Need for Speed

[MotownMachine]

I have had a problem with my second 'A' since I bought it in April. It will not top 40 mph! As soon as you get to 40 it starts missing wildly. It will do the same if you rev it high enough in intermediate gear, but it is not as noticeable as you've usually shifted by then. The car does very well under load though. It will climb hills in high gear all day, albeit under 40.

I've tried a number of things such as swapping the carb, coil, distributor, retiming, but to no avail. Simultaneously there is what I presume, and others have presumed as well to be a valve tick from the front end of the engine. I've put some marvel in the tank over the past few months but that hasn't done much either.

I finally pulled the valve cover to do some checks and measure the tappet clearances. No valves are sticking while turning over. Clearance wise, the exhausts are all a bit big (.016) and the cylinder one intake is a bit small (.008). Unfortunately, the last person to rebuild the engine did NOT put in adjustable lifters.

Are the valves the culprit as I presume? What are your thoughts? Thanks!

 

[Bob Bidonde]

The intake valve with 0.008" clearance is an issue. That valve, when it gets hot may not be seating properly. Also, if you are running a hi-compression cylinder head, is the ignition timing allowing enough spark retard for hard acceleration?

 

[BRENT in 10-uh-C]

I have had a problem with my second 'A' since I bought it in April. It will not top 40 mph! As soon as you get to 40 it starts missing wildly. It will do the same if you rev it high enough in intermediate gear, but it is not as noticeable as you've usually shifted by then. The car does very well under load though. It will climb hills in high gear all day, albeit under 40.

I've tried a number of things such as swapping the carb, coil, distributor, retiming, but to no avail. Simultaneously there is what I presume, and others have presumed as well to be a valve tick from the front end of the engine. I've put some marvel in the tank over the past few months but that hasn't done much either.

I finally pulled the valve cover to do some checks and measure the tappet clearances. No valves are sticking while turning over. Clearance wise, the exhausts are all a bit big (.016) and the cylinder one intake is a bit small (.008). Unfortunately, the last person to rebuild the engine did NOT put in adjustable lifters.

Are the valves the culprit as I presume? What are your thoughts? Thanks!

Sometimes a knowledgeable ear is key to going directly to the source.
I would start by making sure the vehicle is timed correctly. Make sure the Distributor lobes are opening the same amount, and that they are opening 90° from each other.

I doubt the valve clearances are causing any of your issues however the camshaft may be worn ...which would. Start by doing a compression assessment. While a compression test is the first method some use, I prefer to do a leak-down test which will typically pinpoint an area to look.

After you have done that, come back with your information. If you get 30 different opinions and/or suggestions, you are going to quickly lose your baseline when you start performing some of those suggestions.

 

[rotorwrench]

A compression check will tell if there are leaky valves or possible ring & cylinder issues. A differential pressure compression check will tell if a lot of air is passing the rings and which valve in each cylinder might be losing pressure if hissing is detected at exhaust or intake manifolds.

Fuel delivery is gravity only so there can be obstructions that slow the flow. There should be a good flow at the carburetor connection. There can be obstruction in the carb as well. The GAV is adjustable for high end flow. The float valve can get obstruction too.

It sounds like you tried most ignition checks. Just make sure it also has a known to be good condenser and that the breaker points are closing with good spring tension and contact alignment. I don't know what kind of spark plugs you have but they should spark well under at least 120 psi pressure in a bomb tester. I test each one for 60 seconds to make sure it isn't breaking down.

Not everyone has a dual gauge differential cylinder test set or a spark plug tester but some clubs may have members that have tools like this. These tools are commonly used in aviation maintenance. Not so much in automotive maintenance.

You might let the car role down a hill to see if anything is dragging down the drive line. Brake drag or binding in the differential or drive line could be an issue although not all that common. A person can generally feel excessive drag while operating a vehicle but that depends on experience.

 

[Jim Brierley]

You've tried everything that normally causes a miss, the only thing that comes to my mind is valve spring pressure, which is more difficult to measure and cure, good luck! Timing won't cause a miss, neither will lack of fuel.

 

[Brad in Germany]

Check the polarity of your coil. I had a similar problem and simply reversing the wires on the coil fixed it.




Brad in Maryland



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[cadillac512]

You've tried everything that normally causes a miss, the only thing that comes to my mind is valve spring pressure, which is more difficult to measure and cure, good luck! Timing won't cause a miss, neither will lack of fuel.

This is good advice and matches your symptoms. Check each valve spring to make sure none are broken and that they "feel" the same when pried open with a lever. An intake at .008" clearance shouldn't cause your misfire.