Mini Mac ignition(?) issues

[MiteyF]

Before anyone says it, I know... everyone seems to hate these saws. I got this little guy in a group of other old saws, and have about $15 invested in it, and when it's running nice, it's a great little saw. Maybe it will end up in the scrap pile eventually, but I'd like to avoid it. When she's running right, she's a real sweetheart, and I'd like to keep it around when I don't want/need my big heavy 10-10, or to keep at my parents house when they need a little tree work done.

When I got this saw I cleaned it up really well, rebuilt the carb, new fuel line, and right off the bat it ran great. However after doing some cutting and warming up (10-15 minutes) it would eventually sputter out and IF I could get it restarted, it would only run near full throttle. No amount of carb playing would cure it, so I figured it must be a failing coil, maybe condenser (old points saw). I also considered vapor lock, but spark seemed weak when I checked after the engine got hot. Not wanting to dump more $ into it than I need to, I figured I'd check the points and all connections first. Once I got into the points, they were super clean already, and I gapped/timed it to exactly factory specs. Coil gap is spot on. Coil resistance to the plug lead is around 7k I believe. New plug, properly gapped. Getting nice hot blue spark.

However, now once I fire it up, she won't rev, acting like spark is cutting out. Again, carb settings don't really seem to make any difference. Before I go trying to swap parts from my other MM (later, points-less style) I'd like to try to play with the timing once or twice more to see if I can improve it and narrow down the issue. Silly me, I didn't check where the timing was before I dug in, but it's a bit tough to mess up with the marks on the flywheel/coil, so I didn't really bother.

Does this sound like it's too advanced or retarded? Obviously it's a bit of a pain to get in and play with, so I'd like to get a good guess on which way to go before digging back in.

 

[Old2stroke]

Not often you have timing/ignition issues with these little gems, the most common source of the type of problem you are having is fuel delivery from the tank. The tank should always be above 1/4 full as there is no fuel line with a heavy filter to follow the fuel around, instead there is a giant chunk of felt in the output of the tank that acts as a filter and pickup wick. The felt has a habit of getting old and hard and won't flow much fuel. To confirm this, you have to unplug the white nylon connector from the tank and remove the tank from the saw, then with the tank 1/2 full there should be a steady stream of fuel out of the tank, not just a drop now and again. If you can remove the felt filter from the tank, just cut off the compressed part and stick if back in.
Your symptoms could be related to an electronic module if you had one but are not typical of points/magneto. If it turns out to be ignition, there are some simple tests.

 

[MiteyF]

The saw was running and revving fine (until it got warm) before I dove into the ignition, and I haven't changed anything with the fuel system. I did remove the filter, clean the tank very well, and replace the fuel line (and rebuilt the carb) when I went through it initially, and always keep at least a 1/2 tank in it. I'll pull the tank and try the drip test just to make sure.

What "simple tests" could I try for the ignition? I can't find resistance specs for this coil, but I figure 7.5k ohms should put out a good spark (and it certainly appears to when checking it without the saw running), condenser seems to test good with the Fluke (and my FLAPS didn't have one that small that I could try), and the timing seems to be about perfect according to the service manual I downloaded from this site (superb resource).

I have an Eager Beaver powerhead (just a later Mini Mac) which runs but needs a clutch, however it doesn't have points, so I'm not sure if the coil will work. They're different part #'s, and the coil from the EB gives me only about 2.5k ohms on the high voltage -> laminations. I can't swap the later flywheel onto the earlier crank either to use the system as a whole, as the pull start mechanisms don't match.

On the plus(?) side, I bet I can tear down a Mini Mac about as fast as anybody else on planet Earth at this point. Once you do it a couple times, it's not nearly as bad as everybody gripes about.

Like I say, I'm only into it $15, but it's such a sweet little saw when it runs I'd hate to junk it out of frustration.

 

[heimannm]

If you didn't replace the high speed check valve when you rebuilt you carburetor, you probably need to do that. This is especially true for the MDC carburetors but even the Zama's will have an issue from time to time (I had to change one this week). If the HS check valve is not working the saw will act as you describe since it it dumping too much fuel when the engine runs at low speeds.

There were two different electronic ignition systems used on the smaller McCulloch saws. If the flywheel on the EB2.0 has "IGN" molded in near the magnets the ignition will work on the points model. If the flywheel has "CDI" or "BB" molded in the timing will be way off on the IGN flywheel. Another clue is the coil itself, the black ones that say SEM must be used with the CDI flywheels.

For the points coil, the secondary should be 7.5-9K ohms, primary side 1-2 ohms. Condenser will be 0.2 microfarads. Make sure your points are clean, clean, clean and that there is no oil residue in the points box to maintain a reliable spark on the points ignition models.

Mark

 

[Old2stroke]

So just to be clear, are you saying that the saw will start normally and will idle well but will not accelerate to high speed even if you baby the throttle and pulse the choke on and off? Initially when you said it would run for awhile and then die it sounded like it might be the tank vent wasn't letting air into the tank. This no longer sounds like the problem but it's easy to test by just running with cap loose.
When you cleaned the carb, did you verify that fuel was coming out of the idle port and the transfer ports under the low speed welch plug? Did you remove the tiny screen covering the high speed nozzle check valve? There is a tiny rubber disc under the screen that may be deformed and is now blocking the main jet. If it's missing completely, it will affect the idle but should still allow the saw to rev up if you pulse the choke on and off.
Your resistance reading of the secondary is about right and the primary to coil ground should be very low (points open). It doesn't sound like there is anything wrong with the coil but the inability to rev up could be due to a bad capacitor (about the only thing that usually goes wrong with this system) and testing with an ohm meter isn't all that accurate. As long as the flywheel key is not sheared, the timing won't have changed and you would need a timing light to verify it. Only variability in timing is due to the size of the points gap. An electronic module off another saw might work as long as the magnet legs in both flywheels are in the same position. A conversion to an aftermarket module that is designed to replace the points would be the best route.
There is a strange feature with this ignition. You probably noticed that there has been great attention to electrically isolate the coil laminations from the case ground. Sleeves around the mounting bolts and insulating washers under the heads and the coil lead under the screw head but above the washer. No one seems to know what would happen if you left them all out and I've been tempted to do that just to find out but that's yet another tear down. Any chance you have done this?

 

[MiteyF]

The saw will idle, and no amount of choke/throttle work will help it get much past ~1/4 throttle, it sounds, to my ear, just like a throttle limiter cutting spark, it will hit a few times, miss a bunch, hit, etc.

When I cleaned the carb I did remove the screen, did diaphragm and gaskets, but did not remove any plugs. Again, before playing with the ignition, I could get 10-15 minutes of good solid running/cutting before it would slowly die out and not restart. I did try topping the tank off and leaving the cap loose, so it's not a venting issue. Once cool, it would re-start and run well for another 10 or so minutes before slowly cutting out/dying.

When it first did the dying-when-hot, I could get it fired after a dozen or so pulls, but it would only run at high throttle, and would die when I let off. Now, after playing with the ignition, it will only run at low rpm and won't rev beyond ~1/4 throttle.

 

[MiteyF]

Also, when I originally had the saw together, I did not have the insulating sleeves, nor washers installed. Surprisingly, it ran just fine. I have since stolen and installed the sleeves and washers from the parts saw, did not seem to make any difference strangely.

 

[Old2stroke]

What did "playing with the ignition" involve? When you removed the fine screen over the main nozzle, was there a tiny black rubbery disc under it?
You now seem to have conflicting symptoms. If the exhaust note breaks up and you seem to be getting intermittent spark that is a sure sign of coil or capacitor going bad. If the engine runs smooth and cuts good but just seems to lose power and slow down, that is a sign of fuel starvation and you can usually confirm it by going to full or part choke as it starts to die, then when it stops, pull the plug and if it is still dry, it's fuel starvation.
It might be at the point where you have to protect your sanity by replacing parts to find the culprit by process of elimination. AM carb, capacitor from any magneto/points ignition, coil (NOT AM COIL)

 

[MiteyF]

I tried to make it fairly clear, let me try again.

When FIRST REBUILT, the saw ran and cut great through the whole rev range, until it got hot, about 10 minutes in. Then it would peter out, and IF I could get it started while warm, it would only run for a few seconds, up high only. If I let it cool down again, say, the next day, it would start and run the same. Great, for about 10 minutes.

Which made me suspect a failing ignition coil.

I tore it down, cleaned the points (they didn't need it), re-set timing, checked coil gap, and threw in a new plug.

Now the NEW issue since reassembly, is easy starting and good running down low, but it will not rev.

I am fairly confident it is not a carb/fuel problem (never can be 100% of course). I rebuilt the carb when I got the saw, and it worked perfectly well until it got nice and warm. I have played with the carb a bit since the NEW problem started, but there have been no major changes, just playing with the jets, which do not make any difference in its ability to rev.

I suppose I can try the coil from the other saw, but seeing as it's electronic, with a different part number, I doubt it will work. I guess that's the next logical step.

 

[Old2stroke]

Like I said, if the 2 flywheels are the same dia , the magnet legs the same shape/size and the same relationship to the keyway and the same north-south orientation and the mounting holes the same, that's a lot of ands but there's a good chance it will work. My money is on the capacitor. To test it after mounting the coil, just use a reverse drill to turn the FW rather then a full reassembly.

 

[MiteyF]

Well, late update, finally got back around to messing with the Minis today (been using the 10-10). I swapped the coil from the EB onto the MM, and got no spark. So I figured WTH, I'll put the EB back together with the MM clutch (EB was running, but broken clutch spring), and use that one instead. The clutches don't have the same thread. So both are going in the bin. I'm only into the 2 saws for $25 total (including carb kits), but the multiple days of screwing with them turns the cost/benefit against me. I've still got a little Homelite topping saw (that also cost me $8 from the same lot) that I'll dig into.

I hate to toss tools, and I REALLY hate giving up on what should be a simple project, but I'm over these 2 little guys. If anybody is local and wants them for parts, come get them before I make another scrap run.

Thanks for all the help O2S