Rough idle

[Washington Blue]

At my wits end. Rebuilt carb thanks to Bert's. Rebuilt distributer. Mt 29 Phaeton ran beautiful after a careful timing job by the book.5 miles later, it died. The new battery was drained down. Purchased a new 6 volt alternator. charged battery. Idles with a sputter and runs rougher with increase of rpms. It has new plugs, condenser,coil, manifolds. copper manifold and head gaskets. Fuel lines and screens are clean. This A ran great for the first 3 years that Ive owned it. Been down for the last year. Any ideas?

[joncrane]

Dirt/rust in the gas tank?

[Canton_Model_A's]

check your high-voltage wire from the coil.

[Will N]

Check the carbon "button" on the bottom of the distributor cap that makes contact with the rotor. I've seen these cracked or even missing and the car runs like crap.

[Benson]

Old stinky gasoline which has plugged jets and stopped up float valve (gooey gas which makes float valve stick).

I have found old gas which will not even catch fire if poured on concrete and attempted to light with a match.

Idle jet plugged up.

Cap jet and /or Main jet plugged (crappy gas or rust from fuel tank)

(Crappy gasoline that has been setting in a carb for a longtime and turned into "glue".

New Ignition points with over 200 miles which have not been readjusted since installed (rubbing block worn down and gap is MUCH less than .018)

Sparkplugs wrong heat range and are fouled ( carbon covering white Porcelain from too much idling and too many short trips).

Note : Gasoline additives DO NOT FIX bad Gasoline.

[Bob Bidonde]

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You should get in the practice of having a standard trouble shooting routine. Your engine needs these things to run: Compression; Fuel intake; Low & hi-voltage ignition. In my experience, the preponderance of engine performance issues are ignition.

[nkaminar]

An easy check for the ignition system is to measure the voltage at the movable point in the distributor with the point closed and open. When open they should show the same voltage as the battery. When closed the voltage should be zero or very close to zero.

Check all the stuff that Benson and other have mentioned. Plus the condensor. Even if the condensor is new it could be bad.

[Cape Codder]

Benson - You have written info that I do have a question and I quote:

"Sparkplugs wrong heat range and are fouled ( carbon covering white Porcelain from too much idling and too many short trips)."

How are the heat ranges determined? Which is best to use and does Sea Level mean anything in this determination?

Thanks for any info you can send on !

[Benson]

Originally Posted by Cape Codder

Benson - You have written info that I do have a question and I quote:

"Sparkplugs wrong heat range and are fouled ( carbon covering white Porcelain from too much idling and too many short trips)."

How are the heat ranges determined? Which is best to use and does Sea Level mean anything in this determination?

Thanks for any info you can send on !

Heat ranges explained by NGK here:

https://ngksparkplugs.com/en/resourc...lug-heat-range

Larry Brumfield talks about heat ranges and how to test your car.

It is important to run car at speed then push in clutch then kill ignition and coast over to a stop before reading plugs.

https://www.fordgarage.com/pages/bru...kplugnotes.htm

About selecting a plug to use:
I looked at the chart that Larry mentions on Vince Falter's website.

Ford recommended Champion C3 plugs. I did not want to pay those prices so I picked W18 which is similar to C3 in heat range.

I use the car in delivery mode and to work.

15 miles per round trip. Ten miles of 30 to 40 MPH and the other 5 miles is 45 to 50. This is enough driving to keep plugs clean.

If I do a series of short trips with shut down after each trip and a lot to idling ... the plugs will foul or at least start to turn the white porcelain black with carbon.

If I repeat the 15 mile circuit the plugs clean right up. No need to remove plugs to clean them. Actually 30 minutes at 45 to 50 is better.

In the 30s, 40s , 50s this was referred to as "Blowing the carbon out" and was used with all cars of the day.

Modern cars with computer controls do not have this problem.

About sea-level question. Henry thought about this which is why he included the GAV. So you can adjust the mixture like for starting car when temperature is below 40 or so.

I run car from 6000 to 9,000 feet and run the GAV at Ford's recommended 1/4 turn open.

At 7500 feet the compression is 25% LESS THAN AT SEALEVEL so any difference in mixture is not noticed. The higher you go the less power you have unless you have a turbo or super charger.

I have driven Model As, VW type IIIs, Volvo P1800 and Porsches at sea-level and the difference is Amazing! Like having V8!

[katy]

Originally Posted by Benson

New Ignition points with over 200 miles which have not been readjusted since installed (rubbing block worn down and gap is MUCH less than .018)

As noted, check the point gap, if the rubbing block is worn down, check the surface of the cam in the "rebuilt distributor".

[Benson]

(see post #5) - The reason for checking brand new points is that most of the points made today have a rubbing block that is too soft and it wears rapidly the first 200 to 400 miles so they must be adjusted several times in the first 500 miles or so.

The result is that once points close up too much you get a poor spark.

After about 500 miles, the rubbing block becomes "work hardened" then further adjustment is not needed.

In the 50s and 60s and later on I could install a set of points and they were good for 3000-5000 miles or more.

It seems that the bean counters found that they could save 5 or 10 cents on a set of points by ordering points with soft rubbing blocks!

Bean counters do not have ANY IDEA how mechanical things work ... NONE.

Somewhere around the 90s the Bean Counters took over business and this is one of the many not so good results that they have brought us!

[wwirz]

You may have your GAV open too much. Turn it clockwise to close it.

[Ed in Maine]

Take a look at your distributer with the car running IN THE DARK! See if the distributor body or the cap is arcing to ground. Ed

[bobbader]

You've listed lots of new parts. Were these installed before or after you started having this problem? You need to bring down the diagnosis to the smallest variable. Otherwise, we're all going to give many, many, MANY answers in different directions.............. all of which will deal with fuel or ignition "maybe" answers or fuel AND ignition "maybe" answers. Help us narrow down the field by putting some chronology as to whether any or all of these parts were added recently (and therefore in what order) or if they were all on the car when you started 3 years ago.

Also .................. rebuilt Carb thanks to Bert's ??????? Thanks to?????? Meaning you bought a rebuilt carb FROM Bert's or you rebuilt it with the help of parts and verbal assistance from Bert's?????? Same with rebuilt distributor??

[norcalal]

I about gave up, and while the previous suggestions are all correct, the culprit was a cracked intake manifold. I had previously replaced the manifold gasket,and still suspecting a vacuum leak, finally removed the manifold from the car completely and there it was cracked, not evident on initial inspection

[serocontra]

Originally Posted by norcalal

I about gave up, and while the previous suggestions are all correct, the culprit was a cracked intake manifold. I had previously replaced the manifold gasket,and still suspecting a vacuum leak, finally removed the manifold from the car completely and there it was cracked, not evident on initial inspection

how do you find a crack in the intake manifold? i'm hoping there's an easy test.

[johnbuckley]

Originally Posted by serocontra

how do you find a crack in the intake manifold? i'm hoping there's an easy test.

Yes, easy ! Have engine running at idle and squirt Easy Start ( ether based spray) or WD40 around manifold , if it makes any difference you have a leak! [Careful- Easy Start is explosive !]

[CT Jack]

1. Check to verify all plugs are firing by grounding out each one with the engine running. This would indicate an igition related problem if one plug is not firing.
2. Check for a vacuum leak. Check intake manifold gasket, carburetor gasket, carburetor throttle lever shaft are places for intake leaks. Vacuum leaks can cause engine to run rough.
3. Remove rebuilt carburetor and replace it with another one. Sometimes a rebuilt carburetor is not a fool proof solution.