Hello all,
Just finished the re-build of a '71 Commando so thought I note thecarb set up I found works.
When I bought the bike it wasn't great, bent frame, collapsed iso's etc.Bike came with a single carb conversion with 30mm Amal MkI, it didn't run well when back together. Decided to keep the2-1 manifold and fit the rightcarb. Asyou can get a MkII to MKI manifold flange adaptor I bought one and a34mm MkII carb.
After a bit of fettling found the followingset up worked well-
Pilot Jet 20
Slide 3
Needle Jet 106
Needle 2A1 (set to lowest notch, richest setting)
Main Jet 270
air filter- K&N RC-2880 (no need to modify filter back plate)
Pretty standard tuning I think.Main point of this thread is that I had apersistent problem ofoverrun when shutting the throttle. With a cold engineblipping the throttle would cause a steady 5000rpm overrun requiringa hand over carb intake tosettle revs back to tick over. This improvedas the enginewarmedbutan over run was still present when shutting the throttle.Throttle cable resistance was minimal.
After much fault finding figuredthat the Venturi effect of2 cylinders on a single carb maybejamming the slide against carb bodyjust before it hit thestop. I tookoriginal MKI carb spring and combined it with the MKII springto createastronger closingforce. This cured both the cold and the warm engine overrun issues.
Bike runs great and the throttle response is as crisp as Quentin!
Hope this is of some use.
I spent some time fiddling…
- Log in to post comments
Previously david_evans wro…
Previously david_evans wrote:
The SU was sometimes difficult to start, though I believe you can get a priming pump for them. The single Mikuni was good, fit and forget.
I have an SU reconditioned by Burlen, no 'priming' issues at all. Did find the single Mikuni 'on/off' choke a liability if driving straight into stop/start traffic from cold.
- Log in to post comments
I spent some time fiddling with a 32mm Mkll concentric after boring it out to 33mm. The main issue was it sticking open and I put this down to carb icing. I fitted a velocity stack as well and this worked very well. I have since been through SU, Mikuni VM 34mm, and my current setup is Two PWK flat slides supplies by Jim Schmitt in the States. The SU was sometimes difficult to start, though I believe you can get a priming pump for them. The single Mikuni was good, fit and forget. The twin flatslides are easy starters and a solid tick-over with good performance. The single Mk1 concentric runs out of steam at above 4500rpm.