MG Car Repair: 74 1\2 MG rough, cylinder head gasket, rpms increase

Question
Barrie - reading with interest 69 MG rough running - my MG also has signs of mis-fire or no firing on #1 and #2 - when I plunge the rod on the pot on the rear carb #3 and 4 - the rpms increase, but not when I do same for front carb. Also pulling #1 and #2 wire does not cause engine to stumble. Plugs are new and gap is .035 I have good Spark on all cyl - tested jumping from screwdriver to engine block and its strong hot orange so can I assume a fuel problem or compression issue? I will check compression> is there easy diagnostic for carbs? Thanks in advance ! Paul

Answer
Hi Paul.  The fact that the problem relates to cylinders #1 and #2 usually means that either (a) you have a problem with the front carburetter or (b) the cylinder head gasket has blown between cylinders #1 and #2.  If you have a compression tester you can verify the head gasket without taking the head off.  If the compression is OK then it must be the carbs.

If you have the earlier HS4 type of carburetters, then take the lid off the front float chamber and check that the bowl is full of fuel.  

If you have the later HIF4 type with an integral float chamber then it is a bit more complicated.  The fuel supply goes to the front carb and then via a joining tube to the back one.  Since the front one is suspect but the back one is OK you know that there is fuel to the carbs, so the needle valve inside the front one is probably stuck shut.

Finally, if you don't have a fuel filter protecting the carb line, then fit one to stop it happening again.

By the way, if your distributor has points (not electronic ignition) then I would not use gaps as wide as .035 no matter what the book says.  I would close them up to .025 (0.6mm).