I have a '58 Chevy Apache stepside 235 ci which has had an uprated engine and has been fitted with an Offenhauser manifold and twin Rochester Bs. This seems to have caused two problems. The first is that the linkage doesn't return properly when idling so it runs too fast but if a stronger accelerator spring is fitted the pedal won't go down. With one carb the linkage was directly below the carb and so would pull the lever down but now it has to pull both down from a centre point, so it's a clumsy operation. The second is that it runs too rich and the plugs are always black. I've tried carb kits, smaller jets (48s) and now new carbs but nothing seems to work. I've read many times that it should only have one carb and two never run properly. When I adjust the mixture screws they're ready to fall out, and I never need to use the choke. The dual set-up is so popular that I can't believe it's impossible to overcome these problems. Any ideas?