QuestionQUESTION: I've just finished re-building an old A-Series engine fitted to the MG1100. The engine runs when started but not too smoothly. I suspect cylinder 1 and cylinder 3 are not firing because when I detached their HT leads, either singly or even both, there is no noticeably decrease in rpm. Detaching the HT for either cylinder 1 or cylinder 4 does decrease the rpm. I have checked the HT cable, valve clearance, contact breaker set, ignition timing, carburetors (twin) but could not solve the problem. I have even removed the head again to check any blown gasket or burnt or leaking valves but everything seem to be all right. The camshaft is new and so are the followers. Where could I have erred? My next course of action would be to re-check the timing chain gear setting. Please assist.
ANSWER: Hi Yahaya,
If you are talking about the MG 1100 sedan, that engine was factory balanced and you should have put all four pistons in as a set if you changed pistons. It will not affect low to mid range running but will affect high RPM running. You have a different problem so you need to first run a "dry" and a "wet" compression test. You should see 125PSI to 170PSI with little difference between cylinders. Be sure to hold the throttle open when testing. Be sure to have the engine spin at least 4 or 5 revolutions on each cylinder. Run the "Dry" test first and write down the readings and put about 4 or 5 squirts of engine oil in the plug hole and run the test again for the "Wet" test. The wet test will show a higher reading but should not be more than about 10 to 15% higher. These are good tests for the engine itself.
Next remove the valve cover and rotate the engine slowly and watch each and every valve spring to see that every spring compressed about the same amount.
Your cylinder kill test is a good test of cylinder firing but you should also try what is called "jump spark" each of those cylinders that don't have an RPM drop. Just hold the plug wire about an 8th of an inch away from the plug while it is running and slowly move the plug wire away for the plug. At about 1/4 in the spark will quit jumping. The objective is to keep the spark jumping but as far as you can get it to keep sparking. This Raises the KV of the spark to a very high voltage and thus fires a cylinder that has a problem firing. (this can't correct a low compression problem)
If you can't get a 1/4 gap you need to look at the wire, cap and rotor and even check the point gap on each cylinder to check for a bent distributor shaft. Don't skip the simple things as experienced mechanics sometimes do, like is the firing order correct? 1,3,4,2. Try switching the plugs with the two cylinders that run ok.
Since the two cylinders that are failing are separated by #2 this is most likely not a carburetor problem but can still be an ignition problem.
Let me know the results of the compression tests.
Howard
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QUESTION: TQ Howard. I don't have the compression tester yet but I have followed your other suggestions and succeeded in coaxing the engine to a smooth running. By the way what type of compression tester (what brand/make) would your recommend?
AnswerHi Yahaya,
Compression gauges are not very expensive and can be used on any gas engine. I don't know what stores you have in Malaysia, but most any store that sells automotive type tools will have them. Here we would generally consider "Snap-on" tools as the top of the tool suppliers and "Harbor Freight" as one of the lower tool suppliers. A compression gauge is not used by a "DIY" (Do It Yourself)often so it would not be necessary to spend a lot of money on a expensive gauge. For example I was a professional mechanic for many years and have many "Snap-in" tools but I also have many tools I bought from "Harbor Freight".
Howard
