Stage 1 V8

Here's the scenario:
Two tanks, each with its own 24V facet pump. The pumps seem to be working ok, one is NOS about 18 months ago, the other is about three years old.
Each pump feeds under pressure into a two-way shuttle valve (two in, one out) and then from there, one fuel line to fuel filter and then to the two carbs.
A switch on the dash flicks on or off, where the shuttle valve is on (position one) or off (position two).
Another shuttle valve provides the return flow and is wired in series from the first shuttle valve.

The problem:
The engine runs out of fuel on hills. One tank/pump is worse than the other, so that flicking over to the other pump restores performance (and I can happily leave those slow drivers on passing lanes, in the dust).
However, the problem is exacerbated with both pumps now running out of fuel but one is still worse than the other and the worse pump is even starting to run out of fuel on the flat. So the problem is progressing. Changing last pump did not seem to improve the situation.

I am not sure if temperature is an issue but I only see the problem when I am on the open road (the carbs have had the time to run out of gas).

Also, I have SU's and 3.9. So the bowls are a little larger than the original strommies.

So,...
What are the possible causes of this?

IF the problem is fuel;
- creating a vacuum in the tank/s
- blocked filters within the pumps, blocked pick-ups
- dashpots not lifting, usually from the vent hole being blocked...

Hi,

Thanks

map1275 wrote:IF the problem is fuel;
- creating a vacuum in the tank/s

I will try running without a cap and see if there's a difference.

map1275 wrote:- blocked filters within the pumps, blocked pick-ups

The in-pump filters are ok. Just checked them. Not sure about the pickups. I'll have to pull one out to have a look.

map1275 wrote:- dashpots not lifting, usually from the vent hole being blocked...

I'll get them a wipe, but if it were that, then both tanks would display the same behaviour?

Could you possibly have a loose line / loose fitting - that sucks air (more air under more demand / heavy throttle)? This could effect one line (tank) more than the other line (tank)...???

I had a problem like this on my Edelbrock and it was solved by adjusting the float height / needle valve adjustment. I know that the side draft carbs are completely different than the Edelbrock but I offer that tid bit up anyway.

Thanks. I did wonder if the problem might be related to the needle set up. I have been on to a couple of very experienced mechanics and they have both agreed these are fine. I figure too that if it were the carbs, then I would see problems on the vacuum gauge and that is very steady.

I pulled off the worst one last weekend (tank to pump, which is where you'd expect the see an air leak). The fittings look good (clean and smooth around the ferules, etc.) and did not seem to indicate an air leak although my test was less than scientific.

I am wondering if the problem might be the flow rate of the pumps. The pressure is obviously fine.

Check the filters underneath by the pumps. One of mine was full of crap

Did that. No problem there.

I haven't swapped sides but I doubt that would change the situation.

I am thinking it is vapour lock. So how to sort?