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50 Backfires instead of starting

S

Sojak

New User
I've been repairing our old gasoline 50 that we've had since the 1960's and has sat many years. I've had it running in the shop a few times testing and repairing various components like hydraulic lift, pto, waterpump, etc. I finally got everything put together well enough to test it out yesterday. Ran it around the yard and tried all the gears. Seemed to run okay but it you revved up the throttle too quickly it would only run on one cylinder. I tried fine tuning the carb adjustments but didn't make a difference. I tried moving timing back and forth a bit on the magneto and didn't see a difference but then the tractor started running worse and wanting to die. It will almost start at a slow idle once in a while. Now it backfires everytime I try to start it. I've just got a car muffler sitting on the pipe and the backfires have blown it up like a balloon. I ran a compression test and each cylinder has over a 100 pounds.
I switched to a propane carb. which I have run this tractor on before and the backfire / not starting issue stays the same.
I have a strong spark at both plugs, but I am thinking I should order a new condenser, points and rotor.
Could my magneto have skipped a tooth? I only loosened it enough to advance and retard it.

Thanks
 
You didn't some how switch the wires on the distributor cap did you? Check your static timing.
 
On one like that since it is so easy to get the 2 plug wires swapped I would try switching then around. You have at least a 50% chance at one time the plug wires got pulled off and then hooked up 180 out and that sure as heck will cause a back fire
 
Wow. Thanks for all the comments. Backwards wires sounds like something I would do, but I switched them just to check and that must be wrong cause when the starter starts spinning it will do a long pause and then spin again and another long pause, etc.....(in other words appears to be trying to fire while under compression stroke)
I did notice just now that the rotor which looks great is a bit loose, upon further inspection I noticed that one of the spring steel ears that hold it on the shaft is broken and someone has snipped a piece of a beer can to help hold it tigher. Doesn't seem tight any more. Unless anyone has better advice I will order a rotor and condender and post back in a few days after the parts come it.

Thanks to all!
 
Thanks Ben. I switched over to a propane carb. just to make sure there was no fuel issue. I initially thought it was a fuel problem as well.
 
What your say here tells me it is firing before it should so the timing is off some and a rotor that is loss could cause that as will having it way out of time in general. But a loss rotor I would think would cause it to fire late not early. I would pull the cap off and turn it over just a little bit to see which way it turns then adjust the timing so it fires a bit latter then it is now. Or in other words it is firing before TDC which is should not do by very much
 
Remove the inset cover off of the flywheel cover and remove the spark plugs, keeping the wires attached, to prevent the engine from firing. Roll the flywheel over by hand until the impulse for the left hand cylinder trips. At this point the two timing marks should line up. If not, adjust mag until they do. At this point the left hand plug should be firing also. If not, switch the wires in the mag and check again.
 
Okay. After reading prior yesterday tractor's forums and considering all the comments today here is what I did. Replaced my old condenser
with a GM condenser and tightened the rotor by inserting a piece of snipped metal where the broken spring steel was. I also re-timed the
magneto to exactly 5 degrees ATDC. GOOD NEWS. After considerable backfiring (blew the whole muffler up) the darn thing started. It ran rough
and seemed to run smoother on one piston than on both. I shut it down after a couple of minutes and after reading another forum I readjusted
the valves (which were almost if not already perfect). I tried to start it again to no avail with more backfiring. I decided to check my
spark again and I thought I noticed that the spark appeared to get weaker the more the tractor spun. I removed the magneto cap/cover and
discovered that when the engine spins slow the magneto snaps pauses, snaps pauses, snaps pauses, but as the engine picks up speed the magneto
quits snapping and starts spinning without pausing and snapping. When it does this the spark seems much weaker. Shouldn't this WICO magneto
pause and snap at all RPMs????
Thanks for your help!!
 
The reason the magneto pauses is to retard the spark so it will start. When the engine starts, the pause is eliminated to allow the spark to be advanced for running.
 
Short version here because it is late. The snap you from the mag is the impulse tripping. When you turn a mag at engine cranking speeds, the mag does not generate enough power for good spark. So as the mag turns, a dog comes out and engages a stop which causes a spring to wind up. At a set point the dog slips by which releases the spring causing the mag to spin much faster creating a hot spark. This is known as the impulse and the click you hear is the dog releasing the spring. At higher speeds the click goes aways because now the mag is spinning fast enough that impulse is not needed.
 
The pause it the impulse setting up to trip. No trip at low RPM means weak spark. No trip at high RPM is also as it should be because the impulse is no longer needed
 
Thank ya'll for teaching me how the magneto works!! I'm back to square one though. I'm going to go ahead and order ignition components, rotor, cap and points. If that doesn't clear it up I think I will pull my new intake and exhaust manifold and make sure I didn't leave a grease rag in any of the holes when I was waiting on the new one. (Hey even the surgeons do that stuff now and then.) I will also check for mud daubers, etc. Let you know what I find in a few days. Thanks again and best wishes.
 


RULE #1...NEVER make a 2-cylinder JD MAD at ya....!!

They sometimes can be temperamental and my experience during actual Farming with a couple Late Styled JD "B"s, is always remember that Moisture inside the Mag Cover will make for a VERY Difficult starting to a day...!

Many times, no moisture was evident, but 10 minutes with Sunshine on the insides of the Mag and cover would cure the problem..and..

The neighbors could go back to sleep, as we never ran mufflers on our JD B tractors..they burned valves way too often when a muffler was on them..

Man, would they BARK when they Back-fired..sounded like a Cannon..!!
 
That's for sure. I practically had to change my shorts after the muffler blew up while I was standing next to it.
 
I have no idea what the timing should be I just know on any engine it should not fire much if any before TDC
 
Dad gum it. Tried the new points, condensor, rotor and cap. I might as well be throwing rocks cause it did nothing to improve the issue. It looks like it is skipping sparks though. Please see 30 second video of magneto spark. Spark especially skips when engine turns fast enough for magneto not to click.

https://youtu.be/y1IurXAGy1Y
 
Yes Sir, the early 50's came with magnetos. This one doesn't have power steering either, although it does have a power shaft (pto). My understanding that you could order late model 50s with a magneto if customer desired.
 
Kind of hard to tell from the video regarding timing but it does seem to be losing spark at times. Just spit balling here but are you sure you have solid core metal plug wires? Sometimes the old style solid core get swapped out with new silicone/carbon wires which carry more resistance than the solid core. Mags don't work very well with the higher resistance wires.
 
Yes Sir, my old 620 has a regular distributor and it too didn't work worth a darn when I put regular car wires on it one time. For the last 10 years or so I just use regular 12/2 house electric wire. Works great on the 620 and my uncles used to use it back in the 1970s and 1980s on our old 60 without any problem. I feel it carries a much stronger current than any plug wire. Just looks "rigged". I think I will pull the magneto and clean everything up. See if I can find any issues.
 
It wouldn't hurt to check your lag angle. When you remove the mag, on the drive coupling side when you look just above the impulse coupling there is a set of increment marks on the housing. The middle mark represents 14 degrees. There is an indicator mark on the plate that has the stop pawl for the impulse. That indicator should be just over three marks to the left of center. Each one of the increment marks is equal to 5 degrees so with the center at 14 and you go three marks to the left (5+5+5=15) that will put you at 29 degrees and just bit more to the left would put you at 30. The plate with indicator mark can be turned when the 4 retaining screws are loosened.
 
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