Tuning Fuel Pressure Regulator

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Tuning Fuel Pressure Regulator

Cheech67

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Dec 29, 2014
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Dear All,

I have a Fiat 500F which I had upgraded to an "Abarth". Part of the upgrade was to put in a Fiat 126 engine (the 650CC).

Recently I had a problem with the fuel system. It appears that the carburetor for the 126 had a fuel return system. For my 500 there is no such line. So the installer simply put a rubber house on the return coupling and closed it off. This seemed to work okay for about 10 years. However, the coupling became loose and leaked fuel. I had the carburetor rebuilt and had this coupling plugged up.

Now I have a problem with the fuel pressure. The fuel pressure should be about 3.5PSI. I think that my fuel pump is producing more than this. So the car starts up and runs fine for about 10 minutes. Then, when the fuel system is full, there appears to be too much fuel going into the carburetor.

The shop that overhauled my carburetor told me that if this occurs, I might need to install a fuel pressure regulator downstream from the fuel pump. Has anyone had a similar problem? Any idea of what model of fuel pressure regulator I might need? Any clues on how (and where) I should install it?

I heard that there is such a fuel pressure regulator that was originally for a Chevrolet that might work. It was my impression that I would have to cut the fuel line (rubber) insert the fuel pressure regulator, clamp it down and dial it to 3.5 PSI.

many thanks
Robert
Cheech67
 
The most cost effective answer to your problem is to fit a simple 'constant-flow' fuel system, as I did. I put a 'T' piece into the main fuel feed pipe BEFORE the fuel pump (and secured it to the side of the engine bay with a couple of 'P' clips) I then led a return fuel line from the top of the carb to the 'T' piece. I led the return pie behind the air tube and protected it by running it through a section of small-bore water hose (or anything similar, will suffice). This way, any surplus fuel just flows around, taking pressure off the needle-valve in the carb. Keeping the fuel flowing this way also helps keep the fuel a bit cooler. Fiat put a return system in for a good reason, I don't like it being disconnected. I have use this system for about 3 years, on both the original Weber 28 I had on my engine and its replacement, a FZD Dellorto with no problems at all with either the engine running too rich or poor hot-start problem due to fuel vaporisation.
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Let me see if I understand correctly. The only thing that has been changed is that the carburetor has been rebuilt?
The fuel pump is the same as before?
It sounds to me like either the fuel float is set incorrectly or binding. It is not shutting off the flow of fuel soon enough. Or the fuel enrichment piston setting ("choke") is not set correctly.
If it ran fine for 10 years and the only thing changed was the carburetor was rebuilt, then the problem is with the carburetor.
If you are running a stock mechanical fuel pump, I doubt that you are putting out more than 3.0 psi.
John
 
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