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Hagerty Employee

Piston Slap: Highway resistance from a misfiring Falcon

Stephen writes:



Several years ago I wrote you about my ’65 Falcon convertible, asking for options to re-do my suspension. I took your advice and put only upgraded factory stuff on, basically Opentracker A-arms and lower control arms with Bilstein shocks. It’s a very comfortable ride, but now I have a new problem.


When I am driving down the road at 65–70 mph in fifth gear (T-5 swap), the engine loses power and I hear a spitting type sound (fft, fft). If I downshift to fourth, the power seems to come back. This only occurs after the car has been running for 5–10 minutes at 65–70. The car has a 289, Autolite four-barrel carburetor, Mallory distributor, new generic NAPA coil, fresh plugs/wires, and I am using a ballast resistor.


Sajeev answers:

It’s wonderful to hear you chose/enjoy my suggested suspension modifications, as it’s usually in the owner’s best interest to stick with a factory-style suspension, but with upgraded shocks ...


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This happened again Saturday when I was climbing a long hill in the mountains.  It only lasted about 30 seconds.  I have a cheap Carter electric fuel pump.  How would the engine react if the pump was acting up.

Community Manager

I think a fuel cutting out would be smoother than what you are experiencing (probably). And it would likely happen at full throttle too, not just when you try to accelerate in OD.   

Advanced Driver

Good day gents:


Does the engine have the original points type ignition or an  upgraded electronic one like the Pertronix Ignitor series of drop in ignitions?  If it already had the Pertronix in it, you don't need a ballast resistor or resistor wire (GM used a resistor wire) in the circuit.  If you're still using the old points type ignition, I'd double check or possibly change out the condenser.  I believe the coil should have about 1.5 ohms of internal resistance, maybe a bit more, but too much would also cause that problem.

Pit Crew

You've probably checked the plugs already but if it's like my Dad's old 64 Falcon it went through spark plugs pretty quick. It was our driver to get to the cottage and he spent most weekdays working on the car to get it ready for the following weekend. And that was his "Good" car. 🙂

Advanced Driver

One more thought guys:


Why the electric fuel pump?  The Ford mechanical fuel pump is excellent.  It could be the volume of fuel, from the electric pump, is too small for heavy load applications.  Where is it located?  Most electric fuel pumps have very low vacuum, so they need to be as close to the tank as possible, preferably with gravity feeding the pump.  If the pump is under hood, then it's having to draw fuel from the rear of the car to the front and may not have enough vacuum to do that under high load situations.  It could even be the alternator or battery is getting compromised and, again under high load conditions, the battery voltage is falling down enough that it affects the electric fuel pump.

Pit Crew

This sounds like a new problem with my '60 Galaxie. It has a 352 cid V8. When going up hills on the interstate, if I try to accelerate, it coughs once or twice.

I was thinking I need to adjust the 2 barrel carb. Maybe the problem happens when the secondary kicks in? I know the previous owner replaced the carb. Based on the rich smell of the exhaust I don't think he properly adjusted it. Thoughts?

Community Manager

A rich smell and that type of stumbling suggests you do need to get that carb tuned properly. 

Pit Crew

All these answers sound good.  The only thing I’d add is check; spark plug gap, spark plug wires.  Spark plug wires (carbon) can break internally.  When they get hot the break (gap) may expand causing a loss of spark at the plug. Also check the routing of the wires.  
good luck!

Intermediate Driver

Probably too many resistors in the coil circuit but ...   I'd also put in a new vacuum advance pot, my '54 didn't run right at higher speed so put in a new one, nobody make the exact one for the original Y-block dist' so we got one for a '50, cut off the hooked rod and welded it back on, 90 deg' turned, works like a charm now.

Intermediate Driver

Stephen said he added a ballast resister to the system which could be the problem if his is using the factory Ford harness. Ford used a very light gauge wire to the coil to create the necessary resistance and the added ballast might have created too much resistance.

Advanced Driver

I'd like a bit more information on this. The power seems to return when the transmission is downshifted? So that (obviously) means that the RPM increases. This only seems to be noticed after the engine warms up? Is there an automatic choke on the carburetor? This doesn't seem to indicate an ignition issue to me but it does require more information. I would thoroughly inspect the carb base gasket and after the engine is warm and idling, spray some silicone aerosol around the intake manifold where it meets the heads to see if the idle speed changes. While I was at it, I would temporarily install a vacuum gauge in the car and check for a steady needle (no wild fluctuations) under the noted fifth-gear operation. The distributor vacuum advance is only supposed to work during part throttle (ported vacuum source), so a low vacuum signal and low RPM (inadequate mechanical advance) coupled with a high load would cause a slight stumble. Let us know what you find.

Pit Crew

The first thing I'd check is for a cracked or defective distributor cap.

Intermediate Driver

Obviously, you need a new flux capacitor.  

Pit Crew

It sounds like this car still has a points type ignition system.   A relatively cheap upgrade is to convert to electronic ignition.  This will ditch the ballast resistor and make this Falcon more trouble free.

Intermediate Driver

With many of the comments talking about the ballast resistor, why not find out what the true voltage is at the coil?  Voltmeter red lead on the coil + terminal, black on battery ground, and run the engine. If the voltage is around 10V-11V, with 14V at the battery, that would be about right. If you find that you are around 8V or less, try bypassing the external ballast resistor and see what the volts are then. Also, run the engine for long enough to get everything up to temp, as higher temps increases resistance, which lowers both voltage and current. You may start with a decent voltage that drops as wires heat up. TimK noted that Ford used a resistor wire for the ignition. Maybe check the volts both in and out of the add-on ballast resistor.  Good Luck!

Community Manager

This is a great diagnosis, thank you for sharing! 

Intermediate Driver

Certainly could be too many ballast resistors, it sounds like an ignition misfire. Crack one of those new plugs during installation?  Could be an open plug wire also, test them with an ohm meter. Or arcing in the distributor cap. You might see a carbon track on the inside if the cap is the problem. 

New Driver

 What comes to mind--From lots of experience is Ballast Resistor or Partially blocked Carb jets-


I just checked Mallory's website. The Unilite distributors require a full 12v at idle. Much like the pertronix, you need to lose the ballast resistor, or bypass the resistor wire. The resistor wire has a thin strand of wire for a reason. It lets 12v through at startup, but as the voltage continues to flow through that thin wire heats up and creates more resistance (resistance creates heat in an electrical circuit) so that the points don't burn up. My suggestion would be to install a Pertronix 12v relay which will supply a constant 12v, or go with Rocketman's relay setup which will allow you to also have a valet setting on it as well. If you go with the relay, don't cut a bunch of length off of the resistor wire. This way if you go back to a points system, you can just disconnect it and go back to the factory setup.