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Fuel Supply Problem


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Hi,

A couple of weeks ago my 1972 240Z began suffering the classic symptoms of a bad load of gas. I tossed in a can of Sea-Foam, ran through another tank and while the otherwise inexplicable power losses and coughing disappeared, a reproducible problem appeared.

If I run hard in 2nd from 3000 it pulls well until 5000 in 3rd and then just flat out won't go any more and runs rough. It's not a misfire, per se, just nothing. If I push in the clutch at speed for 30 seconds or so, then rebreak from 4500 it will pull to 5500 and then, nothing. In deference to the 200,000 miles under its belt, I use 6000 as the redline in normal use, but it will run up to 7000 in 1st or 2nd, even with the above problem.

The mechanical fuel pump is long gone and has been replaced by a rear-mounted electric pump of unknown origin. I rebuilt the carbs (for the first time -- floats = fun). Replaced the 2 month old fuel filter because it had quite a bit of resistance to a blow test. I suspect it did its job, giving up its own life itself for the good of the engine. Gave it a tune-up. Blew the lines from the fuel pump to the filter. Running great, smoother and a bit more power than it has in months, but still runs out of power between 5000 and 5500 in 3rd.

All the fuel pump measures that I've come across are in terms of p.s.i. I don't have the capability of measuring fuel line pressure, but I did pull the line at the filter and at the carbs (before and after blowing the line) and measured an estimated 8 gallons / hour. That seems, well, skimpy, to me. I figure that at a steady 80 mph I burn through 4 gallons an hour (20 mpg). From my other car, which has a instant gas mileage gauge, I know that hard acceleration can easily chop mileage by 2/3rds or more and don't see why the Z would be any different.

So, is replacing the fuel pump a reasonable next step? Or am I overlooking something?

Much thanks.

Chris

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Well, this probably has nothing to do with your situation, but it is something to check. I recently had a fuel supply issue which resulted in my engine going flat (still running, but no power at all). If I pressed in the clutch and revved the engine for 5-15 seconds it would recover and run normally for a while. My air/fuel ratio gauge would go full lean when the problem occurred. My problem was not easily reproduced like yours, but I also had recently rebuilt my carbs.

I finally traced it to a sticking float valve on my rear carb. The needle was not moving freely within the seat and was getting stuck closed (up) even though the float bowl was empty. After discovering the problem the solution was easy... I merely had to bend that little tiny spring retaining clip that keeps the needle from falling out of the seat... it was dragging on the needle and keeping it from moving freely.

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<cut>

I finally traced it to a sticking float valve on my rear carb. The needle was not moving freely within the seat and was getting stuck closed (up) even though the float bowl was empty. After discovering the problem the solution was easy... I merely had to bend that little tiny spring retaining clip that keeps the needle from falling out of the seat... it was dragging on the needle and keeping it from moving freely.

Fingers crossed, I think I can rule this one out. I replaced the needle valve with a new one and verified that it operated much more smoothly and reliably than the old one.

Doesn't mean I won't go back to it, though.

Chris

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It sounds as if it may be fuel starvation. I had similar intermittent symptoms. I suspect a piece of junk may be blocking the fuel pick-up in the tank at times.

With a fuel pump of unknown origin, it may be worth taking a look at it to see if it has a filter installed between it and the tank.

Just my thoughts - hope they help.

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There are little brass filters in the banjo fittings on the fuel bowl tops. Have you taken them out and cleaned? They are the 'last chance' for foriegn matter and often overlooked.

Bonzi Lon

Maybe?

I'm not sure what a banjo fitting is, but I did remove the large bolt/cap from the end of the fuel supply fitting. I do know what a brass gas line filter looks like and had expected to find one there, but I didn't. So, if that is where the brass filter should have been then the brass filter can't be the problem. Or does the brass filter lurk somewhere else?

Chris

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It sounds as if it may be fuel starvation. I had similar intermittent symptoms. I suspect a piece of junk may be blocking the fuel pick-up in the tank at times.

With a fuel pump of unknown origin, it may be worth taking a look at it to see if it has a filter installed between it and the tank.

Just my thoughts - hope they help.

Nah, no filter there.

I'm pretty sure that the problem lies in the back end of the car, either the fuel pump or the fuel pickup. One is somewhat expensive as a "could be the problem" part while the other involves a largish PIA, again with no guarantee of success.

My thinking now is to put some gas in a container, attach a longer fuel hose, and test how fast gas the fuel pump can pull it out of the container. If it is still around 8 gallons / hour, replace the fuel pump. If it jumps up, pull the tank and clean the sucker out somehow. With my luck, it will pump something like 12 gallons/hour, a nicely ambiguous figure that will leave me scratching my head on how to proceed.

But seriously, isn't solving puzzles part of the charm of driving a 37 year old car?

Chris

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remove your electric pump and see if it is clogged with debris, add a inline filter between the tank and the pump. Basic stuff here.

You only need 3 to 4 psi pressure to supply your engine. the carbs won't tolerate more than 5 psi. If you can find a RX-7 that is carberated it has a great fuel pump that will supply even a stroker with triples. $15.00 at the bone yard. Been running one going on 9 years.

Gary

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Maybe?

I'm not sure what a banjo fitting is, but I did remove the large bolt/cap from the end of the fuel supply fitting. I do know what a brass gas line filter looks like and had expected to find one there, but I didn't. So, if that is where the brass filter should have been then the brass filter can't be the problem. Or does the brass filter lurk somewhere else?

Chris

Banjo fitting: The round fitting on the float bowl lid that the bolt (with a hole in it) goes through. (also used in modern brake fluid applications.)

That is where the filters should be. Bruce at Z-Therapy can supply them. They really are the 'last chance' filters for tank trash, you should have a pair.

Bonzi Lon

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