Don't jump ship on the dual quads yet....I run them in 100*+ heat in Orlando with no issues. First rule: COOLANT TEMPERATURE HAS LITTLE TO DO WITH PERCOLATION. Your car can run at 160* and you can still get percolation. But first things first. Simplest to hardest:
1) Check tank venting...you should have a long rubber tube going from the gas tank to the gas tank filler cap cavity that is your tank vent...it MUST be clear.
2) CAREFULLY check the ENTIRE PCV system - from the special crankcase draft tube hole adapter gaskets (and tightness), the rubber line, the PCV valve, etc..
3) Check the snugness of carb mounting nuts - take a 1/2" wrench and just give them a twist to make sure they are seated well. Take a screwdriver and snug the 18 screws in the top of the air horn on each carb. You have to take the air cleaner-to-carb gaskets off and the little vent cap cover off the front of the carb to get to all the screws...do NOT over tighten. Test drive afterwards.
Check throttle linkage for smoothness and COMPLETELY returning to idle via the return spring - make sure nothing is binding at the ignition shielding.
4) Check engine for vacuum with vacuum gauge and if its fallen off from the baseline or the gauge indicates problems check for leaks. With the vacuum gauge on the car go ahead and reset the idle speed and mixture adjustments.
5) Get a Harbor Freight I/R temp gun for $26 and shoot the carb bowls when this problem occurs...if the temps are heading north of 150* you might well be percolating - if so get back here; there are things you can do to help (which I have done). Its unlikely though if the carbs were running well for a long time before the problems. Make sure the steel carb fuel line coming up from the pump is well away from the engine head - bend outwards if needed (carefully).
6) Check your fuel pump pressure (if you have a vacuum gauge that will do the job); the pressure should be between 4.5 and no more than say 7.5. Check your fuel filter in the glass bowl carefully and also the sintered bronze filters in the carb inlet passages. Any debris or crud in these devices will indicate a problem. Run the carb line into a mason jar with the coil wire disconnected and crank the engine...you should have good flow (this is DIFFERENT than pressure) and also no debris or crud. These issues are also somewhat unlikely if the car is running well at highway speeds though.
7) At this point you will have to open up the carbs and check the float level and drop and inspect/replace the needle valves (possibly) and check for general cleanliness, issues, etc..
I have a large manual on these carbs I can email you (too big to post) if you provide an email address...
Last edited by Frankie the Fink; 08-09-2013 at 07:20 AM.