Help me fix this starting issue.

[msaeger]

Here is the issue.

The engine is a Tecumseh TMV125 on a really old Craftsman rider. I have been looking at the Tecumseh tech manual I found on here and it looks like a series 1 carb. Here is the thread I started with a pic of the mower. http://www.lawnmowerforum.com/small-engine-mower-repair/3345-sears-rear-engine-rider.html

The mower will not start unless I have the gas cap totally off not just loose I have to have it off completely.

If I take the cap off, put on the choke, a pull it will start then I need to quickly take off the choke and put the cap on or it will not stay running. It doesn't matter if the engine is hot or cold. The level of gas in the tank doesn't matter.

So far I have replaced the air filter, the gas cap, and ran a tank through it with some carb cleaning stuff. What should I try next?

 

[Rivets]

Try this and tell me what happens. Put the cap on tight, clamp the fuel line, (vise grips works good, not too tight) remove fuel line from carb, open clamp and see if fuel runs out. If it does the cap no longer needs to be off. Put line back on. Clamp off the fuel line again. Next carefully remove the high speed needle and nut from the bottom of the carb. Try not to dislodge the float bowl. Look at it carefully so that you can tell me what you see. I want to know color, nicks, cuts, rings, corrosion, etc. everything in both parts. Clean with parts cleaner and reinstall, nut first with gasket, tight. Then the needle, loosely seat, then backlit 1&1/2 turns. Take off the clamp and try. Then tell me what happens.