51 B-Why is it running rough? Video attached

ndgregor

Member

I am having some trouble with my 1951 John Deere B. It is not running smooth, and the cylinder firing seems to be erratic. It seems to run rough at either low throttle or full throttle. It also seems to be running pretty rich, as it will start to blow black smoke when it is running rough. Here is what I have done so far:

Pulled carb out and cleaned the passages. I removed the drill plugs, and used some stainless steel wire and brake clean to clear the passages. I have not drilled them out yet as I cannot find the correct size bits in the right length locally.

Confirmed I am getting a good fuel supply to the bowl. I also checked to make sure the float was adjusted properly, which it does appear to be.

Pulled oil cup off air cleaner and it was caked with some nasty yellow oil. I removed all of that gunk. I thought maybe there was an air supply issue, since it was running rich, but it did not seem to make a difference.

Compression tested both cylinders. LH side is 98 PSI, RH side is 96 PSI.

New spark plugs.

So far I have not gained any ground. My next thought is to look into timing, maybe it is not set correctly? Does anyone have any other thoughts? I attached a video of it running to try and help. I really appreciate any feedback.
 
Weak spark? Mag or distributor? Do you have to have the throttle closed to get it to start? The B I have been working on has that goofy Wico XB distributor and the spark is weak. The throttle has to be closed in order for it to start and it will sputter intermittently when it runs. It will only jump a 1/4" spark straight out of the coil when I play with opening the points with a screwdriver. I would like to see a 1/2" or 3/4" spark. Just a thought.
 
How should I best determine if the spark is good? Mine is a Wico C mag. I normally have the throttle fully open when I go to start it. Can you explain the procedure you are using when measuring the spark?
 
With a mag you can't turn on the ignition with the engine not running and play with the points like you can with a distributor.

With a mag about all you can do is pull a plug wire off a plug and hold it close to ground while the tractor is either running on the other cylinder or being cranked over. Spark color doesn't seem to make much of a difference but the length of the spark does make a difference. If it starts with the throttle open I suspect you have a good spark. I'm no mag expert but I'm sure a mag expert will chime in soon to give you better advice.
 
Have you tried removing the intake elbow from the carb? Maybe your air intake is plugged up with mud dauber nests????
 
Okay, thanks for the advice. I will go ahead and try that test and see what distance it will still arc at. I was thinking I should maybe take a look at the points/rotor/condenser. I also have a John Deere M and all of a sudden it started to run very rough for no reason. Ended up replacing the condenser and that fixed it immediately.
 
I have them adjusted to the initial setting in the book. I turned them both all the way in, then back off one full turn. I have tried manipulating them, but it did not seem to make much of a difference.
 
A weak condenser can play games that seem fuel related. I would get new points and condenser whether it's a mag or distributor. Buy quality electric components not the cheapest thing you can find.
 
With that compression the C should run the B just fine if it's working correctly! It COULD have a high resistance secondary winding. Is the problem worse warmed up than cold?

One item not mentioned so far is the C mags have a bushing inserted into their aluminum housing that supports the rotating shaft and that bushing sometimes gets loose in the aluminum housing! This causes irregular point gap/point function if the shaft is wobbling enough. Usually this wobbling shaft problem is speed sensitive in that it comes and goes as speeds change.

Sometimes this loose bushing can be fixed in the C mags. Sometimes it's hopeless. I saw a feller disassemble his mag completely, clean it very well then using a drive pin punch "staked" the aluminum around the bushing then let super glue wick into the space between the bushing & the housing.

Sometimes it's just plain easier to switch mags and see if the problem goes with the mag. I've proved guys had ignition problem many times doing exactly that! If it runs poorly on this mag & runs fine on that mag means one mag has a problem. If it runs the same on both it's most likely something else!

I did not find & see the videdo.
 
I started looking into the mag yesterday. I took the cap off and noticed that the one stud looked really banged up. It almost looked as if it was nicked and did not have a flat uniform surface where the rotor passes by. I am no ignition expert, but it would seem to me that would potentially be a problem. The other stud seemed to be in better shape.

I also noticed that the rotor seemed like it may not be the correct type. The cut out section of the rotor did not seem to match the shape that was on the shaft. It was also missing the tab of metal on top of the rotor, it looked like it had broken off.

I have ordered a new cap, rotor, points and condenser to try. Pictures are attached.
a152140.jpg

a152141.jpg
 
There is a piece missing off your rotor where it contacts the center button.
 
That piece on top of the rotor is what takes the voltage from the mag correct? If it is missing a section of that, it could causes inconsistent spark correct distribution correct? That would be consistent with the behavior I was seeing when the engine would run.
 
In addition, the missing piece could be forcing the spark to "wander" around the cap finding a path to the spark plugs. When it does this, it will create a carbon trail that then sometimes becomes the path to ground. You may find that the the cap is bad as well and should be replaced. The condition gets worse at higher engine speeds as the mag puts out a higher voltage and the higher voltage goes to ground somewhere. Years ago, I spent weeks chasing a missing problem with an old Ford Flathead engine. Replaced the distributor, the wires everything but the rotor which was an integral part of the distributor. It turned out that once one of the wires had come out of the cap and that spark had made a path to ground. At higher engine speeds, the spark went right to ground and would kill the engine. I would replace both the cap and rotor.
CPeter
 
You mention the rotor didn't seem to fit the shaft properly. Is there a metal spring item under the rotor like there is SUPPOSED to be that keeps the rotor on the shaft under spring tension? Sounds by what you said that may be missing. Looks like you have metal dust in your cap as well.

When you get your new points gap them carefully. Mags are sensitive to point gap. Forgive me if I'm telling you something you already know.

Also that screw where your red & black wires attach to the points..... Many new ones have a metal tab on the spring (under that screw) that easily grounds the points if not installed just so. It will NOT spark if that tab hits ground!

From what I see there I'm guessing your B will behave better once you put the new parts into place!
 
It appeared like the recess cut into the underside of the rotor did not quite match the shape of the shaft. Due to this, it seemed like I could rotate the rotor slightly on the shaft. I will check to see if it was missing a spring on the bottom, I am not sure if it had that or not.

I will make sure to avoid the situation of grounding the new points. I am not sure exactly what you are referring to, but I am sure it will make sense once I have the new parts sitting in front of me and when I get the old ones out. Thank you for your help.
 

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