Hitchcocks Forum

Ok, I know a single carbed Connie should have a Monobloc fitted, however the one I had was shot, so on went a "premier" Concentric. The monobloc would have had a 3.5 slide, 300 main jet, needle position 2.
On the concentric I have fitted a 3.5 slide, the main jet is a 210.
The bike (oh did I ever have issues with it) is running reasonably well, HOWEVER when accelerating (racing between roundabouts) the engine bogs down a bit, the engine response (power) improves if I back the throttle of slightly. I could do with a little guidance in the right direction...seems the older I get the more things I forget? Which reminds me: I fitted a bellmouth with a tea strainer/gauze and some foam to it to stop crap getting sucked in, bad/good idea?
Hope you can point me in the right direction, thanks

This is a bit difficult to unpick but here's a couple of ideas: the Connie fitted with a single Monobloc would have had a 320 main jet (I'm basing this on the 1961 model) and I *think* the rule of thumb for calculating main jets for an equivalent Concentric is to reduce size by 20% so your main jet ought to be 254, much richer than your 210. As they go up in 10s you have either a 250 or 260 to choose from. All this assumes your filter arrangements are roughly equivalent to what you had before.

REgards, Mark

Thank you Mark. I knew there was a percentage figure somewhere for converting monobloc to concentric main jets and you've just jogged my memory cheers.

Update: problem sorted! I ditched the "filter sock" that had been fitted to the carb bellmouth (gauze type), an old short amal bellmouth (no gauze) was fitted in its place. A 260 main jet was then fitted. Engine now runs cleanly through the entire rev range, no bogging down, spitting or anything. Plug colour after a chop was perfect.

If anyone thinking of fitting a single concentric to a Connie wishes to avoid the fun & games I went through then the settings are: 928, 17 pilot jet if using a Premier, 106 needle jet, 3.5 slide, needle pos 2, main jet 260, no gauze on intake or air filter. Hope that helps anyone in a similar situation