Bud W

Member
My 6 volt coil shows about 5 volts on the plus terminal of the coil but zero volts on the terminal that connects to the distributor. I'm having no luck starting the tractor but I do have a small spark at the plug terminals. Is it possible that the coil is bad and not delivering sufficient voltage to jump the plug gap? I have 150 lbs of compression on all cylinders , timing appears right on and the carb has been rebuilt. Running out of options??
 
My guess is the weak spark is jumping at atmospheric pressure but insufficient to fire under the compression-pressure in the cylinder. In general the larger the throttle opening, the higher the compression pressure and the more resistance to the spark jumping the gap.
 
When the breaker points are closed, voltage at the distributor low-tension terminal will be very close to zero, the better the points are, the closer to zero the voltage will be.

When the points are open, voltage will rise to battery voltage.

I would wonder if your points are not opening, or if there's a short in that area, however, you state you have at least a "small" spark from the coil, which proves the point are at least opening.

(If you didn't know, spark to the sparkplug occurs at the instant the points OPEN, breaking the primary circuit, and allowing the magnetic field built up in the coil's iron core to rapidly collapse, causing a spark from the high-tension winding.)

Possible causes of weak spark could be a failed condenser, a failed coil, or perhaps someone has installed a coil intended for a 12 Volt system in your 6-Volt system.

If you copy the following link and paste it into your browser, it will take you to a very good tutorial on how "Ketting" ignition systems operate, and how to troubleshoot them.

http://farmallcub.com/rudi_cub/www.cleancomputes.com/Cub/Blue%20Ribbon%20Service%20Manuals/FOS-20%20Chapter%206-Ignition%20Circuits/index.html
 
As Bob says. Also most of the time when you shut the engine down the points will be closed so no volts on the distributor side since the points being closed completes the circuit. But if you where to pull the distributor cap off and turn the engine to the point the points are open you should read battery volts. Also if you take the ignition side wire off the coil and touch it back the coil and do that a few times each time you should see a small spark with the ignition turned on and also get a good blue/white spark out of the center coil wire that will jump a 1/4 inch gap or more each time you pull the wire back off
 
OK I pulled a plug , reconnected the wire and hit the starter with the plug threads grounded. Really looks like a very good spark across the plug gap. I then hooked up a new 6 volt coil from another tractor and still no luck with the starting. So here is what I have done so far:

-checked compression with all plugs out and half throttle-got 150lbs on all four cylinders
-replaced condenser, points and rotor-cap is almost new and looks great
-checked coil and spark across plug gap
-lined up timing mark and checked that distributor rotor was at the number one plug terminal
-checked that plug wires were in correct firing order
-replaced fill needle and seat in Zenith 12119 carb ( tip of old needle had come off )
-disassembled carb , soaked in carb cleaner and blew out all orifices. This was a new carb from Yesterdays Tractor and has less than 5 running hours ( parade tractor )
-emptied fuel tank and added new gas-I have good flow to the carb

I sent my original carb to YT for rebuilding last week and will install when I get it back even through I can't find anything wrong with the one now on the tractor

Is there anything else I should try?-I'm fresh out of ideas

By the way it starts on 12 volts so I do get a great spin with the starter

Thanks for any help
Bud
 
So your saying if you use a 12 volt battery it will start but if you use a 6 volt battery it will not. That tells me you carb is not a problem and your ignition system is not a problem. It has to be either a weak 6 volt battery that is not full charged and should read 6.3 plus volts on a volt meter or the starter is going bad and pulling to many amps when you trying to start it so the ignition system is not getting the power it needs
 
Get an Echlin brand condenser from NAPA. At least they are made in Mexico. The generic ones from China are poor quality.
 

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