Cb500 Four K3 Jetting

[Nashfly]

Hi all,

Carb rebuild time on my CB500 K3 to RC181 Replica project (I'm in the UK, but it's a US model)

I'm planning on running uni foam filters and straight through 4 into 4 exhausts and looking for suggestions as to jet sizes

My normal guesstimate would be to up the Main Jets by 3 sizes (100 to 115), up the Pilot Jet by 1 size (40 to 45) and raise the needle by one notch (from middle to 4th down)

My thinking is that should make it on the safe side of rich and I can tweek it from there

I've searched the forum and have read that keeping as much of the original brassware as possible is a good idea, also that 'Nurse Julie' on this forum is a good source for O rings (other things as well)

Any suggestions as to where to purchase quality jets and gaskets (preferably in UK)?

Thanks in advance for any assistance

Pic is where I'm at with project at the moment....

[McCabe-Thiele (Ted)]

That's a neat looking bike - it will be interesting to see how the build progresses.

[K2-K6]

Lovely looking bike, a great era to replicate too.

I saw a Hailwood 500 (Donington collection) being run some years ago now, standing on the "Dunlop" straight right by the barriers as they properly wound it down there. Absolutely, brilliantly memorable 

A starting point to offer in regards to jets......1 or two larger on idle, standard mains, lift needle (clip down) one notch, run it on a NGK 9 heat range or equivalent to assess and see what you've got.

If possible, use a bellmouth inside the filter, even a very short one can make a significant contribution to help the carbs work well. Thinking just turned from nylon or similar to fit carb stub and be inside foam filter.

[Johnwebley]

A very nice replica

I know of a cafe racer 750 sohc,

He has fitted an early 750 Suzuki kettle double sided 4 leading shoe drum brake, looks very like the original RC181



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[Bryanj]

Not a K3, that had an engine like the 550 with smaller bores. Probably a K2 which never officially came to uk

[McCabe-Thiele (Ted)]

I like the look of those silencers - bespoke I guess.

[philward]

Reference the jetting advice, based on K2-K6's suggestion and after several variations, I went a similar way with my CR750 Rep (race exhaust with basic baffles and foam filters). Ended up with standard mains (did try larger), 1 notch richer (down a notch) and 1 up on slow runner jet size. Having tried several combinations/variations over time, the biggest improvement in 'across the range' performance was the 1 size up on the slow running jet size - seemed to smooth the transition between lower revs to higher range running