Was out plowing with the D, usually an excellent runner and halfway down the field, the tractor suddenly started missing and sounds like it is running on one cylinder. Take the load off and it idles normally. Now I can't get it to take a load without it immediately running on one cylinder. My initial thought was fuel restriction but it acts the same on either fuel tanks. I took the carb off and it appears to be clean. Cleaned out the idle passages with the drill bits. Checked and blew out all fuel lines. I have good fuel flow through the carb and the needle is shutting off. Fuel level in bowl is set at 3/4" but tractor still drops a cylinder as soon as a load is applied. Starts and idles good. I am going to get new plugs to put in it tomorrow, is there anything else I can try? Thanks