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MG MGB Technical - What is that strange hissing sound?

I have had my '69 "B" for about 5 years, and I am still having trouble with the engine cutting out.
After about 20 miles the engine will cut out and then restart again after about 15 minutes. I have replaced the carburettor heat shield, and tuned the engine.
The last several times, the car ran fine but then a flat spot appeared around 1000-1500rpm, getting gradually worse until the engine cut out. After several attempts at starting there was a load hiss and the engine started fine again. Can anyone shed any light on this problem. With any possible solutions

Thanks in advance
Chris Kendal

I'll start the ball rolling by suggesting maybe a spilt vacumn advance tube from distributor.

Im not completely sure, but mine has split twice in a couple of months and can be heard hissing at certain rpms.

This would also affect your running (as mine did), and would quite possilby contribute to the flat spot you experienced....

Not sure about the big hiss, but someone will know....

Cheers ~PHIL
Phil

You might try a new fuel filter....if you have not already.
Steve LaGoy

Wild guess here -- did you recently replace the fuel filler cap? I think there were vented and non-vented varieties, and as I recall a '69 (which is what I drive as well...) needs a vented one. Next time it cuts out, remove the fuel filler cap and see if you get an inrush of air -- and see if it will then start normally!
Rob Edwards

My money is on a clogged fuel tank vent.

As the fuel level in the tank drops, air is supposed to be allowed in to replace the displaced volume.
If not, a vacuum slowly developes in the tank as the fuel is pumped out. After a while, the fuel pump
cannot overcome this vacuum - and then the engine starves and conks out.

One clue to this is when you hear a "whoosh" when you release the fuel filler cap.

The "hiss" sound you heard before the car restarted could be air being sucked back into the tank
via the fuel bowls or a breather line from the charcoal canister. Once the pressure inside of the tank
returns to 0 (zero) vacuum, the hiss stops, the fuel pump can operate normally again and all returns
to normal for a short while, until this cycle begins again.

Trace back all vent hoses and steel lines coming from the fuel tank - all the way forward into the engine
bay. Look for any breaks or kinks, or brittle and crumbly rubber. Check the hoses leading to & from
the round metal separator bottle (inside the right rear fender, near the tail light) as well as all hoses
connected to the charcoal canister. Remove the charcoal canister, unscrew it's base, and check the
filter pad for cleanliness. Clean in warm soapy water if ladened with dirt.

If you have an early model car with a vented fuel filler cap, check this, too.
Daniel Wong

This thread was discussed on 01/08/2002

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