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Allen,
Yours is a slightly different situation than the car that rough-idles only when put in gear. Here is my theory on your car: The float level is set way too high. Back in the 70’s, I had a friend who had a ’67 and it did this exact same thing. Stumped everyone. While it was sitting there idling one day, with me scratching my head YET AGAIN, all of a sudden, the idle sped up, it smoothed out, ran like it had never run before, like a new car, ALL BY ITSELF!! After about 10-15 seconds of this, it completely died. Guess what happened? It happened to have run out of gas. Once it could no longer fill the float bowl in the carb, as the level of the fuel in the bowl diminished, THEN it finally had the right level, and ran GREAT until it exhausted what was in the carburetor.
So here is a suggestion: Disconnect the fuel pump from the supply (INPUT) line. Cap & seal both lines where you’ve disconnected it so that there is no fire hazard. Crank up the car. Let it sit & idle until it runs out of gas. Does it run smoother? If so -- Voila! And there ya go. Maybe it’s set to specs. Maybe the specs are wrong. Lower that fuel level by adjusting the float, and see what happens.
Keep us posted.... Inquiring minds want to know.
--Brooks in Dallas
PS: For the unwashed, here (in my own words) is how a carburetor actually works, hopefully in enough “layman’s terms” so you can understand what’s happening in your Imperial’s fuel system if you are, in fact, experiencing the above situation. The bowl in the carb is just like your commode. There’s a float, and when it gets to a given level (adjusted usually by bending the float arms, just like in your commode), no more fuel can (theoretically) come in. The level at which it is set is critical.
In the bottom of the bowl are brass things called the “main jets”. They screw into the bottom of the bowl. When the engine’s not running, the fuel (theoretically) just sits dormant in that bowl, just like the water in the back of your toilet.
Once the engine is running (and I’m ignoring the “idle system” for right now), air flowing across the “barrels” (which you see if you look into the top of the carburetor) creates a vacuum by passing over these barrels at high speed. This causes fuel to be sucked from the bottom of the bowl, thru those brass jets, up passageways drilled in the carburetor, and into those barrels, which are *ABOVE* the fuel level in the bowl. Sort of like blowing across a straw immersed in a glass of water. Think “uphill siphon, as long as there’s suction”. And I’m clearly differentiating between “siphon suction” here, vs. “manifold vacuum”. Manifold vacuum is not really directly involved in this “siphon suction”, which varies depending on how many cubic feet per minute (CFM) of air are flowing thru the carb, which is dependent on how fast the motor’s turning.
If the fuel level is too high, you’re
constantly getting “more than you bargained for” because more fuel
is able to be sucked through than the engineers planned, or else maybe the
level is getting actually above the barrels – so even if there is no flow
(like at idle), the bowl level is still spilling over into the throat of the
carburetor – causing it to run way too rich. Hence,
perhaps, your 7mpg on the highway. A good test for this: With the engine idling, look down the throat of the carburetor. At idle, these barrels are not being used at all, because the air flow is not great enough. You should see *NO* (= ZERO, = ZILCH!!) fuel dripping/running/anything else from these barrels. If you do, there is a problem, and I’d start with the bowl fuel level.
The “idle system” is what the engine runs on at idle. BELOW the throttle plates, there are holes that lead to fuel. The size of these holes is regulated by those screws on the front of the carburetor. When the throttle plates are CLOSED, i.e., idle, there is a HUGE vacuum BELOW the throttle plates, but not a lot of airflow ABOVE the throttle plates. So that big vacuum causes fuel to be siphoned thru these holes BELOW the throttle plates, and hence you’re running on the idle system.
A QUICK TEST: At IDLE, if you can close both those idle mixture screws and the engine’s still running, then I’d like for you to tell me just EXACTLY WHERE IN THE H*LL the engine is getting any fuel to run on? Because *SUPPOSEDLY* that was its sole supply. If it’s still running, guess what? There’s a gasket leak or fuel coming thru the barrels when it’s not supposed to or SOMEthing’s going on, because if all is well, you just cut off the gas supply and it should have stopped.
As you gradually press down the pedal, opening the throttle plates, this vacuum is REDUCED because as the throttle plates open, there’s less vacuum below them. As that vacuum diminishes, fuel no longer is siphoned thru the idle system, because there is not enough vacuum to do it, but the airflow has now increased across the BARRELS and they begin to siphon thru the jets and atomize it.
To see this – rev up the engine to about 1200 RPM and NOW look down the throat of the carburetor. You’ll see beautiful little spray patterns of fuel being sprayed (siphoned) from the barrels and instantly atomized in the air flow.
This transition from Idle System to “Main system” is where a car can encounter a “flat spot” coming off idle. If the fuel level is too LOW, then there’s a pause between when the idle system stops siphoning, and the barrels begin to siphon. If the fuel level is just right, or TOO HIGH, you won’t have a flat spot. Also, as you press down the accelerator / open the throttle plates, the “accelerator pump” injects fuel into the system to help get over this “transition”.
Another thing that can happen to cause too much fuel in the mix: the seals around those jets screwed into the bottom of the bowl, letting the gas get sucked AROUND the jets instead of THRU the jets. But I think that’s pretty rare….
Obviously another thing: Is the needle and seat sealing when it’s supposed to stop letting fuel into the bowl? Questions to make you look until you find an answer.
Maybe now you can understand a little more on why vacuum leaks are so critical. All this was designed for a “given” amount of vacuum, and if you’re going to be leaking it, then your car is no longer to “tolerance” and all kindsa weird stuff will start happening.
OK, that’s the end of my carburetor theory story for the day. If you read this far, and understood anything or learned something you did not already know, then you get 100 and a gold star. And you can now say “Wow. It’s not a nuclear reactor. It’s JUST a carburetor.”
Keep those Imperials running!!!
-----Original Message-----
I purchased a 67 Imperial convertible in Tucson in February, since that time I have done a complete "Extreme Make-Over" However after the complete engine tune up it just will not run! The engine runs rough, (the carb, points, plugs, wires,....everything were changed out) I drove this car most of the way from Tucson to Palm Springs, and it ran smooth, ( it did seem terribly thirsty, getting 7mpg Hwy,) when all the paint, upholstry, brakes, front end,...well when the car had pretty much been replaced, the engine was targets for new Carb..or rebuilt, plugs points etc.....Now.............It runs like a 38 year old piece of junk. Most of the mechanics in Palm Springs are younger than the car, and are lost at what to do when they can't find where to plug in the computer diagnostic unit.....Just keep on looking...(haha) I have give up, and the car goes on the block at the McCormic Classic Car auction, held in November. I would like for it to run better for that, but have exhausted everything I can think of. Can't find a service manual....any ideas would be appreciated! I realize I am having all the same symptoms as Jay is with his 413. Any thoughts???????????? AGDJ Allen 67 Imperial Convertible |
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