Top cylinder spark puzzle

Hi lads,I need somebodies experience with this one.Motor is 57 12 hp Gale.Have recently overhauled it,new coils points leads plugs,the whole 9 yards and of course the carb.Motor starts first or second pull and runs like a top,ON ONE CYLINDER.Something wrong with your "points/leads/ coils/plugs I hear you cry.Well as I said everything there is OK.Now for the weird part(to me anyway)By the way I have tried three different plugs,all with same result.When the engine is ticking over merrily and I pull the top plug-no change.Pull bottom plug-engine stops.Ah,but wait,when I pull the top plug and hold the lead about an inch away from the plug I am presented with an inch long fat blue spark and the little beast roars into 2 cylinder mode.What is going on here.First I thought maybe the plug insulation has broken down and the spark is grounding to the head.I can't believe this has happened with all three plugs so there must be something I'm missing here and if any wise soul can throw any light on it they will have my undying gratitude.
Geoff69a1

Re: Top cylinder spark puzzle

So when you move the wire around you get a spark on the top cylinder?

I think that could be your answer.

Re: Top cylinder spark puzzle

Does your fuel pump mount to the top cylinder?
I once had a similar problem, fuel pump was leaking fuel into the cylinder and flooding it. When you pull the wire and increase the gap that the spark has to jump it causes the spark to be hotter, clears the fouled plug and it runs properly until it floods again.
Might not be your problem, but it wouldn't hurt to check it out.

Re: Top cylinder spark puzzle

You may have something wrong in your magneto on that cyl. When you pull the plug wire you intensify the spark and then the cyl will start to run. Will the spark jump a 1/4 " open air gap on a spark tester ??

Daviet also has a good idea.

Re: Top cylinder spark puzzle

Hi Daviet. You might be onto something there,the pump is mounted on the top cylinder so it's a possibility.Not sure what you're getting at Capn chaos.
Boobie the mag checks out Ok big fat sparks on the tester jumping 1/2 an inch.

Re: Top cylinder spark puzzle

I was suggesting that your wire may not be seated well, either in the coil or the spike in the plug boot and by moving it around you are able to get a spark.

Re: Top cylinder spark puzzle

OK I see but the only part I'm moving is the end that attaches to the plug via the plug boot
I even removed the plug boot and stripped back the lead and attached it directly to the plug.Same result,but big fat spark when I moved the lead away from the plug.

Re: Top cylinder spark puzzle

Then it sounds like Daviet has the best idea yet.

Re: Top cylinder spark puzzle

That's what I thought but I just whipped the pump off and could find no evidence of it leaking,all dry as a bone and nothing when squeezing the bulb.Guess I'll just have to get my thinking cap on and try again Friday,tomorrow's out as I have to get 15 tons of hay into the barn.Night all.
geoff69a1.

Re: Top cylinder spark puzzle

Daviet's idea of flooding that is solved by a more intense spark sounds like a good possibility. Perhaps the flooding is coming from the carb on that cylinder? Prime the bulb until it gets hard and then keep priming it while looking at the carb. Any leaking fuel will show that this could be the problem. If the bulb never gets hard, then this could also indicate this as the problem. If the carb is bone dry after doing this, then look at the choke. I'm not sure if this old motor would have this, but the fuel enrichment system (prime start) could be faulty, and sending excess fuel to that carb. Also check the butterfly plates to make sure they are opening in sync. If the one on the bottom carb is not opening, that would create a rich condition in that carb that could explain the flooding (if the problem is in fact flooding). If these tests fail, then check the reed valves. A broken or stuck reed valve can restrict air flow into one of the cylinders making it have a rich condition. If there's only one carb for both cylinders, then most of this could not be the problem except for the reed valves. There should be a set for each cylinder, even though only one carb.

Re: Top cylinder spark puzzle

You are right ,this is an old motor and only 12 hp and only has one carburetor which I have thoroughly checked and removed from the equation.The fact that it runs perfectly when I hold the high tension lead away from the plug tells me that fuel is not the problem.As I said earlier,I checked the pump and it was good and not leaking,which brings us back to the electrics.On Friday I will tear the magneto down(again)and check everything.The only thing I can think of at the moment is a fault in the high tension lead or the plug boot itself,though this is unlikely as well.I will resume my quest Friday and will keep you posted.
Night.