The hateful pony motor on my little D2 won't run on one cylinder.  Pulled the head and the valves aren't stuck, good spark, new plug.  What can it be?  A real bitch to get at the carburetor.  One side runs fine.  I'm puzzled. 

Hi, wanglow.
Them pony motors can be temperamental contraptions - 10% temper and 90% mental - I once operated a 2U D8 on which the pony motor would NOT run on two cylinders unless #2 was EARTHED/GROUNDED to some point on the body of the motor. The Cat mechanics checked everything, spark plugs and leads, magneto cap, the guts of the magneto, the lot. And it still would not run on two unless #2 was grounded.

It went to the dealer when traded on a new D7F still with that ground wire attached from #2 plug to the drain screw on the bottom of the carburettor.

And nobody could tell us why it was like that.

The D7F was direct electric start.

Just my 0.02.

Sounds weird but I'll try that.

As Deas states, these pony motors often do have interesting and sometimes frustrating personalities, one of mine will not run on two unless one of the plug wires is loose with about a half inch gap from the wire to the plug, as long it has that it's very reliable, I could spend some time describing the quarks of every one of the pony motors I have or had, best to learn what yours likes and stick to it.
If you do have the mag cap off, be sure to clean the inside of the mag cap, carbon build up from the brushes can cause the spark to leak along small cracks or buildup, called tracking, starting fluid seems to be good for this, be careful the brushes are very delicate.

Hi, wanglow.
IF this is the problem, all you need to do to try it is to short each plug out to the block with an insulated screwdriver. If you're only running on one cylinder, one shorted plug will stop the engine and the other will either make no difference or make it run on two cylinders.

If it makes no difference, that is likely not your problem. IF it makes it run on two cylinders, try installing a ground wire from the plug to some part of the engine body.

Just my 0.02.

josh wrote:

As Deas states, these pony motors often do have interesting and sometimes frustrating personalities, one of mine will not run on two unless one of the plug wires is loose with about a half inch gap from the wire to the plug, as long it has that it's very reliable, I could spend some time describing the quarks of every one of the pony motors I have or had, best to learn what yours likes and stick to it.
If you do have the mag cap off, be sure to clean the inside of the mag cap, carbon build up from the brushes can cause the spark to leak along small cracks or buildup, called tracking, starting fluid seems to be good for this, be careful the brushes are very delicate.
 

Called a "JUMPSPARK" in my old motorcycle days. I have no idea why it works unless there is some kind of inductance generated.

dont use starting fluid to clean a magneto dont use petrol either bad things can happen. use electrical contact cleaner or methylated spirit instead. you can also use a product called shellite just remember dont saturate the windings and let everything air dry well.

what josh describes can be due to a weak magneto or poor sparkplug wire and the gap increases the energy of the spark it was a old trick to use a button on the plug stem before you fitted the lead. you used to be able to these things called spark boosters too.

magnetos rely on good return paths via the engine casting to the magneto clean everything spotless and see if that helps. you can also use a light rub of 2B pencil to improve contact between the mag and the engine apply only to the mating faces. it can also work on the threads of the spark plugs dont get it anywhere else.

Hi. trainzkid88.
What I have been talking about with the 2U D8 was connecting a wire from the TOP of the spark to somewhere on the engine body. I don't think that has anything to do with grounding the spark plug through the engine body to complete the circuit back to the magneto.

What Josh mentioned in his post sounds more like boosting a weak spark but that raises the 'kwestyun' - why only on one side? There could be a number of answers to that.

Just my 0.02.

a lot of diesel expurtsz get lost when dealing with gasoline/ ignition systems....

-spark plugs, plug wires, dist. caps, condensers, points etc are all consumables....they only last so many thousand hours! if you don't know how old they are, replace, eliminate a problem and move on....and toss the old ones out....do not store and recycle....they are compromised to some degree...next thing you know they have an entire spark system that is 80% toast...and it starts and runs like shyte....hmmmm
-an old'timer neighbour of mine would never part with any electrical parts...and was constantly chasing his tail when working on gasoline engines...go figure
-rant done

my grandfather did that would change plugs and not throw the old one out we got the shit one day and tossed em all. he even recycled nails. we were extending the lean to at the back of the shed once dad got the shits with recycled nails and sent me and aunt kathy to buy some new nails we came back with a 2kilo tub each of 2 and 3inch nails. the old bugger still recycled nails! we still have those tubs half full 25yrs later.

deas thats a bit weird it would run with a ground wire to the top of the plug. normally that would stop it running as it would take the power from the plug. like water electric current flows the path of least resistance. what josh described sounds like a bad plug wire or contact in the rotor button/cap.