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As part of a very long process in nailing down what was wrong with the way my 450 idled I threw a bunch of money at a set of Mikuni VM32 carbs. Bought one of them kits from Power Barn which claims to be set up for the 450 and winced mightily at not only the sticker price, but the import duty I had to pay. Swapped them in, and they made absolutely no difference in terms of the problem I was trying to fix, so I swapped them out again and resumed the search for the source of the idle issue.

Fast forward several years - the underlying problem has been identified and fixed. It had nothing to do with the carbs (no surprise there) and the bike runs pretty much fine with standard carbs/settings. While shifting some stuff round the garage, I stumbled over the Mikuni's; they'd sat in a box ever since their first unsuccessful outing. Remembering just how much they'd cost - and knowing that I'd never given them a fair go - I thought I'd try them on again.

Results were mixed. Starting the bike was hard work, idling was stable but the bike kept dying; mid-range seemed very strong and the upper end (6-7,000 rpm+) was somewhat inconsistent. Looking at the plugs, I'm suspecting the idle jets are w-a-y too big - the carbon fouling was very pronounced after even a couple of minutes idling and backing out the air screw didn't really make much noticeable difference. Doing a plug chop after a prolonged run at about 5,000rpm showed a good mid-brown. I've yet to come to any meaningful conclusions about the top end because there were periods where it pulled and ran fine and others where it was choppy. That could be a result of the carbon fouling or maybe a slightly undersized main jet.

Right now, I don't know exactly what my jet sizes are but from other comments I've seen, the idle jets are likely to be 40's out of the box. As I say, I suspect whatever's in there is too large for a road going 450, and the kit came with a pair of 30's and 35's, so I've got options. I've also got three sizes of mains - from memory they're something like 150, 165 and 170's but it's not like they're expensive.

All that to say has anyone dialled in VM32s on a 450 and if so, what jet sizes/settings worked? I've tried searching the forum and as some will already know, the search function is at best unreliable and at worst unusable. I've also tried Googling with the site name first and came up with a long thread on Mikuni's on 350/360's but nothing much on the 450's.

For the sake of completeness, I think the pistons are about 0.5mm oversize (done to clean up bores, not for outright performance gains) and the tops look very clean. Pamco ignition (that's what fixed the original problem) and standard camshafts. No other engine mods. Valve clearances, cam chain and ignition timing all set up correctly. Ignition coils are standard and good. Battery is good and plugs are B8ES.

Thanks in advance.
 

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You know where to ask ;-)

I know Tom runs his CB450 with Mikunis
 
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