Low-Tension Mag Starting


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The following comes from a recent topic on SmokStak, an Internet engine bulletin board (go to http/ smokstak.cgi). As ever, various individuals started, commented on and concluded the following bulletin board thread.

I have a really nice, barn-fresh Associated with loads of character gained through the years. It runs great, but it's a real bear to start. I would like to start it with a battery and coil and then switch it over to the mag. My question is, should I install a two-way switch to switch between the mag and ignitor or should I install a two-pole, two-way switch so I can ground the magneto when the battery is in use? Will I need a resistor in the grounding circuit so the mag is not shorted directly to ground, and what size and value of resistor might I need? -Al

A single-pole, double-throw switch will be just fine. There is no need to ground a low-tension mag while the engine is running. The AC voltage generated by the mag when not loaded will be quite low compared to the spike created when the igniter trips. Bearing condition is critical in low-tension mags, as is strength of the magnets, and it must be correctly timed to the engine. I would guess you have a problem in one of these areas. - Ken

Thanks Ken. This engine is in 'as found' condition and I have done nothing to it. The mag timing is good, but the magnet could be weak. The armature bushings are a bit worn, but not so much as to allow dragging on the fields. This thing is completely original and I want to leave it for the next generation to restore. I am not even going to clean it. I had it mostly apart and repaired the rod bearing and the wrist pin. It is a lot of fun to run and people point and laugh at it. I just love it. - Al

Yes, some of the nicest running engines I have seen were in 'as found' condition. Nothing wrong with that, it shows how these old engines spent most of their working life. It's getting hard to find engines and the old, larger tractors in that condition. -Ken

I just installed a rebuilt gear-driven mag on my small Associated. I measured the voltage from it while the engine was running off the battery and coil and got about 5 volts. This sounded low. The engine will run off the mag if the rpms are kept up and will start a crank if you turn it fast. Does this sound normal? - Mike

Al, here is the diagram out of a United manual (see next page) for mag/battery hookup. Notice how the low-tension spark coil is in line with the ground. - Keith

Keith, looking at the diagram you posted, is the coil wired correctly? I would think the coil would be between the battery and the igniter, not between the battery and ground. Does it make a difference? I have not gotten to the point where I have worked on this type of battery and low-tension coil ignition and am just wondering. - Jim

I think the coil is actually between the battery and the igniter. They have the positive side of the battery going through the coil and to the ground on the igniter. The negative side on the battery goes to the stationary point on the igniter. It isn't the way we are all used to hooking up coils, but I have my Stickney wired this way because it showed it the same way in the owner's manual. It runs great. - Bryan

Positive and negative make no difference with a low-tension coil and igniter. It is wired this way, in this case, to isolate the coil from the magneto. - Ted

If your mag is working correctly it does not have to be spun to start. Just like any other hit-and-miss engine it should snap right off with a pull of the flywheels. - Mike

Mike, no disrespect, but what are you talking about? As I understand this thread, the low-tension mag being discussed does not have an impulse. It's impossible to generate electricity without it being spun at a relatively high rpm (60-100 engine rpm). That's with a good mag with good magnets. That is why there are impulses on high-tension mags, to replace the huffin' and puffin' required to generate enough speed by people power. The point I am trying to make is that an igniter engine can be started on a mag, but it is more than a casual 'flip' of the flywheels. - Mark

Harry has very good instructions on the Associated magneto/ignitor in the 'ignition' section of his web site at:

Pay particular attention to the information regarding the push pins on the magneto and the relationship they have with the trip of the igniter. When the magneto revolves there are two peak times of output. If the igniter is timed to trip at the moment the magneto is at peak you will get a viable spark. It does not require lots of cranking and/or high revs to work. There is also info on a simple method of checking the magneto to see if it is working. Al, check it out before you go to all the trouble of setting up the battery/coil rig. I just finished fixing one that was just like yours, and now it starts off the mag with one easy flip of the flywheels. The timing of the magneto with the push pins and trip finger is critical. - G.V.

I hope I didn't open up Pandora's box. Mike said a mag did not have to be spinning to create a spark. That is true in an oscillating mag. But the spring does the same thing as spinning a mag when it trips. A small engine without a large amount of flywheel inertia is very easy to spin to sufficient speed to start with a simple flip of the flywheels. 1 agree and completely understand what you guys are saying. 1 was talking about getting that same speed with flywheels of 2,000 to 3,000 pounds. It can't be done without some huffing and puffing.

A battery start can be started with only a rollover on compression (or even a flip of the igniter while at rest on compression stroke past top-dead-center). A mag that creates spark while at rest (not spinning or being oscillated) is something new to me, and that is what I was asking about. Low-tension mags are nothing more than a low voltage generator. I was trying to understand how it could generate electricity without being in motion. Can a mag store energy while being at rest, then create a spark some time later? Sorry for the confusion. - Mark

Mark, I re-read my post and you must have misunderstood my description. The mag on my engine is the two-bolt type, same as a John Deere engine. It is nothing more than a generator. It is mounted 90 degrees to the timing gear. By disconnecting the wire lead to the igniter and running the engine on a battery and coil, I can measure the generated voltage between the mag wire and ground. Peak is about 5 volts at approximately 250 rpm. I timed it using the instructions provided in the owner's manual; push in on the detent button until it falls into the depression and set the engine at the igniter trip position. With the mag off and turning it by hand, you can feel a strong magnetic pull as it passes through the field. I was wondering if the mag is putting out the nominal amount of voltage, given that it will not run at slower speeds and is difficult to start without spinning the wheels rather fast with a crank. Thanks for any opinions. - Mike

Mark, I agree with you. As you know, Associated engines are my favorite and I have three four-bolt mags, so I am very familiar with starting on the mag. I can start my Hired Man by flipping the wheels, but the mag is freshly rebuilt and it's very strong. Plus, the engine runs very well, so after a couple of tries it will start by pulling back on the wheels. But it is much easier to just use the hand crank and get the wheels spinning, and away she goes! - Keith

SmokStak is an engine conversation bulletin board with over 50,000 messages on file and is part of the Old Engine series of Web sites that started in 1995 as 'Harry's Old Engine.' Harry Matthews is a retired electronic engineer and gas engine collector from Oswego, N.Y., now residing in Sarasota, Fla.

'Positive and negative make no difference with a low-tension coil and igniter. It is wired this way, in this case, to isolate the coil from the magneto.'