John Deere backfiring

I have a 1951 JD M model.
It suddenly started backfiring. I have read all the advice on this topic here and still can't find the problem. I have had two tractor mechanics look at it and neither of them can sort it out.
What it does; it cranks up and when running at idle or power, it surges, loads up and backfires through the manifold. I know this means unburnt fuel is loading in the manifold. Have replaced entire electrical system from coil to starter switch. Have cleaned fuel lines and rebuilt carburetor. The one thing I haven't done yet is pull the manifold or heads to inspect the gaskets.
One strange thing it does is, when manifold gets hot, water condenses on the outside of the pipe leading from the carb to the manifold.
Anyone have this problem before ?

Condensation is normal.

I would suspect a bad spark, diagnose the ignition.

"[b:654c4848f0][i:654c4848f0]<font color="#6699ff">it cranks up and when running at idle or power, it surges, loads up and backfires through the manifold.[/i:654c4848f0][/b:654c4848f0]</font>"

You might consider adjusting the governor linkage.

Reviewed the [b:654c4848f0]John Deere Model ?M? Tractors Service Manual SM2001[/b:654c4848f0] in the [b:654c4848f0]Engine[/b:654c4848f0] section in the [b:654c4848f0]Group 35 Governor[/b:654c4848f0] information under the [b:654c4848f0]Install and Adjust Governor Linkage[/b:654c4848f0] topic on pages 60-35-4 and 60-35-5.

Take a look at the diagram below.



Note the hex jam nut (Key 8) and set screw (Key 9).

Take s look at the photo below.



?[i:654c4848f0]Install governor screw and lock nut into fan bracket. Adjust stop screw until there is approximately 3/8-inch of it above lock nut. Do not tighten lock nut.[/i:654c4848f0]?

Take a look at the diagram below.





Note the speed control lever (Key 1), governor control rod (Key 6), and bracket (Key 12).

?[i:654c4848f0]Pull throttle control rod lever as far downward as possible, that is, until throw on front end of throttle control rod is against stop on throttle rod front bracket.[/i:654c4848f0]?

Note the plain washers (Key 5), governor control rod (Key 15), spring (Key 14), and spring anchor (Key 13).

?[i:654c4848f0]Slip flat washer over front end of throttle control rod, and slip throttle-control-rod-to-governor linkage into place on rod. Install other flat washer, and finally place coil spring into position between throttle control rod and spring anchor pin. Be sure spring is seated in grooves provided for it.[/i:654c4848f0]?

Note the ball joint (Key 18) and governor-to-carburetor connecting rod (Key 19).

Take a look at the photo below.



Take a look at the diagram below.



Note the lever and shaft assembly (Key 2).

Take a look at the photo below.



?[i:654c4848f0]Insert swivel joint of governor-to-carburetor linkage into governor operating shaft lever from outside. Tighten nut and lock washer securely. Move throttle control rod lever to full open position (throttle control lever on instrument panel as far up as possible).[/i:654c4848f0]"

Take a look at the diagram below.



Note the throttle shaft (Key 8).

Take a look at the photo below.



?[i:654c4848f0]Move the carburetor throttle fly to full open position (carburetor throttle fly arm as far forward as possible). Adjust rod of governor-to-carburetor linkage until it will fit in carburetor throttle fly assembly arm. Make this adjustment by turning rod of governor-to-carburetor linkage in or out of swivel joint. When linkage is of such length that it will just slip into carburetor throttle fly arm, without moving either the arm or governor lever, [b:654c4848f0]turn rod into swivel joint two complete turns[/b:654c4848f0], or until proper governor action is obtained. Tighten lock nut. Slip free end of rod into carburetor throttle fly arm and insert cotter key.[/i:654c4848f0]?

Note the yoke (Key 25) in the THROTTLE, CHOKE, AND STARTER CONTROLS diagram.

Take a look at the photo below.



Note the governor lever (Key 3) in the GOVERNOR diagram.

Take a look at the photo below.



?[i:654c4848f0]Disconnect throttle-control-rod-to-governor linkage yoke from governor lever, and move throttle control lever down as far down as possible.

Push governor lever as far down as possible. Turn adjustable yoke on throttle-control-rod-to-governor linkage until holes in yoke line up with hole in governor lever. [b:654c4848f0]Then unscrew yoke two complete turns[/b:654c4848f0]. This insures that throttle is tight against stop pin when throttle control rod is against stop on throttle control rod front bracket. Tighten lock nut. Insert pin and cotter key to attach yoke to governor lever.[/i:654c4848f0]?

Hope this helps.