-----------------------------
-------------------------------------
The main steps are as follows -
1. Balance the airflow between the carb venturi, this is contentious because the adjusting screws (Air Bypass) are set at manufacture and then blocked with wax.
--------------------------
------------------------
----------------------
-------------------
Ian.
Hi Ian,
If you've removed/refitted the butterfly valves (you've said in an earlier post that the throttle shaft is new), you may have no option but to adjust these air bypass screws to get the airflow into each venturi to become identical.
I've encountered these air bypass adjustment screws on some Weber carbs e.g. 40 IDF Downdraft Double Choke, and they are meant to be adjusted to achieve identical vacuum under the throttle plates at idle, they are not sealed on this carb model.
You mentioned in an earlier post, the option of using a Morgantune type of carb synchroniser to measure vacuum under the throttle plates at idle. Have you heard of carb. spacers/insulator plates which are tapped and have a pipe stub to which these carb gauges can be easily connected. These spacers were originally marketed to allow the fitment of a Vacuum Gauge (to monitor engine condition, air leaks etc. without having to remove and tap the inlet manifold. I can't recall who sold them, possibly SPQR/Paddy Hopkirk/Oselli Tuning/Gunsons? Perhaps you could drill and tap your existing carb spacer plate or fabricate/fit your own design?
Checking mixture at idle - gas analyser or Colourtune device (modern ways).
Old School - remove each spark plug lead (or short to earth) in turn, drop in revs should be identical if throttle opening/mixture at idle is identical across all cylinders. I suspect this won't work on a twin cylinder and might not be advisable to try if engine is fitted with electronic ignition...
Old school - allow engine to idle for 15 minutes. Remove and compare spark plug nose colours - should be identical if mixture is identical on each cylinder.
A vacuum gauge can also be used to indicate mixture strength but afaik it's not very accurate - better than nothing though? So if you connected up a Morgantune to balance air flow, you could then use it as a guide when adjusting the mixture volume screws?
Another thought re. balancing the air being drawn into each venturi.
Have you checked the cylinder compression pressures on this engine.
If the 2 cylinders are not producing the same compression figure, then maybe the two cylinders might draw different volumes of air, so could never be exactly balanced? I've always paid more attention to the initial pressure produced in a cylinder (1st compression stroke) than the final pressure achieved. Or then again, at idle, this might not make any discernible difference?
AL.