[Roadsters] Carb tuning

[stan]

Hello all!
Note I will be using spell check with this email.
 
Anyways, I have been trying to tune my SU carbs for my 69-1600 and seem
to have hit a stumbling block.
 
I understand how to set the idle screws for each carb, as well as sync
them using a syncronometer.
I've don't that successfully.
As far as setting the jet height under each carb I haven't figured that
one out yet.
I turned the mixture screw in all the way and then turned it out 2 turns
as suggested on several of the websites.
>From there no adjustments have been made.
 
Here is what I've run into so far:
 
*	the motor fires up fine and idles so-so
*	when I rev the motor slowly the rpm's increase nicely
*	when I punch the motor at low rpm's the motor gasps for air and
several backfire pops occur
 
What I cannot figure out is that at high RPM's I can punch the throttle
and it will respond quite well.
 
What would cause it to backfire so heavily at low rpm's?  I would assume
non-sync'd carbs but as I stated I balanced them out
quite well.
 
If anyone can help me out I would greatly appreciate it.
 
Best regards
 
Cheers from sunny Vancouver, BC
 
Stan