QuestionQUESTION: Howdy. I have a 1975 MGB with dual SU carbs and electronic ignition. I am not a mechanic. If the car sits for a few days it will not start. I will grind and grind and finally, it will hit and try to start. It trys to run but sounds like it is not on all 4 cylinders. If I keep the gas peddle down all the way, it will run but even makes the doors shake. In about 2 minutes, it will start to get more power and all of a sudden, it runs fine. It will then be fine and will start right up.
ANSWER: Hi Richard. I think you are right. When starting from cold, the engine is not firing on all 4 cylinders, but after a while it gradually clears and runs fine.
I note that you have electronic ignition. When correctly installed, a good electronic ignition system should be able to produce a spark even if the spark plugs are fouled up, wrongly gapped, or worn out.
I have several suggestions for you:
It could be a worn engine. You need to get the compressions checked. The actual readings don't matter, so long as all 4 cylinders are within about 10 psi of each other.
I am concerned that the valve guides could be worn, so excess oil is getting into the cylinders and fouling the spark plugs. However, there are a number of things you can try before taking the head off.
Check the wires to the + terminal of the coil. If the wire is plain white, then that is OK. If the wire is white with a green stripe, then possibly the electronic ignition has been wired wrongly, because your car has a ballast resistor inside the wiring loom.
Check the letters and numbers on the spark plugs that you are you using. If they have a letter R in them, then buy a new set without an R. Also, if they are a number 5 then change to a 6, and if they are a number 6, then change to a 7.
For example, if you have Bosch WR6DC, replace them with W7DC.
Fit the new set of plugs, and my guess is the engine will immediately fire up and run cleanly. If the problem does not come back, then your problem was with the plugs. If the problem does come back a few days later, then there is a problem with the engine that is causing the plugs to foul up.
Get back to me when you have tried this, and could you please tell me the make and type of spark plugs, and the make and type of electronic ignition that you are using.
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QUESTION: Thanks for your reply. Compression OK. Electronic ignition installed 3 years ago by engineer. I have removed a BOSCH GERMANY PLATINUM WR7DP Plug which looks faily clean. Should I replace with W8DP ??????
Richard
AnswerThe symptoms that you describe are typical of an engine with fouled-up spark plugs. I normally recommend a 7 to start with, changing to a 6 if you get running-on, or to an 8 if you are experiencing fouling-up. However, you say the plug looks clean. Please check all 4 plugs, because it only takes one fouled-up plug to give these symptoms.
As regards the WR7DP, these are very good but expensive plugs. The P means they have Platinum tips. I never use R rated plugs in an MGB because the car has resistive plug leads (wires), so resistive plugs plus resistive wires makes for a weak spark. If all four plugs are clean, then I would change them for W7DP. If one is fouled up, then that cylinder probably has a worn-out valve guide.
Remember to check the wire colours on the + terminal, even a professional engineer can make mistakes.
