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Hi all. New member here with a '77 1.8 saloon with twin MGB carbs. Problem is running on. Have set the carbs with a colourtune and done the timing with a strobe. Engine runs sweet. I removed the NGK BP7ES plugs and fitted Champion N9YC's. Running on seems worse. What's the cure?

I remember years ago my uncle had various 1.3's all of which ran on a wee bit occasionally but nothing like this. Any suggestions appreciated.

Cheers,

Derek.

Hi and welcome!

My '76 coupe used to suffer with run on quite bad in all the time I drove it. I ended up putting it in 4th and bringing to clutch up to stall it! Last year I stripped down the head and found a lot of carbon on the valves, which from what I was told stays hot and kind of 'burns' the fuel and keeps the engine going without the spark, hence the run on. Obviously there's no way to check for you without stripping it all down but that's what I've found after I'd checked everything else.

I've just rebuild said engine in the car and had it running at normal temperature for five minutes and it didn't run on when I switched it off, but that's not to say it won't, but it seems promising so far.

Hope you get it sorted though

Alex

Cheers. May have to go down that road but the engine was (allegedly) rebuilt not long ago. You never know 'til you strip things.

The B series engine was known to ‘run on’ in normal use, so much so that the MGB and I think MK 2 Marina had a anti-run on valve which opened when the ignition was turned off, which allowed excess air into the inlet manifold, which in turn diluted/leaned off the air/fuel mixture so much that it wouldn’t fire and the engine would stop.

Hi and welcome.

First off I would check your ignition timing. Is it too far advanced? Is the engine high compression or low compression? If you are running unleaded without an additive it may need retarding. What fuel are you using? standard unleaded or super? with or without an additive?

Ben

The main reasons, for the cause of running on are. Retarded Ignition timing, Carbon in the cylinder head. High engine temperature. High idle speed, Air leaks in the inlet tract. The Actual cause is the hot carbon on the valves igniting the fuel without any spark.
The MGB engine, was always a bit prone to running on made worse on later rubber bumper cars, due to the inclusion of over run valves in the throttle discs.
I would check the Ignition timing running as much advance as possible but not enough to cause pinking. Ensure engine is running at correct temperature not hot is important. If carbs have overrun valves solder them shut or replace with solid throttle discs. Adjust valve clearances. Balance carbs. setting the idle speed as low as possible with even mixtures preferably, slightly rich this reduces combustion temperatures, I would also run one grade hotter plugs than standard again this will reduce combustion temperatures. Check all hoses on breather system for Air leaks and manifold carb gaskets spraying around all joints with carb cleaner with engine running is a good way of finding air leaks as engine will pick up and the carb cleaner is sucked in though the leak. If car has a lot of carbon a good way of clearing some out, without stripping the engine is give it a good run keeping the revs up, ie running on motorway in 3rd gear. If all the above fails you have two options. Fit a Air solenoid in the brake servo line that opens with ignition off. This removes the depression from inlet manifold so fuel cant be sucked in. ( These valves were fitted to very late MGB MG Metro and some Crossflow Fords of the era) After that a lot of carbs had a fuel cut off valve fitted in the carbs as standard by the mid 1980's to prevent the problem. The big solution is to remove and decoke the cylinder head. Hardened valve seats will be required as all previous lead memory will be lost.
Dave

Thanks all. The carbs are balanced and the mixture and the timing spot on. I think maybe the N9YC plugs are maybe too cold. What is the next hotter grade up? I may put the NGK BP7ES back in. Running on wasn't so bad with them. If problem continues, I'll take the head off.

Derek.

NGK's back in, long hot run, hardly running on at all now. Result.