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FCB tech ?'s

This is a discussion on FCB tech ?'s within the Technical forums, part of the Caferacer.net Forums category; steve- see, i'm running 105s and still rich with more disks? it's gotta be the intake then, no? sirius consolidated, siriusconinc.com , has jet kits. ...

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Thread: FCB tech ?'s

  1. #21
    Senior Member ttilghman's Avatar
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    steve-

    see, i'm running 105s and still rich with more disks? it's gotta be the intake then, no?

    sirius consolidated, siriusconinc.com , has jet kits. i see they now have 100s, maybe that'll do it for me?

    -tt



    Edited by - ttilghman on Apr 04 2007 1:47:46 PM

  2. #22
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    tt,

    I had an open meg on my Ducati and went to the dyno. Changed the pipe to a factory reversed cone meg and got 2HP. I couldn't belive a facoryy tuned pipe would make that much difference. I learned 2 things that day. Exhaust stuff and the importance of dyno tuning a race bike. They also fiddled with the timing and carb and I came out of the dyno session with a well sorted and FAST Ducati.

    AHHH I see it....coming out of T12 nhis parks61 has a few bike lead (cause he smokes me T-11-12a) and by the time I get to 2/3's of the front straight I motor by leading ino T1. That has happened more than twice!!!

    Champ

  3. #23
    Senior Member Mc69's Avatar
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    You guys have a place where you order different sprockets from? Will the cb350 sprockets fit other bikes?


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  5. #24
    Senior Member parks61's Avatar
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    and it pissed me off every single time (a lot more than two times). so i try to go faster every place else. champ had great swirl that last 3rd of the front straight. hope he returns this year with duk58.
    parks
    p.s. read Dr. branson closely about gears...at nhis, shorter is better. i picked up a lot of speed as i shortened my gearing a few years back from street bike (17/42) stock to where i am now (16/44)...am considering adding yet another tooth to the rear when there is a front straight head wind...just another variable to make you one-set-up guys bum. turk probabley runs taller than me because he's got about 10 horse power on me. wish i had his swirl.


  6. #25
    Moderator jbranson's Avatar
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    Wow, big differences in gearing between the LWSV gearing and FCB or stockish bikes.... you guys aren't short shifting those things are you. I guess you can tell when they run out of steam better than I can from my seat in front of the computer. Craig's bike is done by Mike Rich...so I know it can turn some rpm.

    Probably the simplist way to jet a four stroke that seems to work is jetting it down till it falls on it's face...then go up about two sizes. And by "fall on it's face" I mean you put it in 2nd gear, crank it wide open for a few seconds...if the main jet is too small it will literally stop running momentarily....but it will be real snappy up to that point. For good power, just like a 2 smoke, you want to be as lean as you can be without damaging the engine...and still be able to run WOT at red line without starvation.

    Intake is important...manifold must be reasonably well matched to the port, no gaskets hanging in the air stream, no leaks of course. A small "step" or obstruction in the intake can totally screw up air flow at WOT.

    Guys like Pete T. can go through a bike and find all the little errors that add up to about 20 percent lost power. If timing just a bit off, jetting is just a bit off, exhaust a bit off, float level a little wrong, carbs not perfectly synced, plugs a little old, wires a little worn, cam chain a little loose...before you know it you've given up a big chunk of your hp.
    JohnnyB


  7. #26
    Senior Member kenessex's Avatar
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    As a case in point about intake stuff. On my 350 at Grattan I was way too lean and couldn't find any bigger mains, so I just pulled my velocity stacks off and then had to lean out the mains even more. The difference turned out to be 100 mains no stacks to 130 mains with stacks. I then put the ugly green uni-ilters over the stacks and it made no difference in jetting.

    JohnnyB,
    Do you think stock CL350 headpipes would be large enough ID to flow sufficient exhaust for the NHIS benefit bike? I still like the look of the dual up pipes.

    Ken


    AHRMA 412
    Vintage racing - old guys on old bikes

  8. #27
    Moderator joe c's Avatar
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    sprocket specialists to get sprockerts. they have a website. the 350 sprockets will fit the 360, but i have no idea what else. there are part numbys on the sprocket site. maybe you can cross em.



    jc


    aaron, send my wheel.




  9. #28
    Senior Member lonewolf's Avatar
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    tt I had noticed that S.C.I. only has the 100 jet in their sets for $35.00. If you haven't ordered yet try mrcycles.com honda part (99202-601-1000) they are only five bucks and change,and you dont have to feel bad about having all those jets you will never use.


  10. #29
    Moderator jbranson's Avatar
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    Ideally you want to run the largest jet you can and still be correct. If you can run a larger jet and still have the right mixture it means more AIR is going into the engine...which of course requires more fuel.
    Using my Helmholtz frequency software you can come real close to the ideal intake track length for a given engine and rpm range. Intake track length is measured from where air enters the carb...so a velocity stack is part of the intake length...that's why they make them in different lengths..they are a tuning tool.
    If you remove stacks and all of a sudden you are very rich...it means you have messed up the timing of the intake charge...less air...which means less fuel is needed. If you add stacks and go lean...that means you are getting to the correct length and intake charge pressure waves are timed properly....meaning more air...requiring more fuel.

    Jetting is supposed to be kind of the last step in the build...when you already know you are using the correct parts and design.

    Ken...actually OEM CB350 head pipes are way too big...even too big for a hot engine. I'd be running 1 3/8 OD if I had a hot 350 engine. Probably 1 1/4" for a stocker. (Of course most people's minds are deranged by a concept like this). But for the bike build...unless someone plans on making custom head pipes...the stock ones will have to do. Often going to something like 1 1/4" from 1 1/2" will also require matching the exhaust port to the pipe size...you don't want an abrupt step down in size. That's why most really hot engine have reshaped exhaust ports...often going down in cross section to increase exhaust gas velocity.

    Once again I'll use our screaming, fast as mofo 72cc Honda single as an example...it has a 3/4" OD head pipe, makes tons of power and turns 13,000 rpm.
    A 1 1/2" OD head pipe has 4 TIMES the cross section of a 3/4" pipe...which means it would be suitable for a single cylinder of 288cc...which would be much larger than one cylinder of a CB350. If you do all the numbers...the equivilant pipe size for one cylinder of a 325cc CB350 would be 1 1/8" OD. Or rounded to 29mm ...how big is the exhaust valve on a stock 350? I'd be a tad leery of 1 1/8", but 1 1/4" would be great for a stocker. (that's 25% larger in cross section than a 1 1/8" pipe). Cross section area goes up exponentially with OD...very easy to get too big too quick. Smaller cross sections keep exhaust gas velocity high with helps extract gasses....of course there is a limit to that.
    JohnnyB



    Edited by - jbranson on Apr 04 2007 8:59:46 PM

  11. #30
    Senior Member mikeo's Avatar
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    John,

    I think I'd like to contradict what you just said. The stock header pipes are just right. If you just measure the outside you'll be misled. The stock 350 pipes are actually two pipes disguised as one. There is an internal pipe inside the one you see when you look at the outside. To really use the stock pipes though it is a good idea to cut off the outside layer except for near the flange (engine) end where you will need a little left on to use the stock exhaust clamps. You also get rid of at least a couple of pounds of weight too. You might then want to wrap the pipes though as one effect of the dual layer pipes is to keep the inner ones hot so the exhaust gas flows well to the megaphone, muffler, what have you. Honest, take a look at a stock set and measure the od and then the ID at the flange end. If you look at the muffler end you will see where the inner pipe flares out to match the outter pipe. For a stock engine the stock pipes are a good choice. If you go bigger you change the flow characterists a bit which can be ok as long as you don't go too big. With Super Trapps you can tune that a bit on a dyno.

    OK John, now rip what I said to pieces. Come on, I know you can do it.

    Mike O.




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