Main Jets

[SixOne]

I took my carbs apart yesterday to give them a clean and a check over, the two outside carbs have #88 main jets and the middle two have #90 main jets. If I'm not mistaken, an L4 should have #95 as standard? Is there any reason why someone would down-jet the bike and then put two different sizes in? Anyone recommend the best place to buy some new main jets? Tim

[parby81]

well, quite often from the manufacturer the jets are a tad rich. secondly the inside two cylinders run hotter than the outside two, hence a richer jet helps an even temperature across all four. I imagine someone has had the bike setup properly on a dyno.

[zimm]

what he said,

+ if it was running ok before, leave 'em that way.

[SixOne]

See that's the problem, below about 4k it's running really lumpy and recently the revs have started hanging every now and then at about 3k when it should be iddling. Am I right in thinking the pilot screws should be 1 5/8 turns out on an L4? There's a lot of talk of 2 1/2 on here? Tim

[gavinfdavies]

minne does the fast idle thing too. does yours do it after slowing to a stop for a junction? I've heard talk of this being the accelerator/fuel pump thing over running, and over fuelling, bbut wouldn't this just over-richen the mixture and make it lumpy, rather than boosting the revs by 1500? Dunno what causes is, bbut a quick dab of clutch to just below bite is enough to slow the idle. now that i think about it mines really lumpy below 4k too. This was massively improved when i took the snorkel & filter out (now pulls like a tractor from 1.5k in a high gear!), but now far to lean in high mid-range, causing power to die by 7-8k.

[extonyg]

revs hanging at idle is a common problem for air leaks, inlet rubbers are a common culprit so i would have a check around them make sure there tight and the seals are good etc. i wouldnt of thought main jets will make a massive difference below 4k but then i might be wrong.

[gavinfdavies]

could be, the last owner was less than scrupulous when it came to then mechanicals (see the zip-ties, missing bolts, and the oil leaks!) so poorly fitted rubbers wouldn't be a surprise! but it doesn't do it all the time, normally after long hard braking, and then only for about 5 seconds. Can help with faster get aways tbh!
you're right, mj's have sod all effect that low, mainly pilot and maybe a little needle.