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What jets for BS34's & 6" Uni's?

Discussion in 'The Garage' started by Clutchslip, Mar 25, 2020.

  1. Clutchslip

    Clutchslip XS650 Member

    Hi all,

    I searched this and couldn't find anything pertaining to BS34's, so don't flame me please!

    I have an '81 650H with stock jetting on BS34's (132.5 mains, 42.5 pilots). It's not currently running but when I go back together with it I'll be using the long 6" Uni double layer filters & no airbox (battery box removed so the long filters should fit I think).

    I'm going to be riding mostly at sea level, not higher than 1000' very often. What jets would you suggest I start with, those if you that have jetted BS34's for Uni's?
     
  2. Jim

    Jim Beyond the edge, is the unknown. Here be Dragons XS650.com Supporter Top Contributor

    I'm a rookie at these CV carbs, so I'm not gonna make a suggestion.... but here's where I'm at. EMGO commandos and the foam filters. I went up 2 on the mains to 137.5 It feels pretty good when I open it up, but I'm under 1K on a fresh top end so I haven't done a long enough wot run to check the plugs. So... not much help to you there. I went up to a 45 on the pilots. I did manage to get a good plug chop on them and they we're black as coal. So I went back to the original 42.5's About all I got for ya.
    I'm at 1,000 ft.
     
    Clutchslip likes this.
  3. Clutchslip

    Clutchslip XS650 Member

    Cool. Those were the BS34'S?
     
    Jim likes this.
  4. Jim

    Jim Beyond the edge, is the unknown. Here be Dragons XS650.com Supporter Top Contributor

    Yeah... 80SG. Have you downloaded the Carb Guide in the Tech section?
     
  5. Clutchslip

    Clutchslip XS650 Member

    Yes, I've read the whole thing. I didn't see anything on Uni's.
     
    Jim likes this.
  6. Jim

    Jim Beyond the edge, is the unknown. Here be Dragons XS650.com Supporter Top Contributor

    Page 19 or 20 I think.

    Untitled.png
     
  7. Clutchslip

    Clutchslip XS650 Member

    Yes I guess I remember reading that. I kind of was hoping to hear from someone who has actually put Uni's on their BS34's. It kind of sounded like the writer of that article was venturing a guess on the 34's when he says "it's been said", "so I would start at least 2 sizes up". Also, he seems to equate going to any aftermarket exhaust to going to any aftermarket filter as if they were equivalent modifications in relation to mixture. I'm not sold on that equivalency. I've read elsewhere, on different bikes that the Uni filter acts much differently than say a K&N style media.

    So, can anyone speak to this with first hand knowledge? If I don't find any better info, I'll just go up 2 on the mains & 1 on the pilots most likely. Maybe keep the stock 42.5 pilots based on Jim's feedback.
     
  8. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, when I wrote that, I hadn't used UNIs yet, only K&Ns, and I didn't have much experience with the BS34s yet. But now I've got both, UNIs on BS34s with a freer flowing aftermarket exhaust. I'm up 2 on the mains, one on the pilots, and one size down on the air jet. That seems pretty close for these mods but I haven't put a lot of miles on yet so that could change slightly. If the UNIs are your only mod, you may get away with only one up on the mains and no pilot jet change. If you feel you need it slightly richer in the pilot circuit but not as rich as a pilot jet size change would make it, change the air jet. It doesn't have as big an effect as changing the pilot jet does.

    Before I installed the UNIs and the new exhaust, the bike was almost stock except for extra holes drilled in the ends of the mufflers by the P.O. Just that small mod threw the carbs off some as the BS34s are borderline too lean right from the factory. I went up one on the mains and down 2 on the air jets. I left the stock pilots in there. The bike was happier and ran better like that.

    I've been using the UNI pods for about 10 years now. On a CV carb, they work better than the K&Ns ever did. I've only ever oiled them with a 50-50 mix of gas and motor oil because I read the actual foam filter oil was too thick for them and would clog them up. The lighter oil-gas mix has always worked well for me.
     
    Clutchslip likes this.
  9. Clutchslip

    Clutchslip XS650 Member

    Perfect, 5twins! Just the info I was looking for! Thanks!

    I have stock exhaust for now, but some rust holes will probably make me a little lean. So I'll step up to 137.5 on mains and stay stock on the pilots, start from there. Won't be long until I order a set of those Emgo Commando mufflers, though. (Thanks for mentioning that Jim!).

    Anyone have Uni's jetted differently on the BS34?
     
    Jim likes this.
  10. Jim

    Jim Beyond the edge, is the unknown. Here be Dragons XS650.com Supporter Top Contributor

    Get 'em from Amazon. They're cheaper than Mikes. And free shipping if you have Prime.
     
    Clutchslip likes this.
  11. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, re-jetting is usually needed on all the 650 carb sets for the basic "pods and pipes" mods, but not huge changes, just a size or two above what was stock. The older sets that came set up richer don't need as much, later sets usually a bit more. But the bottom line will be found by experimenting. That will show you what is going to work best. Mix screw settings on a stock bike with BS34s seem best at a rather large 3 to 3.5 turns out. That can be reduced quite a bit if you re-jet the pilot circuit richer, using 1/2 to 1 turn less. Plugs will run pretty black if you don't.
     

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