40* total timing just reaffirms that the compression is detrimentally low.
No heat with that lil squeeze, losing efficiency there.
Now understand that the power valve is designed so that you can use the leanest cruise jetting possible but under load/xelleration give the proper enrichment.
Carbs in general do not yeild poor milage, however poorly tuned carbs do.
Without a trained right foot the dbl pump can be hogish, but lets look at this.....
A vac secondary has an xellerator pump, so does the dbl pump
A vac secondary has a power valve, so does a dbl pumper
A vac sec is square bore, so is a dbl pumper, so cfm is split between 2 two barrels.
The common dbl pumpers are calibrated for street cars, just like the vac secondaries.
A lot of times the street calibrated carbs get lean in the r's, thats why the designer race carbs use a richer calibration...nt a bad thing cause you want the ideal ratio for max hp and to keep from puttin a hole in the piston.
If you can run 40*, have tried locking out the distributor to 40* and seeing if it will still start fine fully warmed up?
A higher initial timing will help power and milage as well.
I run 26* initial w/340 & 34* total, it wont start with any more initial.
You have too low compression, thats part of the problem.
for comparison-I run a 750 dbl and get 15-16 mpg w/ 280* cam and ported x iron on a 340
