slow cranking power once engine is hot

Ok heres another small issue ive discovered since i have finally been able to drive my Cougar.
If you all remember I had quite a few issues to sort to get it road legal here in OZ but thankfully that is all done now.
So what i have is a 1968 Cougar running a 428cj and C6 Trans what i have found now once it gets hotand is turned off the cranking power to start it is next to nothing almost like a flat battery but once it cools down all is normal.
It would seem to me that the starter motor may be getting to hot ?
Any light on this subject would be appreciated .

Regards Andrew

It could be the starter,but it could be the ground cable from bat. to engine is to small, ground from engine to firewall missing, if rebuilt engine, needs more break-in, to small battery, needs group 27 at least, with hi amps.

I fought this problem for years. Bigger cables, bigger battery (because it started fine with a jump box when hot), heat shield…
Using a ‘modern’ CRX(?) high torque mini starter with a heat shield (blanket type) helped a lot but what cured it was switching to a powermaster ultra torque starter.

It sounds like an old MOPAR gear reduction starter, but she has heat soaked to 230* and spun over like a top.

Here’s the 428 version:

I had the same issue with my 69. Do you have long tube headers? I put a heat shield blanket around my starter and have had good luck with it.

You have the "big block hot slow cranking syndrome’. This can be worse if the block has been bored on rebuilt. All the above mentioned will help the situation. If you don’t want to change anything then retard the timing and it will start better. Of course that could drop the vacuum to the brakes. My best solution was a powermaster hi torque starter, 2 gauge cabling, and a longer bolt through the one starter hole and put a nut with serrated washer on the other side of the bell housing and crank it.

The electrical side of the problem
The way to determine the source of the problem is to evaluate the circuit for high resistance connections and components. If you use a meter and try to measure the resistance across a connection it will probably show either zero ohms or something very close to it. This may be an accurate reading but since very little current is passing through the circuit you really aren’t getting the complete picture. What you want measure is voltage across the connection. You can only measure voltage across a resistance. So if we put the two leads from a volt meter across a big buss bar, no matter how much voltage was flowing through it, it would not measure a voltage. So no voltage means you have no resistance which is the ideal. If you do measure a voltage it is telling you that there is resistance some where between the two test probes.

Resistance equals heat
Even before you get the meter out, the first thing to do is to look for a physically hot to the touch wire or connection. You can of course touch things, but that is begging for a burn if you do find one. I like to use a non contact thermometer that you aim at the connection to read the temp. Harbor freight sells cheap ones that work passably well. Of course to find a hot connection you need to heat it up first so go ahead and try to crank the engine over for a bit. Then start at the battery terminals and follow the cables to the solenoid, and then the starter. Don’t forget that you also have to follow the ground cable as well. If you find a hot connection you need to clean it up, and recheck until it doesn’t get hot under test. You may find that the problem is a cable that is getting hot. There are two reproduction battery cable kits. For many years the only one available was for the 289 /302. This cable is too small for 390 and 428 and at least marginal for the 351. I always recommend using the larger gauge (lower number) replacement regardless of engine size.

If you didn’t find a hot spot the next thing to do is to measure voltage and see if you can find resistance across the circuit. You can actually measure end to end, that it from the battery post to the starter post to get an idea about room for improvement. If you measure very little voltage than you will know that side of the circuit is okay. Keep in mind that you may find several different locations where you are measuring voltage. Make each one as close to zero as possible. What is happening is that there are several resistances in the circuit. Each one drops the voltage passing through the circuit. The speed of the starter motor is controlled by the voltage it sees in the circuit.

About heat soak:
Most materials expand when they get hot, but different materials don’t expand at the same rate. For example copper and aluminum expand more than steel. The internal resistance of most materials also goes up when heated. This is not a big deal if the length of the conductor is short, but if you were to unwind the copper wires inside the starter you would find them to be very long. So heat can increase the internal resistance of the starter motor and that drops the voltage that passes through it and that reduces the speed that it turns, and also the amount of current that it can use to rotate the engine. Engineers know this so they design starter motors to be robust enough to start a hot engine, up to a point. If you change the under hood temperatures by replacing iron exhaust manifolds that make excellent heat sinks with a lot of thermal mass to be heated up, with a set of thin steel tube headers, the starter may become the new heat sink. Keep in mind that heat always moves toward cold. Then you are going to possible have to find a way for the starter to be less of a heat sink by shielding or insulating it, or using a starter that is designed to operate at higher temps.

Wow ! thanks for all the options Guys, lot of food for thought there.
Im gunna try the heat shield option first and see if helps the problem as i can fabricate something myself which will be a quick and a cheap option.
Will keep ya all posted on the results.

Many thanks

Found this result of some research on another forum.

When you stop the engine, the heat causes the fuel to expand in the fuel lines, creating more pressure than the holley needle and seat can hold, forcing fuel from the float bowls into the manifold. This exacerbates the heat soak problem as you have to pump fuel to fill the float bowls

Yes my 428 has heat soak problem

I have always wondered if the right hand drive conversion might introduce some new issues with the move of the steering box to the right side along with the starter. I am confident that the Australians can figure this out as they are among the very best fabricators on the planet, so no criticism, intended. I am just amazed at the complexity involved in the conversions.

Andrew, you really should eliminate the electrical problems first. Most common area for problem it the connection at the starter. Next most common problem is the positive battery post connection. Third most common is the solenoid itself. If you are running headers, then you might want go heat shield first but otherwise this was a system that worked when the cars were new.

Leon, that wouldn’t effect the speed that the starter turns over the motor but it is an interesting possibility. I am not currently running a pressure gauge on the fuel inlet on any of my cars. I wonder if any one that has one could tell what the pressure does after you shut down?

Overly advanced timing can also add to your problems. The first thing I would do is get a baseline in that, your fuel mixture, and take some vacuum readings when you play with the timing. Bill is right about resistance and heat. This is the same theory as when your breaker trips in your home. The circuit is trying to supply more than it can handle and the wire gets hot. If allowed to continue to try and supply more current than the wire can carry, the wire will melt down and catch fire.

Now you also know why you shouldn’t go with a higher rated fuse if you keep blowing them in your car as well.

I would follow Bill’s recommendations on finding your high resistance. I would also look into a heat shield for your starter or a blanket. A carb spacer would also help with percolation. You could be looking at one or all of the possibilities listed. That’s why I would get her hot and double check your timing, then move on and eliminate each scenario one at a time.

Steering box requires a RHD Galaxie exhaust manifold and it is still a tight fit. Probably easier now that TCI produces a RHD rack conversion suitable for a big block car

Conversion is a lot easier and cleaner than fixing the problems created when the previous owner in the US decided to spray sound deadener over the surface rust and crud underneath the car

I would say so!

Just so we all know when a left hand car comes to Australia or New Zealand as long as it is over 25 yrs old no conversion is required and i
believe it is 15 yrs old now.
So my cougar is Lh drive wouldnt of imported if it needed converting its all about keeping the character.

Hi Xr7
Yes your right i will check everything youve suggested first alot of which I have already done with all the other issues i had getting the Cougar to the road legal stage. The 428 still runs the cobra jet exhaust manifolds I couldnt imagine headers fitting in there not much room
I have the correct coil arriving in a few days from USA to go with the ignitor (pertronix) as the orignal one got fryed with the previous issues.

Mine were all imported before 1992, when they had to be converted here in Oz. The rule on LHD didn’t come in until 1998. Now you can retain LHD if it is over 30 years old in Oz

Bugger expensive exercise and for me LHD is the unique part of the classic Cougar or any american car.
You mentioned also about the shonky work people do on these cars i had a few issues to sort when mine arrived just people taking shortcuts and causing the new owner some heart ache.
Its a trust me thing when buying unseen you kinda hope the person ya talking with is honest and up front unless of cause you can veiw the vehcile in person.
Im in Perth there isnt many Cougars this side.

Just a quick update i have fitted a new coil (Pertronix Flame Thrower) which goes with the Pertronix ignitor thats running in my 428cj. Took for a drive today and so far havnt had the heat soak issue that was there so maybe coil was the issue ?. Fingers crossed lol

In your post header it said the problem was slow cranking. Does this mean that it is cranking faster after changing the coil?

Yes XR7g428 once engine got hot the motor would crank over slowly almost if battery was flat but i have now changed out Coil and taken for a drive and havent had the heat soak issues if that makes sense