Mini-Mac no spark

[Scottnc]

1974 McCulloch Mini Mac 35. It ran when parked some years ago. Points look good, gap at .018", coil to flywheel gap is .012" everything is bright and shiny. The flywheel magnets feel strong. I think there may be a problem with the coil but I'm unsure how to correctly bench test it. Here's what I have checked: The coil lead going to the points has continuity to ground whether the points are open or closed, I believe that is wrong. With the condenser wire unhooked it is open to ground which I believe is correct. With the coil lead to the points and condenser unhooked it shows continuity to ground. Ohms across the coil primary terminals is zero. Has the points side of the coil failed via an internal short to ground?

 

[Vintage Engine Repairs]

Hope this video I recorded for someone else helps you with testing- the specs I mention are for this ignition coil, so check your manual for your specific model.



To test your condenser ground it against itself to discharge (it may spark) go on the capacitance setting on the multimeter. Hook up the wires and check it’s close to spec

To test points, connect a multimeter probe to the ground wire and the other probe to a cylinder fin or other ground. When the points are open the resistance should read OL or open loop. When they touch closed it should read a very low reading 0.1 ohms for example. The point at which the points break (changes from 0.1ohms to OL) is when the magnetic field collapses and the secondary coils are charged. Technically if the points are gapped and the timing is correct, your magneto edge gap should be correct too, but worth checking. That’s the distance between the trailing edge of the N pole shoe and the magnetic pickup. This spec will be in your service manual, ignore the measurements specified, image is just for representation.



Pop some grease on the felt wiper pad too. Keeps the contact area of the points against the cam from wearing down unnecessarily.

Also worth mentioning 18thou is too wide for points, try 14-16thou

 

[Scottnc]

Thanks, that helped a bunch. Looks like the same values apply for a battery or mag ignition. Found the problem - points. Tiny spot of oil.

 

[J D]

If you have the ability to I'd check the condenser while you're in there. Capacitors can "dry out" & fail over time. That won't necessarily stop the points working, they just won't last very long

 

[Old2stroke]

If you have the ability to I'd check the condenser while you're in there. Capacitors can "dry out" & fail over time. That won't necessarily stop the points working, they just won't last very long

The capacitor is VERY necessary, provides a "ringing" circuit with the primary inductance when the points open which boosts the secondary voltage, won't get high enough secondary voltage without it.

 

[Scottnc]

It's running, or was. Fired right up - only to expose another problem; fuel line is cracked at the tank barb. The search is on for a replacement.

It's nothing short of a total joy pulling the engine from the case just to access the carb. And that throttle linkage, what were they thinking?

 

[Vintage Engine Repairs]

It's nothing short of a total joy pulling the engine from the case just to access the carb. And that throttle linkage, what were they thinking?

Sounds like the Stihl 020 AV. I absolutely hate working on them, though it is a charming saw both it running and its appearance.

 

[J D]

A friend of mine describes the mini mac's as "the first of the disposable saws" not because of their quality, but due to the time it takes to fix anything on them

 

[Old2stroke]

Yeah, it's a real pain to have to tear the whole damn thing apart just to get at the carb, with practise you can do it in 15 minutes. Takes longer to put it back together. Throttle linkage isn't a problem when reassembling, if you hook an elastic band on the end of it and stretch it forward over the oil pump, keeps it where you can find it after sticking the engine back in the case.

 

[KASH]

Make sure you clean the oil completley off the points using carb cleaner brake cleaner or contact cleaner.Any oil film on the points cause them to burn out on the contact.I fixed a few Mini Macs years ago but now changing a spark plug is about as deep as I will go into these engineering marvels.
Kash

 

[Scottnc]

Make sure you clean the oil completley off the points using carb cleaner brake cleaner or contact cleaner.Any oil film on the points cause them to burn out on the contact.I fixed a few Mini Macs years ago but now changing a spark plug is about as deep as I will go into these engineering marvels.
Kash

It's been sitting in a box for more than 10 years. The threaded clutch end of the crank had snapped off, since it had been my dad's saw I fixed it. Ran it maybe one more time. There was no oil under the point cover or on the point plate so it is kinda odd there was that tiny splotch dead center on both contacts. Couldn't really see it at first, ohm meter showed zero continuity so I yanked them and then saw the problem. Waiting on some carb parts. Have a bunch of too tall peach trees that need to be reigned in, wanted to give this little saw a try instead of holding an 036 overhead all day.