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ever think the three screws that hold the tank to the frame are to tight or missing the orings ...........try to super glue the fuel line to the tank ass. wasn't thinking when they put a rc10gt tank in a touring car

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YOU MAY HAVE AN AIR LEAK YOU SHOULD SEAL THE MOTOR. WONT HURT ANYTHING IF YOU DO.ALSO CHECK THE FUEL INLET NIPPLE IT MAY BE CRACKED AND LETTING AIR IN. I ASSUME YOU CHANGED THE FUEL TUBBING.

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You said that the problem was with the air in the fuel line, originating at the carb end... It could be that you overheated the engine/carb, that happens to me, I run the NTC3 as well. When the carburator overheats, it's a bitch to crank up the engine up. Maybe check out the tuning? I would also try checking the pressure line (the one that goes from the tank to the pipe); it's recommended to run a long pressure line on the NTC3, but NOT TOO LONG; try running a shorter pressure line and see if it solves the problem.
Apart from the normal problems on the car, how bout the condition of the engine? Ever tried running the engine in another car to troubleshoot? Just in case the problem lies withing the engine.
Keep us posted!

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The fuel inlet at the carb may have a hairline crack, since it's plastic.

Are you running a slide carb or a rotary carb. The novarossi rotary carb is a pain in the butt. The slide carb is just fine. If you are using a slide, there is no reason to switch to an OS. If you run a rotary, the OS is a big help. I know that isn't the nswer to your problem, but I wanted to answer your queston regarding the carbs. I have been using OS carbs on my offroad engines (rotary carbs) for many years now. For onroad, I use slide carbs and the stock nova carbs are just fine.

I think that you are having a issue with heat like Tommi said. I once had a issue where I would get bubbles in the fuel line and the fix for me was to run the fuel line over the clutch to the fuel tank. This kept the fuel line well away from the crankcase and head. This stopped the bubbles. I think that it was boiling the fuel in the line making the bubbles that I would see.

Simple thing is an overheating carb. You can try running a richer setting on the high end needle. Stock LSN setting works really well. I've had the same issue with the MT12 and just opening it 8th of a turn fixed my problems. Also do consider tuning wisely for the late spring/summer as it heats up fast.

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