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MG MGB Technical - Flooded engine - Do the plugs get wet?

Hi, all. I had the opportunity to help a friend of mine this weekend work on a '77B (not his) - standard everything except a manual choke on the 175CD. We found a torn diaphragm and replaced that, but it was still squirting fuel up the jet into the carb body. Long-story-short, we found a float absolutely full of fuel on one side, so ordered more parts.

Now, here's the question: the car would run for a few minutes until the carb got absolutely full of fuel, and gasoline began to run out the carb throat, then the car died and wouldn't restart. But, pulling the spark plugs NEVER showed any wetness, just fluffy black soot. Every other car I've ever worked on under similar conditions always showed wet plugs. Am I missing something here??? The plugs were warm, but not uncomfortable to hold barehanded.

Very confused - comments warmly solicited - - Alec

Alec Darnall

The fuel tried to burn, but the fuel mixture is too rich.
Kimberly

If the car does not fire then the fuel will be there wet and unburnt. If it has fired and choked then there will be a lot of carbon left. It may be tracking across this that stops the negine
Stan Best

Thanks - the car appears to die from the extremely rich mixture, since - if left to stand overnight - will fire back up the next morning with no problem, only to die again in a minute or so, with fuel pouring out of the carb.

My major problem is, "Where does the extra fuel go?". If into the cylinders, it would have to get splashed onto the plugs, then drain into the crankcase, but that doesn't seem to be the case. I'm wondering if it somehow gets sucked into one of the cannisters?

I was hoping someone had already puzzled this out - - Alec
Alec Darnall

Alec Because the engine fired up in the first place "most" of the excess fuel would be blown out the exhaust. Denis
DENIS H

The plugs will only be wet if they haven't fired, like flooding on startup where it hasn't fired yet. Excess fuel (not running) will run down the bores, washing off oil, causing rapid wear when the engine *is* run, diluting the oil, and an explosion risk. I don't know the carb you mention but normally a bad float or float valve should cause fuel to pour out of the overflow before the jets, but it does depend on the relative height of the vent and jet top. Where the vent is lower than the jet it only pours out of the jet if the vent is blocked, and it will do this even if the float and float valve are fine. If the vent is connected to a charcoal cannister an overflow will soak the charcoal first, then pour out of the canister, but it takes a while unlike where the vent has a pipe hanging down past the engine.
Paul Hunt 2

Thanks, Paul, Denis, and all - that's about what I had guessed. The Zenith-Stromberg they used over here for emissions reduction is not my favorite carb, but seems to run well once properly set up. It's similar in many ways to a SU type HIF, but just enough different to be exasperating.

This carb has two floats on a horseshoe-shaped yoke, and with one float sinking, fuel was actually squirting up the jet around the needle, as the electric pump pushed it past the float valve. I just couldn't account for where it was all going!

Sincere thanks for the help and suggestions. Looks like an oil change is a good idea, too - - Alec
Alec Darnall

Paul is right, except that on both SU and ZS, the vent is substantially above the jet. This is easy to see, as the correct fuel level is just below the jet top, and the vent is substantially above the fuel level. The key here is that the "vent" is a VENT, NOT an "overflow". The fact that it may sometimes ACT as an overflow doesn't change the design function. Any of these carbs will be dumping fuel through the jet if there is fuel coming out the VENT pipe; the amount of fuel from the jet will be small and not noticed as you run about trying to shut it off and find the fire extinguisher. If the car is not running and the mixture needle is all the way down, the needle blocks the jet so that there is excess fuel to overflow through the vent. If the piston and mixture needle are removed, most or all the fuel will come out of the jet.
Paul & Denis are right on the plugs - if the engine has been running, most of the fuel is burned, and any wetness on the plug is dried off by the hot plug before you get it out. You only find wet plugs in cylinders that have not been firing.
FRM
FR Millmore

Thanks, Fletcher, for the confirmation - I appreciate it - - Alec
Alec Darnall

This thread was discussed between 31/07/2006 and 01/08/2006

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