CB Perf 34mm solexs, Bigger venturi's

[RSorak 71Westy]

I have a pair of CB Perf's 34mm Solex's on my bus. They have the 22mm venturi's.....CB now offers 28mm vent's and asst'd jets....Do you think my engine would benefit from the larger vents, and if so, how might this change my jetting needs? My heads have been pocket ported and I have header......

[Amskeptic]

I have a pair of CB Perf's 34mm Solex's on my bus. They have the 22mm venturi's.....CB now offers 28mm vent's and asst'd jets....Do you think my engine would benefit from the larger vents, and if so, how might this change my jetting needs? My heads have been pocket ported and I have header......

As a general rule, the size of your intake valves is the best guide to venturi size. Porting and profiling valve guides will let you go slightly higher, but please remember the important effects of air velocity, you want velocity for best mixing of fuel charge and velocity will also give you better torque due to the bigger "bang" when the valve closes (which gives you a return pressure wave when it opens the next time.

Consider the difference between torque and horsepower.
Torque is the twist force. Horsepower is the rate of rotation X twist force.
Torque gets you off the stop. HP gives you the pull over a longer period of time as you run up the rpm range.

The stock engine gives you a decent little band between 1,700rpm and about 4,400 rpm. Anything you do to improve torque in this band is what I call daily driver useable. Better launch steeper hills in 4th, etc.
You want the ported polished extra valves/lift/duration/bigger venturis/headers/etc if you are planning on extending your useable horsepower past 4,400 rpm, like pulling 3rd gear to 55 mph, at which point you will be visiting screaming generator and fan, ovalized main bearing saddles and fatiguing valve spring harmonics.
Colin

[RSorak 71Westy]

As a general rule, the size of your intake valves is the best guide to venturi size.

By this rule I definitely want them....As I have 22mm vents and like 38mm intake valves.....

The heads needed the pocket porting very badly, I'll show you the pics, when I see you at the end of the month...even for low RPM flow....

[RSorak 71Westy]

Doing some more research, I think I have 33mm intake valves and I found this quote on aircooled.net "Venturi size (the narrowest part in the carb throat) should be about 3mm smaller than intake valve on stock or mild engines, and about SAME size as the intake valve on high output engines. "

So by this I want 30mm vents but will have to settle with 28's.