The intake manifold gets plenty hot w/the exhaust manifold right underneath the intake manifold, esp, like John said in Ca, you really should not need carb heat, I never ran it.
It does sound like you have a vacuum leak as John stated.
Also,,, what color spring are you running in the vac ,secondaries? you stated a mechanic put a lighter weight spring in it? I would think,if it's a lighter spring, the secondaries will open too soon, causing it to possibly to bog?
I have an Offy intake if you want to buy it?
It will make a night & day difference in torque!!!
