704/K161T Won't Start

[rmaynard 15,458]

Let me start off by saying that I bought this tractor last summer, after the big show. It was not running, but seller says that it ran when he got it, was going to fix it up for his kid, but never got around to it. He put it in a tent garage but couldn't start it again. It's not a fuel problem, but a spark problem. Previous owner put in a new plug but that's all.

 

Before I start to pull my hair out, I need to know how to test one of these 'no longer available' magnetos. 

 

 

I have not worked on an old Kohler magneto in years, and my memory is going.

 

Thanks in advance for the help.

 

Edited December 28, 2022 by rmaynard

[953 nut 55,224]

Bob, first step would be to remove all wires from the condenser, put tape on the ends, put the flywheel back on to see if you have spark. If the spark returns then check the external wires for grounds, pull the flywheel and test the condenser.

[rmaynard 15,458]

Do you know if there is anywhere to find what the resistance is supposed to be on the primary and secondary of the coil?

[squonk 41,104]

I'm sure you already checked but I had a 704 that lost spark. The mag ground wire was run close to the steering shaft and had rubbed through and was laying against the shaft grounding it out.

[rmaynard 15,458]

8 minutes ago, squonk said:

I'm sure you already checked but I had a 704 that lost spark. The mag ground wire was run close to the steering shaft and had rubbed through and was laying against the shaft grounding it out.

Thanks. Checked.

[rmaynard 15,458]

Before I get too deep into the magneto, maybe I should check the points.  Two years of sitting can do wonders to the surface of points. 

 

Will report back tomorrow. 

[953 nut 55,224]

10 hours ago, rmaynard said:

maybe I should check the points.

Good idea, I figured you had already done that.   Section 8 of the Kohler manual in our manuals section is sort of vague as far as resistance values or methods of testing a mag. 

The question of testing a Magneto has come up on Red Square from time to time so I did a bit of surfing and the simplest explanation I have found was on a site devoted to those Green tractors (sorry about that) I have a NOS magneto coil and a couple of used ones on the shelf and will let you know if my results are the same.

 

The resistance of the secondary should be around 8k to 10k ohms. The resistance of the primary should be around 1.0 ohm. You need a good digital meter to tell the difference between a shorted primary and a good primary. If it really is shorted (unlikely) check the wiring harness where it enters the stator plate and under the clamp for a pinched or bare wire.

Both the primary and secondary coils surround an iron core and are grounded through the core to the stator plate. While the points are closed the rotating flywheel magnets induce a voltage and current flow through the primary coil which is made up of a few hundred turns of heavy wire. This current flow creates a magnetic field. When the points open the current flow stops and the field collapses through the secondary coil which is made up of thousands of turns of fine wire. This collapsing magnetic field creates a very high voltage in the secondary due to transformer action and the high voltage arcs across the electrodes in the spark plug.

The resistance of the primary is measured between ground (engine, or stator plate if the stator is dismounted) and the wire from the points and condenser. Resistance of the secondary is measured between ground and the spark plug connector at the end of the plug wire

[squonk 41,104]

10 hours ago, rmaynard said:

Before I get too deep into the magneto, maybe I should check the points.  Two years of sitting can do wonders to the surface of points. 

 

Will report back tomorrow. 

That same 704 quit running 2 days after I came home from the show with it. Points were shot. Year later chafed wire. 

[rmaynard 15,458]

24 minutes ago, 953 nut said:

The resistance of the secondary should be around 8k to 10k ohms. The resistance of the primary should be around 1.0 ohm. 

 

Thanks Dick. That's just what I was looking for.

[rmaynard 15,458]

The primary resistance is 0.8 ohms 

The secondary is open. 

Looks like I found my problem. Now to find a magneto. 

[rmaynard 15,458]

Well, I took the coil out and the plug wire fell off. No big deal, most do. But I could see the pin that the wire slips over, and I did a meter test from there. Still open. Went online and found that the complete magneto is NLA, but there is a separate part number for the coil. Coil is NLA but they are replaced with a different coil. Found one on eBay NOS, open box. What the heck? For $36 I'll risk it.

 

[ri702bill 8,319]

I replaced a bad coil on a K161T a couple of years ago - got one for a 6HP Tecumseh for about $12 - had to extend the black wire - no issues.

Bill

[rmaynard 15,458]

RESOLVED

 

Found the insulation on the input lead to the magneto coil had a bare spot that was touching ground.  Didn't find it until I'd purchased a new coil. 

 

Replaced the coil, plug wire, condenser, points (gapped to.020), cleaned carburetor and replaced the float valve and gaskets. Runs great. 

 

Thanks for all the help.