MG Car Repair/MGB engine turns nicley, but does not fire
QUESTION: I have a 1970 MGB (US Model) that turns normally but does not fire. I have begun trouble shooting and noted the following:
-I loosened the fuel hose entering the carbs into a cup and confirmed fuel is flowing (aftermarket square fuel pump continuously pumping once ignition is turned)
-While cranking I checked the spark plugs and all four are sparking; I removed and cleaned sparkplugs--only grayish to tan discolorations that the manual said were normal.
-I removed the distributor cap, discovered it has an electronic ignition module hooked up to a 12V non-ballast coil. The earth lead inside the distributor is stripped of insulation, taped it up (could this be the source of the problem?). I replaced cap, ensuring wires were in correct firing order for MGB. Cranked it, but not change observed.
-Do I need to check the timing or since it is not firing at all can that be eliminated?
-How can I further troubleshoot the ignition system? I tried assessing voltage flowing out of coil via Multimeter, but I'm not sure the best way to ensure accurate readings.
-Could another system be an issue--fuel air mix flowing from the carbs? If so, how do I approach the situation?
Thanks for your time.
ANSWER: Hi Tom.
You have started your troubleshooting correctly. For an engine to run it requires fuel, air and a spark. You have eliminated fuel and if you can see that the accelerator cable is still working, then you can eliminate air. That leaves the spark (or rather a lack of one).
Does the distributor have an internal module with two wires coming out of it or three? If it only has two wires then the internal bare wire that you taped up is a ground wire. It should provide a ground for the plate that the electronic ignition module is screwed onto. If the module does not have a good ground then it will not work and there will be no spark. It is a very flimsy wire and they often break.
If that doesn't solve your problem then you need to check for a spark at the plugs. PLEASE do not try to test the output of the coil with a volt meter. Most volt meters only measure up to 750 volts and with electronic ignition a coil can generate 40,000 volts which is enough to give you a nasty shock. You must use a stroboscope to test for a spark and to check the timing of the spark. With electronic ignition there is no other way to check the timing.
Let me know how you get on.
---------- FOLLOW-UP ----------
QUESTION: Thanks for your response. I decided to double check the fuel side of the house, removed the air filter and applied starter fluid to the carb throats. The engine started for a few seconds until the starter fluid was consumed and then abruptly cut off--no arrangment of throttle pedal or manual choke keeps engine running.
Does eliminate ignition system trouble? What next steps would you recommend either to address weak spark or carb issues?
Hi Tom. That alters everything. Sounds like the ignition side is OK, although I would still fit a new ground wire inside the distributor.
It sounds as though you have gas to the carbs, but there is no fuel mixture reaching the engine.
First off, remove the 3 screws holding the lid of the float chamber on the rear carb and lift the lid. Is the bowl full of gas? It should be. If not, then you have a problem, either with the cut-off valve in the lid or the feed pipe itself. There is a possibility that the feed pipe is blocked, so attach a rubber tube and blow through it. If the bottom of the float chamber is dirty, then you could have a build-up of sludge which is blocking the outlet pipe underneath the float chamber. Clean the sludge out with care, the plastic pipe to the jet is quite fragile.
I assume you have two carbs, so you could also check the front one. The lid of the rear carb has a through-flow arrangement so the rear carb feeds the front carb.
If you do not have an in-line fuel filter fitted to the fuel line, then I would advise you to cut the line and insert one. They only cost a few dollars and they are see-through, so you can see whether they are full of dirty fuel and they can be changed easily.
If both the carbs have fuel in the float chambers and they are clean, then get back to me with another question.