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MGB-GT Help with my 1967 MGB GT running very rough


Senior Member
Hello gentlemen,
I have sold my healey 100 and now own a beautiful GT. I am trying to diagnose why it it has all of a sudden one morning it started running rough.
1. very hard to start and to warm up
2. loss of power when driving, like running on 3 cylinders
3. took the air filters off and 'seems' to run a bit better
4. On revving the car and the air filters off - the front SU piston does not lift up as much as the back piston ( is a sign Ive blown the head gasket?)
5. seems to runs and rev more freely when warm, but still loss of power.

Do I have a carbie issue?running too rich? or have I possibly got a 'slightly' blown head gasket

Any help will be appreciated - thank you in advance


Great Pumpkin
Country flag
Carbs are not balanced when pistons do not move together.


Great Pumpkin
Country flag
Go to the dizzy first. Make certain all is in order there. Clean & gap points, set timing, etc. You may have a bad condensor, may be points crudded up. If it ran well then after being parked runs 'funny' on restart, the carbs would be last on the list of suspects.

Head gasket problems would manifest as exterior leaks, blowing clouds of white smoke out the tailpipe, wet (water) plug tips, oil and water mixing in crankcase (oil would look like chocolate mousse) or rdiator (water would show oil as a foamy light brown "fluffy" residue). A compression test or leak-down test can be helpful in diagnosing that.


Senior Member
Radiator coolant looks OK. Carbs are definitely out of synch and I am thinking there a probelem within the dizzy as suggested.
However, could it be possible that Ive blown the head gasket between the first and second cylinders?



Jedi Trainee
This is a long shot, but it's what happened to me. I had a massive vacuum leak occur when for some reason the sheetmetal plug on the front end of the intake manifold fell out (or got blown off due to a backfire). The car ran horribly, was very hard to start, and it took me forever to figure out the problem. The manifold is basically a long pipe with plugs in each end. If the plug has fallen out (you should be able to see the hole) you will have a huge vacuum leak that would definitely cause the front carburettor piston not to rise up as far. However, as I said, that is a long shot. Similar effects could result from a bad gulp valve (but I think your car is too early for that) or a loose manifold (check the nuts),.

If the above isn't the case, I would start off by checking the compression. If you have one bad cylinder then you might have a burned valve or similar. Two adjacent low cylinders might indicate a bad head gasket. If you have one, throw a vacuum gauge on there and see how badly it oscillates. If all cylinders test good and your vacuum is good then I'd start looking at carburettor or ignition problems (check dwell, timing, plug wires, plugs, cap, rotor). However, that "one low piston" issue sure makes it sound like a non-electrical problem.


Great Pumpkin
Country flag
Easy way to check for vacume leaks is spray a bit of carb cleaner around when engine is running, not in the carb though.
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