Originally Posted by Wirespokes
How did you achieve the 12:1 CR? Is it dual plugged? I'm curious about the specifics it took to make it run well. It was running the stock cam previously with the 1050 pistons, large valve heads and 40mm Bings? Any mods done to the advance and timing?
12:1 (185psi measured cold) is achieved by using a pair of '77 RS heads modded to suit the 1050 Venolias. Specifically - the Venolia's flat periphery. Think squish band as it was meant to be. (BMW's famous "squish band" top ends of the '77 S's and RS's are over 0.100" deep. ie: nice idea but practically useless).
Heads are machined for dual plugs, but I don't use them. I've had problems in the past with reliability from Dyna and Accel coils, so am a little shy about running dual plugs. I'm convinced it would run better at the bottom end with dual plugs, but the squish band is so effective it runs fine with only single plugs. Pre-ignition is not an issue, though I do need to run cooler plugs over the stock set up.
Previously, with the stock cam, modded heads and 1050 Venolias it ran great with 40mm carbs modded slightly richer - increases in torque and power were dramatic. Made off-road and trail riding especially easy with it's huge gains in low end torque/power. It'll idle up walls. Also, fuel efficiency increased to a fairly consistent 55mpg (imperial) which is better than the best of 44mpg it achieved in stock form.
Ignition system is completely stock except for a Harley Davidson coil I fitted in a pinch to replace a failed Bosch piece. It's been there 40k kms now and works without any discernable issues. Though "in theory" it's not idealy suited to the system.
muter mutter mutter...