This is going to sound strange but the MT12 does not work right in a NTC3. The rod in an MT 12 is so long it causes additional shake to be induced into the chassis causing (and the chassis is weak because of the pull start hole behind the engine, why they never made one without it I will never understand)the fuel in the tank and lines to have a absolute cow. If you really want the car to work you need to use a K factory tank, with one of the newer short rod novarossi type engines (like a mugen X12) and relocate the pressure line from the tank to the back chamber of a 2 chamber pipe. you will have to fill the old hole in the pipe with an epoxy like jb weld or with a set screw screwed in it. then drill a new hole in the last 1/4 inch of the pipe in the exhaust baffel to put the pressure line into.

This will fix your issue. I have been there lived that and suffered through it. If you cant get the K factory tank do everything else. this will solve 90% of the problem. PS the OS is a long rod motor and will vibe like the MT but I have seen guys getting away with the old TR's with some success.

Dennis Richey
Richey Racing engines