danhar707 Posted April 9, 2013 Report Share Posted April 9, 2013 Just acquired a beta 250 rev 4t 2008. All good apart from poor running on full throttle mid speed....does what id call 'baulking', i think going too lean. Feels fine off idle when just hopping around. Tried adjustments to the air screw and mixture screw with no success and only managed to make the engine speed occasionally stay high on drop to idle.... this along with the bike's lack of use made me think the carb might be gummed up.... With the carb out (ball ache to remove), all looked very clean inside and out but i went to town with the carb cleaner anyway. The mixture screw had been replaced with a red Jitsie one. Whilst pulling this out a tiny disintegrated o ring appeared from nowhere. I couldnt see how it fitted and it was too damaged to refit anyway. Wanting to get out on the bike that avo, i decided to go ahead without it. Result was even worse running, which was overcome by taking the mixture screw quite far out and this gave a reasonable afternoons running. Can anyone confirm that o ring came from the mixture screw and how to get a replacement? Any other tips for sure fire success with this carb? Ive read all sorts of ideas. Hoping for the definitive step by step guide to 4stroke sweetness! Quote Link to comment Share on other sites More sharing options...
zerorev3rev4 Posted April 9, 2013 Report Share Posted April 9, 2013 the seal is from the screw and without it it would suck air making it leaner you might after get a new screw from jitsie with the seal on unless lampkins can provide one but carb parts for the rev 4 are limited you might get one from an o ring selection box to fit ,, it sounds like it needs a larger main jet or try lifting needle with a washer or small seal , i take it theres no mods to air intake or exhaust ? an iridian plug new plug cap and hi octane fuel could also help Quote Link to comment Share on other sites More sharing options...
neil king Posted April 10, 2013 Report Share Posted April 10, 2013 Do yourself a favor and get the R & D remote screw. Its for an FCR but it works fine in the BSR. Quote Link to comment Share on other sites More sharing options...
pindie Posted April 10, 2013 Report Share Posted April 10, 2013 I second the new seal on the screw. If it don't solve the issue I'd look at needles etc as this is where the work is done once of the pilot (throttle closed to 1/8th open) turns into the needle jet at larger throttle openings. Getting the transition right between these two is key for four strokes or you'll get spitting and popping with flat spots and coughs. There is a online guide to set up 4t engines. I think it may be on the Scott Summers website. It is for XR600s but the principles are the same for all carbed bikes. I wouldn't worry to much about the wide open throttle settings unless you do a lot of road work etc. Quote Link to comment Share on other sites More sharing options...
copemech Posted April 11, 2013 Report Share Posted April 11, 2013 http://www.partzilla.com/parts/detail/yamaha/YP-3H1-14147-00-00.html Quote Link to comment Share on other sites More sharing options...
axulsuv Posted April 11, 2013 Report Share Posted April 11, 2013 Gee Cope , I didn't see Beta in the bike list ... Cool location for a tiny o-ring though .... And danhar 707 , when you get a new o-ring ,give it a shot of silcone spray before you install it on the mixture screw , just a wee bit will extend it's life greatly . Glenn Quote Link to comment Share on other sites More sharing options...
copemech Posted April 11, 2013 Report Share Posted April 11, 2013 (edited) Gee Cope , I didn't see Beta in the bike list ... Cool location for a tiny o-ring though .... And danhar 707 , when you get a new o-ring ,give it a shot of silcone spray before you install it on the mixture screw , just a wee bit will extend it's life greatly . Glenn Thought you would like that Glenn! Actually I have no idea what is on them, but if true as Neil says it is a Mikuni BSR, (or just a MIC) seems common o-ring for application, and yet strangly enough, the part number seems to have been deleted(no longer supplied) in many applications from Yamaha. There should be some out there, but I am sure it is a little POS that is hard to match up! Edited April 11, 2013 by copemech Quote Link to comment Share on other sites More sharing options...
nzralphy Posted April 11, 2013 Report Share Posted April 11, 2013 Yes you will need the oring, and yes it will make the bike sick without it. Check the end of the Jitsie air screw has the tapered needle still attached - mine stayed in the carb when it broke off.... so I got a replacement. I run mine about 2.5-3 turns out. The bikes are shipped from the factory lean in the midrange to pass emissions standards. I, and a heap of others, have raised the needle right up (put the clip on the bottom notch) for an improved bike. Do the same mate and see how it runs.... it will have a heap more power. Ralphy Quote Link to comment Share on other sites More sharing options...
neil king Posted April 14, 2013 Report Share Posted April 14, 2013 It's a BSR 33-79. There's not much info out there on them but some Yamaha big bore 4 wheelers used them. Quote Link to comment Share on other sites More sharing options...
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