Rough idle - Vintage Mustang Forums
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post #1 of 14 (permalink) Old Yesterday, 03:08 PM Thread Starter
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Rough idle

I have a 66 mustang with a 200 straight 6. I was just wondering how the idle is suppose to be. Mine is rough. Not too much that it is un drivable, but enough to aggravate you. New plugs, new wires, new Petronics, new carburetor, actually this is the fourth carburetor that I have tried. Timing is good, no vacuum leaks that I can find. Any ideas or suggestions that I can do?
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post #2 of 14 (permalink) Old Yesterday, 08:08 PM
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What kind of distributor do you have, Adtrivett? Does it have a single vacuum advance port on it?

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post #3 of 14 (permalink) Old Yesterday, 08:24 PM
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I have the same and mine idles very smooth until I turn on the A/C (Belt slap). I assume you have adjusted the idle mixture screw? Low compression will cause a rough idle. If you think the carb is adjusted and working OK, I would start with a compression test.
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post #4 of 14 (permalink) Old Yesterday, 09:21 PM Thread Starter
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I have the original distributor, with a Petronix. The compression on each cylinder is between 180 to 185. The distributor has a single vacuum advance and when setting timing, it seems to work ok.
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post #5 of 14 (permalink) Old Yesterday, 09:22 PM
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The 200 should idle like a fine sewing machine.
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post #6 of 14 (permalink) Old Yesterday, 09:39 PM
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Double check the firing order, the static and dynamic ignition timing, and carb idle and mixture screws. If no luck, pull the valve cover and manually turn the engine and look for a broken valve spring or rocker, a valve or lifter sticking, maybe a bent push rod.

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post #7 of 14 (permalink) Old Yesterday, 09:48 PM Thread Starter
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Got it, thanks.
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post #8 of 14 (permalink) Old Yesterday, 10:18 PM
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I didn't see coil on your replacement list. If its old it would be on the list to be swapped out too with the carb, plugs and wires. I had a coil that would cause misfires which made a car run rough so that is what they do before they fail completely.


Spark plug gap could be wrong too which might cause it to misfire randomly.


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post #9 of 14 (permalink) Old Today, 04:55 AM
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"The 200 should idle like a fine sewing machine."

+1 to that.


)

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post #10 of 14 (permalink) Old Today, 09:05 AM
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From the first semester of Automotive Technology, circa 1973...

Step 1 - Cranking Compression Test. This will establish the engines basic ability to run (properly). It is performed when the engine is "warm" by removing all spark plugs, grounding the coil wire, blocking the throttle wide open, making sure the battery is fully charged (connecting a battery charger throughout the test helps) and cranking the engine a minimum of 6 compression cycles. Record the readings. Repeat the test after inserting 2oz of fresh engine oil into the cylinders. The difference between highest and lowest cylinder should not exceed 15%. If it does, you have an issue that needs to be corrected before the engine has a chance of running smoothly.

Step 2 - Assuming you've passed Step 1, the next step is to verify the integrity of all IGNITION COMPONENTS. Use your ohmmeter to check resistance of the coil primary and secondary, ignition high-tension wires and spark plugs. Check the distributor cap for carbon-tracking, cracks or worn/corroded contacts. Check the distributor rotor as well. Disconnect and test the condenser and, finally, inspect the breaker points for wear/pitting, proper gap/dwell and the condition of the rubbing block. Finally, set the ignition timing to specification.

Step 3 - Assuming you've passed or fixed issues in Step 2, move on to the fuel system. Check the condition of all vacuum connections, hoses and operation of the PCV valve. Inspect and/or replace the fuel filter and check the fuel for contaminants. Adjust the carburetor choke, idle mixture and curb idle speed.

Step 4 - Check MANIFOLD VACUUM with a suitable gauge. Observe the needle reading/movement. It should be steady and, at or near sea level, should be around 20-21" hg.

Step 5 - Road Test.

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post #11 of 14 (permalink) Old Today, 09:56 AM
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Quote:
Originally Posted by Woodchuck View Post
From the first semester of Automotive Technology, circa 1973...

Step 1 - Cranking Compression Test. This will establish the engines basic ability to run (properly). It is performed when the engine is "warm" by removing all spark plugs, grounding the coil wire, blocking the throttle wide open, making sure the battery is fully charged (connecting a battery charger throughout the test helps) and cranking the engine a minimum of 6 compression cycles. Record the readings. Repeat the test after inserting 2oz of fresh engine oil into the cylinders. The difference between highest and lowest cylinder should not exceed 15%. If it does, you have an issue that needs to be corrected before the engine has a chance of running smoothly.

Step 2 - Assuming you've passed Step 1, the next step is to verify the integrity of all IGNITION COMPONENTS. Use your ohmmeter to check resistance of the coil primary and secondary, ignition high-tension wires and spark plugs. Check the distributor cap for carbon-tracking, cracks or worn/corroded contacts. Check the distributor rotor as well. Disconnect and test the condenser and, finally, inspect the breaker points for wear/pitting, proper gap/dwell and the condition of the rubbing block. Finally, set the ignition timing to specification.

Step 3 - Assuming you've passed or fixed issues in Step 2, move on to the fuel system. Check the condition of all vacuum connections, hoses and operation of the PCV valve. Inspect and/or replace the fuel filter and check the fuel for contaminants. Adjust the carburetor choke, idle mixture and curb idle speed.

Step 4 - Check MANIFOLD VACUUM with a suitable gauge. Observe the needle reading/movement. It should be steady and, at or near sea level, should be around 20-21" hg.

Step 5 - Road Test.
A vacuum gauge is an excellent tool to diagnose engine problems. Here is an excellent "tool" to use to interpret vacuum readings. Vacuum Gauge is an Invaluable Tool when Diagnosing Engine Problems
Also, make sure the distributor shaft is not worn "loose" in the housing causing the dwell to be erratic. A dwell meter is the best way to set points as it shows the dwell (point gap) while the engine is running. It should be rock steady. If not, the points assembly is defective or the distributor needs to be replaced.

Dave
'68 GTCS
8R01J
C4/PS/PB/AC
2/8/1968
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post #12 of 14 (permalink) Old Today, 10:51 AM Thread Starter
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I did another compression check this morning. Number 1and 6 were 155 and the rest were 175. Would this cause all my problems. Distributor shaft is tight. Dwell is steady and plugs are gapped correctly. Took it out last night and ran it hard for about 50 miles. Well, interstate hard. Had football practice. The plugs this morning were black. Oil on the threads. Valve seals? I think it is time to take head off for a complete valve job. Anyone else agree?
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post #13 of 14 (permalink) Old Today, 11:34 AM
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Quote:
Originally Posted by Adtrivett View Post
I did another compression check this morning. Number 1and 6 were 155 and the rest were 175. Would this cause all my problems. Distributor shaft is tight. Dwell is steady and plugs are gapped correctly. Took it out last night and ran it hard for about 50 miles. Well, interstate hard. Had football practice. The plugs this morning were black. Oil on the threads. Valve seals? I think it is time to take head off for a complete valve job. Anyone else agree?
Does it smoke when you first start it in the morning? Plugs black with oil or soot?
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post #14 of 14 (permalink) Old Today, 12:28 PM Thread Starter
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Yes, it starts right up then starts running rough and dies. You have to keep your foot on the gas for a few minutes. I know this is a choke problem and I’m working on it, but nothing I do seems to work. It has a strong gas smell when warming up. Plugs have soot and oil.

Last edited by Adtrivett; Today at 12:30 PM. Reason: Wrong spelling
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