39 A spark problem

[Peyton]

Finishing up a restoration of a 39 A; can't get it started. I believe the problem may be the gap setting for the magneto points. I have new plug wires, new autolite spark plugs (set at .030) and new condenser and points in the mag. The magneto is a Fairbanks Morse magneto; I believe it must have come off an unstyled A - not original to the tractor. When the new points were installed a few weeks ago they were set at .020. It ran once yesterday - rough, but needed ether to get it started. The timing is perfect and there is a spark - bluish white - every time the mag fires. Using new 93 octane gas. The carb has been completely rebuilt, cleaned, and the jet and all passages are clear. Changed the gap on the magneto points to .015 - seemed worse if anything - one of the plugs was wet after that round of trying to start it. Anyone know what the right gap is for a Fairbanks Morse mag? The JD operators manual says .015, but that is for a wico mag, I believe. Any other ideas - pretty sure it is a fire problem. Hand crank by the way, so my buddy and I are pretty sore from pulling on that son-of-a-gun all day! Thanks. Peyton

 

[J.Wondergem]

Aren't the plugs that run off a mag. set at 25?

 

[John T]

I'm handier with Wico Mags, if its a Fairbanks Morse DRV type Mag, the point gap is 0.012. The start/impulse timing (critical for good hand starts) needs to be right at TDC, if too late shes hard to start if too fast she can kick back. If the plugs are soaked wet thats too much gas or choke and they stop firing but bone dry isnt enough gas WELL DUH. I set my throttle at slow idle when hand cranking and do NOT overchoke. Once you get light fine vapor out the petcocks thats about right, if its excess raw gas out she can wet foul the plugs.

Id insure the static impulse timing is at TDC, pull start it then fine tune adjust the carb, then let it die by turning the gas off n see if she will re start and DO NOT overchoke and set throttle at slow idle.

John T

 

[PopinJohn Co., LLC]

If the shaft that opens/closes the points has too much side play, the dwell angle (aka gap setting) will not be stable and cause your problem.
Also, I use no more than 7/8 choke and 1/4 throttle setting when trying to hand crank. But that's my experience.

 

[rlp in Co.]

Make sure that the spark plug wires aren't crossed! It can happen. Take a plug out and crank and hold your finger in the hole to feel for compression. Lay the spark plug on the block and watch for the spark. Make sure you hear the impulse as the flywheel crosses the mark on the transmission on the compression stroke. This is not top dead center. I believe it is 22 degrees before top dead center, but that is where it is supposed to be. If you can't move the mag enough to get the timing right you are off a tooth on the timing gear. Anyway that is how you time a B, and the A might be the same. A wet plug means it is flooded. Dry the plug with a match or a torch and try again without the choke. If it still floods, something is wrong with the carburetor. Don't just keep mindlessly cranking. A John Deere that is in good shape will try to fire after one or two pulls if everything is correct. I stopped by a friend's house one day and he had been cranking all day on his B and was about ready to drop. He was off a tooth on the timing and I had it started in about 15 minutes.

 

[John T]

RL, in response to your statement... "Make sure you hear the impulse as the flywheel crosses the mark on the transmission on the compression stroke. This is not top dead center. I believe it is 22 degrees before top dead center, but that is where it is supposed to be".

With all due respect and NOTTTTT to argue but so the poster wont get confused: Actually, if the flywheel is on the correct crankshaft spline, when the flywheels Left Hand Impulse timing mark is lined up with the flat mark on the tractors side at 3 o clock

THAT IS INDEED TDC OF THE LEFT PISTON AND NOTTTTTTTTTTTTTTT 22 DEGREES BEFORE TOP DEAD CENTER

AND THATS INDEED (as you correctly state) WHEN THE MAG SHOULD TRIP/SNAP/IMPULSE TO FIRE THE LEFT PISTON AT TDC (NOT 22 BTDC) AT STARTING/CRANKING RPM's

IF THAT WAS NOT TDC (when mag snaps/impulses) BUT 22 BTDC AS YOU MENTIONED, THE FLYWHEEL WOULD KICK BACK SOMETHING AWFUL OUCHHHHHHHHHHHHHH AND SHE SURE WOULDNT START

Now, the static start impulse timing is set primarily by the mags rear lag angle stop plate adjustment and secondarily by the mags rotation in the governor flange adjustment window/slot. If you rotate the mag in the governors open adjustment slot that changes BOTH the static start impulse timing PLUS the dynamic run timing. However, adjustment on the rear stop plate AFFECTS START/IMPULSE TIMING ONLYYYYYYYYYY

I think where you may be confused is that after 250 RPM or so, the mags wind n trip impulse mechanism ceases operation, so the run time advance after that could indeed be 22 BTDC as you mentioned. The run time advance is primaruly a function of which drive cup is used (like a 25 degree cup) and secondarily by rotation of the mag in its open window/slot.

To see where shes actually firing at over 250 RPM when shes running, you have to use a timing light and you can mark off say 22 BTDC (if thats where you want it) on the flywheel and rotate adjust the mag as necessary (to the extent possible based on your drive cup and adjustment tolerance) but dont forget when you rotate it youre also changing the start/impulse timing

Hope this helps yall understand so post back any questions and Ill try my best to answer. Duane (recently deceased) and Pat Browning have more detailed descriptions of this on their websites, I havent taken the time to to prepare a canned presentation for this...

God Bless n best wishes

John T

 

[Anonymous-0]

If your mag is a fairbanks DRV you have to be sure they are timed internally first. They have a firing pattern of fire-fire-miss-miss. The point gap on a magneto is actually related to the rotor edge gap. If you vary a few thousandths to get a hotter spark that is fine. If its a Fairbanks J2 then you probably have your wires switched. Also if its a DRV they are are geared internally and the rotor turns the opposite direction of the drive end so make sure your plug wires are in the correct position.

 

[Lee B]

No, I&T manual clearly calls for .030 spark plug gap no matter what source of the ignition.

But you still bring up an important point in that Magnetos do not work well with larger spark plug gaps than the 'standard' .030. The fire tends to start shorting around under the cap if you don't let it get thru the spark plug easily.

I suppose it's possible that a weak mag might see a benifit with .025 gaps on the spark plugs? Wouldn't cost much to try either.

 

[Peyton]

Thanks for the explanation, but I"m afraid I don"t understand much of it. I am a complete novice at this. The mag is a FMJ2 (the letters are unreadable after 2). Mainely magnetos sold me points, which fit it exactly. After experimenting with the gap, .020 works - at least it now starts. The setting mentioned below (7/8 choke and 1/4 throttle works best also - thanks PopinJohn). My friend who restored a B helped me with the timing. I had the governor off during the restoration, so we set the timing gears to line up (governor to camshaft gear). We also adjusted the angle of the mag - the LH impulse is EXACTLY when the fly wheel passes the mark on the side of the tractor.

But... She runs rough. Appears to miss. The plug gap is still set at .030, which I am pretty sure is correct. Can there be a timing problem if the LH impulse is dead on? Doesn"t seem logical to me. I think I just have a carb adjustment problem now.

Thanks for all the help and thanks for everyone"s patience.

Peyton

 

[John T]

Peyton, REMEMBER, Im handy on Wico C & X butttttttt nottttttttt so much on FM mags:::: Hopefully the mag repair poster man above (who knows FM Mags better then lil ol me) can confirm what I say below is correct??????????

You ask "Can there be a timing problem if the LH impulse is dead on? Doesn"t seem logical to me. I think I just have a carb adjustment problem now".

ANSWER IS YES: Even if your start/impulse timing is correct (essential so she starts easy) THAT DONT MEAN YOUR RUN TIME ADVANCE IS CORRECT and how she runs is critically tied to if your run timing is correct or not !!!!!!!!!!!!!!!!!

For gas engines (unlike if using low octane distillate tractor fuel) I like the run timing to be around 25 BTDC and NOTTTTTTTT as much as 35 like some old all fuel tractors used, nor too slow like 15 BTDC

The way to see what your run timing is requires use of a timing light because over 250 or so RPM the impulse stops working so run timing is then a function of the drive cup and points gap and wear n tear n slop PLUS THE ROTATIONAL ADJUSTMENT OF THE MAG IN THE GOVERNOR FLANGE, forward CW retards it backwards CCW advances it butttttttt that also changes strat/impulse timing ya know, so if you get run timing where you want it (say 25 BTDC) then you may have to re adjust the impulse stop plate to again achieve start timing at TDC.

YOU NEED A TIMING LIGHT TO SEE THE RUN TIME ADVANCE and our guessing just isnt gonna get it and again just because your start/impulse timing is right at TDC DONT MEAN YOUR RUN TIME ADVANCE IS RIGHT and thats whats important as far as how she runs DUHHHHHHHH

The points gap is actually a secondary parameter, if all is new and theres no wear n slop its a good approximation (like if it calls for 0.015 etc) that will render the best edge gap adjustment, but if you get maximum spark energy (but how would you actually know that???) using 0.020 who am I to say its wrong.

Once you get it running then you can fine tune adjust the carb, I initially set mine for rough in at 1 turn out for laod and 2 for idle then adjust once running. I hand start mine at slow idle on the throttle and maybe 7/8 choke, depends on temperature.....

John T Although Im NOT any FM mag man, I still believe the above to be true

 

[36 Coupe]

A JD with gas fouled plugs will never start.The spark runs up the insulator in thin streams instead of jumping the plugs gap.You have to have the plug out and firmly grounded to the block to see this or check the plugs with a model T coil on the bench.I keep a pair of plugs in a covered jar of lacquer thinner ,that gets rid of the fouling.PROLONGED idling will gas foul plugs.

 

[rlp in Co.]

You are absolutly right, that the mark is TDC. What was I thinking?

 

[Peyton]

Thanks for the help. Got email from Danny who told me to set the mag points at .018 and the plugs at .025. I did that last night and she started right up - third pull. She runs beautifully. Nice to see (and hear) the fruits of one's labor after 15 months of restoration work. Thanks again.
Peyton