Fuel Consumption Issue......

[222s]

For reasons that it is currently (& annoyingly) keeping to itself, my Amazon has very recently developed an enormous appetite for unleaded. I initially thought that the fuel gauge was on the blink until circumstances proved otherwise - less than 90 miles from a fully brimmed tank to being bone dry....

Also one of my colleagues has mentioned being able to smell petrol when the car is parked up. However there are no drip patches under the car, whereas in the past when I have had a fuel leak, it has been very obvious.

I have completely reset the mixture & balanced the carbs to eliminate that and checked the engine bay for fuel leaks (couldn't find any) but the fuel consumption is still horrendous. Aside from a small & therefore unobtrusive leak, all I can think of is that the fuel pump is leaking (into the sump...) but that doesn't explain why there is sometimes a smell of petrol.

I'm 99% sure that it isn't being siphoned out as I have a locking fuel cap & also because I've noticed the level shown on the fuel gauge dropping when I have been driving around.

Anyone have any suggestions as to what I should be checking & how to sort this out?

Thanks!

[TomTom]

Lets assume that it is leaking, but mostly when driving, and not when standing. Have you asked someone to look under the car while you rev the engine? It must be bubbling over or coming out somewhere.

The only other thing I can think of is that your choke isn't adjusted properly, but that normally wouldn't give you that bad consumption.

Tom

[Steve121]

I remember having the same symptoms once, which was fuel coming out of an overflow on one of the SU float chambers, caused by dirt in the needle valves holding them open, and this would not show up when the car is parked and engine off, and fuel would evaporate immediately on the hot exhaust when driving.. not very safe!!

[222s]

Originally Posted by TomTom

Lets assume that it is leaking, but mostly when driving, and not when standing. Have you asked someone to look under the car while you rev the engine? It must be bubbling over or coming out somewhere.

That's my next line of investigation, especially since it looks like a driving related issue.

Originally Posted by Steve121

I remember having the same symptoms once, which was fuel coming out of an overflow on one of the SU float chambers, caused by dirt in the needle valves holding them open, and this would not show up when the car is parked and engine off, and fuel would evaporate immediately on the hot exhaust when driving.. not very safe!!

I've checked everything under the bonnet when the engine has been running & it's not the float chambers. Had that one the other year so getting quite good at playing with them!!! Seems to be running very nicely (haven't tried it out on the motorway since the last adjustments though) after new plugs, dizzy cap etc & a carb tuneup / balance over the weekend. This had no effect on the fuel consumption though

[222s]

Been thinking about this some more & I vaguely recall that there is an electric thingy hidden right up under the rear seat pan inboard of the rear chassis rails. Would that be a secondary booster fuel pump?

[222s]

Originally Posted by 222s

Been thinking about this some more & I vaguely recall that there is an electric thingy hidden right up under the rear seat pan inboard of the rear chassis rails. Would that be a secondary booster fuel pump?

Just had a closer look earlier this evening & it's actually something to do with the rear brakes

[Pigeon]

Fuel pump diaphragm leaking. Does your oil smell of petrol? Though you seem to have lost a heck of a lot for it all to have drained into the sump and not been noticeable.

Or a rust hole in the top of the tank so it sloshes out when you're driving but nothing happens when stationary.

[devilishdesigner]

shouldnt have this much effect, but i had a scarey incident at the start of this year, was checkign all my fuel lines, and noticed that the metal shaft that the fuel hose clamps to going into the fuel pump had come loose from the pump and was just sitting in place!

[222s]

Originally Posted by Pigeon

Fuel pump diaphragm leaking. Does your oil smell of petrol? Though you seem to have lost a heck of a lot for it all to have drained into the sump and not been noticeable.

Or a rust hole in the top of the tank so it sloshes out when you're driving but nothing happens when stationary.

Nope, oil doesn't smell of petrol & nor is the level suspiciously high! (Must have been an entertaining sight yesterday evening for passers-by seeing me sniffing the dipstick.....)

Can't see any sign of tank leaks (would I expect to see anything less obvious than suspicious looking damp patches?), even when checking immediately after driving around.

As the only pressurised section of the line is in the engine bay, presumably any other line leaks would be apparent all the time (rather than just when the engine & therefore also fuel pump) is running - however I can't see any.

No sign anywhere in the engine bay of leaks from the fuel hoses either, even when the engine is being revved.

However it has now started running a bit rough & the fuel smell is still very obvious from outside (through the exhaust perhaps?) although not under the bonnet, so will check the carbs again, in case the issue is an internal one there. First stop will be the plugs, as the carbs pass the 'lift the float chambers' mixture test.

Thanks for all the advice so far everyone! However when it comes to what to check though, for this issue please feel free to assume that my mechanical knowledge is very little, as I suspect that there is probably something glaringly obvious that I have overlooked.....!!!

[austingipsy]

I think one of your carbs has a sticky plunger, maybe a rough spot on the inner face of the carb or the damper is making it hang up and your carb is drinking vast amounts of petrol because the needle's not sat at the right level......maybe, or perhaps your brakes are binding a bit, has there been a change in performance. If it's sounding a bit rough is it struggling to pick up as you drive away?
Keep at it mate, you'll run out possibilities eventually. As Sherlock Holmes would say " When you've eliminated the impossible, whatever remains, however improbable must be the truth"