UK Car Repair: rough idling, coolant level, vacuum leak


Question
I have a 1977 MGB.It has been running good til recently it started idiling very rough. Itshakes so bad I can't hold a screwdriver on the idle adjustment. I have been moving my choke rich & lean lately due to the weather change. I changed plug wires, replaced plugs(Bosch), tried to adjust the idle,,al to no avail. I finally pulled each plug wire off while eng running and found when I pulled No.2 wire, there was no change in idiling. Could this be valve problem? THE CAR RUNS GREAT AT ANY SPEED ABOVE IDLE. I have good oil pressure (50plus),and good power. I have the original Stromberg 175 Carb.

Answer
Hi Charles,
The cylinder kill test you did may have found the porblem so you now need to do a compression test on all cylinders (throttle wide open)to see if #2 is low. If it is lower than the rest do a "wet" test by squirting 3 or 4 squirts of engine oil in that plug hole and quickly do another compression test. If it comes up to what the others were dry then it is a ring problem. If it don't come up with a wet test then it is a valve or head gasket. If the first compression test was OK, then I would look for a vacuum leak near #2 intake port. Keep in mind that that model "B" just about won't run at idle if the air filter is not in place correctly. If you determin it is a valve or gasket remove the valve cover and check the valve clearance on #2 valves. If you find #2 low on compression and the intake and the exhaust valve clearance is OK, then get a fitting to attach an air hose to that cylinder. The fitting is inexpensive and most auto parts stores have them. Put #2 piston at TDC with both valves closed (on the compression stroke)put the trans in high gear and brake on hard then apply air pressure to the cylinder. With the air connected listen at the tail pipe and at the intake with the filter out and throttle open. If either valve is leaking you will hear it hiss. At this time you can open the coolant filler cap and note any bubbles or riasing of the coolant level with the air applied. If you remove the oil cap you will hear a hiss in there but you would have to have listened to many to know what a good or bad hiss is in there as they all hiss there.
Good luck,
HMF