Hi Eddy,
I am contemplating fitting a 28 or 30mm Mikuni to my Comet but have NO experience with setting them up. Are you able to share with me the configuration, jets selection etc?
Many Thanks,
Martyn
Dear Martyn,
When I first built my 1948 Rapide from a pile of mostly worn parts I fitted two second-hand Amal Monobloc carburetors. (I was on a tight budget back then.) The bike ran quite well on these and once I'd got the bike registered and on the road (late 2008) I completed a few thousand miles. Although it started easily and ran well the slides would often stick if If went over half throttle. I tried a lot of "fettling" but never really got it sorted. This is not a condemnation of the Monoblocs, they were very second-hand but served the purpose of getting the bike up and running. Stuck open throttles can lead to interesting experiences that are probably best avoided!
I thought about having the Monoblocs sleeved but when I looked at how much this would cost it was as cheap to get two new Mikuni VM28 carburetors. Originality is not important to me so I've used the Mikuni carburetors for most of the 50,000 miles I have put on the clock.
The settings as supplied by Motocarb were:
Main Jet 190
Needle Jet P-6 (182)
Needle 5FL 14
Slide 2.5
Pilot Jet 30
Air Jet 2.0
That is what I used and suggest that these would be a good starting point that you could test and adjust to the engine configuration on your Comet. I put the needle in the middle slot and experimented from there. There are lots of different needle types for Mikuni carburetors but for a Rapide it doesn't seem critical. I reduced the main jet to a 160 but now that I am now running 8:1 pistons and a straight-through Armours silencer I will probably increase the main jet size once the Covid 19 restrictions allow us to get out on the road again to do some testing.
An observation on the Mikuni carburetors is that they seem to be very well made. The aluminium body casting is of good quality, possibly LM24. The slides feel quite weighty so are probably brass and they are chrome plated. There doesn't seem to be any significant wear.
I've no recent experience of recently made Amal carburetors but would think (hope) that the materials used now would be better than the built down to a price of the originals.
I did have a new Norton Commando in the early 70's that had two Concentric carburetors that worked OK. As a teenager, I had a BSA Gold Star with an Amal GP carburetor and my brother used Amal TT carburetors at times when we were vintage racing. The GP and TT types seemed well made with better materials.