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Discussion Starter #1
Hey,

Would a weak mechancal fuel pump cause the rear carb to constantly run dry? I am getting gas into the bowl on the front carb but the rear is getting none. I've pulled the float and all seems to be well there. I checked the pump and it is doing it's thing but seems slow. I'm stumped.


Chris
 

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I have and original 70 240Z with the old mechanical pump, I had near the same problem except for the fact that mine would run outa gas when I stepped on it. The solution to this problem is to replaced it with an electric fuel pump. There fairly inexpensive I haven't had a problem yet.
 

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Could be a weak pump.

Could be the float tab on the float in the float chamber set incorrectly enough that no fuel can get in no matter what the level in the chamber.

Could also be a problem with the fuel inlet valve inside the float bowl cover. If you take the float bowl cover off and disconnect the fuel line, you should be able to blow through the inlet. These little nozzles can be adjusted, there is a little spring inside that you can compress with pliers.

Most likely is the float tab. I would check that first before going to crazy with anything else.

You know, you can put a pair of vicegrips on the return line and this will cause pressure to build up in the fuel line. I would only do this temporarily as you run the risk of over pressuring the fuel inlet valves and flooding the carbs badly. If after partially blocking it the fuel starts to build in the rear carb chamber, then there is a good chance your pump is weak. Fuel pressure gauges aren't expensive and are easy to plumb.
 

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Could be the inlet screen!

If it's plugged even before the float cahmber, nothing you do inside the float chamber will help. Little fine screen in the inlet fitting. They do get gunked up.
With two other filters, you can eliminate them, but do so at your peril.

I had a similar problem on my 260, after I did a valve lapping! two cylinders had bad seals on intake valves, and before the lapping it would rev to 7000 no problem. After the lapping, fell flat on it's face at 6000 to 6500 (soometimes as early as 5500!) Pulled my hair out thinking I jumped a link on the timing chain, but when that all checked out I just had to sit and think.
Eventually, I reconnected the original stock 260 electric pump at the rear of the car, and the problem went away. I guess before it wasn't drawing down enough gas because of poor performance on two cylinders so I had no problem. After lapping them and having the car pull hard on all six, the thing couldn't keep gas in the carbs under hard pedal mashing to redline! Make sure of your pressure near redline before you start changing things! Cheap diagnostics save money in the long run.
Weird and wierder!
 

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Or the fuel line gallery....

You might have crud in the supply line. Disconnect all the rubber lines and blow compressed air a some carb cleaner through the inlet and returns, making sure to plug off all opening but the one you want to clear out.
 

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Discussion Starter #6
This problem has been slowly getting worse. All of the lines are clear and the float passages are clear. Could the float level change on it's own? Can anyone suggest a make and model of fuel pressure gauge as I think that I will see what the pump is putting out before I replace it.

Chris
 

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Can get a simple vacuum test/fuel pump test gage setup at most auto parts stores. Maybe $15-20. I think Sears has them also, or NAPA stores.
 

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"Float Passages"?

Does that mean you pulled the inlet banjo fittings, removed them and checked the screens, or only inspected the needle/seat assembly?

Something progressively getting worse means something going bad/getting plugged.

I agree with JohnH240, get a fuel pressure gauge and QUANTIFY what you are seeing. If you have good pressure in the rail, and are running low, then something between the point you checked and the jet assy is restricted/not right.
If you got low pressure, then a-troubleshootin' the feed line from the tank, pickup, fuel pump, filter assy you will be a-doin'!

Filter before the pump normally, and if it plugs...
 
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