[Farmall] 1951 Cub won't start when very hot

[Mike Sloane]

Going by my memory (which is not to be trusted lately), the instruction 
leaflet that comes with the Pertronix ignition module calls for a 
resistance between the two terminals on the primary winding to be 3.4 
ohms. So you need a fairly good DVM to get an accurate reading in that 
range. If your coil shows up as reading, for example, 2.0 ohms, then you 
need to add a ballast resistor in series with the coil that is 1.4 ohms. 
Note that resistance increases as the temperature rises, so it is a good 
idea to put the ballast resistor somewhere in the air flow from the fan, 
not bolted to the back of the block or near the exhaust manifold. The 
ballast resistor is usually a "sand filled" wire wound resistor rated at 
at least 5 watts, although the higher the rating the better. I have seen 
them at auto parts stores (many older cars use ballast resistors, and 
some cars used a resistance wire). If you can find them at an auto 
salvage yard, bring your meter and check them out before you bring them 
home.

The above is for a standard coil in a 12 volt system. You can also buy 
"12 volt" coils that have a resistor built into them. But either way, 
you still need to have the 3.4 ohm total resistance.  (or whatever the 
number is that Pertronix specifies)

Mike

On 8/30/2010 2:23 PM, Dave Maynard wrote:
>      I have the same problem on my 50 cub, especially if i have been working
> it hard[cultivating, mowing, and running the buzzsaw]. What Mike is saying
> makes a lot of sense, because I have tried all of the above. When my sons
> and I restored it down to the last wheel bearing and seal we converted it to
> 12 volt and I don't remember doing anything with a ballest resistor. We also
> went completly through the engine, milling the head a little to make it flat
> and changing out the pistons to crater domes and using the bigger carb, all
> which are making more heat. It gets hot and won't restart. Where can I find
> the proper ballest resistor?
>       If anyone is interested the pistons and rings, as well as the carb are
> available at cub cadet and were used after 1974. The pistons, bigger carb
> and increasing the high idle speed at the governor to 1850 rpm, change the
> rated horsepower from 8.5 ro 9 to about 17. The parts were not inexpensive,
> but if the tractor ran 50 years the 1st go round, it should go 50 more and
> last longer than me, so I figured spend it now and double the horsepower
> forever. It is a very strong running cub and it will be nice to finally not
> have this problem any more.
>     Tanks Mike.
>
>
>
> On Mon, Aug 30, 2010 at 1:25 PM, Mike Sloane<mikesloane at verizon.net>  wrote:
>
>> If it is running very hot, I would check the timing as per the manual.
>> You need an old fashioned strobe timing light to do it right (and a 12
>> volt battery to power the light). Also check to see if you have a lot of
>> grass stuck in the radiator and blow it out from behind.
>>
>> If you have converted the electrical system from 6 volt to 12 volt, you
>> need to make sure that there is a proper ballast resistor in series
>> between the ignition switch and the coil. Otherwise the coil will
>> overheat and cause the problem you are having.
>>
>> I am sure others will other suggestions,
>>
>> Mike
>>
>> On 8/30/2010 12:44 PM, jimbon88 at aol.com wrote:
>>>
>>>
>>> Hi Folks,
>>>
>>> I've been lurking on this list for a while to learn about my Cub but
>>> have never written in.    Now I have a problem that I can't solve and
>>> would like to know if anyone has any suggestions.   My Cub starts
>>> right up when it is cold or hot, but after running it so hard that
>>> the coolant is boiling in the radiator and it is VERY hot, it won't
>>> start.   It turns over just fine so I don't think it's the starter.
>>> I installed a new coil with no change.    Then I thought it was a
>>> fuel vaporization problem, but I can smell gas at the exhaust pipe
>>> and choking it makes no difference.  I have a pertronix ignition in
>>> it now, but it acts the same as when I had the points ignition system
>>> were in it.    If I let the engine cool down for an hour, it starts
>>> right up as if nothing was wrong.   Does anyone have any other
>>> thoughts as to what might be causing this no-start condition?
>>> Thanks for the help.
>>>
>>> Jim Gearhart
>>>
>>>
>>>
>>>
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>>>
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