It runs! BUT.... fuel starvation under full throttle....

[angello]

My cursed '79 Spider 2000 has a fun new problem.....

I've finally got the thing to run without overheating... or dying... or all the other ailments it's had.... and been able to actually drive it. This is a WIN

BUT, before long it was dead again.... After a good deal of head scratching and testing I have been able to establish that I am suffering fuel starvation after a period of full throttle....
I know this, because I've had the top off the carb immediately after the symptom and the fuel level in the float bowl was below the jet orifice. When running normally the fuel level is higher in the float bowl, so it looks like the pump isn't keeping pace with demand. Question - what level should the fuel be at in the float bowl in normal operation and where do you measure it from?

The mechanical fuel pump has been replaced with an electric one. These can flow plenty of fuel, so I don't know why it isn't keeping pace in this situation. I don't know if it's pumping it back to the tank via the return line and just not filling the float bowl or what's going on!

I've had the float valve out of the carb and can't see any issues with this. I have observed bubbles coming into the fuel filter, which is odd and suggests maybe there might be an air leak in the supply pipe? but there are no signs of fuel anywhere.... I guess it could be the pick up in the tank?

Any thoughts or ideas would be welcome.... I'm determined to get this Spider running properly and back on the road where it belongs!

[Nut124]

I recommend you get a 0-5psi pressure gauge and plumb it into the fuel line just before the carb. Install it in a long hose so you can attach it to windshield wiper and observe while driving. Fuel pressure at the carb should be in the 2.5 to 4psi range.

Watch the gauge at sustained full throttle.

Possible causes: crud in the fuel lines. Loaded fuel filter(s). Weak pump. Check for filters in places you may not expect, in the trunk, under the car.

[angello]

I should have known....

0-5 psi pressure gauge duly acquired and plumbed in.... fuel pressure only about 0.5 psi, so that looked to be the issue. As a quick remedy, I plumbed the original mechanical fuel pump in series with the electric, so the electric pump acts as a lift pump and the mechanical one does the heavy work.

BUT, as with everything on my car, it wasn't that simple, oh no! Even with both pumps fuel pressure at the carb remained at a lowly 0.5 psi.

Scratching head, I started experimenting with raising the pressure by crimping the return pipe, which proved that the mechanical & electric pumps have enough output to comfortably exceed 2.5 psi pressure - indeed the electric pump on it's own has more than enough 'oomph' to exceed 2.5 psi in pressure. So the pumps produce enough pressure, but I'm not seeing that pressure at the carb as the fuel is being returned to the tank via the return line.... So that's problem no. 1 - how do I resolve the low fuel pressure problem as fuel is returned to the tank?

Problem no. 2 arose when I tried to raise the fuel pressure, as the engine started to bog down.... With the air cleaner removed it was clear to see that as soon as the pressure rose beyond the roughly 0.5 psi it had been running previously, fuel began to trickle into the venturis.... So it looks like either my fuel height in the carb is wrong or the float valve isn't sealing... or something else!

So I'm now stuck in a position where it looks like I have low enough fuel pressure that the engine will run well but suffer fuel starvation under moderate throttle and that if the pressure is raised to overcome the fuel starvation I then get flooding of the carb....

Honestly, this car!!!

[Nut124]

Well, you seem to be making progress troubleshooting.

I run no return line. I run an electric pump in the trunk and a pressure regulator in the engine compt.

Perhaps the previous owner adjusted the carb to cruise around with 0.5psi fuel pressure. Webers are supposed to have about 2.5 to 3.5psig at the carb. They can usually handle more, 4-6psig with synthetic float needles. Check the float needle for all brass or synth cone. If all brass, stay below 4psig.

I would consider removing the carb cover and checking, adjusting the "float level". Possibly, the float has been adjusted to work OK at low rpm with inadequate fuel pressure. There are specs for these. Then, perhaps try blocking the return line. I have had multiple Fiats and never ran a return line. Not sure what it was supposed to accomplish.

You will need no less than 2-3 psig at the carb for any performance. This needs to be maintained at sustained WOT, at least 2psig.

With an electric pump running, the float needle must be able to hold the gas back at idle and when not running so the engine does not flood.

[angello]

Progress
Checking the float valve shows it's brass and sealing perfectly when operated manually. Checking the float showed it was way out of adjustment...... With it reset I appear to have cured the leaking of fuel through the venturis and I can now shut the car off and run the electric fuel pump without it flooding the carb. I'm going to call that a win for now.

As a useful bonus, rectifying the leaking float valve has upped the fuel pressure to around 1 psi. Less than ideal but I'm going to see if that is sufficient to resolve the fuel starvation problem. If not, I'll see if I can get away with closing the fuel return line and just run full pump pressure to the carb all the time or alternatively add a restrictor / pressure valve to the fuel return line. I'll have to see what works.

Next job: road test.... and that should be the fun part Getting closer!