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Fuel system HELP


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If your running mikuni 44s on a l20b do I need to hook up a return line? I have a 1978 datsun 620.


I'm asking this cuz my truck runs good when it's cold but when at normal temp it starts to feel a little weak. What I'm thinking is that the fuel tank has a lot of pressure and its not giving good flow of fuel.


Correct me if I'm wrong. And thanks for the help.

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Nissan installed the fuel return line mostly to circulate fresh cool fuel from the tank past the carburetor. The 'new' L20B runs hotter that the previous engines and in the summer under hood temps can get very high and the fuel in the pump, lines and the carb can boil from radiated exhaust heat. This is known as vapor lock and it makes re-starts very difficult. Any boiled gas is an emission of hydrocarbons (bad for the environment)and a waste of fuel.(bad for the wallet)


Secondly and more important (I think) is that over time all the fuel in the tank is recirculated and that means it has to pass through the fuel filter. This keeps the tank and the fuel lines clear of sediment and any rust.


Some suggest that it reduces fuel pressure but I don't think so. The return has a pin hole restriction in it so the pump has to work against it to make pressure so the gas can push into the carb and not just take an easy path back to the tank. I will say that when the engine is shut off pressure between the pump and the carb does drain away unlike the L16/18 engines that would spurt fuel when removing the carb fuel hose. This would prevent the possibility of over filling of the carb after shut off.

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If your fuel TANK has a lot of pressure that's a whole different issue.   Tank should be vented through the carbon canister.  There should be little to no pressure or vacuum at the tank, irregardless of if the return line is hooked up or not.


If it runs good cold (like, with the choke engaged) but runs weak warmed up (with the choke open) that's usually a jetting issue.  But it also can be a vapor-lock issue if it's extreme, which could be solved by routing the fuel lines away from hotspots or running a return line, but the return tee has to be as close physically to the carb inlet as possible to be effective.  With twin mikunis the stock return setup wouldn't meet that, though you would run part of it.

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Yes im running a fuel gauge and the fuel pressure is at 3 1/2 psi. Also i notice when i downshift, it back fires only when its a normal temp. but when its cold its fine. Is this normal?


My carb jets are 45 pump jets, 150 main jet and 200 main air jet

Could it be the carbs getting hot from the exhaust?


thanks for the help

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Are you using a cast iron exhaust manifold or a header?


I have a heat shield template that we used to run on the race cars. It is a generice template and mounting it is different for each type of intake manifold. It is basically two sheets of aluminum with some asbestos sheet sandwiched in between. You used to be able to get the sheet from Mikuni, but I don't know if it's still available.


I will try to take a pic of the template and post it up.

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