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John Deere Tractors Discussion Forum
Show Parts for Model:

H mag question

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bill finn

08-12-2017 19:42:58




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After a complete tractor restoration, we installed a wico x mag on our hand start H. It has good spark when turning mag by hand and has good click of the spring, but does not like to start tractor. If we pull it 5 feet it starts and runs like a watch. My question is did they have a different impulse spring for hand start verses electric start? also, when verifying my timing can i use a timing light to verify correct firing at the lh impulse mark, or will this mark only work for static timing.

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dan_41jdh

08-14-2017 09:35:26




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 Re: H mag question in reply to bill finn, 08-12-2017 19:42:58  
The part number (6274) for the 25 degree impulse drive cup is sometimes stamped on the outside of the cup. Is the "25" you mentioned something that someone may have marked on the cup, or does it look like a factory mark? I found a picture of a 25 degree cup with the 6274 part number stamped on it.
Anyway, if your magneto is mounted and rotated properly so that the LH spark plug fires at (or slightly after) top dead center as indicated by the illustration showing the "LH Impulse" marks on the flywheel aligning with the marks on the cover, that is what you must have for starting. Once the tractor starts and the magneto is spinning around 220 RPM, the impulse mechanism, which retards timing necessary for starting, quits working and the run timing automatically advances to 25, 30, or 35 degrees, depending on the impulse drive cup and/or lag angle settings.

So again, make sure you have spark at the plugs and that the start timing (set by rotating the magneto on the tractor) is correct and you can be confident that the magneto is not the cause of your starting problem. The impulse drive cup only affects the running timing and has no effect on the starting timing.
Although the earliest "H" tractors were fitted with 35 degree impulse drive cups, they were changed to the 30 degree cups during the 1940 model year, and 25 degree cups can be found but weren't furnished during factory production. Actually, when running gasoline (instead of distillate), the 25 degree advance is probably best for power and economy, but you really won't notice much difference for a parade and show tractor. And the cup lag angle won't affect starting whatsoever.

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bill finn

08-14-2017 04:18:51




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 Re: H mag question in reply to bill finn, 08-12-2017 19:42:58  
we found that there is a 25 on the mag drive cup, will this work for our hand start H or maybe this is our problem.



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NoDakInMN

08-13-2017 16:35:43




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 Re: H mag question in reply to bill finn, 08-12-2017 19:42:58  
Dan_41jdh covered the timing pretty well. As for the plug wires being reversed, if you pull the left hand plug and inspect the spark and it also is sparking on L H impulse, you are fine and not 180 degrees out of phase.



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dan_41jdh

08-13-2017 19:28:44




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 Re: H mag question in reply to NoDakInMN, 08-13-2017 16:35:43  
A comment regarding reversed plug wires - - you'll know pretty quickly because there'll be a nasty backfire explosion every other time a plug fires.... If the plug wires are correct and at cranking speed, each plug will fire when its piston is at top dead center (or slightly after); If the piston is at the top of the compression stroke and is at TDC, the spark should create a power stroke of the piston and the tractor "should" start. That same plug will spark again 360 degrees later when the piston is again at TDC, but now the piston is at the end of the exhaust stroke (and the exhaust valve is still slightly open), and the spark doesn't do anything.

But if the plug wires are reversed, each plug will fire at Bottom Dead Center - and if the piston is at the bottom of the intake stroke, the cylinder will be full of an explosive mixture of air and fuel vapor. And if a plug fires at the bottom of the intake stroke, the intake valve is still open (it doesn't close until 50 degrees AFTER Bottom Dead Center), so not only will there be an explosion in the cylinder, but the explosion will travel back through the open intake valve, back through the carburetor (assuming the throttle is open somewhat), and will blow oil back out of the air filter - along with a loud BANG!

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bill finn

08-13-2017 10:43:26




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 Re: H mag question in reply to bill finn, 08-12-2017 19:42:58  
i do have blue in the spark, what about the timing light? should i disregard it and stick with mag snap on lh impulse? also how will it run if wires are backwards? thanks for the advice on the mag. That's just what i needed.



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dan_41jdh

08-13-2017 11:16:23




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 Re: H mag question in reply to bill finn, 08-13-2017 10:43:26  
If you have proven you have a good spark at the spark plug gap when rolling the flywheel as NoDakInMN mentioned, your impulse mechanism is almost certainly not the culprit. And regarding the timing light, keep in mind that when the tractor is running, the light should flash with the "LH Impulse" letters 25 to 35 degrees before the timing mark on the transmission cover on the side of the tractor (depending on the drive cup of the magneto). If the impulse is snapping during starting or extremely slow running, the spark advance is "0". And this is necessary so that the plug fires at Top Dead Center, or just a tiny bit after top dead center when starting. But when running, the magneto will spark at the intended advance which was 30 (early models) or 35 degrees for the "H". But there are some 25 degree impulse cups, also.

"H's" seem to rather finicky about starting - almost all of them I've seen start best with little or no choking, and a closed, or nearly closed, throttle. They flood easily so don't overchoke. And they are very sensitive to a loose and sloppy throttle butterfly and/or shaft. Those must be in good shape for good hand starting.

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NoDakInMN

08-13-2017 10:18:00




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 Re: H mag question in reply to bill finn, 08-12-2017 19:42:58  
I should also add that that the gap in the plug should be to spec of 30 thousandths. Excessive spark gap can cause the coil in the mag to overheat.



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NoDakInMN

08-13-2017 10:15:02




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 Re: H mag question in reply to bill finn, 08-12-2017 19:42:58  
Regarding the extra wrap of the spring on the impulse, before you take it apart, try this check. Remove the plug and check the spark by rolling the flywheel. It helps to be shaded when viewing the plug. If the spark has a good sized spot of blue in the middle of the spark arc, you should be good. If it is just yellow across the length of the arc, the spring is weak or not tight enough.
To tighten the spring you will need to remove the cap from the mag, take off the rotor and then secure the shaft so it cannot turn. then remove the nut that holds the impulse cup, slowly lift he cup so that the edge clear the pawls. Hopefully both ends of the spring are still attached otherwise you will have to correct that, then give the cup one complete turn counter clockwise and set back down and reinstall the nut. Reinstall the rotor and cap, put mag back on the tractor and check timing. Now when you check the spark it should have blue in it.

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Chasing_deere

08-12-2017 23:03:18




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 Re: H mag question in reply to bill finn, 08-12-2017 19:42:58  
Sounds more like a fuel issue make sure your nozzle in the carb is clean.



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bill finn

08-12-2017 22:51:06




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 Re: H mag question in reply to bill finn, 08-12-2017 19:42:58  
I have not wrapped a spring for a mag before my only experience has been points. that being said I can disassemble it and try to get another wrap on the spring. Carb is clean as whistle. on the timing, we set it on lh impulse and it runs pretty good, but timing light shows it to be off.



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RandyB(MI)

08-12-2017 20:55:40




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 Re: H mag question in reply to bill finn, 08-12-2017 19:42:58  
Many times the spring needs one more wrap. Some just don't think they can get it back together if they go one more.



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greenmech

08-12-2017 20:13:21




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 Re: H mag question in reply to bill finn, 08-12-2017 19:42:58  
Not a different spring for hand start, could be needs more tension if it doesn't have a good snap. When timing set so it trips the impulse right at left hand impulse. You will hear it snap. Have you checked the carburetor out? Hope this helps you get it to start better.



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