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Carb jetting

Discussion in 'The Garage' started by dlabkeeg, Dec 29, 2009.

  1. dlabkeeg

    dlabkeeg XS650 Addict

    I heard you have to change the jets, when you replace the original air box with the pod type air filters. If yes, suggestions please. I'll probably just get carb rebuild kit. I've seen a few different kits, anybody have a favorite? Oh yeah, they're stock 78' carbs.
  2. inxs

    inxs xx

    - thats all going to depend on where you live, altitude, air pressure, other engine mods, riding style etc so better to get a range of jets, both main and pilot, in general a size bigger works well...i like to swap in new needles too...

    - remember too that kits are often put together to service several different bikes so often contain parts you will never use
  3. Travis

    Travis Staff Member Staff Member

    A lot of times when people think they need to re-jet, they go way too big. Especially if you only change one thing and not the other (intake and exhaust). If the carbs are clean and working fine now, you might not need to do that much. Probably a size up on the pilots and a size or two on the mains. Then play with the needle heights if you can raise and lower them (can't remember which carbs have adjustable needles) and obviously set your mixture screws. I wouldn't buy carb rebuild kits unless your carbs are dirty and I also wouldn't buy a jet kit. Just figure out what jets you have and buy the next sizes up. That'll only cost like $20. If they are dirty and leaking, then buy the rebuild kits.
  4. KevC

    KevC XS650 Addict

    Read the carb guide in the tech section , then print it off & read it several more times :D
    make sure you are using decent air filter pods, the cheap ones can block the intake holes on the carbs & cause lots of problems. Use K&Ns or XS performance.
    Im running 1977 BS38s with XS pods and one off exhausts, went up one from standard on pilot & 2 on mains, dropped needle one notch.
    Not perfect but close :bike:
  5. Gordon

    Gordon XS650 Junkie

    I do belive that '79 was the last year of the 38's. In '80, the EPA mandated that the carb's be 'emission compliant'. Remember the plug's on the carb's for automobile's? Same law. So that's why people do not like the BS34 carb. However, I've found them to be a good carb, once an understanding of their operation is comprehended. I think he'll be able to get a good 'feel' on his carb's once everything is installed, engine tuned ( cam chain set, valve's set,point's & timing all set), then he can ask some more about what's going on.:doh::yikes:
  6. xsBUTCHER

    xsBUTCHER XS650 New Member

    Ok, so I have I question for the all knowing XS gods. I recently brought a long neglected 81 xs650 heritage special back from the dead (got it back to 1 kick ;) ...). Im going to be cutting the mufflers off at the headers leaving about 3ft of open pipe. Im assuming the carb internals are original and Im already running pod filters with no issues at all. 1st question, how do I know what jets I already have and 2nd question what would yall recommend I upgrade to?

    Thanks in advance,
  7. xjwmx

    xjwmx XS650 Guru

    Hi. I have an '81 as well. The jet size is stamped on the jet in tiny numerals. You have to remove it and look with a magnifier. You might not need to remove the main jet to see its; can't remember. On the pilot jet use the right size screwdriver and one with vertical sides like from a set of bits, or you'll bugger it. Don't try to tighten it back too much or you will damage it also. Just snug plus a little.

    As for what size to use with your new exhaust, I would say that since the exhaust is isolated off during the intake stroke it's irrelevant, but that's not at all what I read is necessary. :) Maybe someone will explain why it is because I would like to know too.
    Last edited: Aug 27, 2010
  8. 5twins

    5twins XS650 Guru

    There's no telling what may be in your carbs now. Previous owners are notorious for changing things. The only way to know for sure would be to take them apart and look. Stock jets for your model are 132.5 main, 42.5 BS30/96 type pilot, 135 air jet, and a fixed nonadjustable needle. The usual re-jetting for minor mods consists of going 1 to 3 up on the mains and 1 or 2 up on the pilots. The exact sizes you'll need can only be determined by testing.
  9. XSLeo

    XSLeo XS650 Guru

    xjwmx, When you run an open exhaust there is much less resistance to air flow out of the engine. This increases to air flow through the engine.
    An increase of air through the engine requires modifications of the fuel deilvery to accomodate the increase of air. An air leak, both between the air cleaner and carb, and carb and engine will do the same thing. Introduce extra air into the engine.
    The exhaust is not isolated from the intake stroke, there is valve opening overlap, A time while both valves are open. This is needed to help the exhaust to pull more air into the engine and help the intake of air to expell more of the exhaust.
  10. xjwmx

    xjwmx XS650 Guru

    Thanks, that would explain it.
  11. gggGary

    gggGary Cara Mia!!

    Non expert warning: I have NOT rejetted non stock XS's
    The canard that less restrictive exhaust must equal bigger jets is tired and untrue. It is not unusual to have to lean the jets to get the best engine operation. The carbs provide an air fuel mixture. In general more air through the carb automatically equals more fuel. If that wasn't true your engine would not be able to accelerate from 5K RPM to 7K RPM under WFO As the engine pulls more air at increased RPM it also pulls more gas. It's the very definition of carburetor. Now that said there is a limit to how much gas can flow through the jet no matter how fast the air flows. So IF your exhaust (and other mods) allow your engine to make more horsepower at some point you will need a bigger jet to get enough fuel through the engine to make that horsepower. Straight pipe open exhaust likely makes noise not power, megaphones and other "extractor" type exhausts that are properly engineered to the engine can need richer jetting. So after all that my advice is...
    #1 leave the blasted mufflers ON. Good god the world is plenty loud already.
    #2 when you cut them off ( I know, you will) leave the jetting where it is and do chops. Full throttle run, kill the engine, stop and read plugs. Decide THEN whether you are rich or lean go up or down a jet size, repeat.
    After three or four or more tests like this the main jets will be close then you have to work on the mid jets and needle jets.
    A knowledgeable tuner could probably get it done well in a day or two. A backyarder waiting for parts, a few weeks to months maybe.
    This is a very involved, work filled, subtle process that is not easy for a non expert.
    #3 buy pipes from someone who has done the work to give you an idea of jetting for them. Then at least you have somewhere to start from.
    #4 Drill the pipes weld on adapters and install EGT gauges. That will reduce the time to arrive at correct jetting.
    Pipe length is critical, the length of pipe from engine to muffler (or end) will change the power characteristics of the engine. Much of this work on the XS650 has been done for you a little googling will reveal ideal header pipe lengths for different engine power bands.
    KC 120 likes this.
  12. Morgantician

    Morgantician XS650 Member

    I'm gonna throw my situation into the discussion here, because this is exactly what I'm trying to figure out.

    I have a stock '82 I just got, and I also replaced the air filters with pods and cut the pipes down (just cut the mufflers off, so the pipes end right below the pegs). While the bike sounds better (to me), I'm getting backfiring on deceleration, and often at idle right after start up. As well the carbs are slow leaking gas...but I'm guessing that more has to do with there not being any gaskets in them.

    I figure the bike is running too rich, should I go up or down in jet sizes in the main? Maybe throw a couple numbers on where I should start. Also, should I get an adjustable needle (I took apart the carbs and still has the stock single position need in both carbs) to help this as well?

    Something else to consider: I'm at high elevation - Denver, Colorado to be exact. So 5280 ft and higher.

  13. gggGary

    gggGary Cara Mia!!

    IMHO stop the leaky carbs, synch the carbs and then start working on your jetting.
    If you have the stock BS34s replace the O-ring that seals the float seat to the carb body. I do that on every set of o-ring equipped carbs I open up. The trick is getting the seat out without burring it up. As you know altitude matters, you might want to see what jets are in the carbs now. High altitude dealers often rejet(ted) their bikes to compensate and improve running.
  14. xjwmx

    xjwmx XS650 Guru

    When it's hard to get out, it might not need a new o-ring, since that's what's holding it in. Mine, the seats practically fell out themselves and was leaking past the rings.

    your tip of Cycleorings,com was an excellent one. There's also a hardware store replacement, and in Britain apparently a car parts store replacement.
  15. gggGary

    gggGary Cara Mia!!

    I keep rings on hand and they are so cheap I won't chance not doing it. I am rebuilding a set of 34s today and the seats came out hard, one o-ring was all cracked and came apart in chunks, I had to wire brush rubber out of the groove before I put the new one in. The little screens behind the seats had done their job, one carb had a wad of crap it had held back. The float bowl just had varnish in it. I opened three sets of 34s this week one had 132.5 mains one had 137.5s and one had 145s. One leaky float, one idle screw that would not come out. I finally broke the carb body on my third try with an easy-out after drilling the screw. I got the other one out by recutting the screwdriver slot deeper with a tool I made with the idle screw in place!
  16. xjwmx

    xjwmx XS650 Guru

    That makes sense. Cheap insurance. Re: the mixture screw, I did that once. Used a Dremel tool to make something of a screw slot in it. Strip, make a new slot, strip, make a new slot, until it was finally out.

    Harbor Freight has a 1/4" mini impact driver that might be good for that job.
  17. Morgantician

    Morgantician XS650 Member

    Okay, so I fixed the leaky carbs. All it was was the petcock not sitting correctly.

    But here is something to add to the equation that might make more sense as to some possible friendly advice: I was making a new battery mount and had to pull the air filters off to squeeze everything out. Well i forgot they were off and fired it up when I was all done and it didn't backfire at all. So I took it for a short ride, and it didn't backfire once. It sounded amazing!

    Now I know I don't want to be running the bike without air filters, but I'm guessing taking those off is allowing more air in and offsetting the carbs running rich on account of me chopping off my pipes. Any simple fixes anyone can think of that would give a similar affect, in the jets or otherwise?

  18. gggGary

    gggGary Cara Mia!!

    Make sure the air filters aren't blocking ports on the intake bells of the carbs. Common problem.
  19. Morgantician

    Morgantician XS650 Member

    I assume these are the small ports on the inside edge of the bell that the air filters slip over? I don't have much experience, so pardon the possible laymen question.
  20. Testify brother!

    What people forget is that in vacuum slide carbs, more airflow = more raising of the slide therefore more fuel anyway. Yes, there will be a "cutoff" as the main reaches it's full ability to pass fuel, but those who cut their pipes off rarely rev up to where they'd actually *need* bigger jets. So voila - no need to rejet.

    BS34's need one thing: the cdn needle jets and needles (on the middle/3rd slot) to do 99% of whatever riding you're going to experience. Everything else stock.

    If you go to a 2->1 exhaust pipe, *then* you need to start thinking about upping the main, because scavenging becomes a serious consideration with amount of valve overlap these things have.

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