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Antique Gas Engine Discussion

FM Magneto Problem


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  #1  
Old 07-28-2005, 08:12:47 PM
Smoke Smoke is offline
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Default FM Magneto Problem

Hi Folks, I am having a hard time getting my FM ZC 208 to stay running. When I turn it over, its hits & fires but will only turn a few revolutions when the impulse on the mag stops snapping therefore no spark. I'll start it again it will fire then stop snapping. What is wrong here ? Its a FM-J Magneto. I don't know much about these impulse mags. Thanks Again.
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  #2  
Old 07-28-2005, 08:44:14 PM
Neale Behm Neale Behm is offline
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Default Re: FM Magneto Problem

Perhaps a weak coil but I'd first file the points and clean them thoroughly and then replace the condenser if the points don't fix it. Let us know what you find.
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Old 07-28-2005, 09:21:01 PM
David Greenwalt David Greenwalt is offline
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Default Re: FM Magneto Problem

Hi Smoke, I believe the impulse should quit after the engine fires. There is a centrifical weight in there that stops the impulse from working once you get the engine started. On my 6 with the Type R the impulse quits as soon as it fires the first time and picks up speed. The impulse is just for starting, then it should work like a mag without the impulse. This is where I'm on shaky ground, but I'll say it anyway. I believe the impulse advances the spark? So perhaps you have the gears off one or two teeth and your timing is not right? David.
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Old 07-28-2005, 09:48:41 PM
BobRR BobRR is offline
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Default Re: FM Magneto Problem

Smoke Im with Neal clean the points good.2nd make sure they are gapped right.This also changes timing. 3rd David maybe right about timing being off but I think the impulse retards the timing. BobRR
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Old 07-28-2005, 10:47:51 PM
Rudy Rudy is offline
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Default Re: FM Magneto Problem

Smoke, make sure the mag is timed to the engine. the fm j mags on these engines have a fixed lag angle and can only be adjusted by changing the impulse cup. the lag angle is the point the mag ouits impulseing. also on the fm j mags test the coil. these mags have to spin quite fast to make and break magnetic flux. if the coil tests ok then remove the armature and clamp one end in the vise and grab the magnets. if there is any movement it's no good. this is common on these mags. it causes real havoc on timing if the magnets are loose on the shaft.you can repin the magnets to the shaft or toss it and get a new one. the engine should start fairly easily on this mag. if you recall the fairbanks style ZD engines which use the same mag ,you have to spin the crap out of them to get them to start. that is why they they went to an impulse to make them start easier.
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Old 07-28-2005, 11:09:42 PM
Allan Wright Allan Wright is offline
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Cool Re: FM Magneto Problem

Smoke, one other thing you can try, is to clean and oil the impulse latch or weight. If it is not moving freely, it could cause the mag to quit impulsing too soon.
Good luck with it.
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Old 07-29-2005, 08:06:10 AM
Smoke Smoke is offline
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Default Re: FM Magneto Problem

Thanks for the replies fellas, I will try "ALL" of your suggestions. Turning 600lbs. of Flywheels by hand was really working on my arms. I'll let you know how I make out. Thanks Again
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Old 07-29-2005, 01:22:14 PM
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Forrest A Forrest A is offline
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Default Re: FM Magneto Problem

Smoke, If the engine fires on the impulse but not under normal RPM the magnets could be weak. True, clean points and a healthy capacitor (condensor) are a must but if you have weak magnets there will be problems like you have state. There are people around at some of the shows that can charge these magnets for you, some do charge a fee however.

As for timing, the impulse cupling retards (ATDC) the spark point so the engine won't kick back during starting. Once the engine is running the impulse stops working and the spark is then happening BTDC. The impulse also provides the needed extra speed to allow for a powerful spark at the low RPM of starting. The impulse isn't where the timing should happen, the engine should be timed for it's running RPM when the impusle is not working. Even though there are marks to use while the impulse is working the wear on parts of the impulse will cause the running timing to become closer to TDC. Using piston location and a timing light you can restore perfect timing if that is what you want to accomplish.

Forrest A
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Old 07-29-2005, 02:29:26 PM
Jim Gorter Jim Gorter is offline
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Default Re: FM Magneto Problem

Smoke, I just finished my 208 last month. I had the same problem with the impulse quitting. I put in a new set of points and cleaned up and reoiled the impulse spring assembly, runs great now. Good Luck, Jim
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  #10  
Old 08-10-2005, 08:14:47 AM
Smoke Smoke is offline
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Default Re: FM Magneto Problem

Just an update here fellas, I took the mag off of the engine and took the impulse latch screw out. Spun it with a drill and it fired great, so the problem was not the mag after all. I checked the fuel and check valve, everything was good. Plenty of compressioin. Everything was in time. Runs great now. The best part, it pulls that 10-20 Tractor Chassis without even flinching. Anyone want to take a guess at what the culprit was ?
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Old 08-10-2005, 10:23:42 AM
Dan Hodge Dan Hodge is offline
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Default Re: FM Magneto Problem

Did you put in a new spark plug?
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Old 08-10-2005, 04:59:34 PM
Joe Morris Joe Morris is offline
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Default Re: FM Magneto Problem

When setting the time on an engine with an impulse magneto you always set it so that the impulse snaps when the piston is at top dead center. Then, when the engine starts the magneto advances the time to the proper degrees before top dead center. If an engine with an impulse mag tries to kick when cranking the time is either set too high or the impulse is not working and it is causeing the engine to fire before top dead center by staying in the advanced position. I have a small Novo with a Wico impulse mag that at times the impulse don"t " unlatch " when I shut it off and the next time I try to start it will kick like a mule because the spark is still advanced. So I have learned to move the flywheel back and fourth to get it working as it should before cranking it again. Joe Morris
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Old 08-10-2005, 08:47:52 PM
Smoke Smoke is offline
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Default Re: FM Magneto Problem

Here's what it was, while turning the flywheels, I noticed bubbles in the mixer while on the compression stroke. I pulled on the intake valve and the bubbles stopped. I did a poor mans valve lap and replaced the spring. It ran fine after that. Just a weak valve spring, go figure.
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