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Antique Gas Engine Discussion

Mag--advance/retard


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  #1  
Old 03-13-2005, 02:02:30 PM
Ihorse Ihorse is offline
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Default Mag--advance/retard

My question is---on a impluse mag--with no manual advance/retard--it works on impluse till the engine starts--and then the impluse kicks out--how much does the advance/retard change then--does it get advanced??- Iam convinced it does--but how much??
thanks d,j,
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Old 03-13-2005, 05:48:33 PM
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Default Re: Mag--advance/retard

Well.... It depends on the Lag Angle of the impulse coupling. Some are adjustable like the Wico C or X. Some are fixed. I believe John Deere prefers 25 or 30 degrees. Rumely's are more like 35 I believe... But they used water to control preignition. It would also depend of the speed of the engine and other variables... There must be a certain amount of lag or it will Kick Like a Mule or run Late after it's started.... Also some mags have a spark advance lever which would in effect change the lag angle...
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Old 03-14-2005, 01:08:18 AM
JoeFisher JoeFisher is offline
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Default Re: Mag--advance/retard

when the engine is running, the lever that causes the impulse to 'wind up and fire' does not catch and the mag fires at the position where the impulse just touches. therefore it is advanced. hope this makes sence. warm in texas-Joe
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Old 03-14-2005, 10:48:28 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: Mag--advance/retard

First, are we talking about high tension mags with a mechanical impulse retarder? or a mechanical impulse mag, such as the American Bosch AB 33 and 34, or Webster low tension types?
Most, if not all, impulse magnetos will latch off at Top Dead Center, when in the retarded mode. On the automatic type of magneto, the magneto has a mechanism that holds the magneto shaft until a certain resistance is overcome, Once this is done, springs suddenly advance the mag shaft, creating a more intense, yet retarded spark. The faster a magneto spins, the more intense the magnetic build up and release of energy is obtained. You get 2 benifits - better more intense spark, for cold starts, and spark in the retarded timing, which translates to no broken bones when cranking! As the engine speed increases, a weight within the retarder mechanism is forced to the perimeter of the retarder, and disengages the latch off. This allows the magneto shaft to be locked up with the impluse, and they spin as one componant. Because the shaft is no longer being held back. the timing is advanced. There are a lot of factors on how much advance you will actually need, such as compression ratio, Maximum load, fuel (kerosene, gasoline, alcohol etc), RPM, and more. Advance can be anywheres from 20 t0 45 or better degrees! Each manufacturer has their own specification, for each engine. If you use a mag with too much advance, pinging may be the least of your worries! Ever heard of detonation? This can lead to holed pistons or destroyed bearings.
The other type of impulse mag, has a manual mechanical mechanism to alter the timing. Most of the time this is usually moved by the engine operater to advance or retard the engine timing. In the case of the AB 33 and 34 mags, on the F/M 6 and 10 HP engines, the mag timing is supplied by a counter weight mounted in the cam gear. As the engine RPM increases, the timing is altered by the movement of the counter weight.
One note on the automatic retarders - I have seen several cases on engines that have been sitting for a long time, where iol has varnished the interior of the impulse assembly, and the impulse does not allow the mag shaft to retard. Usually this is evidenced by severe kick back when cranking an engine over. If you note this severe kick back, pull the mag off the engine (NOTE TIMING MARKS!), and check to see if the mag is latching up.
Andrew
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Old 03-14-2005, 12:03:46 PM
Ihorse Ihorse is offline
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Default Re: Mag--advance/retard

The magneto in question is the one I am useing on my hart-parr 50 stationary power unit--it is a model X wico--it does not have a manual advance/retard--I have the engine where it trips just after top dead center when cranking over--that is a bit retarded--does it go advanced when
running--and how much??? thanks for the replys.
d,j,
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Old 03-14-2005, 01:56:14 PM
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Forrest A Forrest A is offline
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Default Re: Mag--advance/retard

If you defeat your impulse trip(I use zip ties on the wieghts) and mark on the flywheel where spark is seen and then allow the impulse to work and mark the location of the spark again you can get a picture of the retardation. This will also aid in knowing when your running spark is happening.

Forrest
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Old 03-14-2005, 05:42:56 PM
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Default Re: Mag--advance/retard

Well... The Wico X is adjustable. Most are in the 25 to 30 degree range. If it is a Base mount and you take the tin cover off the impulse end there will be some marks and a line where you can loosen 2 screws and adjust the lag angle. I would geuss just offhand it could be changed from about 15 to about 35 degrees. You can also tell by watching for the points to open by turning the engine backwards then forward after the impulse trips. When the points open the spark will happen. If you are fussy you can degree out the flywheel and get the advance just how you want it.... Have Fun....
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Old 03-14-2005, 08:56:04 PM
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Default Re: Mag--advance/retard

You can probably get by with that Fairbanks if it has a 30 degree advance. The 12-24 and 28-50 are the ONLY two of all the Hart Parrs that will run well at full advance at all speeds. The SPARK mark on the flywheel is the point at which the points JUST BEGIN to separate. Let the mag impulse, then turn the flywheel backwards a bit (so the impulse doesn't pick up) and turn the flywheel forward again to the spark mark and see what the points are doing. Don't fiddle with the impulse setting if you are already set at SIGHTLY past TDC.
You really SHOULD have the correct mag for that engine though---it's WORTH it.
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Old 03-14-2005, 09:57:58 PM
Ihorse Ihorse is offline
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Default Re: Mag--advance/retard

I am useing the mag that was with it--really don't know what it uses--I know it should have one with a advance retard--because it has a lever to operate one--
I have other mags--I can try one of those.as far as checking behind the tin cover on the impluse end for a adjustment--there is no tin cover--just the drive dogs--this mag is a 4 cyl-base mount--the engine runs--
d,j,
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Old 03-14-2005, 11:37:48 PM
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Default Re: Mag--advance/retard

Well.... Here you see the inside if a 4 cylinder Wico X After removing the TIN COVER. You will see that there are marks along the top side and there is also a mark on the plate with the stop dog. IF you loosen the screws you can rotate the plate and control how much the mag will advance after the impulse kicks out. I assume this is what you are trying to do. It will then be necessary to retime the mag as adjusting will change the point at which the impulse trips...

Many, Many of these mags were sold aftermarket to replace malfunctioning original ones so that is why they are so adjustable. Yes the original one with the advance lever will be better but most engines will run well with one of these. For example, my 16 -30 Rumely which is later model don't have the advance retard lever where the early ones do....
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Old 03-15-2005, 12:23:15 AM
Ihorse Ihorse is offline
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Default Re: Mag--advance/retard

Ken--so I take it off---remove the cover and rotate it a bit --how does one retime the points-is there a small adjustment somewhere i don;t know about? as you can tell mags are not my bag--give me a part to mfg or weld anything--but not electric stuff!
thaanks-d,j,
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Old 03-15-2005, 12:39:57 AM
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Default Re: Mag--advance/retard

Well the adjustment in there will change the point at which the impulse trips. Then... if you retime the mag so the impulse trips slightly after TDC the running timing will be later or earlier depending on which way you move the plate in there. There is no need to retime the points. The Difference in where the impulse trips and the running timing after the impulse is off is what they call the lag angle. Some mags have this stamped on the coupling if they are not adjustable.

The reason I am familiar with these as I made some adjustments to a JD D some years ago to get it to run Just Right.... Then I sold it and the guy applied his own adjusments and it never ran decent again... BUT he was Happy I geuss...
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Old 03-15-2005, 01:20:47 AM
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Default Re: Mag--advance/retard

DJ, the magneto has a 2 piece magneto drive flange. It is held by three cap screws located on the backside. Loosen the screws (metric) and you can rotate the drive flange through a rather large range. Be sure to tighten them TIGHT of the hammering at impulse might cause a timing shift which is why you see a big dent in the left fender of EVERY SINGLE HART PARR.
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Old 03-15-2005, 11:59:04 AM
Ihorse Ihorse is offline
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Default Re: Mag--advance/retard

Craig--the DENT in the fender would be a crank dent--or maybe a person dent. or both-thanks guys.
d,j,
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