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FM Z 3hp help


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  #1  
Old 05-21-2005, 08:20:03 PM
TSmith TSmith is offline
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Default FM Z 3hp help

I'm stumpped! I purchased a nearly complete (less mag, tank and governor parts) and very stuck FM Z 3hp (throddle controlled). After 2 months of working the piston free, replacing valves, bearings, head work, derusting and painting everything, I started looking for a mag/governor bracket assembly. All I could find was the earlier cast iron unit (that I preferred over the original pot metal). I then find the FM "R" type mag won't mount to it without modifications. I then bought a Bosch mag that works with the mount. It's still fighting me. I have timed the exhaust cam opening 35 degrees before BDC, closing 5 degrees after TDC as per manual. The problem is the boss mechanism that trips the mag is firing the mag way early and there isn't any way to change the ignition timing relative to the cam that operates the exhaust valve without remilling a keyway for the gear on the camshaft.

I thought that maybe the earlier engines might run in the opposite direction but if that's the case, the gear driving the governor would be driven to unscrew and I'm certain the design would be to turn it so it wouldn't be prone to loosen.

I've got way more money in this thing than it's worth, but I'm having a ball with it and I guess that's worth something.

What am I doing wrong here? I guess I could buy a repro bracket asembly and use the "R" mag that can be adjusted for timing since it's gear driven.

HELP!!

Any assistance greatly appreciated.

Tom
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  #2  
Old 05-21-2005, 09:28:52 PM
KidDynamo KidDynamo is offline
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Default Re: FM Z 3hp help

A cam gear for an ABB-33 magneto would incorporate an "early and late device" (to use Fairbanks Morse terminology) on the 3 and 6 horse models. This spark retarding device involves a counter weight and an odd hooked "finger that trips the mag much earlier when the engine has a bit of speed. Conversely, when starting, the timing is retarded to reduce kick back.

I would guess that the Fairbanks "R" magneto for this engine would have an impulse mechanism (please correct me here, if I'm wrong, somebody) to accomplish this retardation at start-up and consequently wouldn't need the counterweight, et al.

If this is the case, you might have to find a gear with the "early/late device, but that shouldn't be too hard. You'd have to get the right teeth pattern but I think there were only two choices and I would try to get one that has it's original camshaft so you don't have troubles with the locating screw, etc. I've had these with a woodruff key and the screw tapped into the shaft and gear end joint and with only the screw.

Pulling the gear repeatedly is not a picnic- you might as well get parts that work without a hitch. I suppose you might modify your gear to retrofit it with the advance weight, etc, but that wouldn't be original to the serial number of your engine, but, come to think of it, neither is the AB-33.

Boy, aren't I a lot of help !! LOL
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Old 05-21-2005, 10:12:35 PM
TSmith TSmith is offline
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Default Re: FM Z 3hp help

Thanks KidDynamo! You actually are a lot of help. You explained the odd looking setup and at least I know what it's suppose to do now.

The "Early and late devise" gear came with the cast iron assembly and that assembly is what I'm using with the Bosch mag. You would think it would have the right cam timing position relative to the ignition timing. There is only one position that the timing gear can mount on the camshaft.

When I say it's firing early, what I mean is it's firing like 40-50 degrees before TDC on the compression stroke. That seems a bit excessive even for a modern high performance engine with premium fuel.

Like I say, I'm stumpped.

Thanks and have a nice weekend.
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Old 05-21-2005, 11:10:37 PM
Larry Larry is offline
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Default Re: FM Z 3hp help

It iS Possible Your AB-33 Mag has the Wrong TRIP ARM on it for Your Engine...As A shorter And or /longer One may be in order..Hope this is Of help! Larry
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Old 05-22-2005, 11:25:21 AM
Rudy Rudy is offline
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Default Re: FM Z 3hp help

As Larry mentioned in the last post, having the proper trip arm is VERY important. No only the right arm but not one that is worn. I see a lot of worn arms on these mags that come into the shop and this will cause you a lot of grief. Heres why. If the arm is worn or wrong the mag will not make proper spark at the right time.if you take the points cover off the mag watch what happens at the points when the arm trips off. If you look in the Bosch AB-33 manual it will tell you how to time this mag to the engine. If it is not right you can raise the mag with shims to change the timing. also the magneto has to be timed to itself.this means that the contacts should be fully open when the rub block is in the center of the pip mark on the body. if this is off you will have a hard starting engine. also check the piece on the cam gear that hits the trip finger,making sure that it is not worn. hope this helps. Rudy
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Old 05-22-2005, 04:01:33 PM
TSmith TSmith is offline
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Default Re: FM Z 3hp help

Hi Rudy,

Thanks for the info. I guess I'm both a little confused and ignorant. It appears to me that the trip arm extends straight down under the mag and the trip devise on the cam gear contacts it when it is essentially vertical and hits the mag trip arm. Like I mentined earlier, this is about 50 degrees before TDC nm the compression stroke. It appears its going to take more than a minor adjustment to get it firing anywhere near TDC. A "L" shaped trip arm that extends down and angles in from the side might give me the delay I need. Does a arm like this exist?? I guess I could fabricate one.

Does anyone know what the ignition timing should be relative to TDC? The manual gives valve timeing but not ignition timing..

Thanks for taking the time to help!

Tom
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Old 05-22-2005, 09:37:14 PM
TSmith TSmith is offline
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Default Re: FM Z 3hp help

The mag is a Bosch AB-34 and I checked a little closer and its firing 54 degrees before TDC on the compression stroke.

Tom
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