D2-5U pony motor woes…

Hello everyone,

I am stumped by issues with my pony motor. I will lay out what I have done, and the symptoms. Perhaps someone here can offer insight.

1951 5U with electric start pony:

When I originally purchased the tractor, it was non running, having sat for roughly 25 years. As such, I have no first hand experience with how the pony ran, prior to bringing the machine back to life.

I cleaned and rebuilt the carb, had the heads magnifluxed and surfaced, lapped the valves, honed the cylinders and added new piston rings and new rod bearings. New seals and gaskets, and OEM Cat parts were used throughout.

After assembly, it occasionally ran well and at other times coughed and sputtered as it ran on one cylinder.

I pulled the carb and confirmed everything was within spec. Settings were per OEM manual and all jets, passages and needle/seat were flowing properly. I did discover the OEM replacement float was dragging on the mounting tabs, and corrected the issue.

I remounted the carb, added fresh spark plugs and there was no change in performance. Sometimes it ran perfectly and at others…it sputtered as before. There was no consistent pattern other than it usually appeared to start and run better, then cold. (And also on cold days) I suspected cold/dense air might be the magic secret and leaned both jets to replicate the air fuel ratio when warm. Unfortunately, there was no improvement in performance or consistency.

I pulled the magneto to confirm timing also and confirmed points are clean and gapped at .016”, I added a new OEM condenser, OEM brushes and springs to the magneto cap, new Brillman plug wires and another fresh set of plugs. I have tried both NGK and Champion.

It appears cylinder 2 (right side) is the problem child, so I pulled the head to see if a valve might be sticking or there was some other visible mechanical issue. All appears in order, with the only concern being the lips of the valves being a bit thin.

I am stumped at this point and not sure where to look. Again, some days it works perfectly and others…not so much.

Short of drilling the bellhousing for an electric starter, does anyone have any advice?

thanks much!

A couple of possibilities. Check the mag cap carefully for a small crack or a carbon streak. A cracked cap is hard to detect and can be a lot of grief. You have already replaced the condenser which eliminates that. Another thing you can do it run a compression check on the pony. With the electric start pony you have that luxury. I don't know the number for the compression on the pony but both cylinders should be reasonably close.

Good luck.

Hi Breadler, one thing I've been offering to folks lately is to loosen or remove the pony oil fill cap, and the pony gas tank cap. I'm not suggesting that these might be part of the problem but they're about the easiest thing you can do to see if either is involved. The D2/4 pony is quite sensitive to an overfilled crankcase due to the way the breather is designed (whether it's overfilled with oil or diluted oil) and can make it pretty hard to keep the pony running. A partial vacuum in the fuel tank due to a non-venting cap can also play havoc. Jim's suggestion is another that is pretty easy to check but very hard to imagine if one hasn't considered it before. The other thing you can try if you have access to them is swapping out components from known good running engines e.g. carb, magneto. You can also install your carb and magneto on a known good running engine as a counter-test. Only change one variable at a time.

Assuming the cam timing and magneto timing is correct, it's really only the fuel and spark that are usually at fault. These engines run on a pretty low compression so if you've done the valves and rings as described, then your compression is "probably" fine but it wouldn't hurt to check it regardless. One other thing you can try is to time the magneto 180 degrees out and swap the plug wires from one side to the other. If #2 starts being the reliable cylinder and #1 goes south, that'll point to the magneto and/or plug wires.

Of all of those items, any of them other than the compression could possibly be a cause. Very interested in this case to see what you find out.

One other "dumb" check is to confirm that the intake manifold passages are free and clear. More than one person has been confounded by a rag stuck down an intake manifold runner

Check your pony air cleaner. The cup was loose on mine, after I tightened it up I had symptoms you described. After cleaning it runs fine.

I did the plug wire swap prior to the head removal. It wouldn’t run at all, in that configuration.

I ordered a replacement head gasket and intake gasket today. I will try to source a replacement magneto cap to try, after I get the head back on, and before I resort to pulling the intake. I am intrigued by the possibility of crankcase pressure or a clogged oil vent/passage.

Interesting on the wire swap. Might suggest that a weak spark combined with a weak cylinder (for whatever reason, be it low compression or a failure to get adequate fuel/air mixture to it). That would lend strength to the argument about the magneto cap.

Crack your fuel cap I believe you have a clogged vent hole in the fuel cap.

have you replaced the plug leads. they need to proper copperwire plug lead. and crimp on ends a bad wire could be a issue. check the cap for fine cracks clean it with shellite or methylated spirit. clean the plug insulators too any conductive grime will stop em working properly.

Do you have a switch on the circuit? Ground wire on the magneto?

I would eliminate those possibilities!

Points must make and break, that is key to providing the primsry winding power in the coil.

Condensor failure eliminated as you have changed it.

As suggested, ensure the cap has no tracking or cracks.

I see no mention of the rotor. Very critical component as it tranfers the high voltage to the cap. Cracks, tracking, corrosion or even sloppy fit (loose) on shaft will cause issues.

Wires, ensure they are not "leaking" voltage, bad ends, etc

This will eliminate most of ignition issues.

Your symptomd of one cylinder acting up. Suggest an ignition problem

Then I fall back to Dad's rule---

When you are positive it is fuel, check ignition or vice versa - it usually is accurate

Cts