Starter issues

[EastYorkStang 63]

Went for a drive in my 70 yesterday. Once stopped I had to restart it. Starter lugged very slowly so I boosted it. Battery was at 12.5  volts but is in the trunk. Started. Went about my business for a few hours. Started no problem and drove an hour home with no issues. 

The solenoid is a factory piece mounted in the stock location. Does this act like a knife switch meaning only on off or is there resistance in it ?

How can I test the starter while on the car ? Pitfall of having a lowered car with headers hinders the removal. 

What else can I look into ?

[aslanefe 333]

Is the starter slow/lugging when the engine is hot and you do a real short stop? If so, your starter may be getting heat soaked. In that case, you need a shield around the starter (or wrap the headers) to keep the heat from headers etc away.

[EastYorkStang 63]

I’ll try that. Thanks. 

Its at this time I wish it was a 4x4

[1969_Mach1 333]

You might need to get away from a stock style starter and move to a permanent magnet gear reduction starter.  They're smaller in size so are less prone to heat soak from close proximity to headers.  Powermaster usually offers several options.

[jmlay 80]

Long cables will add resistance and voltage drop between the battery and the starter. What is the voltage at the solenoid static and while cranking? 

[EastYorkStang 63]

Will look into both of those.

Thought of putting in a 1993 Mustang starter.

[SM69Mach 68]

I would check your grounds too to make they have good connection.  

[Vicfreg 771]

I have a massive ground wire going from the frame to my block, and it is attached to where the starter mounts to the engine.  I also have a mini starter, and have it wrapped with one of those starter insulating kits.