A 720 pony saga

jimsmt

New User
Jim’s 720 Pony Motor saga

First I have some questions about the pony motor:

-Do pony motors have an oil pressure switch? The schematic shows one, but I have not found one on this pony motor. It is from a low serial number 720.

-I would like recommendations on the rebuilding of the water pump. The seal is still available but I am apprehensive about performing the rebuild. Suggestions? Water is getting into the pony motor oil when the pony motor runs but not when the diesel runs. I removed the pony motor water pump, put it in a vise impeller up so I could cover the impeller with water. I rotated the pump by hand and let it set. I rolled up a paper towel and put it in the weep holes. It has some dampness from the weep hole that exits to the exterior of the pump. I am guessing this is the source of the water. I did not notice leakage from this when the motor ran but I didn’t know it was there and there was a lot of oil and dirt around the base of the pony motor.

-What oil is recommended for the pony motor? I figured a 15W-40 which is what I figured to run in the diesel but I have seen recommendations for maybe 10W.

-Does anyone know of a good source of pony parts or maybe a salvage pony? I know the easiest and probably cheapest direction is to go to electric start but I really like the pony system partially due a 720 pony start that a late friend use to let me run.

-What are the recommended tappet clearances?

-My nephew is also interested in going to a narrow front end from the aftermarket wide front end. Suggestions on where to find one?

Now here is the saga for your entertainment:

My nephew bought a 720 diesel that had recently been started after setting for numerous years. He plans to use it for his pumpkin patch and Christmas tree business. The pony did not run so the diesel was pull started. From the condition and the little info received from the previous owner relatives I am guessing the pony had not run in 30 years. The diesel ran OK but it ran out of fuel and we found the sediment bowl and shut off valve completely plugged with fuel that had turned to putty. The diesel had been running on the fuel in the filters. We cleaned this all and the diesel ran fine.

The pony starter would run but not engage. We removed the starter and fixed a problem with the bendix, dumped some gas in the carb and spun it over. No luck. My nephew had other things to do so I took the pony on as a project.

My goal was to spend the minimum possible to get it running and determine the cost to get it in good shape. There was no spark so I checked the coils and determined both of them were bad. I then wired up four 12 volt coils separately from the tractor electrical system and powered the system with 12 volts. I used 6 volts for the starter. I got the plugs firing, dumped some gas in the carb and got it to pop. I removed the carb and determined it was a mess. I drilled out the lead plugs and cleaned out all the passageways which were plugged with corroded aluminum. I also got a main jet from a Cat pony from a salvage yard. I got it to sort of run but the Bakelite points did not like the 12 volts and one of them fell apart. I adapted a set of Ford points for the failed set and hooked up a Ford condenser in addition to one of original condensers. I got the pony running up to governor speed but saw the other points were smoking. I hooked the ignition up to the 6-volt battery and the pony ran fine. I then attempted to engage the pony with the diesel decompressed but all it did was grind. I found the clutch was stuck. On a whim I engaged the pony gear into the flywheel gear and with the diesel decompressed, started the pony which also rolled over the diesel. The pony came up to speed, I pulled the level fully in for the pony clutch and compressed the diesel and it started. I did this a couple of times and then the other set of points fell apart. I decided it was time to get new points, condensers and coils. After I installed these the pony would fire up fine. I then pulled the pony clutch in partly and compressed the diesel in an attempt to break the clutch lose. No luck at this. I also determined water was getting in the pony oil when it ran. I decided to remove the pony from the tractor.

After removing the pony, I checked the water pump as stated above. I also removed the pony transmission. The clutch was rusted but cleaned up nicely. The throw out bearing looks good, the transmission turns smoothly and the one-way clutch works properly. I filled the pony transmission with diesel fuel to soak.

As I stated, I used four 12 volt coils, one was from a Chevy, one from a Ford, one from a Mercury marine engine and one of unknown origin. They worked just fine on 6 volts. I have read info about coil resistance and voltage but from what I found in this instance mixing worked just fine. Attached is a photo of the ignition setup.
 

Attachments

  • 720 pony adapted ignition.jpg
    720 pony adapted ignition.jpg
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Early ponies do not have the oil pressure switch. It's a good addition. I especially like them when it has original coils as they burn up quickly if the switch is left on. I also run it with an oil pressure gauge just to make me feel better.

The waterpumps are pretty easy to rebuild. The impellor can be rough but probably smoothed out. The condition of the water pump shaft can be a problem if it's really corrodded. I'd open it up and find out. Definitely rebuilt it, water or antifreeze (especially!) in the SMALL amount of oil in a pony is VERY bad

There are suppliers of used pony parts, the Lind brothers is one supplier. https://www.lindbros.com/

Some new engine parts can be found at H+J Machining http://handjmachining.com

Brandon
 
I do not have a shop manual for the 720, would someone give me instructions on how to rebuild the pony water pump?
 
Brandon,
Thanks for the info. I had already talked to H&J and am thinking about sending the pump to them. I could also get the shim gaskets from him. To me the impeller looks excellent, no signs of corrosion or erosion.
Jim
 
Brandon,
Thanks for the info. I had already talked to H&J and am thinking about sending the pump to them. I could also get the shim gaskets from him. To me the impeller looks excellent, no signs of corrosion or erosion.
Jim
The pumps do not really take much to rebuild. All you need is a press. Any impeller erosion problem would be on the backside where the seal wears. We can help here with any questions, and I might be able to help with the manual if you email me [email protected]

Brandon
 
There are some changes to the pony motor, I think they were made at the black dash change, 720 serial number 7214900. The later ones had a serial number tag riveted to the block, about 4 inches above the nose of the pony starter. Since these pony motors have sometimes been moved, between tractors , you could have an early one on a late tractor, or a late pony motor on an early tractor, the riveted serial number tag is the defining clue. I don't think there is any change to the water pump. The valve train changed, and may affect the clearances, I only have the late model pony motors.
 
You might check out Squatch253 on YouTube. He has a playlist on Caterpillar D2 Starting System Rebuild. Cat starting engines are not exactly the same, but are very similar.
 
You might check out Squatch253 on YouTube. He has a playlist on Caterpillar D2 Starting System Rebuild. Cat starting engines are not exactly the same, but are very similar.
The Cats are closer to the R pony. though the carb is the same as a Deere V4
 

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