QuestionHave a Mk1 MGB that recently started missing badly at idle. Sounds like it's running on 2 cylinders. Will rev, but slowly and missing does not change. Also will occasionally backfire when it never did previously. Thought it was bad/leaky carbs, so I changed it out with another pair. No difference, although the backfiring is diminished. At the same time, I am now hearing a metallic clank in the area of the fan that was never there before. May not be related, obviously. Increases with engine speed. Plugs look good, jets are at proper height and carbs are balanced with oil in dashpots. Needles may not be centered though, could this do it? And could loose fan bolts be making the metallic noise?
AnswerHi Dave,
You may or may not have two different issues with the miss and the noise. There is a standard procedure to go through with a miss.
A miss in the engine is a common complaint that I would receive when I worked in MG dealerships and I found it necessary to follow a set procedure to diagnosis the problem.
It is no use looking for a specific problem thinking it may be this or it may be that. Testing is the only 100 sure method of diagnosis.
It is no harm looking for obvious faults like a plug wire off or such but further than that you are just wasting time.
First you must determine if the engine is good. (at least the combustion chamber area)
This is done by a compression test. 130 PSI to 180 PSI on all cylinders with little difference between cylinders. (throttle open during the tests)
If any cylinder is low adjust all the valves (cold and set to .012 not what the book says)
Retest the compression and if it is not up to par it is no use proceeding. A miss can not be corrected on a bad engine.
If it is good check the carbs by adjusting and balancing (which it sounds like you have already done)You didn't say that you checked for a manifold vacuum leak. There are several ways to do this. With the air filters off, spray a little starter fluid, Propane or even WD-40 into each carb while idling and note any RPM change or engine smoothing. If no positive results go to Ignition.
So look at ignition. I had that easy in the dealerships as I had access to a scope but most people don't have that equipment so you need to look at other methods.
The cylinder kill test is the first. The 66 MGB did not have plug covers so if yours is still original you can easily short each plug one at a time when idling to note RPM change. This tells you which cylinders are dead. Look closely with a strong light at the inside and outside of the distributor cap for carbon tracks. (thin black lines) Next (key on in neutral and brake on) plug a piece of plug wire into the coil and and hold the other end (with insulated pliers or such) about 1/8" from the center of the rotor button and spin the engine with the starter solenoid button. If you can jump a spark at all to the rotor, it is bad and must be replaced. Next (key off brake off gearbox in 4th gear roll the car and look at the contact points to see that you get a .015" gap opening on each cam lobe as it rotates.
If you have the original plug wires it is no use looking at them as they are multi strand copper wire. The ends are resistors and need to be either checked with an ohm meter or you can just unscrew them and switch to a known good firing cylinder to see if the same cylinder is dead. This NEEDS to be done with the plugs also as a good looking plug can be bad. (that very thing just caught me on my own car just this last year.)
DO NOT CHANGE THE SEQUENCE OF THESE TESTS NOR SKIP ANY ONE OF THEM NO MATTER WHAT YOU THINK IT IS OR IS NOT GOOD!!!!!
Let me know. Also read my "Tech Tips" on my web site
http://mg-tri-jag.net
Howard
