Power drop when under load/accelerate

[Gerald]

I have a V8, ex-military engine (retrieved from Land Rover ex Gulf War), 20k miles, in my Land Rover. Retrofitted an Edlebrock carb. Worked fine, then started running poorly. Fault traced to bronze gunk clogging fuel filters (dervied from the reaction of modern fuel with the miltary bronze filter in the fuel tank). Cleaned out. Ran ok. Idles perfectly but under any load and it has not get-up-and-go. Changed fuel pump which helped (Ex Rimmer bros. Fuel Pump Electric V8 and 2.6; PRC3901) and fitted large capacity 50mm Fuel Filter 100 Micron fuel filter pre-pump (tank side) and fuel pressure regulator (set a 7psi) before carb. No improvement. Is it fuel starvation? Or ... vacuum loss resulting in no advance under load?
What test steps could I try? Anyone near Brighton (Hurstpierpoint) who can assist?

[GDCobra]

Can't really help much as carb's are not my thing but what I can comment on is:

Or ... vacuum loss resulting in no advance under load?
What test steps could I try? Anyone near Brighton (Hurstpierpoint) who can assist?

You don't get advance under load you get advance under no load. When the throttle is closed the, post throttle plate, inlet tract will be under vacuum and can advance the spark. When you open the thottle the inlet pressure will rise towards atmospheric and advance will reduce.
One caveat to this is that most systems now connect the ignition's vacuum advance to a port which is closed when throttle is completely closed so you only get advance once the throttle starts to open (something to do with emissions I believe).

You could check this by disconnecting and re-connecting the vacuum feed to the distributor while pointing a timing light at the front pulley and seeing if timing advance changes. If this doesn't work at idle due to the closed port issue I mentiond above you could open the throttle a bit (will have to be constant) to perform the test. You could also test the vacuum advance using a vacuum pump, possibly even a large syringe (no toolbox is complete without one). You can either check if the timing alters as above or just look for movement in the distributor with the cap off.

Personally I'd think if you've recently been tweaking/changing things in the fuel system there is much more chance that the problem is located there such as delivery flow, it's easy to have enough pressure to feed the engines needs at idle but when more fuel is demanded the flow can't keep up.

[scudderfish]

Personally I'd think if you've recently been tweaking/changing things in the fuel system there is much more chance that the problem is located there such as delivery flow, it's easy to have enough pressure to feed the engines needs at idle but when more fuel is demanded the flow can't keep up.

Wouldn't the float bowls keep it running well until the pump weakness showed up? I don't know carbs so this is idle speculation.

[GDCobra]

Personally I'd think if you've recently been tweaking/changing things in the fuel system there is much more chance that the problem is located there such as delivery flow, it's easy to have enough pressure to feed the engines needs at idle but when more fuel is demanded the flow can't keep up.

Wouldn't the float bowls keep it running well until the pump weakness showed up? I don't know carbs so this is idle speculation.

Well as I say I'm not too au-fait with carb's, many years since I had anything to do with them and never liked them then!
However my thining is that if the flow out of the float bowl is higher than the flow in then the level must drop and when it does the mixture will lean out and lose power.

Bear in mind that the delivery pressure has no (great) influence on the internals of the carb' that only serves to get the fuel to the float bowl, none of this pressure is 'seen' inside the carb'. There may be a difference in float height as the force required to close the inlet valve may vary with pressure but this shouldn't come into play if there is sufficient flow.

[Ian Anderson]

Have you blown through the fuel lines? Could the bronze junk have settled in and be part blocking the fuel line.

These pumps like to push fuel, not suck it so is the pump near the tank (good) or in the engine(not as good)

Is the pump mounted low /at chassis level? Yes…good no not as good as not liking to suck uphill

Most of those pumps have an inbuilt filter on inlet side….has this become clogged?

Could the new pump be marginal at best so it copes in normal running but cannot keep up when asked to bring more fuel in the under load condition?

You could check and see how long it takes to pump a litre into a container and then check you are getting sufficient gallons per hour flow…..although a Google did not say what flow they are rated at. A Facit silver top is 26 us gallons / 30 U.K. gallons per hour so you should get close to this.

Ian

[ChrisJC]

I would stick on a temporary fuel pressure gauge and see if the pressure remains OK.

Chris.

[Gerald]

Thanks for your comments.
Pump is near tank.
Pump filter clean.
Fuel lines blown through.
Will recheck and determine pressure.
Will check vac. as suggested.
See how we go.
Thanks again.