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Thread: How to create a fire breathing GN250

  1. #1
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    1st February 2004 - 18:17
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    How to create a fire breathing GN250

    Hollowmen asked so lets figure this one out:

    INCREASING COMPRESSION
    borexstroke = 72x62.1
    so displacement = 252.8cc
    divide by 8.9 = combustion chamber volume = 2.8cc
    we want 10:1 so ideal combustion chamber volume = 2.5cc
    difference of 0.3cc is taken out of barrel so as to not change combution chamber shape.
    So i ended up with 0.767mm shave to achieve 10:1, because of increased compression 96 would be used, flame propagation would be faster so therefore more controlled
    Because of the head being lower the slack would have to be taken out of the cam chain as well.

    ADVANCE IGNITION
    simple as making the woodruff key thinner and rotate the flywheel, would need to be done on dyno.

    EXHAUST
    Ditch it from the footpegs back for some straight pipes. Probably lose power but the diapproving looks from old ladies makes it worth it.

    CAMS
    throw a few $$ at a cam grinder and see what he comes up with, I'm thinking you want stage3+ and you'll need to replace the valve springs at the same time with something a bit more heavy duty.

    CARBS
    Will need some serious work with these mods, specialist work with that high tech system.

    TURBO/NOS?
    You know you want it.

  2. #2
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    21st May 2004 - 09:25
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    Why would you want to?

  3. #3
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    1st July 2004 - 11:19
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    Quote Originally Posted by Dr Bob
    Why would you want to?
    well I was just wondering how to get it to 120 easily to pass those twits at 90, but without taking up the whole passing lane getting up to speed...

    although it would be good for a laugh...

    Although I've heard of someone down here who raced GN125 by shaving the head and putting GN250 carbs on it...

  4. #4
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    25th February 2004 - 07:36
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    I reckon a small turbo would be the go, if you're half arsing it anyway just chuck in a decompression plate, I guess you'd need a suck through system and some hefty jets, do they have cams or push rods ? coz you'd probably want to change the overlap a bit... might get you all of 60hp on boost and about 20hp off ( till it popped) ... seriously though, why would you do it ?

    Just switch a GN400 engine in if you really have to ?

    Sigh... suggestion 2 is to get a GN250 and set fire to it.

    Sedge.

  5. #5
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    21st November 2004 - 23:26
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    How much slack would a 0.76mm shave off the block make anyway?

  6. #6
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    12.3mm..............

  7. #7
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    Quote Originally Posted by FzerozeroT
    CARBS
    Will need some serious work with these mods, specialist work with that high tech system.
    Just biff a Mikuni competition 36 odd mm (or stock XR500 sucker if ur poor) flatslide, smoothbore, pumper carb
    Quote Originally Posted by FzerozeroT
    EXHAUST
    Ditch it from the footpegs back for some straight pipes. Probably lose power but the diapproving looks from old ladies makes it worth it.
    Nah dude itll ruin all the work youve done!.. Must get the exhaust just as precise as the rest of the beast. rip out the old exhaust chart and work some fine magic!

  8. #8
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    best would be tuned length straight-pipe (x2 one for each valve same as standard), I don't have a 2T tuners handbook handy, but you just need to figure out the speed of sound in hot air and it has to travel to the end of the pipe (+ a little extra travel to suck the next lot through) and back in 8.5 x10^-3 secs (7,000 rpm?), 330m/s at 25degrees, 1.414 m, so probably start off at 1.6M and cut off 2cm lengths. Of course the diameter of the pipe will come into play as well , guess it would be 1&1/4"? that seems awfully long though faster sound in hotter air might bring it back under 1m though?

    Guess when you take the pipes off you just use the same diameter as the manifold/port?

    Has anyone ridden both a (1981)CB250RS and a GN250, whats the power difference like? I might have to sort out a dyno

  9. #9
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    dead stock CB250RS is up on the GN250.

    I had a fully worked/tuned CB250RS thats where all my info comes from. And yeah about the pipes... but to save weight (altho make calculations harder) your better off just joining the 2 header pipes straight together ASAP after exhaust ports, without disrupting flow too much... its not like they are individual cylinders and their timing is going to affect one anothers backpressure... so the only reason they made them twin outlet i beleive was to get more exhaust port area to let it out.

    i think. And 1981 CB250RS is 1978 XL250S motor from memory 30odd HP? ProX piston kit number 01.1500.000 std, same as XR250RB.

    valves 622.1014EX and 1014IN from memory? Should you want to rebuild one

  10. #10
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    Quote Originally Posted by Kwaka-Kid
    ProX piston kit number 01.1500.000 std, same as XR250RB.

    valves 622.1014EX and 1014IN from memory? Should you want to rebuild one
    Nerd! Shamed!

  11. #11
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    no no shade. nerd would then be if i told you the Superin Gasket set number was 19-59444, that they have the same rings as a TRX300, number 624.060R std... and have a 74mm bore stock (i THINK, or 74.5?), Rodkit 03.1255(i know thats wrong, its close to that though.. 1285?)

    Not to mention they use SHKIT-1 SH bearings, FSK-003 fork seals... then the easier stuff like tubes/520 chain etc etc, rear brake shoes 92-39150 superin number (i suspect) front brake pads are possibly GF020?(thats a hard one, cant fully remember what they look like. Master cylinder rebuild kit MSB-102, camchain is 82RH102L? ahh crap hes rambling again.

    The better info anyway is that the CBX250RS 6 speed gearbox goes straight in with a little shaving off the casing.

    ohh myyybad on this post.

  12. #12
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    i still cant beleive that some one with way too much time on their hands would want to even do this let alone admit to wanting to!!!
    hang on ill just find my caring face.....

  13. #13
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    just get a faster bike
    "Read not to contradict and confute, nor to believe and take for granted, nor to find talk and discourse, but to weigh and consider."

  14. #14
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    I'd suggest going for 28mm Id headers of the same curve /length as stock and feed it back into reverse cone megaphone
    To see a life newly created.To watch it grow and prosper. Isn't that the greatest gift a human being can be given?

  15. #15
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    Best mod for the buck is a 280 big bore if you can still get 'em.On a single a megaphone is the best thing you can do on the exhaust,with a reverse cone,way better than a straight pipe,and twice as loud.Bigger carb and a hotter cam.
    Drew for Prime Minister!

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    www.prospeedmc.com for parts ex U.S.A ( He's a Kiwi! )

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