Ford 172 stationary power unit

JOB

Member
I have a 172 cubic inch Ford engine on a brush chipper. It has no spark. I look at the points and they are tarnished. I shine them up and re-install with a .025 gap and the engine is running. I run the engine till it warms up, maybe 30 minutes. Shut off engine to change oil. Try to restart and no spark. Shine points again and install new condenser, still no spark. I check voltage at coil, 11.75 volts coming to coil and 11.75 volts going to distributor.

Anybody have any suggestions? Am I not seeing something? I had the same problem last summer I fooled around with points and condensers and it started working until this spring. I can't honestly tell you what I did to make it start working last summer.
 
This engine gets used less that 10 hours a year, the points and condenser were put in last fall (echlin) has a new condenser now. I am leaning towards a bad coil but am not sure how to test it. I do not want to buy one if that is not the problem.
 
You wrote " I check voltage at coil, 11.75 volts coming to coil and 11.75 volts going to distributor. "

Is that with the points "open" or "closed" If the points are "open" those voltages would be normal. If the points are "closed", the coil primary terminal connected to the distributor should be art ZERO Volts. If it is NOT, the points are not making contact, which is what I suspect is happening.

HOW did you "shine up" the points? If you used any type of sandpaper or emery cloth, it is VERY likely that the points are the problem.

Post back and let's work this problem out!
 
I am not electrically smart when it comes to
ignitions systems. I did not know it would make a
difference in voltage if the points were open or
closed. The numbers above is with the points open.
With points closed I get 5.91 volts on the side of
the resistor going to the coil. On the switch side
I have 11.22 volts. On the distributor side of the
coil I get .079 volts. Isn't that kind of low on
the distributor side of the coil?

To shine up the points I used an already used
piece of sand paper. I use the same process on the
points in a wico Mag that I have not changed the
points in, in over 30 years. It still runs great,
but only gets used 10 to 15 hours a year.

I am leaning towards a bad coil, I don't want to
buy a new one if that is not the problem. I would
like to know how to test the coil. Last summer
when I had this problem I was running the machine
and all of a sudden it sounded like someone turned
the key off. So I was thinking now that the coil got
hot and failed. I should have thought of that last summer. Just a guess. I should have
checked for spark out of the coil wire immediately
when the engine shut down. But did not know any
better.
 
You wrote "On the distributor side of the
coil I get .079 volts. Isn"t that kind of low on
the distributor side of the coil?"

NO! Not with the points closed.

Connect a spark tester (spare spark plug) to the secondary terminal (tower) on the coil and using an insulated tool, open and close the points.

You should see a small spark at the points each time they open and a good, hot spark coming out of the coil each time.

If not, the coil MAY be bad and the condenser is a possibility.

Here is a link to the John Deere FOS manual for ignition systems, as posted on the Farmall Cub site. That MAY give you some further insight into this.

http://www.cleancomputes.com/Cub/Blue%20Ribbon%20Service%20Manuals/FOS-20%20Chapter%206-Ignition%20Circuits/index.html
FOS 20
 
Thanks for the help Bob, I Tried the opening the points with a screwdriver and having a grounded spark plug, and could not get a spark.

I borrowed a coil off a similar engine and put it on the 4 cylinder engine and had the same luck with the screwdriver. I then took the coil wire and held it a 1/4 inch or so from ground and spun the engine with the starter motor. Had plenty of spark. I am now up and running but one coil short.

I now understand how the ignition system works sort of. The points act as a switch, but how does the opening and closing of the switch make the coil fire? Thanks again Bob I need that chipper off and on all summer.
 
(quoted from post at 19:25:30 05/02/13) Thanks for the help Bob, I Tried the opening the points with a screwdriver and having a grounded spark plug, and could not get a spark.

I borrowed a coil off a similar engine and put it on the 4 cylinder engine and had the same luck with the screwdriver. I then took the coil wire and held it a 1/4 inch or so from ground and spun the engine with the starter motor. Had plenty of spark. I am now up and running but one coil short.

I now understand how the ignition system works sort of. The points act as a switch, but how does the opening and closing of the switch make the coil fire? Thanks again Bob I need that chipper off and on all summer.

The buildup of a magnetic field in the coil when the points are closed and it's collapse was points open, causing a spark from the secondary winding is explained in the first 3 pages of the previously linked manual FAR better than I could!.
 
I looked at that link. If I would have done that
when you first posted it I would not have had to ask
so many questions and take up your time. When it
mentioned John Deere I thought it pertained to John
Deere. I should have known all ignition systems work
basically the same way. Thanks Bob
 

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