Mechanical fuel pump?

[Greeks]

Hi

I've recently adjusted the curve on my 123 tune distributor and seen a reasonably good improvement in both performance and economy. Nice.

However, on two occasions now, after pushing about 5500 rpm in 3rd gear, the old thing gets a bit wheezy the revs die and it takes about 30 seconds before I get full power back.

Scratching my head a bit. It's almost like hitting a rev limiter, but it feels like a fuel starvation issue. I'm still running an AC mech pump, no signs of any leaks.

Is it possible i'm needing more fuel than the pump can manage? Or am i missing something simple?

One other thing. When I manually operate the pump there always seems to be resistance, but then afterwards there's a noise. Could that be a valve failure?

[TedTaylor]

Greeks wrote:

One other thing. When I manually operate the pump there always seems to be resistance, but then afterwards there's a noise. Could that be a valve failure?

That is as it should be.  Resistance as the line to the pump pressurises than when float(s) full pressure release takes over and you get that funny noise and no more resistance.

MUT

[bxbodger]

Cheapest test first- before changing pumps, etc, just make sure your fuel cap is breathing properly....at 5500 rpm you'll be pulling a lot of fuel out of the tank, you may have a vacuum sucking it backwards. Next time it does it, pull over quickly and whip the cap off and see how much air is gulped in.

[Greeks]

Did it again today on a steep hill at lower rpm...also noticed the pump keeps resistance regardless of how many times it's pumped as long as I pause a second between each stroke. The pre-carb filter isnt filling completely.
Couldn't pull over to check vacuum where I was though I thought there was a separate breather on the tank.

[nang]

Go for a drive with the fuel cap off and see what happens. I'd have thought that with 1/4" fuel lines you should be able to fuel a V8. How about carb float valves ?
Tony.

[Greeks]

Yeah good thinking, will try that tomorrow. Tanks under half full now so less likely to lose any fuel.

I'm also gonna tweak my curve again to flatten it out above 3500 and see. Though I had a blast in 1st gear and was ok at 5500, so I don't think it's ignition.

Will have a look at the floats after that if it's still not playing ball.

[Greeks]

Fiddled around a bit today.

Can get some serious wheel spin with the latest curves... but in second and third WOT eventually lose power for a while.

Tried removing fuel cap. Made no difference.

Lifted the bonnet when I got home and the fuel pump was near empty but starting to fill under gravity, and the glass fuel filter up the line was empty.

Next stop carb floats.

[nang]

Wonder if something is floating around in the tank and partially blocking the fuel outlet? Had this once with a bus, after trying every thing took the tank out and was full of rubbish.
Tony.

[Nick Moore]

I wonder if there isn't some debris in the fuel line partially blocking the flow? If you blow compressed air back down it, you might push the blockage back out. Only a temporary fix, as the crud will soon get sucked up again, but worth a try as a fault finder.

[DougBGT6]

I, unknowingly, had a fibre washer fall into my tank. When I put my foot down there was a greater flow of fuel out the tank and the washer got sucked over the outlet, fuel starvation, engine died. Left it for a bit, the washer floated off, engine started. This happened a dozen times until I finally found the washer and hooked it out with a wire coat hanger.

I have also suffered with crud in the fuel line which I sorted by dyno rodding the pipes with plastic strimmer cord.

[Greeks]

Thanks guys a blockage seems plausible. Although weird it seems to occur at the same spot.
However yesterday it happened on  cue on the short drive to the station.  But didn't on the way home and I sat at 4k. Nothing this morning either.
If it comes back again I'll look closer.

[RobPearce]

I had a similar problem with the Toledo shortly after I bought it. The solution was simple - take the top off the fuel pump and clean the gauze filter.

[Greeks]

Already done that Rob.

[CRAJ]

I had similar with the pi, until one day it wouldn't clear.
I took the tank out and found the remains of a fuel can rubber seal stuck in the outlet pipe that bolts into the bottom of the tank. It must have been fine until the level in the tank got low or demand got high then it got sucked back in. Needed twezers to pull it out, the Lucas pump had done it's best to pull it through the line.

Colin.

[Nick Jones]

Tank outlet pipe is a bit small to begin with.  Add 35 years accumulation of fossilised hydrocarbon crud (which goes weird when introduced to ethanol) and you really don't need much more to restrict flow to the point where it is inadequate.

Also possible that one of more of the rubber hoses in the system is breaking up internally - they don't much like the ethanol either.  Replace with "SAE J30 R9" coded stuff.

Nick

[Greeks]

When I cleaned out the fuel pump gauze it wasn't too bad, really. The rear carb feed was a bit worse for wear so I replaced that. No ethanol fuel has been used since 2008 - they have to tell us here so it's easy to avoid.

Anyway, i've had another trip where it ripped up the road like a scalded cat, so fingers crossed it was a blockage that's cleared itself for now  :-/

[trumpy]

That's good to hear Greeks. A blockage would be the obvious answer if the fuel pump is in otherwise good nick. I have the AC mech pump, and it seems to have no trouble feeding 3 SU's at all sorts of revs. Its a rising rate pump being driven from the cam, and is up to the job, as far as I have found!