good then weak spark

I am working on an int 340 tractor. installed new plugs, wires, coil, points, condenser. when I turn the motor over I get a nice blue spark across the points but then it quickly turns to a lower red spark then nothing. I have tuned up tractors before but this is aggravating me. originally it always started great. went to try to start it and it wouldn't start. we had some pretty good winds with rain and it looked as though the distributor got wet. finally managed to get it running and put it in the garage. then it wouldn't start. I installed the coil with the plus side to the resister. neg side to the distributor 12 volt coil with resister just before the coil. gapped the plugs .023. points .023 or there about. not sure why I have a good spark at first then she goes away. all thoughts welcome. thanks norm

On a 12 volt tractor IF you have a full true 12 volt rated coil you DO NOT also use an external ballast resistor or else the spark will be weak.

NOTE a coil labeled "12 Volts for use with external ballast" is in reality more like a 6 volt coil and DOES NEED the external ballast (just like it says) or the points will burn quick and the coil overheat HOWEVER a full true 12 volt rated coil that may be labeled "12 Volts" or "12 volts no ballast required" etc is what should be used on a 12 volt tractor.

NOTE a 6 volt coil has somewhere around 1.2 to 2 ohms LV primary winding resistance as measured between its small + and - terminals while a 12 volt coil has more like 2.5 to 3.5

IF ITS A 12 VOLT TRACTOR WITH A FULL TRUE 12 VOLT COIL TOSS THE BALLAST !!!!

NOTE some tractors in lieu of a traditional external bathtub porcelain/ceramic looking ballast used a thermistor wire ballast device which resistance changed once warmed up. If you have that instead of a regular ballast you may have a 6 volt coil. If you have a temperature compensating thermistor type of ballast it may be faulty.

NOTE if its NEG Ground the coils - output goes to distributor and + gets power from the ignition switch perhaps via a series ballast. If its POS Ground reverse that.

I have seen even brand new condensors go bad when once warmed up they fail or grow weak and lower the spark energy IF YOU HAVE THE RIGHT COIL/BALLAST TRY A DIFFERENT OR THE OLD CONDENSOR

I have seen coils go bad and fail ONLY AFTER warmed up !!!

Suspects...???.Wrong Coil/Ballast combination????..Faulty Condensor???..Faulty Coil

Hopefully the other gents have other ideas ????

John T

"a nice blue spark across the points" Actually you are supposed to have a nice spark at the spark plugs and not very much sparking at the points at all. If you had a lot of sparking at the points, the points would be quickly eroded. The main function of the condenser is to prevent sparking at the points.
Try another condenser.

A big spark at the point is not good. A big blue/white that jumps a 1/4 inch gap or more at the plugs is good. To me I say take that new condenser out and put in the old one and clean the points. Good chance you got one of the 50% bad condensers. Now days you have a 50/50 chance the condenser is bad right out of the box. I have not changed a condenser in over a decade

Bad condensor was the first thing I thought of too

I agree, there should be none to very little spark across the points, indicative of bad condenser.

if I take the resister out will it burn the points up faster.

That is a yes no or maybe thing. All depends on if you have a true 12 volt coil or a 6 volt coil that needs a ballast resister. Most coils now days will say on them 12 volt coil ballast resister needed or 12 volt coil no ballast resister needed.

Do you have a volt/ohm meter?

Check the ohms across the coil, out of circuit. If 1.5 ohms, it needs a resistor. If 3 ohms it does not need a resistor.

Go by the readings, not what was on the box or the part number unless it is clearly labeled on the coil itself. Many times coils get reboxed, shuffled from store to store, as they are no longer a fast moving item.

Next check the voltage at the ignition switch side of the resistor. It should be near battery voltage, switch on or cranking. If it drops below around 9 volts while cranking, there is not enough left to make a good spark. If the engine spins normally could be a bad ignition switch or high resistance connection between the switch and the resistor. If the engine is turning slowly, could be low battery, bad connection, bad starter, too small battery cables.

If that all checks good, move the volt meter to the - side of the coil. with the ignition on, the points open, should have battery voltage to ground. With points closed should have zero volts to ground. If the points are closed, and have any voltage at the coil - terminal, the points are not making connection, or a bad connection from the coil to the points, or the distributor is loose and lost it's ground.

The answer in pure theory would be YES REGARDLESS because less current the points have to switch the longer they last DUH

HOWEVER my answer above remains true. If its a 12 volt tractor and a 6 volt coil YOU MUST USE THE BALLAST or coil overheats
PLUS points burn fast

BUT if its a full true 12 volt coil DO NOT USE A BALLAST as the spark is weaker

John T Retired Electrical Engineer

That makes 2 or 3 of us who also thought that lol maybe were right??? But there are plenty other possible problems grrrrrrrr

John T

Try another condenser. I burned up a starter on my A-C G thinking it was going to start but didn't quite get there. I had put new points and condenser. Put the old one back in and it started. 20 years ago and it's still running.

I have seen this a number of times, for Me it was the connections between the breaker plate and distributor causing it,, all I had to do was remove it and clean all connection areas well, I also wipe them after with oil to "prep" the metal so it does not rust up again as fast
cnt

My JD crawler wasn't getting spark to the plugs. I checked the distributor plate as CaseNutty suggested and it fixed the problem. Thanks Tom.