Dear Ole Mechanic;Fouled Spark Plug

I recently purchased a 1949 John Deere Model B.  While I did operate one almost identical to this one many years ago, there is a lot that I either did not learn or have forgotten.  My problem is that the tractor runs fine for a while, and then it fouls both spark plugs.  They come out completely covered in black carbon.  They are not wet, and when the tractor is running, there is no blue or black smoke coming out the exhaust pipe!  The oil level stays right up to the full petcock--for what little time I have run it.  This leads me to believe that the plugs are being fouled by too rich of a gasoline mixture, but it idles OK, accelerates with out any hesitation, and pulls OK when the RPM’s are up to the governor limit.  I have not really put a working load on it such as plowing or belting it to a feed grinder, and I don’t intend too.  What is causing the plugs to foul?

Proud John Deere Owner


Dear Deere Owner;

Your diagnosis has taken you in the right direction.  It sounds like you are getting gas-fouled spark plugs.  Normally, most people would start with the carburetor adjustments, but there are some other things that should be checked first.  One is to make sure that you have the correct spark plug and plug heat range in the tractor.  I do not have that information, but a phone call to any John Deere tractor dealer should get you the correct information about which spark plug you need.  Don’t forget to tell them about your light load usage, as that may call for a hotter heat range plug than would be appropriate for hard pulling, like plowing.

OK, if you do have the correct plug and heat range, then it is almost time to go to the carburetor.  First, start with the air cleaner and the tubing to the carburetor.  If there is anything restricting the air flow, then it would be like running the tractor with the choke on.  That would foul plugs in a hurry.  Even too thick of an oil in the oil-bath air cleaner could cause plug fouling.  Also, the possibility of a vacuum leak must be eliminated.  The simple way to do this is with an oil can and a rag to wipe things up.  Squirt oil around all connections between the carburetor and the head.  If the oil is sucked into any connection or if there is a change in engine speed, then you have found a problem that must be corrected.  A vacuum leak will cause more problems at idle and part throttle light load conditions than it will with the engine at full load.  When the throttle plate is not wide open, then there is a higher vacuum in the intake from the throttle plate all the way to the intake valve.  With a vacuum leak, air without fuel will enter the system.  More air without fuel will be drawn in at idle due to the very high vacuum.  The idle mixture can be adjusted richer to compensate for the extra ai,r but then the mixture is wrong (too rich) as the throttle is opened.  Check very carefully and correct any vacuum leaks.

With no vacuum leaks, then it is--at last--time to start with the carburetor.  First, grab the throttle shaft and check for any looseness between the shaft and the carburetor body.  Any looseness will cause an inconsistent vacuum leak.  If the shaft is loose, and the hole for the shaft is worn oblong, then there are three ways to correct the problem. The most expensive is a new carburetor.  Not quite as expensive is a rebuilt carburetor, and the cheapest way--although still not cheap--is to have the housing bored out and a bushing installed to correct the looseness.  With any looseness corrected, or if it was not loose to start with, then it is time to adjust the carburetor.  You will want to check the service manual or consult a Deere dealer to make sure my memory is correct, but I think the adjustment on the left (flywheel side) is the idle mixture adjusting screw, and the one on the right (clutch/belt pulley side) is the high speed mixture adjusting screw.  With the engine warmed up and at a slow idle, start turning the idle mixture in (clockwise) to lean out the mixture.  When the engine speed starts to slow down, then unscrew the mixture screw about one quarter of a turn.  Should the engine speed get faster then reset the idle speed to a slow idle and repeat the idle mixture adjustment.  To set the high speed mixture, the engine should have a load on it.  If a load is not available, then seat the high speed mixture screw lightly and back it out one and one half turns.  With the engine warm and idling, open the throttle quickly.  If the engine accelerates with out any hesitation or backfire, then lean the mixture a quarter of a turn at a time until it does hesitate or backfire.  Then, richen it a quarter of a turn.  If you ever decide to do any pulling or put any other working load on the tractor, then find a dynamometer or a Baker fan and adjust the high speed mixture for maximum horsepower with a working load on the engine. 

Somewhere in the above rambling, you should find a correction for your plug fouling.

Herr Professor Nuzanbolts