wenley
Junior Member
Posts: 36
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Post by wenley on Aug 4, 2008 0:49:47 GMT -5
hi guys just a question im a bit confused on which jet to use some say 320 n others 330 on stock carb on a 78sr
or is it a trail n error process
i have straight through exhaust n knN filter
thanks
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Post by andy on Aug 4, 2008 6:08:23 GMT -5
Hi Wenley,300 is stock,opinions vary greatly on mainjet sizes but I'm positive 300 will be fine for your set-up,maybe experiment with needle height settings to get the mixture right but thats all.I'm running a new re-placement vm34 mikuni [non-pumper] and thats got a 240 mainjet and runs great.Like yourself I'm running a similar set-up and reckon its only when you go to the next stage of tune i.e incresed comp ratio , different cam,headwork that the carb becomes more of an issue.Yes it is a bit trial and error but dont get to bogged down and confused,stock carbs are fine if there not worn out then they can be a pain,thats why I changed mine to a much simpler carb that is so simple and easy to tinker with.Regards.
Andy
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Post by caferacercarl on Aug 6, 2008 4:25:28 GMT -5
310 main, lift needle one clip, one size up pilot, 2 and three quarter turns mixture, to start with, main jet size on other Mikuni's depends on needle jet size which is why a two stroke mikuni is a waste of time. the pump is there for a reason. and thats for free. 3 decades of R&D.
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wenley
Junior Member
Posts: 36
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Post by wenley on Aug 6, 2008 19:07:44 GMT -5
just opened the carb this morning found a 330 main jet in side
ummm i might try a smaller size as its not running right atm.
thanks guys for ur help..really appreciate it..
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Post by andy on Aug 7, 2008 9:42:13 GMT -5
Hee Hee,like I say,opinions vary greatly on carbs and jets,don't they Carl!!!
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Post by wotavidone on Aug 7, 2008 18:10:07 GMT -5
I started with a 310main, needle up 1 clip, and my bike (hi compression, non-stock air cleaner and muffler) ran like a dog. Ended up with 98 octane fuel, needle back to stock position, and 330 main.
Dam I wish you weren't so far away Carl. I'd love to get my bike over there for you to look at.
My R&D consists of a couple of years of the seat of my pants. Not the same at all, unfortunately.
I have to agree on the pumper carbs. My xt has the non pumper, and it seems to be waaay much more sensitive to changes in temperature - if its really hot say 40 degrees centigrade plus or really cold, the thing loses its edge - bit flat spotty, etc. The SR with the stock carb seems to be much more forgiving.
Mick
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Post by wotavidone on Aug 7, 2008 18:15:59 GMT -5
P.S. My main indicator was the plug. Started out very sooty with the needle up one, then very white when I dropped the needle back to standard. So I went up on the main - 320 worked, the plug started showing a nice tan colour, but the jets were very cheap so I went up one more to see what happened. What happened was - nothing much, I could not see/feel any difference, but I couldn't be buggered changing the jet back. So I justified my laziness by saying "better to be on the slightly rich side", and left it all alone.
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