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Discussion Starter · #1 ·
hey gents!



just a quick quiz, trying to tune a 25mm mikuni that comes on most of these chinese bikes.



I ripped the head off this thing, reshaped the inlet and exhaust, blended the bowl, lightweighted and swirl polished the valves, indexed the spark plug, matchported the inlet manifold.........



As expected the motor runs of of tune now BADLY. Anyhow, at WOT the revs fluctuate approx 1000-1500 rpm. Does this mean lean or rich?





Anyway, the pic is from my parts bin special. EVERYTHING on the bike is second hand/ex warranty. Tyres are brigestone bt601ss on 1.85 x 12" front + rear. Should be a bit of fun when sorted.



Any responce appreciated.
 

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Discussion Starter · #2 ·
....alright, i did a bit of a sherlock holmes, and reckon she's lean as a bean. It originally had a 95 main and ran like a bucket of sh#t. Stuck a 105 main in it same dice. Any idea's of jet size? (air temps round here 20 degrees C. Close to sea level.)



Thanks again!
 

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i dont know anything about that carb and jet sizes, sorry



what i can say is, as long as you didnt polish the intake you should be able to run a bit rich to keep things safe. with something getting that much air you really dont want to run it lean at all. thats my stance, others may vary



and get an oil cooler. motard= sustained high rpms = lots of heat
 

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Discussion Starter · #4 ·
definitely didnt polish the intake side! I'm no guru at all with carbs, so no idea what size increments to go up with jets... still would love to hear any suggestions



Scouting round for a cooler at the moment, can get a chinese cast up alloy thing and machine all the excess alloy off it to get it to actually cool...



Thanks for the info z50king appreciate it!



While im asking questions what gearing do most people use? Im on 16/37 at the moment, seems very w-i-d-e...
 

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I don't know much about 4-stroke carb jetting...but if it's the same as 2-stroke then you need to start with figuring out exactly where your biggest issues occur.



First, you can always go richer...you're not going to hurt it that way!



So, if your bike is idling badly and bogs or coughs as you're cracking the throttle to start going, or at low RPM then work on your pilot jet. Next, if your bike is having issues at 1/8 to 3/5 throttle open and mid range power, raise your needle a clip or two. Lastly, full throttle high RPM is all main jet.
 

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two stokes and four stroke carbs differ because has the 4 has an air screw, and the 2 has a fuel screw. they do the same thing, but dont confuse them when your turning it one way to try to richen it when your really leaning it out. just be careful



as for the china coolers, i dont know how thick those are, so be careful machining the all the crap off of them
 

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Discussion Starter · #9 ·
thanks for the imput gents, apreciate it!



Yeah she idles perfectly, can rev it OK when colder, but once hot it will only idle and boggy all the way to the top with 1500rpm (approx) fluctuations at WOT. Assuming that I can play with the screw to get the mids a little happier, no idea where I'd get a pilot though...



I'll keep you all posted... Cheers!
 

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Discussion Starter · #12 ·
... hmmmm carby's are fun!



1 Million Rubles to the first person that can guess the make and model of the bike in the background !..?..!



ps- what sprockets you guys generally run with 12" - 12" combo....?
 

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Discussion Starter · #15 ·
yep nsr250 andy !



As for the bt601's, there hard as sh#t to get in Australia, the boys at Brighton Bikes and Bits (VIC) know someone at Bridgestone that can get a hold of them..... for memory theres some laying round out the back for sale.
 

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Discussion Starter · #20 ·
get them to order them for you andy! quite expensive though...$170 approx each..



Cheers mello, im starting to think that I've got the jet sorted now, but the problem is (i think) mechanical... I've tried 2 carbys and the bike will rev until you get to the main jet (WOT) and it dies in the ass.... aaah well im stumped!
 
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