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Airtex 8012s electric fuel pump on a 1972 240z with 3 screws su carbs


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#1 jalexquijano

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Posted 11 February 2018 - 12:36 PM

My mechanic left the airtex 8012s electric fuel pump on the on position without cranking the engine and now the fuel pressure at the carbs has lowered from 4.3 psi to 3.2. Question? Could the filter that is besides the pump be clogged and thus causing the decrease in psi or could the pump be partially damage due to his negligence on not craking the engine immediately.

#2 Stoffregen Motorsports

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Posted 11 February 2018 - 01:05 PM

It depends. If he left it on for a long time, say, all night. I could see that being a problem, but turning on the fuel pump before cranking the starter wouldn't hurt the pump.

 

Do you have an adjustable psi regulator? Does the pump still pump fluid? Why not just crank up the PSI and see what happens?



#3 datzenmike

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Posted 11 February 2018 - 01:17 PM

The SU's don't need a lot of pressure. Just over 3 is plenty. If worried about the filter, just change it. But don't expect a change.

 

The pump is self regulating and pumps only when needed as the pressure lowers from gas drawn into the carbs during use. 

 

If you have a fuel return line it would have chugged along all night... the same as if you drove all night on holiday. The return line has a restriction so that pressure will build on the carb side or else all the gas from the pump would just return to the tank. The restriction has a small pin hole in it so that cooler gas from the tank is constantly recirculated past the carb and warmed gas from under the hood is returned to the tank.

 

Here's the restriction and pin hole in the L20B return line...

 

11aMYIJ.jpg


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#4 jalexquijano

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Posted 11 February 2018 - 04:20 PM

Its weird i used to have 4.3 psi and now only 3.2. Car hesitates and backfires between 3000 to 4000 rpms. Once i put in a new pump it raises to 4.3 and no more hesitation or backfire.

#5 datzenmike

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Posted 11 February 2018 - 05:02 PM

Well fuel pressure on a carb engine shouldn't matter if withing range. But if it fixes the problem...


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#6 jalexquijano

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Posted 12 February 2018 - 06:19 AM

Well when the pump was new the pressure was 4.3 and i had no hesitation or backfire between 3000 to 4500 rpms going from 2nd to 3rd gear.

#7 Crashtd420

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Posted 12 February 2018 - 07:54 AM

Well when the pump was new the pressure was 4.3 and i had no hesitation or backfire between 3000 to 4500 rpms going from 2nd to 3rd gear.


I can't help but think that the extra psi is masking a tuning issue....
Now that it's running good how do the plugs look?

I think Mike touched on the su carbs wanting 3psi... and a quick search say 1.5 to 3 psi....
With that said I run a single weber side draft... initially the recommended carter pump was outputting around 6 psi. That was causing a rich condition, it would foul my plugs and make it hard to start the next day...Once I added a regulator and lowered the psi to 3.5 my weber was much happier. And No more plug fouling...

#8 Stoffregen Motorsports

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Posted 12 February 2018 - 09:59 AM

Have you checked that the fuel pump is still pumping fuel?

 

Do you have enough fuel in the tank?



#9 tr8er

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Posted 12 February 2018 - 03:41 PM

Hard to believe 3.1 is an issue. Generally people have the opposite issue with pump pressure too high. If pump pressure is the issue, all I can speculate is that your fuel return is not circulating as much fuel, thus delivered gas temperatures will be higher. Perhaps when really pushing your engine this could cause an issue. Pretty speculative though