Tune Up Notes

[jchamberlin]

Time for a spring tune up.

Engine won't turn over, spraying ether doesn't help, must be spark. Probably the points.

Points seem to be OK (open and close).

Got 12 volts on positive side of the coil.

Coil is OK (low Ohms across the primary; high Ohms from secondary to hot side)

Spark Plug wire Ohms out.

Plug looks OK . . .

After cranking, the meter is reading 2 volts and slowly drops? Conclusion: the "condensor" is being charged --"condensor" is OK.

If the meter is connected to the positive side of the coil, and is set to read Hz (or AC voltage), there isn't any reading? Conclusion: the points aren't working.

Electrically speaking, the points open to break the current flow through the primary winding of the coil (which electrically speaking is a transformer) and thus induce a voltage in the secondary winding. The ratio of turns in the secondary winding, compared to the number of turns in the primary, results in a stepped up voltage at the spark plug. But . . . if the the points aren't conducting current when they are closed, the circuit is open and the only thing that is happening is that the condensor is getting charged. (That is to say, the condensor is "blocking" the DC currrent.)

Action: Clean the points and make sure there is continuity across them when they are closed.

Note: It took a feeler gauge to scrape them, some sand-paper to rub them, and a good "snap" with the points in the closed position to achieve the desired continuity reading.

Started right up (after the cleaning)

Lessons learned: (1) Physical appearance doesn't guarantee good performance. (2) Points can fail "open" as well as closed.

(I'm thinking they should be called "maker/breaker" points because they need to both make and break the circuit in order to do their job of converting DC to AC so the coil can do its job.)

Technical Note: Transformers (the coil in this instance) only work with Alternating Current (AC); they are useless with the DC provided by points which are stuck open (or not making good contact, as above) or the total lack of voltage as when the points are stuck closed.

Just though I would pass these insights on to those who might find themselves facing similar challenges this season.

 

[eford]

Thank you Jeremiah! I just went through this with a buddy on his Allis Chalmers 720's Onan. Thing runs like a champ after we methodically went through ignition when no spark occurred as you just spelled out.

 

[kide]

Not sure about using AC VOM settings, a rolling over,but not running battery ignition engine is really see in interrupted DC, not AC.

Also for cleaning points, the best tool I've ever seen is a relay point burnishing tool, which has a bumpy,surface, but no cutting action. Use caution using sandpaper, some abrasives are conductive and can make things worse, unless you get all the debris off.

 

[glippert]

Thanks for sharing that, Jeremiah (and Gerry for your thoughts). My helpful tip/reminder is about good grounding. My 1450 didn't want to start - the starter turned the engine over fine, but it wouldn't fire, and when I released the key the engine would usually backfire. Cleaning the points didn't help, so I removed the coil and condenser from their mounting location on the engine tin, to check connections, etc. Turns out the "L" bracket that connects the tin to the head bolt was broken at the bend. And the tin had completely broken away at the lower, rear corner where it bolts to the engine block, so there wasn't a good, direct ground. Got both repaired (along with a 2" crack in the tin) and put it back together, set the points at .020 & she fired right up.

It's almost as if the condenser needs a good ground!

 

[jchamberlin]

Gerry: You're probably right about the reading on the AC volts, I need to verify whether I get the reading on a running system. I included the item because it was at that that point that the light bulb went off in my head and I had an "Aha moment."

A better diagnostic is to make sure there is continuity across the points when they are closed.

Note: I really wanted to use my old analog VOM (set to DC) but I didn't have a set of leads with clamps that would fit it, so I tried to use a fancy DMM with a "Hz" readout. Again, any way that the pulsating current through the primary can be verified should work. The value of checking it at the coil is that a broken wire or bad connection can be isolated as well as dirty points.