Moss Motors, Ltd

my tr4 with 175 zs carbs  starts and idles great.  however, after about a mile (less if driven harder), it seems to run out of gas.  after coasting for a few seconds, it restarts and will run fine if the throttle is kept less than 1/2 open.  has new fuel lines, new fuel filters(2) , new 2-4 lb electric fuel pump, and complete carb rebuilds including throttle shafts and bushings, needles and jets, diaphragms.  open discharge gas flow at the carbs is 0.31 gallons per minute.  this is about 5 times the amount of fuel required to cruise at 65 mph.  gravity discharge just before the pump inlet is actually a little better at 0.36 gpm.  both inlet float valves have been checked and cleaned several times, but no debris has ever been found in them or in the bowls.  any help or ideas will be greatly appreciated.  thanks

What happens after it restarts and you try to hold the throttle MORE than half open? Does it die again? Does it just sputter and continue to run? I don't want to speculate until we've got a lot of information, but this is starting to sound like one of the ninety percent of fuel problems that's actually an ignition fault.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

after the car restarts, it runs fine until driven at over 1/2 throttle for a half mile or so.  then same thing happens--sputters and dies.  left in gear with the engine continuing to turn over as it coasts, will always restart itself before car completely stops.  then can repeat the cycle.  ignition is pertronics unit for positive ground.  coil is new high end unit from napa.  cap and wires are new .  distributor plates and wiring appear to be in good shape.  vacuum and mechanical advance seem to work well.  thanks for your help. if more info needed, i'll gladly supply.

If you have confidence in the ignition it's time to indeed consider fuel issues. Unfortunately, we can't easily watch fuel flow under the circumstances which surround this failure. It sounds like the fuel supply is not keeping up under driving conditions, even though it is more than adequate under static test conditions. So what's different? The fuel is being mildly agitated in the tank. The fuel pump needs to actuate at a much higher rate. Certainly the demand for fuel is higher than at idle. I wonder if the fuel pump is "sticking" at the higher rate of actuation. This is unusual, but I have seen it happen in the past. The other thought that occurs is that perhaps something in the tank is being sucked into the feed tube until the engine dies, whereupon the suction lessens and whatever was blocking the egress of fuel floats away. Also not that common, but it has indeed happened before.

I'm not convinced any of the above is what's going on here, but it's something to inspect. Some electronic ignition failures will also mimic this behavior, but they usually worsen quickly and stop refiring. How long has this been going on? 

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

been going on only for 20-25 miles, most of which were test runs.  fuel tank was brazed to repair a leak near the top (had rusted from the outside to inside).  after that it was cleaned with an abrasive chain, rinsed several times with solvent to dissolve any remaining internal rust, then coated with a gas tank coating and dried for several months.  i don't remember seeing a vent anywhere on the tank.  should there be one?

If the cap isn't vented it MUST have a vent to prevent it from going into negative pressure, which would certainly starve the system for fuel. Have you tried running with the cap off or loose?

Hmmm...

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

yes, tried with cap open--no difference.  also gravity discharge is higher than pump output. 

gas cap is the standard flip-up chrome unit.  how do i tell if it is vented?

does anyone know if larger needle valves are available for float bowls on zs 175 carbs?  my competition book states the standard size is 0.175 in, but a 0.225 in. should be used for high fuel draw situations.  any ideas where these can be procured?