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MG MGB Technical - SU HIF Flooding

I read some past threads from 2008 about flooding in SU carbs. I have a 1980 MGB and removed the Zenith carb and replaced with 2 x SU HIF. The front carb is fine, but the rear floods constantly so I can't get the car to run properly. needles, seats float levels set, etc all checked and seem fine. Most of the emission control "stuff" has been removed. The two lines that come off the crankcase breather have been connected to the carbs at the angled connections and the overflow line to the charcoal canister.

I read on the SU website that the breather lines can cause flooding if blocked (Mine are not but there does seem to be more oil coming through than I would expect - is that normal?). Has anyone solved this or come across this? I'm really stuck as to what to do to fix the flooding. Could it be related to the removal of the emissions hoses except for those that I kept.
D SINCLAIR

Only when the apprentice connected the fuel delivery hose to the overflow/breather pipe on the carb and not the inlet pipe!
Paul Walbran

Well now. That's a thought but no, the apprentice didn't do that!
D SINCLAIR

Don - Dave -?
HIF's are well known for sinking floats-
Might pay to get him out and check, make sure there's no fuel in the float
willy
William Revit

The breather lines won't cause flooding if blocked but the overflow/vent lines can. A blockage stops the fuel level rising in the float chamber to lift the float and shut off the valve, the pump keeps pumping, and fuel comes up the jet into the intake manifold.

First step (if that is what is happening) is to take the hoses off the overflow/vent ports on the carbs. If it floods out of there then the float isn't rising and shutting off the float valve, for whatever reason.

You can get excessive oil in the crankcase breather if the front tappet chest oil separator/flame trap filter is clogged, or with excessive blow-by.
paulh4

Further to the good advice you have received so far, there is some scope for the choke mechanism to leak fuel after the knob is pushed shut.
The first and most basic check is to make sure that the choke control does actually close the choke completely.
The second check is to see whether the engine continues to run when the main inlet is fully closed. If the engine even struggles to run with one carb fully closed, you have a simple age-related O-ring problem in the choke mechanism. It's very easily sorted but it does involve removing the carb from the manifold and its twin. Then remember where the choke spring sits - remove the screws and withdraw the circular mechanism to see the offending O-ring. If the tube shows any signs of corrosion - clean it up and make a note to be careful about which fuel is most likely to be water-free.

Good luck
Roger
Roger Walker

Roger - thanks a million. You hit the nail on the head. Worn O ring on the rear carb. Fixed now and all good.
Also checked the other suggestions and did find that the breather was clogged so I washed that out while the carbs were off. But the primary cause seems to have been the worn O ring.
D SINCLAIR

This thread was discussed between 20/07/2022 and 13/08/2022

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