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MG TD TF 1500 - SU Vent/overflow down pipes

After driving the car and getting it nice and warmed up, what if anything should come out of the overflow/vent tubes after stopping the engine if everything is working correctly?

I ask this because I am getting an intense fuel smell in the garage after driving the car. I have checked everywhere for leaks and there are none. It seems to me as if fuel in the float chambers is evaporating out of these tubes.

Are they just open, so the float chambers just vent through them into the atmosphere or should they be closed off by the float? I have tried googleing this without much success.

Since the image shown I have replaced all the seals and washers in the fuel lines and replaced the bent copper line between the carbs with a correct mesh pipe. Also replaced the seals on the float chambers.

Thanks much

S Griffin

If any thing would come outofthe ovetflow tubed, it would be gasoline in liquid form, which would be noticable as drips.
Have you checked the fuel line from all the way back at the tank??
SPW Wincze

People have a hard time sealing the sending unit on the fuel tank. That leak can create quite a gas smell.

If your carbs are very over-rich, or floats set too high, you can dump enough fuel into your engine oil to make the oil in the sump, and your garage, smell like gasoline. Sniff the oil on your dipstick to check.

Evaporating fuel in the bowls is not going to make an "intense fuel smell" by venting out the overflow tubes.

Gasoline that has "gone bad" has a very strong and objectionable odor. Simple test for bad gas is to open the tank filler cap and see if the fuel smells like normal gasoline or something else. Most people can smell a few drops of "bad gas" from 50 feet away, while a few drops of good gas is hardly noticable 5 feet away.
D mckellar

What, people don't normally smell fuel in their garage? <grin> Actually the fuel tanks are vented to the atmosphere so some vapor will escape regardless.

With 3 MGs and 2 moderns in my garage, fuel smell is entirely normal. I have gas and vapor sensors in there and they never go off, so nothing is reaching dangerous levels, but my nose sure picks up on their presence.

The float bowl vent tubes are there in case the fuel inlet jams open or a float leaks and sinks. Unfortunately they are usually made redundant by the seals in the tickler pins having gone missing or perished, so they always leak first. But I doubt this is the issue here.
You may be able to smell fumes from the vent pipe that runs down from the tappet side cover. This is supposed to vent engine fumes to atmosphere and is meant to project into the airflow so that when the car is moving, gases are extracted. Maybe it is positioned wrongly and / or the engine is running very rich. Some fuel odour is inevitable with cars of this era.
Dave H
Dave Hill

This post opens a can of worms. The new Burlen SUs I purchased for my TF were supplied with no provision to fit overflow tubes.

I made some inquiries regarding the necessity to install overflow pipes and took the option to not fit them.

The limited mileage the vehicle has been driven since restoration has revealed no negatives in my decision to take this option.

G Evans

My experience is much the same.

If I drive for some distance and then take the car straight into the garage and switch off, the smell of fuel is very strong.

The secret is to let the car cool down before driving into the garage.

Investigations have shown two things, immediately after stopping the engine, the heat from the exhaust pipe causes the fuel in the bowl to boil and push some fuel out of the overflow pipe (mine are long and run to the front of the engine mounting plate, to avoid dripping on the exhaust pipe).

Secondly the seals around the jets are leaking needing replacement. The evidence for this is that on restarting, even after a very short run of the engine, and then a waiting period before re-starting, the SU pump gives a good few ticks showing the bowls need refilling.

Ian Bowers

Thank you everyone, some excellent lines of investigation for me to follow. I know it is not a fuel leak anywhere in the system, or bad gas as I have been driving a lot. It may be perfectly normal and I just have not become used to the smell. But it is intense after driving and lingers for 24 plus hours. Then fades away if the car is left alone. The garage mates are a 77 MGB and the wife's Chrysler Minivan! Time for a thorough tune up, which I have not done since the car arrived.
S Griffin

I see that you have a copper pipe joining the two carbs instead of a flexible hose. Could the union at one of the carbs be cross-threaded or not fully seated?

Engine heat could push fuel out of the union after shut down.


LM Cook

in his opening post, Simon says:

>>>>Since the image shown I have replaced all the seals and washers in the fuel lines and replaced the bent copper line between the carbs with a correct mesh pipe. Also replaced the seals on the float chambers<<<

SPW Wincze


Gotta start reading posts a little closer.

LM Cook

I don't have a fuel smell...and yes there is gas in the tank..heehee

Regards, Tom
tm peterson

I had a terrible fuel smell in my garage when I brought the TD home from Dad's. Turns out the fuel pump was seeping around where the diaphragm is. Sent it to Sanders for rebuild and now there is no fuel smell. Just something else to look at.

S Grimm

There is some confusion here, do you mean Giovanni Venturi or Bernoulli? In any case, I think the point is well made, there has to be ventilation for the float bowl to operate properly.
Dave H
Dave Hill

I ran the car yesterday and same deal, fuel smell much worse than I would expect. I aired everything out and my this morning all was clear. I just turned on the ignition with the hood up so I could see the carbs. As the fuel pump clicked away there was a sudden small squirt of fuel from the base of the carbs where the choke is attached. This stopped on its own as if the pressure sealed it, but there is a clear seepage of fuel from this area. A tiny bit of research suggests incorrectly set float and or a bad cork seal. Thoughts?

S Griffin

Very common problem. You have a choice of fitting new cork seals that have been properly soaked in oil before use, or fluoropolymer seals / O rings, or the new seal kit from Burlen / SU.
Dave H
Dave Hill

Thanks Dave, I have ordered the upgraded seal kits from SU in the UK. Seems like a good approach. Time for me to get more familiar with SU's. I have a pair of HIF's to go on the MGB to replace the current Weber and of course the Austin Healey 100 has a pair. It has probably been 30 years since I messed around tuning that car! The photo is of what it looks like now, so a while before it is back on the road, but at least it is heading that way!

S Griffin

A final update to close the loop. I arrived home from a trip to Europe to find the new seal kit from SU. I installed it today and can report that this has completely cured the problem with the fuel odor. The upside of this is that I also now have the carbs tuned much better. Thanks all for the suggestions and input.
S Griffin

This thread was discussed between 08/02/2017 and 06/03/2017

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