Small Engines (Lawn Mowers, etc.): Tecumseh carb issues, tecumseh carburetors, vacuum leak

Question
I have an MTD tractor mower with a Tecumseh 15.5 HP mower
Engine model OHV155
Spec 204509F
Engine Family YTPXS.4902AA
Serial Number is not on the tag with the above numbers

Engine would only run with part throttle and 1/2 choke.
I figured a vacuum leak.
Couldn't find any intake manifold to cyl head bolts were tight enough not to leak.
Took the carb apart, didn't see any problems except for some crud on the float needle. Scraped that off, blew carb cleaner from a spray can through all the holes, blew them out with air.
After reassembly engine will not run with the jet/seat for teh anti-backfire solenoid in place. It doesn't get any gas as it will run on starter fluid.
The solenoid works fine when the ignition switch is on. Haven't tested to see if it works with the switch in the start  mode.
If I take out the spring and the jet/seat the motor will run but very, very rich with no power.
Local shop says there are supposed to be two O-rings on the top and bottom of the emulsion tube. I only see a place for one at the bottom. The top has a shoulder that seats against the bowl gasket.
I didn't have a rebuild kit so I'm using teh old gasket and O-Ring. So if the shoulder isn't sealing well against teh gasket, maybe I'm losing some low pressure from teh venturi so that the gas won't pull up the emulsion tube.
Alternatively it looks like there is a fixed idle jet that perhaps I didn't clean out at the bottom of the bowl the first time.
Ordering a carb rebuild kit tomorrow but beyond that I'm out of ideas.

Have you got any?

Thanks Jay Stannard

Answer
Jay

Tecumseh Carburetors are simple units. The float model carbs are easy to rebuild/repair. Before you start, one thing you will need is a factory repair manual.
There are some common problems with carburetors that should be checked before you start a rebuild. Many times the following items will correct the trouble your engine is having. You will still need a repair kit to replace certain gaskets and seals that should be replaced whenever disturbed.
One of the more common problems is the flooding of the carb. If this is your complaint, check the float first. Remove the float bowl, float pin and remove the float. Be careful that you do not loosen the float needle valve. Now shake the float and listen for gas inside. If the float sloshes, replace it with a new one. Replace the float needle valve seat and adjust the float per the factory manual. Replace the float bowl gaskets.
If you are experiencing performance problems and suspect the carb, you will have to remove it from the engine to correct the problem. After the carb is off the engine there is one thing you need to check before going any further. Remove the float as above. Now hold the choke and throttle butterflies so that you can shake the carb without them making a sound. Shake the carb and listen for it to rattle. If you do not hear the rattle, the emulsion tube is stuck. This tube is sealed into the body of the carb during manufacturing and can not be replaced. The carburetor will need to be replaced. Also if the aluminum body of the carb is corroded inside, any rebuild will only be a temporary fix. Once the corrosion has started it can not be stopped.
If everything checks out up to here, a rebuild and cleaning should fix the carb. However, there is one more tricky thing you will need to do. You must remove the Welch plug on the side of the carb. This can be done by driving a sharpened small punch or screw driver through the plug and popping it out. Take extreme care when doing this procedure. There are two jets cast into the carb body under this Welch plug. If you damage them the carb will have to be replaced. With the welch plug removed and the main jet(in the bowl nut) out the three holes that need to be cleaned are exposed. Using a piece of soft tag wire clean these holes. Now clean the complete carb with carb cleaner.
Re-assemble the carb. Make sure that the needle valve seat is completely down into its bore. After driving in the new welch plug, paint it with finger nail polish to seal it. Adjust the float height using a #4 twist drill as directed in the repair manual.
Now that your carb is rebuilt don't drop the ball on the 90 yard line. Replace the gaskets between the carb and the engine, and the carb and the air cleaner. One common mistake made is to fail to replace the fuel line between the carb and the fuel filter/tank. If the carb was full of dirt or varnish the fuel line will also be. Plus when the carb or fuel line is removed, the inside of the hose usually is damaged. Small fragments of hose materials can break loose and flow into the carb. This will ruin the job that you just completed.
After the carb is remounted to the engine, it must be adjusted correctly. You will need a repair manual to do this. Remember that the idle circuit must be adjusted first. Without the idle circuit working correctly you will never get the main jet adjusted correctly