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18 HP L-Head (2 cyl flat head) no spark

Posted by jerry_nj (My Page) on
Sat, Oct 2, 10 at 16:24

My 1993 Craftsman GT6000 has a B&S 18 HP twin cylinder opposed, L head, that appears to have no spark. I pulled both plugs to help the low battery condition (not parked near the garage) and was able to turn the engine over at a reasonable clip.. I could clearly see the spark plug and in the open air with a wire from the threads to ground. I assume that if working correctly the spark would be very strong as it was trying to jump the 0.030" gap in open air (no compression).

I will dig out the Sears owner manual, but thought I'd bounce my test off his forum for suggestions and comments.

I believe this engine uses a magneto, no points or condenser, but this I need to check. Main need I have is what is the next step in checking the ignition? Trace the high voltage leads back to the source of high voltage? The wires look pretty good, but being 17 years old I suppose one could be leaking the HV to ground, next test is to test the other side, if both are out I'd say the problem is something common to the two wires, not the wires themselves. ... just thinking out loud

Follow-Up Postings:

RE: 18 HP L-Head (2 cyl flat head) no spark

  • Posted by canguy British Columbia (My Page) on
    Sat, Oct 2, 10 at 21:05

Disconnect the ground wires to the mag and give er a spin.If it sparks, there is a problem with the interlocks. If no fire, replace the coil

RE: 18 HP L-Head (2 cyl flat head) no spark

Thanks, I've now looked (and understand) the schematic in the Sears Owner Manual. It show the Coil (called Ignition Unit) has a kill ground wire coming from both: 1) Ignition Key Off, and 2) electric clutch (PTO) engaged and no one in operator seat. Any other grounds on this line would also kill the spark. I think this is what your good advice says. I'll try to get at look on Sunday.

If the kill wire is open and there is no spark it seems to me the problem could be either the coil (Ignition Unit in parts list and schematic) or the trigger-drive on the flywheel. I think I know how to test that if the coil and kill wire pass tests as good.

I assume the coil dc resistance (ohm meter) is only a few ohms for the winding connecting to the flywheel trigger/magneto. The schematic doesn't show this specific doesn't even show the Ignition Unit is connected to the flywheel pickup/drive/trigger.

I could be off base on some of my thinking, but in any case your advice will be followed and that may uncover the problem to be something simple on the kill wire side.

RE: 18 HP L-Head (2 cyl flat head) no spark

Posting the Sears 917.xxxxxx number is always helpful.
They have too many different schematics, many with slight variations, that it helps to be able to look at the "same picture".

BTW, none of the 5 GT 6000's in my home made database list a Briggs. Mostly Kohler, either Magnum or Command or ONAN.
Doesn't mean they didn't come with Briggs, but?????

RE: 18 HP L-Head (2 cyl flat head) no spark

Thanks, I have taken the cowling (fan and flywheel cover) off and see the Ignition Unit is the whole ignition, except for the spark plugs and the magnet on the flywheel that swings past the Ignition Unit package with the laminated core for the rotating magnet to induce the ignition electrical charge into the spark plug wires. It also has a kill wire, which I disconnected, and found there was still no spark. I then measured the resistance (ohm meter) from each plug back to the laminate/ground and found an open circuit. As this is the secondary of the "coil" it should measure in the neighborhood of 3K ohms, per an on-line reference. I have ordered a new Ignition Unit, expect it will arrive tomorrow. It came for about $35 delivered, a good bit less than for an Onan I'll bet. I end up with a completely new Ignition with this one part. I'll post here if this fixes the problem, if it doesn't there must be something wrong with my trouble shooting method or with the magnet/installation. I have also found the gap should be a bit over 0.01" or the thickness of a business card.
As for the authenticity of the GT6000 claim, the manual shows 917.255970, no mention of GT6000, and the hood has "Diehard" and "Gold 18 HP" which refers the the B&S engine. The front, below the "grill" is a lable: GT6000.
The tractor was purchased in April 1993, and was not the top-of-the-line, which I think came in a dark gray color and had an Onan engine.
The engine has about 1500 hours on it, and is "tough as nails" as is the 2x3 manual transmission (6 forward, 2 reverse), and the frame. It has been a good tractor; this and a fuel diaphragm replacement are the only engine repairs to date. It may be on its third set of plugs, and the full air cleaner components were replace a couple of years back. It has a washable prefilter.

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