling this bike once I'm finished with my cafe project, so I'm not looking to put a bunch of money into it that I won't be able to get back out of it when it's time to sell.

Best,

ichael

xsjohn

Never mentioned changing the engine.......I dont recommend that anyway.....just making it run better.....

xsjohn

weekendrider

cooltouch, I was talking with a fellow that says, freer flowing intake leans the mixture and less restrictive exhaust richens it. This was given as a genaral rule of thumb. I would say you've probably opened the exhaust.
What do the plugs say?

Cooltouch

weekendrider wrote:

cooltouch, I was talking with a fellow that says, freer flowing intake leans the mixture and less restrictive exhaust richens it. This was given as a genaral rule of thumb. I would say you've probably opened the exhaust.
What do the plugs say?

Haven't pulled them since the mod I did. Usually they're almost white.

The general rule you mention seems counter intuitive to me regarding the exhaust, but then I know that often reality is counter-intuitive. I've just ordered a set of XSJohn's needles, and I'll be ordering the jets tonight or tomorrow. We'll just have to wait and see. I'll know more, looking at the plugs once I've made the mods, and will take it from there.

I'll be sure to report back my findings to y'all.

Best,

ichael

xsjohn

With straight open exhaust or very free flowing a few of my customers have found the custom 46 pilot I came up with is the best..........not available but I will tell you how it is done......have to have a real micrometer......Chinese one may or may not be accurate...

can try reducing the air jet from 135 to 132.5 but trying it myself I found the slightly bigger pilot was better staging from the pilot curcuit to the cutaway is minimal in the BS34....being a flat bottom with no cutaway.....and mileage was better......

BS34 deceleration ............popping free flowing exhaust relief............

Local music store......the extra guitar string box......look for 18 plain steel guitar string.....package will lie so you have to measure.......find one that is .018 thousands.......exact is best......this give you a 46 pilot.........just debur the end of the string and push it through a smaller pilot jet...bit of a challenge but it will go with a little persuasion.....little valve grinding compound or comet to get them both the same.....them set mix screws at 2 1/2 give or take turns......decel will be clean but stock needles will still suck no matter where you raise them............

xsjohn

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

I would suggest you try 1 to 2 sizes larger on the mains and one on the pilots along with John's needles.

xsjohn

Be careful with the BS34 and a 137.5 main jet which can easily wash the rings on heavy acceleration especially on grades requiring holding the throttle or extended periods………for the BS38 this may be a solution ...should only be used with the BS34 if you know exactly what you are doing with other mods absolutely requiring the extra fuel up there…big mains will not correct decel popping…that is a low throttle problem.....…..big mains are a poor choice to cover for poor stock jet needles no matter where they are positioned with the BS34 or replacements that are not positioned correctly………….don’t believe me just give it some time…...then you can have washed rings and opened bore clearances………..and just when you thought your new jetting was cool.......been there and done that.....

xsjohn

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