Datsun Roadster Forum-311s.org

Still having the same issue and scratching my head trying to figure this one out. Someone please check me on this. When the carb fuel bowls are topped off the car will start and run till the bowls are dry, as verified by pulling off the covers. I'm not running the fine mesh carb filters, as I know they are prone to plug, but I have two clear filters between the tank and fuel pump that are always clean. When I pull the hose off on the carburetor side of the fuel pump, the pump seems to be working, quickly filling a glass jar. The fuel pump was re-built about two years ago using Dean's kit and both the float valves are brand new. I've taken the float valves apart to check for any obstructions and there are none. The only thing that makes any sense to me is that maybe the fuel pump diaphram has developed a small leak that that lowers the fuel pressure enough that it is unable to overcome the float valves. It does not make sense to me that both new float valves would fail at exactly the same time. There's nothing other than the float valves between where I am getting gas and where I am not. What am I missing??? Obviously I could start throwing parts and money at the problem, but as important as fixing the issue is understanding what's happening. Any other thoughts?

I always advocate checking for "invisible" leaks, particularly with old fuel lines, and clamps. Clamps can be TOO tight, creating cracking and thus letting air in. The air leaks are particularly tricky as they let air in, but fuel does not necessarily leak out. It took me a long time to find this problem on my 67.5. It was a piece of fuel line at the fuel filter that was letting in air. Changed it and all good. May not be your problem, but cheap and easy to remedy.

I can second the air leak issue in the past... it was the cool but leaky glass fuel filter assembly (67.5 2000) that would allow the system to pump air, not fuel, and would leave me gasping for fuel at high demand. Check all your lines and if need be try going direct to the carbs with a temporary line to see if you can eliminate areas that may be leaking air.

BTW, I use a Mr Gasket fuel pump as a pusher, mounted to the frame near the right rear tire. Ford escorts have an inertia switch in the trunk that one can easily locate and remove that will turn off the pump in case of an accident... I have one, but it is still in my tool box drawer...

Should have also indicated I tightened all the hose clamps on fresh hoses, so rule that one out.

The chances of both fuel bowls running dry is odd. So like you said floats are probably good. Sounds like the fuel is siphoning back to tank. Are you positive the inlet/outlet fuel connections are correct? Some other owner recently swore they were correct but turned out it wasn't giving him issues. Some years back my soninlaw broke down on the Frwy and a Good Samaritan stopped to help. Turned out FP was bad so after they got a replacement he got him on his way only to breakdown some miles later. Got it towed to my place and I found the 2 lines reversed.

Sounds like you have an early style fuel filter with the glass bowl. Frequently the cork gasket goes bad and you draw air instead of fuel. The test to prove that is the problem, is to remove the filter from the system and see if bypassing it makes the problem go away you have found th culprit. You just need to put a hose in with clamps temporarily and if the car does not have any more problems your good to go. Just buy a replacement gasket. There one other that can happen, people over tighten the wire frame on the filter and distort the zink casting and it again leaks air.
Dave

I will plumb around the glass filter tomorrow. It's certainly worth a try. Car had been running flawlessly when this issue started and my car was freshly restored about a year ago. I'll report back tomorrow. Thanks!

Thought I'd update you all on what I finally discovered after pulling out all my hair. I borrowed a used mechanical fuel pump from a friend and it pulled enough gas from the tank to fill the carb bowls and start the car, but it was running like crap. Decided to forego my mechanical pump and wired and plumbed in an electric one. Now I was getting great fuel flow but the car acted like it was running on three cylinders. After pulling the plug wires off one at a time I found that pulling number #3 and #4 made no difference in the way the car ran. That led me to check the float bowl on the rear carb... empty. The car had been running on only two cylinders. That left only one thing, the float valve. I had discounted the float valves as an issue as the pair were purchased new from Z-Therapy 1 1/2 years ago and I couldn't believe they were bad. Took the valve apart thinking there was an obstruction but there was nothing visible. After re-installing the valve I blew into the fuel inlet on the float bowl lid while manually engaging the needle in the valve. No mater what I did I could not get air through it (or obviously fuel). After substituting a used one I had laying around the car again runs flawlessly. The only thing I can surmise is that the composite tip on the float valve needle was somehow compromised by the alcohol in the fuels we have here seasonally in AZ.

The ethanol can really gum things up. Some carb cleaner should help or some use a little Marvel Mystery oil in the gas.
Glad you found it
Linda