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I'm running mikuni BST 36ss carbs from an '89 GSXR 750 on my '86 GSXR 750 with mods. I've got it jetted as good as I can and it runs perfect, but if I open it up from the bottom of a gear it runs super fat in the midrange (stock needles and tubes for '89 GSXR 750) those carbs are jetted rich from factory for the short stroke configuration of the '89 motor, the '86 longer stroke draws in more fuel and thus needs to be jetted leaner. The '89 uses a 5FZ91 needle that I cant find specs anywhere for, so I have nothing to compare it to in order to find something much leaner. Im afraid if i were to change the emulsion tube (forgot the series number, but its a Y-5) itll throw off the pilot circuit jetting. Please advise
 

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Don't know off hand, but I have an .xls spreadsheet of Mikuni needle dimensions. Forum won't allow .xls upload so pm your email & I'll forward a copy.

Hope it helps.
 

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PM'd, or rather I began a conversation as I couldn't see the option for private messaging. I went to Mikuni's website and Jets R Us and couldn't find any info on these OEM needles
 

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I played around with my settings a bit today. I took out those nylon spacers that go under the circlip on the needles to lower them. It cleared up that super rich midrange flat spot quite a bit but its still there. I developed a lean flat spot at 1/5 throttle that was fixed, or almost fixed, by turning the mixture screws another 1.5 turns out which puts them at 4.5 turns out and its still there. When I get more free time I'll just take those needles out completely and plug up the openings which I know will fix my lean spot issue, lol. soooo, word from the wise: DO NOT put 36mm slingshots from a short stroke 748 motor onto your long stroke 750 thinking it'll work without changing much, because they won't (granted my setup is quite a bit different than the average 750, but it still shouldn't be that far off)
 
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