Hopefully this will help someone in the future. Restoring a 550 several years ago resulted in a fine tractor, with new rings, complete head
rebuilt etc. ran great to start even with a badly worn Zenith carb. After 3-4 hours it started only with two or three cylinders. First I reset tappits,
then rebuilt the distributor, replaced carb with New Zenith. Each bandaid helped, but problem still resurfaced. Intake valves were sticking.
Removed the head, intake valves were coated with a black gummy tar, ( like old permetex). Intake manifold inside was totally coated as well.
Even the air intake on the new carb and throttle throat were coated. At operating temp. The gunk softened enough to run fairly normal.
When I restored the tractor I had replaced a cracked rubber tube from the air canister to carb. 11/2 inch about 10 inches long. Could not find an
exact part, put new 11/2 radiator hose on. It's only air right!? Turns out the new hose was decomposing with gas and heat. The fumes, ( I know
it sounds weird), were being sucked in the intake, solidifying and coating all parts with this rubbery tar. Pulled my hair out for a few years
before I figured that one out. Got seven restored tractors, never heard of that. Neither had the guys at "Korves"
rebuilt etc. ran great to start even with a badly worn Zenith carb. After 3-4 hours it started only with two or three cylinders. First I reset tappits,
then rebuilt the distributor, replaced carb with New Zenith. Each bandaid helped, but problem still resurfaced. Intake valves were sticking.
Removed the head, intake valves were coated with a black gummy tar, ( like old permetex). Intake manifold inside was totally coated as well.
Even the air intake on the new carb and throttle throat were coated. At operating temp. The gunk softened enough to run fairly normal.
When I restored the tractor I had replaced a cracked rubber tube from the air canister to carb. 11/2 inch about 10 inches long. Could not find an
exact part, put new 11/2 radiator hose on. It's only air right!? Turns out the new hose was decomposing with gas and heat. The fumes, ( I know
it sounds weird), were being sucked in the intake, solidifying and coating all parts with this rubbery tar. Pulled my hair out for a few years
before I figured that one out. Got seven restored tractors, never heard of that. Neither had the guys at "Korves"