Missing Concentric Idle Jet?!


Well Known and Active Forum User
VOC Member
I'm sorry if this has been covered before, but I have just spent an hour looking for it using the 'Search' facility to no avail. There's some great Concentric input by Martyn, Craig and others but nothing that helps me with my conundrum.

I am working on a mate's Shadow and, unknown to him, he had a massively weak mixture on the front pot only as evidenced when I removed the exhaust pipes. It didn't take me long to discover not one, but two, big air-leaks in the 'orrible two-piece clip-together inlet manifold. I've replaced that with a nice alloy 30mm double-flanged one from the from the VOC Spares Co Ltd.

So all that is left is to strip, clean, check and reassemble the 930 Mk1 Concentric carb. A new experience for me after 45 years of Vincenteering (keep 'em standard I say!). The carb was nice and clean (quite new I think) but what is that unused threaded hole in the body - and where is the idle jet? Photo attached. The threaded hole is at 1 o'clock; is that where the idle jet is supposed to be?! In which case, what size is recommended? (Incidentally it has a 190 main jet and No.3 slide - comments?).

Thanks in anticipation,

Peter B
Bristol, UK.
STILL WANTED: Series 'A' Front Frame.


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Chris Launders

Well Known and Active Forum User
VOC Member
Only the very early ones had pilot jets and these are probably scrapped by now, if you take out the air screw you should be able to look down the hole and see the restrictor, if you go to thevincent.com there is a full explanation about this and what to do. My twin has 32s and I couldn't get to tickover properly (too weak), followed the guide, drilled out the restrictors and fitted 30 pilot jets, ticked over much better straight away.

Bill Thomas

Well Known and Active Forum User
VOC Member
Hello Peter, Mine are 28mm L/ning engine, I need 30 jets, Ron's Comet, Runs better with a 25 But starts bad, So he has took it out !! Sounds like another case of Suck it and See, Cheers Bill.