Carb problem frustration level 10/10

[Cleveland Crush]

By the way Cribbs, what fuel pump are you running?

 

[Big Red Mach 1]

Adding a fuel pressure regulator was one of the best things I ever did. Sounds silly, but even being 1psi higher than your carb likes is all it takes sometimes. I also don't think the cheap fuel pumps they sell now are bench tested at all anymore.

 

[Cribbs74]

I spoke too soon. now I am having the opposite problem. Not enough fuel from the idle circuit. I went through the carb 3 times today and it just won’t idle. All air bleeds are clear. There is no reason this carb shouldn’t be working.

I even had a friend stop by today and he couldn’t get it working either. I am about to abandon this carb.

Any suggestion on a new carb that will fit under the stock air cleaner?

 

[Cribbs74]

fuel pump was a Delphi unit. The pump that is currently on there I have no idea

 

[Cleveland Crush]

I know you did a cranking fuel pressure test but did you do a running pressure test? Also did you replace the fuel filter? If you perform a running pressure test for say 3-5 minutes that will at least help verify where the problem is. Point is don't go for a new carb until you're certain the fuel problem is not "upstream" of the carb. Good luck with it!

 

[Cribbs74]

Thanks for the reply, I think you may have missed a post or two. The one thing I didn’t bring up that you mentioned was the fuel filter. Running two inline filters. One at the carb and one before the pump. Both new and both clear.

I didn’t do a running pressure as the carb no longer floods after I swapped pumps.

 

[72vert]

I think I am currently having the same problem. Could a mechanical fuel pump going bad cause pressure to increase suddenly?

 

[Cribbs74]

I wouldn’t think so it’s a rubber diaphragm. It could only get weaker or fail completely.

 

[72vert]

That’s what I thought but wasn’t sure if it could be something weird.

 

[ROCKET70]

Years ago I ran into that problem and it was the fuel filter before the fuel pump I removed it and it ran fine. Just a thought!

 

[mrgmhale]

I know you did a cranking fuel pressure test but did you do a running pressure test? Also did you replace the fuel filter? If you perform a running pressure test for say 3-5 minutes that will at least help verify where the problem is. Point is don't go for a new carb until you're certain the fuel problem is not "upstream" of the carb. Good luck with it!

You know... If your fuel starvation problem happens "a while after running," especially at higher speeds, you may have an incorrect fuel cap on your tank filler neck. If you have an older, open vented fuel tank and cap, but if the cap is not venting correctly, or more likely you are using a fuel cap designed for a non-vented fuel system that harnesses a charcoal fuel vapor collection tank, drawing out liquid fuel will create a vacuum in the fuel tank. At some point the vacuum in the fuel tank will overcome the fuel pump's ability to pull more liquid fuel. If this is suspected you can go to the filler cap when the fuel starvation is occurring and open the cap while putting your ear near the cap. If you hear a whooshing sound of air rushing into the tank you found your problem.

If your Mustang does have the fuel evaporative control system you could be running into a few different problems. First, the vapor return line (steel tube and/or rubber hose section) is plugged or kinked, and the fuel tank will be unable to pull air in from outside the tank to compensate for a lower liquid fuel level as the engine is running. Hence, vacuum will build up inside the fuel tank in a short period of time. The more full your fuel tank the more quickly and prominently the symptoms will appear.

Another possibility is "someone" has seen a rubber hose on your fuel evaporative canister that should be connecting to the fuel tank vapor/vacuum control line, and assumed it was connected incorrectly - then connects the steel fuel tank's steel vapor line to engine manifold (or ported) vacuum. That builds up an immediate and significant vacuum inside the fuel tank, which will cause problems with fuel starvation very quickly - immediately in fact. Although some folks will find that laughable that anyone would do that, I have seen it happen. Especially on pre-73 Mustangs (in the world of Mustangs) as the 1972 and prior shop manuals do not show how their various emission control systems are supposed to be hooked up, much less show how they function. Attached is a PDF file that contains an excerpt from the Forel 1973 Mustang Shop Manual Volume VI that shows how the various emission control systems work, and how they are connected.

If you have any questions or concerns re: the fuel evaporative system let me know (PDF file pages 18 - 21) . Both of our 73 Mustangs have them connected properly. That said, I did take some photos of our 73 Mustang Convertible's fuel evaporation system a while ago for someone else having connection issues. Those files are already attached to this post also. I can take more photos if needed.

A quick aside, I have also seen the fuel evaporative charcoal canister hoses disconnected and blocked, which will also cause vacuum buildup problems with the fuel tank. Please note, the fuel evaporative vapor control system does not cause any engine performance problems when it is working correctly. It is akin to the PCV system, where having those systems do nothing but provide benefit when they are connected and working correctly. Trying to defeat either system will cause problems. In the case of the fuel evaporative system, when not properly connected it is likely you will end up with fuel tank excess pressure or vacuum issues. Or, you will have a garage needlessly filled with fuel vapors.

 

[ROCKET70]

Did you remove the fuel filter before the fuel pump? I also had a friend with a 64 pontiac and he would drive the vehicle about 5 miles and it would die would have to waiit about 20 minutes and it would be good for about another 5 miles. He put in a new distributor, carb, plugs and wires.
When I was leaving a car show saw him broke down and he explained the problem to me so looking at his car noticed a fuel filter before the fuel pump
which was new and I had some hose in my car so removed it and has run fine since. that was 4 years ago.

 

[Big Red Mach 1]

Did you remove the fuel filter before the fuel pump? I also had a friend with a 64 pontiac and he would drive the vehicle about 5 miles and it would die would have to waiit about 20 minutes and it would be good for about another 5 miles. He put in a new distributor, carb, plugs and wires.
When I was leaving a car show saw him broke down and he explained the problem to me so looking at his car noticed a fuel filter before the fuel pump
which was new and I had some hose in my car so removed it and has run fine since. that was 4 years ago.

The only filter is the sock in the tank unless someone added one.

 

[Cribbs74]

Sorry for not replying sooner. I have been in Guam for over a month.

so I was able to mess with the car today finally. Installed a new pertronix unit in place of the points. Everything is solved now. I can’t explain it, but it cleaned up everything. Runs smooth, fuel is not soaking the gasket anymore. if I was to take a guess it was unburnt fuel coming out of the exhaust and also fuel coming up into the intake. Strange one, but it‘s working.

 

[ROCKET70]

That's one to remember if it happens to someone else. Glad you got it fixed.