Homelite XL-12 attempted Prestolite blue coil replacement

[oneoldsaw]

Hi forum!

So I figured I'd try to get my dad's old saw running again. No spark. A little googling turned up "blue Prestolite coil of death", and suggested baking the coil in an oven. That actually worked... for about a minute. A longer bake (24hrs @ 115C) and it worked for 10 minutes. Long enough to confirm the rest of the saw is fine, anyway. (9980 rpm free running, 149psi tested warm)

So, on to replacing the ignition. I didn't have a donor saw handy with a "points" coil, so I ordered one online based on manufacturer and appearance. It is physically very similar, and I was able to swap the core from the blue coil onto the new one and fit it into the saw just fine. The nova module fits in a nook under the starter housing, as per the photo.



Here's the curious bit: Pull the cord, no spark. Flip the kill switch to "stop", and now there's a spark! How the heck? The kill switch connects to one of the spade connectors at the top of the coil, and as far as I can tell those spade connectors are a short to the black points wire coming out of the bottom of the coil, so with the kill switch closed there is continuity from the chassis, to the switch, to the tab, to the points wire, to the + terminal on the nova module. And the - terminal on the module also goes to the chassis. So, there is continuity across the terminals of the nova module via two points in the chassis and whatever is going on inside the coil, yet the saw runs!

Put the switch into the "run" position, and no spark. Switch tests fine. I also tried the nova module in reverse polarity, and it sparks just the same though I didn't run it that way.

Anyone seen this before, or care to guess what's going on inside that mystery coil? Is it just wound in the opposite direction?

Anyone have a known compatible points coil for a reasonable price, or a part number for one?

Is anything likely to blow up if I just rotate the stop switch 180 degrees and use the saw?

 

[LegDeLimber]

Some circuits ground or short circuit the power to kill it.
Others simply disconnect the wire to cause an open circuit
and thus an incomplete path.

One directions swing/setting of the switch closes the contacts.
The other direction/setting opens them.

Functionally speaking; You can flip the switch around with no harm to the saw.
Just look for any flat sides or little notch and tabs
that hold the switch in a "standard" position.
IF there are any locators, You'll have to decide if you're willing to file off or otherwise modify them or the hole.

and lastly: Be very careful to not get the wires in positon that will cause them to rub through the insulation on something like a bump or corner, etc.
You also need to make sure no moving parts like the flywheel or throttle and choke linkages, carburetor shafts, etc, will be rubbing or scraping the wires.

This may sound like a lot of fussing with it,
But a simple chaffed wire is all it may take
to mess up your day in the wood.

 

[a. palmer jr.]

Why didn't you just find another module or coil from a Homelite saw and put it in? A friend who owns a lawnmower shop told me long ago that if it will physically fit there's a great chance it will work. The last Super XL I owned had the dreaded blue coil and I swapped it for a black Homelite coil and although I had to elongate the mounting holes, it worked fine, no other changes necessary. I also see no reason why you can't just turn the switch around so that run is in the up position to avoid any confusion.

 

[oneoldsaw]

Some circuits ground or short circuit the power to kill it.
Others simply disconnect the wire to cause an open circuit
and thus an incomplete path.

One directions swing/setting of the switch closes the contacts.
The other direction/setting opens them.

Functionally speaking; You can flip the switch around with no harm to the saw.
Just look for any flat sides or little notch and tabs
that hold the switch in a "standard" position.
IF there are any locators, You'll have to decide if you're willing to file off or otherwise modify them or the hole.

That's good to know. I did measure the polarity at the plug, and that's still chassis positive (pluglead negative) with the nova module hooked up the normal way (as per photo). I may just flip the switch bezel over.

and lastly: Be very careful to not get the wires in positon that will cause them to rub through the insulation on something like a bump or corner, etc.
You also need to make sure no moving parts like the flywheel or throttle and choke linkages, carburetor shafts, etc, will be rubbing or scraping the wires.

This may sound like a lot of fussing with it,
But a simple chaffed wire is all it may take
to mess up your day in the wood.

The lead off the coil is already very short, and I have reduced the chassis lead to minimum as well. The only remaining danger is the little arm I mounted the module on tipping into the flywheel, but it looks to me like it'd grind the lead away and kill the ignition rather than do anything catastrophic if that happened.

Why didn't you just find another module or coil from a Homelite saw and put it in? A friend who owns a lawnmower shop told me long ago that if it will physically fit there's a great chance it will work. The last Super XL I owned had the dreaded blue coil and I swapped it for a black Homelite coil and although I had to elongate the mounting holes, it worked fine, no other changes necessary. I also see no reason why you can't just turn the switch around so that run is in the up position to avoid any confusion.

I would have used such a coil if I had found one. As it is, the core was almost identical, but I think it was intended for a slightly narrower flywheel as there was a slight wedge shaped gap, so I swapped the cores. That should have helped keep the timing consistent as well. If anyone knows the part number for directly compatible coil/core assemblies, though, by all means post them for the benefit of anyone else who is searching.

Anyway: Vroom Vroom! That saw sure sounds like it means business! Unfortunately I tried tuning the carburetor, and now it's hard as heck to start. Probably I dislodged some old gas goop, so that's the next part to clean up...

 

[a. palmer jr.]

The older saws don't thrive on leaner fuel mixtures like the new ones do. You have to give them a little fuel to get them to run right. A too lean mixture will do little but make them hard to start and possibly burn a piston up. Turn the H and L screws out at least a full turn, sometimes more.

 

[oneoldsaw]

The older saws don't thrive on leaner fuel mixtures like the new ones do. You have to give them a little fuel to get them to run right. A too lean mixture will do little but make them hard to start and possibly burn a piston up. Turn the H and L screws out at least a full turn, sometimes more.

It starts consistently if I squirt some gas into the cylinder first; I just can't get the needle settings back to where they were before I started fussing. Pretty sure I dislodged some crud and blocked an orifice, so I've ordered a carb kit. (appears to be Tillotson HS-179 on this Canadian UTNo 50130) I'll definitely bias toward a rich mixture before I consider the saw ready for use, though.

 

[a. palmer jr.]

It starts consistently if I squirt some gas into the cylinder first; I just can't get the needle settings back to where they were before I started fussing. Pretty sure I dislodged some crud and blocked an orifice, so I've ordered a carb kit. (appears to be Tillotson HS-179 on this Canadian UTNo 50130) I'll definitely bias toward a rich mixture before I consider the saw ready for use, though.

You might try starting out with both the H and L screw about 1 1/4 turns out from lightly seated and see how it does. This would change if the inlet needle height is maladjusted. You can go to Tillotson's web site and see how to adjust everything.

 

[oneoldsaw]

You might try starting out with both the H and L screw about 1 1/4 turns out from lightly seated and see how it does. This would change if the inlet needle height is maladjusted. You can go to Tillotson's web site and see how to adjust everything.

Even adjusting for richer than when I started, I can't smell gas anymore after pulling it over a bunch of times. I suppose I could unscrew the needles completely and spray some seafoam up into the seats, but that would probably just delay the rebuild... Also ordered a gas line, since that seems to be a common thing to break when servicing the carb.

 

[a. palmer jr.]

Probably either a stopped up place in the fuel system or a cracked or broken fuel line. Might as well put a new filter in it also when you rebuild the carb. I wouldn't spray Sea Foam into the carb, I would use carb cleaner. I think Sea Foam is basically like putting kerosene in it. If you think the carb is stopped up somewhere it would be nice to disassemble it and soak the parts (except for the metering lever spring) in carb cleaner then flush them in hot water and dry them with compressed air, at least that works for me..

 

[oneoldsaw]

Probably either a stopped up place in the fuel system or a cracked or broken fuel line. Might as well put a new filter in it also when you rebuild the carb. I wouldn't spray Sea Foam into the carb, I would use carb cleaner. I think Sea Foam is basically like putting kerosene in it. If you think the carb is stopped up somewhere it would be nice to disassemble it and soak the parts (except for the metering lever spring) in carb cleaner then flush them in hot water and dry them with compressed air, at least that works for me..

Hmm. I'm pretty sure seafoam is indeed kerosene plus a little light oil, whereas carb cleaner is basically solvent designed to strip oil away. I guess either in moderation should be OK, but carb cleaner definitely seems harsher to me. Anyway, let's give it a go - it'll be a week before the parts show up.

 

[oneoldsaw]

Mail arrived! Carb kit was 90% correct, though the block gasket on this particular saw is more oval rather than the diamond shaped gaskets in the kit. Anyway, with a squirt of gas to help it start, it idled fine for a few minutes, but died quickly when gunned. Now no spark. New nova II module...

OK, kinda sorta runs, but isn't pumping gas reliably. What's the procedure for checking crank case seals?

Hoping to avoid further overpriced ebay parts purchases... Also hoping this saw starts working nicely soon, or I'm going to develop some embarrassingly asymmetrical arm strength.

 

[oneoldsaw]

Half this saw wants to run, and half just wants to sleep. It probably was pumping gas fine after the carb refit, but the electrics were failing. Now there's no spark again. Either the mystery coil killed the second nova module almost instantly, or the new coil itself has failed.

So, back to the start. Anyone have a part number or link for a coil that's known to be compatible?

 

[a. palmer jr.]

You can check the coil with an ohm meter. The primary side, measured from ground to the points wire end, is a low reading, probably just a few ohms and the secondary, measured from ground to the sparkplug wire end should be several thousand ohms. These are general measurements, a few ohms one way or another isn't a problem but an open circuit or a shorted one (zero ohms) is. You should check the parts before buying a new coil, I don't know a reliable way to check the Nova II except replacing with a known good one but the coil definitely can and should be checked.