Tractor Repair: AC D 17, spark plug wires, vacuum leak

Question
My D17 is running very rough. Changing plugs and points didn't seem to help. I noticed that the area where the carb feeds into the intake manifold gets very cold ( frost forms ). Is that an indication of anything? How do I trouble shoot this problem?

Answer
Hello,

   It sounds like you have a vacuum leak between the carburetor and intake manifold.  To verify, leave the choke part way out and see if it will run better.  You can also try spraying carb. cleaner or gas on the area between the carburetor and manifold, and between the manifold and engine and listen for changes in how it runs.  When it smooths out, you found your vacuum leak.  Do it carefully though, don't start a fire.  Safer to do it while the engine is still cold.  It could be that the nuts or bolts just need tightening, or it could be that the gaskets need replacing.  When the weather is very cold, sometimes it's normal to see frost there until the engine warms up, so it could be something other than a vacuum leak there, but I would check that first.  Make sure you have the new points gapped correctly at .020    Make sure the ignition timing is correct.  If the distributor cap and rotor and spark plug wires are old, replace those also.  I have seen a bad condenser cause this problem.  Check the distributor shaft and cam for excessive wear.  Make sure the firing order is correct.  Make sure the carburetor and fuel supply are clean. Make sure the valve clearances are correct.  And if none of these helps, then the last resort is to do a compression test on the engine to see if it needs some internal work.

                    Good luck,

                         Arnie