1966 rough start and idle

[nobadlie]

I bought 1966 thunderbird 390 engine which starts rough and after a few minutes it warms up it is still rough but not as bad. It drives well for a couple of miles then when I come to a stop it feel like it want to choke out, but it seem to be fine I step on the gas. I’ve replaced the carburetor with an Edelbrock 1406, PerTronix 2 and a Pertronix coil. Any suggestion on what might be the cause for the rough start and idle.

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[jack martin]

I bought 1966 thunderbird 390 engine which starts rough and after a few minutes it warms up it is still rough but not as bad. It drives well for a couple of miles then when I come to a stop it feel like it want to choke out, but it seem to be fine I step on the gas. I’ve replaced the carburetor with an Edelbrock 1406, PerTronix 2 and a Pertronix coil. Any suggestion on what might be the cause for the rough start and idle.

Check the Power brake unit check valve or a leak in the hose connecting to the PB.

 

[74 Harley]

After you check for vacuum leaks, go ahead and do a compression test. Do you know how many miles are on the engine?

 

[64ZCODE]

Sounds like a fuel/air mix problem. You can eliminate possible vacuum leaks by disconnecting and plugging vacuum lines that connect to the carb or the intake manifold. You could have a manifold intake gasket leak.

Was the carb you installed new or used? An old carb with air leaks or out of adjustment could cause what you're experiencing.

 

[Ward 57]

Did you have the problem before you replaced the carb and is that why you replaced it? It sure sounds like a vacuum leak to me. The trick is to trace it down. Mine turned out to be a broken vacuum line at my wiper motor and it was so brittle it just broke off when fussing under there.
Get a can of carb cleaner and when idling spray around the base of the carb and intake manifold. If it smooths out you know where to look.
Ford loved vacuum controls until electric servos became reliable. As stated above, try to disable any vacuum lines at their source and see if that helps. Then trace any rubber vacuum lines, the weak spots are at the connections.

 

[Lineforker]

Bought this car about 3 months ago. It ran rough when it got here from Nevada. I replaced the timing gears and chain. replaced wires, plugs, distributor cap, coil and put pertronix ignition in. Set the timing. It's rough when it idles and gets worse under load. It backfires when applying fuel. The plugs are black with carbon every time I pull them. I checked compression and it is 135-140 across the board. I checked the vacuum with a gauge and only have 12-13". I have checked for vacuum leaks and blocked off all ports with no solution! Also sprayed the base of carb and intake manifold with start fluid with no change to problem. I am at a loss. I also puled valve covers to check for a weak lifter and they are all tight. Any help or suggestions are greatly appreciated.

 

[Lineforker]

I bought 1966 thunderbird 390 engine which starts rough and after a few minutes it warms up it is still rough but not as bad. It drives well for a couple of miles then when I come to a stop it feel like it want to choke out, but it seem to be fine I step on the gas. I’ve replaced the carburetor with an Edelbrock 1406, PerTronix 2 and a Pertronix coil. Any suggestion on what might be the cause for the rough start and idle.

Sounds like vapor lock. I have heard of people making a home made shield between the intake and carb. Hope this helps.

 

[inspector]

Maybe your gasoline has water in it.

 

[doug7740]

Bought this car about 3 months ago. It ran rough when it got here from Nevada. I replaced the timing gears and chain. replaced wires, plugs, distributor cap, coil and put pertronix ignition in. Set the timing. It's rough when it idles and gets worse under load. It backfires when applying fuel. The plugs are black with carbon every time I pull them. I checked compression and it is 135-140 across the board. I checked the vacuum with a gauge and only have 12-13". I have checked for vacuum leaks and blocked off all ports with no solution! Also sprayed the base of carb and intake manifold with start fluid with no change to problem. I am at a loss. I also puled valve covers to check for a weak lifter and they are all tight. Any help or suggestions are greatly appreciated.

A normal vacuum reading should be between 17 and 22 inches of mercury. Keep in mind, this reading is ideal for stock engines running at sea level. Higher elevations may cause slightly lower readings. For every 1,000 feet above sea level, you can expect the reading to be about one inch lower.

You mentioned that you checked the engine vacuum with a gauge and only have 12 to 13" of vacuum. A steady low reading between eight and 14 inches of mercury is usually indicative of incorrect valve timing. An extremely low reading, which holds fairly steady, means the engine is producing less power and, therefore, less vacuum. Common causes of a steady low reading are blow by due to worn piston rings or possibly late ignition or valve timing. An extremely low reading could also indicate an air leak at the intake manifold or carburetor.

doug7740
1955 Thunderbird Blue