Car starts one click races to 2000 rpm will not idle down! Runs smooth as silk at high rpm as i turn key off it back fires with a loud boom! I assume choke on first issue but boom. I am Afraid to start this beast now. Any crazy suggestions?

 

Have you checked your timing?

 

Have you looked for vacuum leaks??

 

Actually, what fuel pump are you using? What is the fuel pressure at the carb inlet?

 

When you turn the key off at 2000 rpm all you are doing is removing the ignition source. The engine is still operating as a vacuum pump as it winds down, consequently it still draws fuel through the venturi. The boom or explosion you are hearing is the hot exhaust igniting the fuel within the exhaust system. Sometimes even enough to blow a muffler off.

 

Automatic? Vacuum modulator on trans bad? That will cause very high idle.

 

what year, did you set the choke high idle and idle speed.

 

Does the carburetor have an electric choke? If so, an electric heating element inside the choke spring assembly heats the spring and causes the choke to open and the idle speed to step down from cold idle, aprox 1800 rpm to the normal warm idle RPM, 650- 850 rpm. The element heats the spring at a rate comparable to the engine warming up.
There should be power and ground to the choke spring with the engine running. If there is no power or no ground the spring will never warm up and open the choke and the engine will idle at cold idle speed. Power to the choke spring should pass through an oil pressure switch. This is so if the car is idling cold and unattended and the engine stalls the spring doesn’t continue to heat up and open the choke making it difficult to restart the engine.
If no power at the choke spring, look for a 3 prong oil pressure switch, usually screwed into the block above the oil filter, or screwed into the block near the distributor. With engine running and making aprox 5-6 psi oil pressure the switch will close and send power to the choke spring. The choke spring ground wire can run to any reliable ground location (engine intake manifold usually).

 

What year car and what kind of carburetor do you have ?

 

Sounds like a vacuum leak to me

 

It is another carb rochester i rebuiltToday while i rebuilt my original because the float was sticking! Also the float was to high so i adjusted the level and now the carb i mentioned is giving me heart burn as did the other. It all started with a leak at the inlet fuel fitting i normaly use teflon but the problem was the nylon O-Ring they are not as durabel as NOS. From here on i will use liquid teflon on all fuel joints , just a sure leak proof joint! Today i will adjust the choke to see if i can get idle to come down on my divorced choke!..There are no vacumn leaks and the timing i will double check today also!

 

Look at the base idle screw and see if throttle lever is touching it . If not the fast idle linkage and cam from the choke is engaged , , if it is free and not sticking the fast idle screw may need adjusted

 

Many 'high idle' conditions are due to system vacuum leaks which FORCE you to open primary throttle plates more than should be required. Normally, the primaries should be almost closed. But excess air intake (due to vacuum leaks) will require further opening the throttle to get a decent fuel mixture...then the idle is too high.

You might also have excessive 'high idle' adjustment or poor choke system adjustment, so that the high idle never drops off to 'curb idle'.

Other possible causes are 1) throttle plate binding with the carb gasket or manifold so that throttle will not fully close; 2) improperly adjusted throttle cable system; 3) inadequate throttle return spring or binding in that system.

 

I think your fuel pressure is too high.