i've had a couple over the years mate, both at the time were on my daily drivers too (couldn't afford 2 cars back then). one had 'proper' mechanical secondary tunnel ram carbs, the other had vac' secondary carbs which when you were driving normally was effectively like having a single double pumper as only the primaries got used. it was also better on fuel than the mech' secondaries were. driveability was fine with no bogging or flat spots. eventually i'll fit 2 impco 425 lpg 'carbs' and do away with the petrol system all together. using the fork lift type propane bottles it only costs about 1/4 the price per litre compared to petrol so if the car does 10mpg then for the same cost money wise it's the equivalent of 40mpg. that and lpg is 110 octane so i can run a bit more timing advance without pinking so more power too, win win.
neil.