My lovely MGA roadster, pictured below, has been running rough at idle for a couple of months now. The engine seemed to smooth out at higher rpm’s, so in my inevitable optimistic way, I did nothing. Err, that is, until several weeks ago when, on a short trip from shop to house, the engine suddenly developed a significant miss. Clearly from the sound, the engine was just running on three cylinders.
I got home, and started troubleshooting the issue. Of course, the first suspect might have been a fouled spark plug, and, sure enough, I did find most all the plug electrodes were pretty heavily coated with deposits. So, off to the local parts store for a new set of plugs, which I was sure would solve the problem. Unfortunately, fter installing the new plugs, there was no discernible improvement in operation, and the engine was clearly not running on one cylinder. By removing one spark plug at a time, it was soon obvious that the #4, or rear cylinder, was the culprit.
The next suspected cause for my problem was a too-tight tappet clearance. So, off came the valve cover (it’s held on by just two bolts.. thank you, British engineers!). Sure enough, the #4 exhaust valve clearance was nil, in fact it looked as though that valve was being held partially open at all times, which would surely have contributed to the symptoms I was experiencing. After a quick adjustment to specs (ya gotta love those non-overhead cam, quick adjusting rocker arms!), the valve cover went back on for what I hoped would be the final test. Alas, even with the correct valve adjustment, again there was no improvement in performance.
Finally, I resorted to a compression test to check on the health of that rear cylinder. As I had feared, while compression in the front 3 cylinders was healthy, that rear cylinder registered pretty much zip on the compression gauge.
At this point, there was nothing left to do but remove the head and assess the situation inside the combustion chamber. This little BMC engine is so easy to work on, and small enough that there is always plenty of room to access every component. Removing the head is about an hour’s job: Disconnect the radiator and heater hoses, along with the oil pressure and vacuum lines, remove the intake and exhaust manifolds via 5 nuts and just move the whole assembly out of the way; remove the rocker tower, then loosen and remove all the head stud nuts working from either end to center. With that, that little lump of cast iron is ready to be removed.
It took about 30 seconds to discover the problem once the head was off! A hole in the aft cylinder’s exhaust valve, about the diameter of a cigarette, was the obvious culprit. Exhaust valves depend on that brief moment in each combustion cycle of being solidly seated against the valve seat to dissipate heat. When the valve is held permanent away from the seat, as appeared in this case, eventually the valve face heats up to the point of failure, and you get this result:
I blame myself for not checking my valve clearances more frequently, however, Jeff Winn from Texas Engine Machine in Llano pointed out how far that valve had receded into its seat compared to the next cylinder over, and explained that I had been a victim of our modern unleaded fuels, which contributes to premature valve seat wear. It seems that in any case, even if I had been more diligent with preventative maintenance, that head would have had to come off so Jeff and his team could do their magic, installing new hardened seats, valves, springs, guides and seals on all cylinders.
It seemed like just a few days after dropping off the head that I got a call from the shop saying my head was ready to go. I was amazed at how reasonable the price was.. they performed a complete valve job as described above, and resurfaced both lower and upper surfaces of the head. With all new parts and the machine work, the total came to just a hair over $600. About the cost of a single valve for a Ferrari! That’s why I love these little hard-working engines! And, with a new model-correct paint job, the head looks just like new again:
Finally, having the head off gave me the perfect excuse to order that complete stainless steel exhaust system I’ve been oogling over for awhile:
Now my favorite driving car is ready to hit the backroads again.