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  1. #1
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    Default Help: Mikuni BS34 CV Carbs.

    I couldn't really find an answer to my particular question in the previous Posts.

    But my question is, Can a pair of Mikuni BS34 Constant Velocity carburetors be properly Jetted for pods and a more open exhaust, or do i HAVE to use some sort of air box?

    Im not doing straight pipes but i do want a lower, meaner exhaust with a high flow baffle.

    I have the ability to make a new more low profile air box if needed, but i would prefer the performance and look of pods. Plus it just seems like it would be easier and take a little less time to just Re-Jet carbs to get the right performance.

    I have been told by a few people to just find a better fitting, low profile air-box and an exhaust to match the flow of the air-box and to fine tune the idle from there and i will be fine.

    Or would you guys say otherwise?

    Thanks, JoeBob.

  2. #2
    Jetblack
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    All carbs can run open/pods/velocity stacks or box. Just know when you open air flow you just enriched the air mixture...so to create balance, I suggest one of two things. Open the pilot mixture screws another turn to about 1.5 more or less out to allow more air fuel mixture, or leaving that alone and bumping up the main jet one or two sizes higher than stock.

    Most likely one size up since you are not running fully open pipes, but two sizes if you change to open. Two sizes may be best because you can just adjust the A/F mixture screw to compensate if too much.

    Of course if the pod is really the only change as in pipes still have restriction, and the EPA nanny shit is staying, then just slap on the pods and put your A/F screws to 2.5 turns out and see where she stands. If lean back the screws out a bit if rich turn them in.

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    Thanks. That is the answer i was looking for.

    So if i have pods, and a little higher flowing exhaust than the stock, then adjust the pilot mixture screws out a bit to increase the fuel in the A/F mixture and then just fine tune the idle with the idle screw from there?

    JoeBob.

  4. #4
    Jetblack
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    Yep,

    Adjust idle on a well warmed up bike.

    If you have any stumble or hesitation on wide open throttle that the A/F screw cant fix then, try popping a washer under the needle or go down a notch if it is e-clipped, if it still can't WOT at peak without issue bump the main jet up a size or two, and readjust the A/F mixture.

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    Cool. Definitely going to try that this next weekend.

    And where would you recommend i get the main jets for such a task? Is there going to be a particular one I'm going to need?

    And do they have any sets that are incremented? Like a set of 4 that each go up a size from each other?

    Thanks, JoeBob.

  6. #6
    Jetblack
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    I'd stay away from premade jet kits those are an over priced option and are rarely plug and play, the "stages" where created to just bilk out more cash...stage 1 to 100 or whatever can be done all at once...no need for stages.

    That being said, I find that this place linked below sell jets cheap and with none of the nonsense involved to mark them up in price.

    You can go local too if you want them quicker. Find out what the stock main is then get one two sizes up example if yours is a #98 a #100 would be one size up etc. and maybe one size up for pilots for future mods...if this is it then just the up 2 sizes on mains. Many people go way way more than they need too on upping jets if stock calls for a #98 going to a #128 is just silly and harder to tune around and actually can hurt performance.

    http://www.z1enterprises.com/ShopByC...ory=0102020202

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    Can i get some help on finding the stock Main Jet size?. I have been searching for a couple of hours and can't seem to find a definite answer. Ive seen some say they are as low as 122.5 and as high as 145. So im confused on where to start... They are from a stock 82 Suzuki GS450L, so they are 1980-84 model stock Mikuni BS34 CV's.

  8. #8
    Jetblack
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    Going by the carb number seems logical to find the jet sizes; but you have to go by bike model. Simply because the same carbs were used on different bikes, and they were jetted to match the needs of that particular engine.

    For your bike model that carb had factory jet settings of 17.5 pilot and 115 main. I'm not sure what it has in it now however...it's a lot of years is a long time for a bikes carbs to go without being cracked and changed at some point, stock jets may be in it but may not be.

    The type of jets needed for this carb are pilot Mikuni VM22, and main is a Mikuni Small round.

    Research into this model's head design and into a few boards bring good pod jetting brings it to a 40 pilot and 137.5 main...for best performance and fuel economy. If you don't care so much about fuel economy but lean towards more performance then a 50 pilot and 145 main.

    This of course takes into account of removing the epa crap etc. usually up two sizes on the main is a rule of thumb when going to pods/pipes/and emission deletes to run well. But the porting and flow characteristics in the head design can change that rule of thumb drastically which in this case it does...mainly because it was later ported to the 500 model.

    Typically any bike platform that was ported to another model using the same carbs companies just increased those jet sizes, so going by the larger platforms stock jets numbers then finding the best numbers for unrestricted can yield the best jets over all for the motor. That's why these numbers are so drastically apart, from the standard rule of thumb of up two sizes...the head porting never changed between models.

    But those are my suggestions, based on what you want out of it...a performance oriented speed demon that brings MPG down to about 45 or still fast but easier on the wallet in regards to MPG.

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    Sweet! I will definitely do a little more research and find out what the exact info on my carbs are and will go from there. And i am going to be taking the carbs apart on Wednesday to check to see if everything is in fact stock cause i did not do that before when i changed the main gasket out and cleaned some of the jets.

    Would the Jets have the type/size etched on them so i can tell what size they are? or will i have to guesstimate that?...

    Thanks again. You have been very helpful.

    JoeBob.

  10. #10
    Jetblack
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    They are usually stamped on the top side, sometimes on the edge. Many times on dirty carbs they are too tarnished to read. Dip a cotton swab in a little acetone and rub it on the jet it'll take that shit off quick and easy to see if that's the case.

    If you have no idea then you can pick up a gauge tool, it looks like a feeler gauge but it has wire to find the sizes. Of course the cost of that tool might fall the same as new jets so it doesn't matter if you're buying new ones anyway.

    I always keep a little jam jar filled with acetone, first thing I do on a carb dismantle before soaking is drop all the brass in it and give it a swirl. After carbs soaked and acetone squirted through all the tiny air passages, the brass is usually spotless enough to go right back in.

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    Cool.

    And i know its a long shot but can Jets be drilled to the right size needed?

    I was searching around and found a guy who said he just drilled the old jets to the size he needed. Is that right or was he just blowing smoke out his ass? It just sounded somewhat probable to me, but im new to the tuning carbs world. I have always just bought pre-jetted carbs for my Dirt Bikes to fit what i was running.

    Also. I think i actually found the size of my carbs without having to tear them down yet. Looks like the stock factory jetting is 17.5 pilot and 115 main. The guy that posted that i guess owns like 3 '82 Suzuki GS450's and recommended that if you want pods and a high flow exhaust to jump those up to 40 on the pilot and 140 on the mains. He said the reason you have to make such a dramatic jump in size is because the BS34 CV's were very restrictive on suction and flow.

    Does that sound about right?

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    Default Jets and such

    If you are gonna run free flowing exhaust and pods you should start with stock jetting and work your way up from there.

    You might end up going up 1-2 sizes on the main and maybe 1 size on the pilot jet.

    Your carbs come stock with 132.5 main jets and 42.5 pilot jets. Your air mix screw is "trapped" under a brass plug to prevent you from changing the mixture from factory settings. Drill a small hole and screw a standard sheetrock or deck screw into it and yank it outta there.

    There are several rubber parts that should be replaced when doing all this. The rubber is usually crusty and old.

    Read this:

    http://www.amckayltd.com/carbguide.pdf

    It will give you everything you need to know about your carbs and how to clean and tune them properly.

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    Quote Originally Posted by CupidsDeathDay View Post
    Cool.

    And i know its a long shot but can Jets be drilled to the right size needed?

    I was searching around and found a guy who said he just drilled the old jets to the size he needed. Is that right or was he just blowing smoke out his ass? It just sounded somewhat probable to me, but im new to the tuning carbs world. I have always just bought pre-jetted carbs for my Dirt Bikes to fit what i was running.

    Also. I think i actually found the size of my carbs without having to tear them down yet. Looks like the stock factory jetting is 17.5 pilot and 115 main. The guy that posted that i guess owns like 3 '82 Suzuki GS450's and recommended that if you want pods and a high flow exhaust to jump those up to 40 on the pilot and 140 on the mains. He said the reason you have to make such a dramatic jump in size is because the BS34 CV's were very restrictive on suction and flow.

    Does that sound about right?
    You cannot drill them out. And the BS34 carbs were used on many models over many years. Its the jetting that makes them unique to a bike.

    I would also recommend using UNI foam filters instead of the corrugated K&N type. You cannot get a stable air-flow from the K&N type and thus will make it difficult to tune your bike.

  14. #14
    Jetblack
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    Jets cost around 2 bucks drilling them out is not a proper gauge to know what size you are going too...the flow characteristics will change and an hour of my time fucking around trying to find the correct bit size and drill is worth more than 4 bucks.

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    drag pipes and foam unis..im runnin 142.5 on mains and ill check my pilots for u tomm...read and re read the linl pmworks posted

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    Okay. I have read through about 1/3rd of it. But i only have time to read it at work but i knock out a few pages a day. So i should be done reading it by the time my week is over.

    But on the 142.5 mains your not running too rich at all?..
    And if not then I'm probably going to assume your running a 40-42.5 on the pilots then?

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    Quote Originally Posted by CupidsDeathDay View Post
    Okay. I have read through about 1/3rd of it. But i only have time to read it at work but i knock out a few pages a day. So i should be done reading it by the time my week is over.

    But on the 142.5 mains your not running too rich at all?..
    And if not then I'm probably going to assume your running a 40-42.5 on the pilots then?


    45 piliot
    130 air
    in regards to the mains..ours are 135 stock.remember they went to the bs34 during the gas crunch so they are jetted pretty small from the factory.so 142.5 isnt really a big jump..i started at 140 .they were lean,i went all the way to 147.5 just cause i had em layn in my box..very rich..i bought 145 ,and they were also rich but not to bad...i put in the 142.5 n they are perfect ...for me..your experinces may differ..

    some more info for you here.
    http://www.mikesxs.net/faq.html

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    So on the Air Jets i should go with the B42/55 instead of the BS30/97 ones, or does that matter?

    Other than that i think i have it now. I need the Main jets to be the 142.5 Small Rounds, and a 45(With bleeds) on the Pilots to get the proper increase in air and fuel on the mixture since im increasing the flow on the intake and exhaust.

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    the air jets in the bs34's are the same as the main jets (small round type)used in the 38 series..


    about half way down the page is the jets ull need for ure air jets.
    http://www.mikesxs.net/products-38.html#products

    i been looking all mornin for B42/55 , BS30/97.i think that u were looking at the wrong air jets..try the ones in the link i provided.n update me if im wrong ,plz.
    Last edited by 3tc; 04-01-2013 at 7:57 AM.

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    That's funny. I checked that out and you are right. They are the same as the main jets on the BS34 CV carbs but the #130's, i need, are out of stock!... xD Figures.

    Thanks for the help though. I went ahead and ordered the Pilots and now waiting on the Main jets to get back in stock, or just might order them on Jets-R-Us.

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