The Triumph Dolomite Club - Discussion Forum

GrahamFountain wrote: ↑Wed Jul 22, 2020 5:43 pm It may be worth a mention that a painter replaced the rubber round where the filler comes through the rear wing. Don't think he had the tank out.

It is not necessary to remove the tank but said individual may have removed the filler pipe.
The joiner hose is another suspect, they are not a tight fit and rely on wire hose clips to seal.




Ian.

So, as it's not raining, I emptied the boot and took the left side panel out, etc., dusted everything with talc, filled the tank again, and chucked it about a bit - speed bumps and everything.

Of course, there's not the hint of a leak or enough petrol to cover the parfum in the talc. Maybe I didn't over fill it as much, but I did put some effort into doing that.

I guess I just keep a nostril out, and remember who said they have a tank.

What would happen if the vent in the filler cap wasn't releasing internal pressure - it must be letting air in; the tank's not flat.

Graham

GrahamFountain wrote: ↑Fri Jul 24, 2020 3:34 pm
What would happen if the vent in the filler cap wasn't releasing internal pressure - it must be letting air in; the tank's not flat.

Graham

I can tell you this one! What happens is the car develops an allergy to motorway cruising! It's OK for a bit then gradually the vacuum created in the tank overcomes the ability of the fuel pump to suck fuel and the engine starves and dies. Leave it half an hour or so and some air will get back in (the cap seal is NEVER perfect) and the car will restart. This cycle will repeat till you get off the motorway and onto slower roads where the demand is lower.

This problem plagued one Sprint's owner to such an extent, defying several efforts to find it, that the owner left it in his garage for 13 years!

I only found it by accident myself, the owner had given it to me to recommission after it's long layup and related the problem to me in considerable detail, along with "cures" that had already been tried. (couple of new fuel pumps and filters and many sets of points, condensers, leads, rotors, dizzy caps and coils)

So I went through the remaining elements of the fuel system with a fine tooth comb, draining and cleaning the tank (not forgetting the pipe in the bottom) and replacing both hard and soft lines along the length of the car. (I checked all the electrics and replaced where necessary as well of course) So it fired up on a couple of cans of fuel , first running direct from the can, then from the tank. I left it idling whilst I went round to the petrol forecourt next door for more petrol and when I got back, (still running happily) I switched it off and immediately removed the aftermarket locking cap, to be greeted with the unmistakeable "pssssht" sound of vacuum releasing.

It turned out that the locking cap was for a MKI Triumph 2000 which isn't vented. It had been on the car since before the current owner took possession and may have been there from new, given the large number of previous owners for what is a tidy, all original steel and low mileage car! Locking petrol caps were a dealer fit accessory and somebody in the stores may have made a mistake! A new vented cap was fitted and it drives perfectly now!

Steve

The storeman (Rory) at the local BL dealer when I lived in York (don't remember the firm, but it were a big place on Lawrence St) sold me a US fuel cap for my TR7 (A TR7 16V the first). It was a little while before I realized that it was taking less and less fuel to fill the tank - and I was getting fewer and fewer miles out of a full tank, and, I just remembered, it got hard to get the cap off.

At this point, I looked under the car to see the pump had sucked the tank virtually flat!

Fortunately I still had the receipt, he accepted his error, and they got and fitted a new tank for free.

But what happens if it lets air in, but not fumes out?

Graham

GrahamFountain wrote: ↑Sat Jul 25, 2020 8:34 am
But what happens if it lets air in, but not fumes out?

Graham

I can't see how this could ever happen, it would need to somehow produce a one way valve effect. Since all the vent is, is a tiny hole, I can't envision it. If it lets air in, it WILL let fuel vapour out. The car I referred to above, showed no signs of pressurizing the tank, though that would require some very hot weather I guess. Also, when it came to me, the tank was bung full of fuel, which would have lessened the effect. It probably saved the tank from corrosion though, after I drained it, it was clean as a whistle inside.

Steve

I've seen a flake of rust form an intermittent and partial valve between the carbs on a TR7 (A TR7 16V the second). That was a difficult problem to find, but fixed with a coat hanger. I suspect that would have been one-way.

It fixed itself for a while whenever there was no pressure on it, then came back later. So, whenever I went to check the carbs were set right, they were. And when I checked fuel levels and flows, they seemed okay. And then it would drive okay, sometimes for days. But start giving it some right foot pressure, and the pipe would block and starve the front carb.

I guessed it must be fuel level when pulling the choke out when revs were needed would mostly fix it. But it still took a while to find and lots of taking apart and putting back together, new fuel pump, etc., which always made it go away for a while. When I worked out it was the just the front carb, and after taking that apart and putting it back together a couple of times, I finally rodded the line - it's not supposed to have moving parts, so how could it have an intermittent fault? When I did that, a load of crap came out, the problem went, and it stayed gone.

The rust had come from the tank, because that car (drop head with the small filler) had a locking fuel cap with the keyhole uncovered at bottom of a small well. If you think about where the cap on a TR7 is, every time it rained, at least that amount of water drained through the keyhole into the tank. Another problem that took far too long to work out the cause.

Graham

So, the leak appearing to have stopped, having filled up on Friday with no sign of leakage, there's petrol in the boot well again.

There's still no signs around the join in the filler or the breather. And I can't see anything round the outlet or sender.

I did take the horde of the things out for a meal in the wilds of Marton and there was some going over bumps and round bends four-up.
So I wonder if the leak is high up at the front or the back of the tank, where I can't see, and the sloshing about is how it's getting into the well under the tank? I think the only thing is to pull it out and look.

And if I'm doing that, I may as well have a spare ready to go in if needed, and sell it on if it's not. I'll lose at least the cost of transport, but that's got to be less than the labour cost of putting it back and taking it out again later when I do have one.

Graham