QuestionQUESTION: Howard,
I have a 1973 MGB, GHN5UD310054. The engine tag 18V884AEL005532, which I believe represents an engine for a 1977. The cylinder head has a stamp CAN1106 which would represent post 1974. The motor has SU HIF4's and a belt driven fan. Should I assume the motor is actually as the stamp says or is there another way of identifying it?
Unfortunately this will be a multiple part question. I acquired the car last August and rewired it over the winter. The rewiring finished three months ago (everything worked). The car ran really well up until about a week ago. It started missing slightly when ran a constant rpm of approximately 2,000. I pulled the plugs and the front two indicated a slightly rich mixture. I leaned the front carb slightly and checked by lifting the piston looking for the rpm change. It appeared to be fine. Then suddenly (no warning) it began to really miss and backfire out the exhaust. I checked the static timing and it was approximately 8 degrees before TDC. I then put an advance adjustable timing light on the motor. At 1500 rpm's it was showing 15 degrees of advance. The light was not truly stable maybe moving off a degree or two and occasionally jumped way off to where it looked like to me around 35 degrees before TDC. The light would only flash once there and then return to 15 degrees. This jump occurring every 10 to 15 secs apart. The car will start and idle but will not tolerate being placed under any load without backing firing. I checked the coil and the resistance was 3.5 ohms between the primaries. The wires were all tight except for the one between the distributor and coil. It was attached but showed no resistance to being pulled off the coil. Could that cause this much of a problem?
I then borrowed a compression tester and tested the cylinders. The results were 150, 130, 150 and 90 on number 4. Unfortunately I have no numbers for before the engine stated acting up. Could this cause the missing and backfiring? I pulled the valve cover and couldn't find anything bent or broken. Is my next step to do a leakdown test, pull the head, get new a new wire set or perhaps a new coil? Just sign me lost in South Carolina.
ANSWER: Hi John,
Moss Motors info agrees with your year of car # and engine # Except neither the car nor the engine came with SUs in the US but could be from the UK or Canada.
As for a loose wire between the coil and distributor, "Yes" that could be a problem so you need to crimp the spades to make them a tight fit and try it again.
However, the 90 PSI compression test on #4 is a "Dead" cylinder. The fact that you had back fires into the intake can mean an intake valve problem but first check the valve clearance on #4s valves. Then run a second test and if you still get a low reading put about 4 or 5 squirts of oil in the plug hole and run a "Wet" test. If the compression don't come up very much it is a valve. If it comes up a lot you may have a ring problem.
Howard
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QUESTION: Howard,
I did a wet test. Cylinder 4 went from 90 dry to 135 wet. Cylinder one,for reference, went from 150 to 200. As the gain for cylinder four was 45 lbs and the gain for for cylinder one was 50 lbs is it possible to draw the inference that the piston rings are in similar conditions in both cylinders and that it is a valve problem? The engine has never used any oil. Which make me question the bad rings possibility.
AnswerHi John,
Generally, a 10% to 15% raise in a compression test is an indication of a ring problem. However the reading of 200 PSI is very unusual which tells me you may have put too much oil in the cylinder. The only way you could get a 200 PSI reading is if the combustion chamber is full of carbon, the engine is modified with very high compression pistons or you used too much oil.
Put some mineral spirits or something like WD-40 in the cylinders and spin the engine to get some of the oil out and run the test again (throttle open). My guess is that you just have a valve problem and too much oil was used. I have done compression tests on hundreds of MGBs and never seen 200 PSI.
Howard
