I've upgraded my CB77 to 350cc and increased the valve size to as large as practical and while it has given a good improvement I believe that the original carbs are limiting? I've seen 2 alternatives which are Mikuni VM28mm or a pair of the carbs that were fitted to the K0 CB750.
A new area for me and I'd appreciate any advice on what may work without ruining the nature of the bike.
Carbs for 350cc upgradeThe inlet valves are now 38mm and the ports have been gas flowed to blend in the increased diameter otherwise the ports haven't been opened up. The advice I was once given regarding carb size was that they should be around 75-80% of the inlet valve diameter which would make the existing carbs a size too small for 350cc and the bigger valves. I believe that there are quite a few bigger bore CB77's around and possibly a few that have the bigger valves too, I just wondered if anybody had gone to the next step of increasing the carb size to match? I've heard that the Mikuni VM28's work well as do the carbs that were fitted to the K0 CB750's also Amal 625's? The 'big' valves I have seen for race use are from Race Engine Components and the inlets are 33mm. I have some Keihin CRs (29 or 30mm I think) on my 350 race bike and have some SL350 carbs (similar to 750K0) to try on another engine I am building.
http://www.gsvalves.co.uk/race-engine-components.html  38 is quite a big step up from standard so you are probably in new territory although Jerry Kooistra has used some pretty big valves in his "500" CB77 based racer. G
'60 C77 '60 C72 '62 C72 Dream '63 CL72
'61 CB72 '64 CB77 '65 CB160 '66 Matchless 350 '67 CL77 '67 S90 '77 CB400F
I've got a 350 big bore kit from BoreTech in my CB77, I'm using Amal 626s copies from Wassel - I believe Vince Lupo is using Amal 626s as well. My head is stock however. Who did the valve work and where did you get the valves from? How did you come up with the new sizes? I've currently bought a used head from Ebay to try and fit a Megacycle street cam to, so I'm wondering if I should consider opening up the valves - and if so, to what size.
Hope this helps BTW, with the standard head and the big bore kit, the Amal copies and a Sachse electronic ignition, the bike really scoots now! The valves are based on valves that were used on good CB77 racers. They were supplied by Bernard Saunders (01451 822397). The work on the head to accept them was done by Race Techniques (Mike or Andy on 01242 245640) who have a SERDI machine which I was told was essential for accuracy. I've had 2 bikes done, ones running and on the road and the second one's almost ready. Both are waiting for upholstered seats as they had the wider, longer optional extra seats that were available in the day. The question I have long term is if there's any advantage in upgrading the carbs too which are possibly now a restriction. No hurry though because the bikes run beautifully as a 350 with the standard carbs.
Thanks for the info! Are using either bike on the street or are these vintage race bikes? Also, I assume that you had to rejet the original carbs, do you happen to have the jet and needle specs for what you're running? In general I've been happy with the Amals, but I've got a little low to mid-range softness and some backfiring off throttle that I've been trying to clean up, but the Amal's adjustability is problematic. I've been thinking of putting on the old Keihins to see what I can do with them, I've got a jet kit for them that I've never used.
Thanks again for the info, Jack
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