Fuel starving/ stalling above 50mph

[TakemuraShuu]

Hi hello, I'm attempting to drive from California to Georgia in my 1979 Datsun 200sx, L20b, Weber 38/38. I'm currently using both the mechanical pump and an electric under the tank. Whenever I try to go above 45-50mph (or going up hill), the carb apparently runs out of fuel, and stalls. I have to pull over and let the electric pump feed more fuel to move again. New fuel filter and electric fuel pump, new fuel lines, I can't understand why this continues to happen.

[Dave 240Z]

Are your fuel filters clean?

If evaporative emission controls don’t vent the gas tank, it can create a vacuum that the fuel pump can’t overcome. (Flow guide valve on the old ones) Rare occurrence but easy enough to check by running it with the gas cap off or loose.

[TakemuraShuu]

1 minute ago, Dave 240Z said:

Are your fuel filters clean?

If evaporative emission controls don’t vent the gas tank, it can create a vacuum that the fuel pump can’t overcome. (Flow guide valve on the old ones) Rare occurrence but easy enough to check by running it with the gas cap off or loose.

Interesting, I'll give that a shot. Thanks for the reply

[datzenmike]

20 minutes ago, TakemuraShuu said:

Hi hello, I'm attempting to drive from California to Georgia in my 1979 Datsun 200sx, L20b, Weber 38/38. I'm currently using both the mechanical pump and an electric under the tank. Whenever I try to go above 45-50mph (or going up hill), the carb apparently runs out of fuel, and stalls. I have to pull over and let the electric pump feed more fuel to move again. New fuel filter and electric fuel pump, new fuel lines, I can't understand why this continues to happen.

 

Lets assume the filter is ok.

 

Why electric AND a mechanical pump?????  Was the electric pump installed to try and fix this problem???? Need more info. Maybe the electric pump is the restriction.

 

 

Pull the fuel line off the carburetor and direct it into a suitable container. Take the coil wire off. Turn engine over a few times. What you should see are strong pulses of fuel.

 

Yes.... strong fuel delivery. Check float level in the 38/38, may be set too low.

 

No.... weak flow. Connect fuel line around the mechanical pump and try again with only the electric pump......

 

 

Yes... strong fuel delivery. Problem fixed.

 

No.... no change. Blockage in fuel line, sediment? Take the cap off the fuel tank and blow compressed air back into the tank to clear. 

 

 

[TakemuraShuu]

16 minutes ago, Dave 240Z said:

Are your fuel filters clean?

If evaporative emission controls don’t vent the gas tank, it can create a vacuum that the fuel pump can’t overcome. (Flow guide valve on the old ones) Rare occurrence but easy enough to check by running it with the gas cap off or loose.

No dice, same issue.

[TakemuraShuu]

1 hour ago, datzenmike said:

 

Lets assume the filter is ok.

 

Why electric AND a mechanical pump?????  Was the electric pump installed to try and fix this problem???? Need more info. Maybe the electric pump is the restriction.

 

 

Pull the fuel line off the carburetor and direct it into a suitable container. Take the coil wire off. Turn engine over a few times. What you should see are strong pulses of fuel.

 

Yes.... strong fuel delivery. Check float level in the 38/38, may be set too low.

 

No.... weak flow. Connect fuel line around the mechanical pump and try again with only the electric pump......

 

 

Yes... strong fuel delivery. Problem fixed.

 

No.... no change. Blockage in fuel line, sediment? Take the cap off the fuel tank and blow compressed air back into the tank to clear. 

 

 

Thanks so much, there is definitely a block of some kind in the feed tube from the tank. Any tips on clearing said block?

[TakemuraShuu]

2 hours ago, datzenmike said:

cap off the fuel tank and blow compressed air back into the tank to clear. 

 

 

Got it haha, thanks again

[carterb]

Another possibility.

I had a small split in my fuel on the suction side of the pump.  This killed the pump efficiency (since it was drawing in some air, not just fuel) and was unable to keep the float bowl full when pulling a hill - the longer I'd climb, the more my power would drop 'till I was just sputtering at reduced speed.  Once I crested the hill and the load was taken off the carb, the bowl would fill back up and performance would return to normal.  Identifying and replacing the split fuel line was easy and once the pump was no longer sucking air, it had no trouble keeping the float bowl topped off.

[TakemuraShuu]

13 hours ago, carterb said:

Another possibility.

I had a small split in my fuel on the suction side of the pump.  This killed the pump efficiency (since it was drawing in some air, not just fuel) and was unable to keep the float bowl full when pulling a hill - the longer I'd climb, the more my power would drop 'till I was just sputtering at reduced speed.  Once I crested the hill and the load was taken off the carb, the bowl would fill back up and performance would return to normal.  Identifying and replacing the split fuel line was easy and once the pump was no longer sucking air, it had no trouble keeping the float bowl topped off.

Thanks for the tip! But datzenmike got it! I used an air compressor in the feed line and the car drives perfectly now! I'm currently in Texas, thanks again everyone for the help! 

[Stoffregen Motorsports]

If there was a blockage in the line, you just blew it back into the fuel tank. Ultimately that junk will find its way back into the line, so it might be wise to have the tank cleaned and blow the lines out again before connecting to the tank or pump.