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MG MGB Technical - Flooding engine

A funny question I know - but under what circumstances can you flood the 3br motor - I mean really flood it. (this is a hypothetical, but someone told me once that they had "hydraulic-ed"an engine with fuel)
If the SU carb floats are working properly, then the fuel should shut off and any excess should flow out the overflow pipes.
If the engine is not started, the needles should seal off any fuel wanting to get into the head unless there is a problem with the needle or the jet or some imputities in the fuel.
Could you fill the cylinders if you left the ign on with pump running, or will the fuel always have "bowl"pressure only trying to force it through the jet and not pump pressure?

John
John Minchin

John,
An engine may flood if the fuel/air/temp/spark don't produce ignition/combustion. Most commonly on start/restart on account of temperature or ignition/timing fault. It can occur with all i.c.e s.

The SUs/strombergs respond to vacuum and 'release' fuel vapour during each induction stroke - the amount depends on the amount of vacuum / the operation of the choke and not the carbs' shut-off valve (effectively a safety valve to prevent uncontrolled carb flooding).

If you leave the ignition on with the engine stopped and accelerator in its 'home' position - there is no vacuum and no demand. The shut-off valves close and the pump produces the fuel pressure to stop it. There is then an occasional 'tick' to maintain the pressure in anticipation of demand.

HTH

Roger
R Walker

Of course you can never flood an engine if everything is within its design parameters, a manufacturer wouldn't sell many otherwise. But outside those design parameters anything can happen.

As well as the 'failure to start' scenario cranking a cold engine with the choke out flooding the plugs, if a float chamber vent gets blocked with the float valve open then when you turn the ignition on the pump will pump fuel into the float chamber. But the float cannot rise to shut it off because of air pressure above the level of the fuel due to the blocked vent. The pump will continue to supply fuel but it will be pumped up the jet and into the cylinders. Leave it long enough and yes you will fill the combustion chamber.

The useful corollary of this is that by blowing into the vent with the air-cleaners removed and the piston raised, you should see fuel bubbling up the jet if the float chamber contains fuel and the jet and pipe are not blocked.

Allowing the float to rise is the real purpose of the vent, it's use as an overflow in the event of a stuck float valve is secondary.
PaulH Solihull

It's extremely difficult to create a hydraulic lock with gasoline. It is very thin and tends to dissipate quickly when exposed to air. It will also run down the cylinder walls between the piston rings and the wall itself. However, I had a customer ask me to look at the 4 cylinder engine in his boat because it would'nt crank over, even with 3 batteries connected to the starter motor. When I removed the spark plugs, water ran out of the cylinder head. The exhaust manifold has rusted through and allowed copious amounts of coolant to enter the intake. This had filled up all 4 of the cylinders and the engine could not be cranked over with the spark plugs installed. Upon replacing the exhaust manifold and cranking the engine over with the plugs removed, the engine started right up. This was after replacing the starter motor as the original had been melted by trying to get the engine to turn over with 3 batteries attacked to it. RAY
rjm RAY

That last story of your rjm ray, reminds me of a lady whom got confused about topping up the radiator. she topped up through the oil filler! That 'locked' up the engine alright! Mike
J.M. Doust

I bet she was wondering why the fluid level hadn't risen in the radiator. Just when you think you've seen everything, another "I can't believe it" comes along. Like the priest who changed a flat tire on his Cadilac. The car had wheels that were painted gold, on one side, to match the color of the car. On the back side, they were painted black. You guessed it. He mounted the wheel with the black side facing out. He was so proud that he changed the flat all by himself, I didn't have the heart to tell him he had mounted it backwards. RAY
rjm RAY

Thanks Guys
I shall check carefully that the needles/jets aren't leaking fuel when the ign is on (ie fuel pump pressurising) and motor not started. If not OK I will check for "sunken"floats in fuel bowls

John
John Minchin

With the ignition on but the engine stopped the pump shouldn't click more than once every 30 seconds - once it has done the initial charging of the float chambers. If it clicks more than that either the float valves are leaking, or the non-return valve on the pump inlet is leaking back. On cars with charcoal canisters it can take a very long time before any fuel is visible on the ground, so remove the hoses on the carb vent ports to check for overflow. The pump valve doesn't cause leaks, just reduces the maximum flow rate, but there is at least 100% over capacity on a 4-cylinder.
PaulH Solihull

Bugger

Once I fixed the fuel fountain in my engine bay (a split brand new fuel hose!), the fuel pump now clicks away happily and slows to about one click every 2-3 seconds.
So it is either leaking back through the pump, or not shutting off at the float valves. There is definitely fuel entering the carb throat - i can see and feel it when i lift the pistons. So I need to -
- check the overflow pipes are not blocked
- check the floats and float valves are working or not (eg sunken floats of dirty seats?)
Anything else I should check?

BTW I couldn't detect any fuel coming out the overflow pipes

John
John Minchin

If a vent/overflow port is blocked the pump will continue to click (but more often than that I would have thought). Fuel will be being pushed up the jet and into the engine in this case. It will also happen if the fuel and vent pipes have been reversed. Ordinarily the pump shouldn't click more than once every 30 secs. If it does, and there is no fuel flow from vent/overflow or jets, then the pump inlet valve is suspect.
PaulH Solihull

This thread was discussed between 23/10/2011 and 15/11/2011

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