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MG MGB Technical - Petrol from VentTube (80 Roadster)

I posted recently about a problem with hot running. I finally got a chance to tinker today and found petrol dripping from the vent tube of one of the carbs whilst not running. I assume that this would point to a problem with the float either not shutting the valve or possibly stuck open. It is not pouring out (steady drip) but will obviously need to be resolved. Any thoughts on cause?
P M Gregory

Float valve not shutting off fully, or maybe the float set too high such that normal heat-soak expansion when switched off is overflowing, which will cause a rich mixture particularly on hot starts.

The former can be ascertained by turning on the ignition when cold (disconnect the coil to stop that overheating) and listening to the pump. With the engine stopped it shouldn't tick more often than once every 30 secs. If it ticks more often it could be a float valve leaking, in which case its carb will eventually overflow from the vent pipe, or it could be the pump inlet valve leaking back in which case it won't overflow.

If it doesn't overflow, and it could take quite a few minutes, then it is more likely to be incorrect float height. HS carbs are easy, turn the ignition on until the pump stops clicking rapidly, turn off again, and remove the lids and check the relative fuel levels. If one is high check and adjustthe float height by inverting the lid. HIFs have to be removed and all you can do is measure/adjust the float height inverted. You will also need new bottom seals for HIFs.
Paul Hunt 2010

The HIF is reknowned for this. I had it for years and it drove me mad, grose jets tried, floats, set and reset by myself and a specialist classic MG garage never cured it. In the end I bought a pair of second hand HS4s with no trouble since.
I've got some thoughts on the problem. If the car has a head where it needs Lead replacement additive, and it is driven only occasnally. The petol in the bowl evaporates but not the additive thus gumming up the float needle.
c cummins

One of my HIFs also did it several times, and through a couple of new float valves, until I realised there was fuel in the dark-brown float. Since that was dhanged I haven't had a problem with them.
Paul Hunt 2010

Right I thought, nice day, got the gasket kit, I'll make a start. On the 2nd (rear) carb the overrun valve on the throttle disc has been glued shut, not with common or garden glue but what looks like the old 'solder in a tube' stuff. I assume this isn't standard! Was this a common bodge to resolve a known problem or someone at the end of their tether. Can I assume that if I free it, clean it up and set up the carbureetors according to Haynes/SU I won't introduce another problem further down the line? I'll attach a photo for reference later!
P M Gregory

Leave it how it is, the poppet valves have a tendency to stick open and cause a racing engine. It doesn't affect the setting up, just give slightly higher emissions on the overrun, which isn't picked up on the MOT.
Paul Hunt 2010

As i just found out! I went ahead and desoldered it (yes I know!) put them back and now can't get carb 2 to stop running fast! Back apart to resolder it now, should I do both of them so that conditions are the same for both units?
P M Gregory

If you mean you unsoldered both, then yes resolder both.
Paul Hunt 2010

Just to finish this thread...All work now complete, finally balanced and set the mixture. Amazing! a car that idles at 8-900 (was 1500 & lumpy) It pulls well in all gears and doesn't run on as much either. Many thanks to all for the help & advice.
P M Gregory

This thread was discussed between 17/04/2010 and 04/05/2010

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