Long-term fuel saga

[Gorpon73]

Perhaps good to burn away a few random minutes, for your amusement and technical consideration I present to you my long term fuel/running bugs. I’m interested in learning any potential relative technical options.

Background: Running Stromberg carbs on an S4 SE with electric ignition and fuel pump. A couple years ago the car developed a hiccup/stutter/stall when running up very long elevated grades. The car was relatively new to me and I live in a mountainous high altitude, so I assumed it was time for a full carb / ignition refresh. Fully rebuilt the carbs, new airbox, air filter, air hose, gaskets, ignition coil, distributor, wires, plugs. Car idles and runs great without discoloring the plugs at any temperature even in very hot summer weather. This did not solve the issue of stuttering when going up very steep grades. I assumed this as a fuel delivery issue because upon stopping briefly or descending the grade, the problem went away.

I replaced the crusty old electric fuel pump and all the fuel filters. This greatly improved the issue. I could drive up very long steep grades up the mountain passes without any stalling. However, recently during a very long drive at highway speeds (no elevation changes) there was some stuttering again. Here again, if I pulled aside or idled for a short while, the issue went away for the most part.

I’m assuming that this is still a fuel delivery issue and that overall, everything else is fine. The issue may be when running for a very long time at 70+ mph, the fuel pump just could not keep up with the fuel consumption? I’ve read that some distributors can fail at high temps. I am currently thinking this is not the issue as the car cools/drives great in high temps. For example, the car did not stutter until after 45 minutes while driving at 70 mph in 95 degree heat.

So, my possible corrections would be:

Still a fuel delivery problem? Upgrade to the higher flow rate electric fuel pump? But would this possibly overdeliver fuel when driving at slow speeds?

Revert back to a mechanical fuel pump? I’m not sure of the relative merits/drawbacks of doing this? The mechanical fuel pump technically makes more sense to me overall.

Change my float height? I’m assuming this may not help on a long run because eventually, the fuel delivery does not keep up with consumption, despite having more fuel available at the start?

Any insight greatly appreciated.

[h20hamelan]

Add a wideband o2 sensor?
And a fuel pressure gauge?

Seems like it has to do with pressure or heat

What fuel are you running

[Gorpon73]

Fuel pressure gauge is a good idea, I'll consider that.

Unfortunately, I've been running premium, but has not been clear gas. I typically only fill up with clear gas at the end of the season when I won't be running for a while.

I feeling like its not heat. I've been into a drive an hour before the issue popped up. I'm going to get up a 5 AM tomorrow and go out and flog it to see if I can get it to reproduce the problem in the cooler air....

[Craven]

Maybe blocked or partially blocked air vent to the fuel tank.

[mbell]

I'd be looking at fuel starvation and overheating ignition components, leading to weak spark.

It can take quite a while for of ignition components to get warm enough to show issues. I recently restored my car to a ballast coil system and mange to miss wire the two supply lines by passing the ballast when the engine was running, it took well over a hour driving in warm weather for the coil to get hot enough to start showing problems and I was still able to drive it home with only minor issues. So I wouldn't rule heat out just because it takes ~hour for it to happen.

If the car make it home fine after 30 min run but struggles after 1 hour, that would lean me away from fuel to looking at ignition.

Where did you source the ignition parts? Quite a lot of reports of issues due to poor quality replacement parts. So can't assume that fitting new parts means they are ok.

[sprintsoft]

Hi Gorpon,

I didn't read that you cleaned out the tank, is it possible there is sediment build up in the fuel tank or the line?

you said you replaced "all the fuel filters" - how many have you got? ...more than one could restrict flow

Iain

[Billelan]

Hi Gordon.
Maybe a long shot but I had a very similar problem many years ago with my S4. I was on a trip down to the South of France from the UK when the car started to play up. The problem got worse as the ambient temperature increased. I suspected fuel starvation. After a lot of head scratching it became evident the mechanical pump was pulling in air somewhere between the pump and fuel in the tank.
The fuel line and tank were fairly new so didn’t think these were at fault. In the end it turned out to be the push connector between the fuel line and tank. This connector was getting soft with the higher ambient temperatures and allowing air to sucked in. A bit of twisted copper wire to tighten the connection and problem solved. No further issues and it lasted until I could make a proper connector back in the UK.

[Gorpon73]

Thanks for the feedback.

I hadn't yet addressed thoughts of the tank being contaminated with rust and/or other particles. It was refreshed within the last 10 years, but I suppose that doesn't exclude it. I was assuming that if I didn't see anything in the in-line fuel filter, it was ok. Perhaps its time for a dump of the gas to look if there is anything in the gas when drained.

Could be the vent, had not considered that. I do have 2 inline filters, which I also was wondering if it was overkill and restricting flow. I didn't think it was from air getting in the line. The line is clear in the front of the car and can't see any bubbles where it leads upwards, so was assuming no leaks. All lines are new and fixed with hose clamps.

I'm still hoping its not the heat. Took it out for a quick errand this afternoon ~92F ambient temp. Very briefly warmed it up then head out. It actually sputtered a bit for the first 10 minutes on the road this time and then drove fine for another 20 minutes (head scratcher that one).

Thanks again for the feedback.

[steve.thomas]

+1 for the fuel tank. I had similar symptoms a few years ago when the car would falter after approx 30 minutes and I suspected overheating of coil or electronic ignition. It turned out to be a partial blockage of the fuel line where it joins the tank.
Good luck
Steve

[oldelanman]

A couple of fuel related things you may not have ruled out already :-
Are the Stromberg temperature compensators operating and set correctly?
Have the carburettor mounting nuts been overtightened so the mounting is no longer "flexible" ? .... might lead to fuel frothing in the float bowls under certain conditions.

[Seamus]

I had a similar problem with my S4 and it was caused by a blockage in the fuel filler cap vent, I cleared it by pushing through the tiny vent hole with some steel lock wire.

If you do go for a high pressure pump then you will need a pressure regulator too, mine is set to 2.5psi (I've got Webers).

[69S4]

I had a similar problem with my S4 and it was caused by a blockage in the fuel filler cap vent, I cleared it by pushing through the tiny vent hole with some steel lock wire.

If you do go for a high pressure pump then you will need a pressure regulator too, mine is set to 2.5psi (I've got Webers).

I have an electric pump on my Stromberg S4 plus a pressure regulator just before the carbs but what I set it to doesn't seem to make much difference to how the Strombergs perform (mine anyway). Maybe Webers are more sensitive to fuel pressure.

[Gorpon73]

Thanks again for the input.

Ran it today at 5 AM when the air was cool and it ran perfectly.....until I approached 2 grades I've gone up recently and it sputtered and stalled. Pulling aside and idling for a moment corrected the problem.

Strombergs are set up properly (says the amateur) and the mounting nuts are assembled with the specified gap and thackwray springs.

Looks like I'll need to look at the fuel valve and pressure regulator. After that, may be time for full fuel tank and line refresh as it happens most frequently on a grade, so I suspect some sort of flow issue....

[StressCraxx]

Where is your electric fuel pump located? By the tank or in the engine bay?

Electric pumps do not like to suck fuel (suction lift). They like to push fuel, as in locating the pump close to the tank and at or near the fuel level so the pump primes easily.
If your pump is in the engine bay, you may have a fuel line leaking air in to the line while trying to lift the fuel from the tank. That's why it happens going uphill. Check your fuel hose at the tank for cracking or leakage. Same at the carb end of the fuel line in the engine bay.

[Gorpon73]

Thanks StressCraxx. Yes, the pump is located on the bulkhead under the carbs. Clearly I'm losing something in the connection. The hoses on both ends are fresh and tight. But does have to lift the fuel slightly which seems no so optimal, so a relo is in order.

I'm going to relocate the pump to the trunk and install a new pressure regulator and see how it goes before tearing the gas tank apart.

Cheers