The Question
My 1973 MGB GT is in good mechanical condition having been fully serviced in the summer with the twin SU carbs being balanced/tuned last month. The battery is also in good condition with spark plugs, HT leads and distributor cap being replaced during this summer's service.
My problem is that despite all of the above, my MG is a very poor starter from cold. If I haven't used the vehicle for a couple weeks and the weather is cold or damp, my vehicle is a real pain to start.
My starting procedure is that I turn my ignition key once, listen for the rapid fuel pump 'clicks' until they slow and cease, pull the choke out fully, wait a further 10 seconds and then turn the ignition key fully (without touching the accelerator pedal) and attempt starting it.
When 'cold' it invariably just doesn't start. However, in fairness, the vehicle starts better in the summer months and also, once started it will start readily first time throughout that day - even after several hours of being turned off during that day.
Therefore, please could you advise/suggest what I could do to make my MG actually start whatever the time of year and whatever the weather.
The Answer
By far the most common issue behind a poor starting engine in cold conditions is that the mixture is not rich enough because the choke mechanism whilst apparently pulling out fully at the dashboard is not actually fully applying the choke at the carbs. Check this out by pulling the choke fully at the dash and leave it out, then go to the carb linkages and see whether the choke mechanism can be opened any more. If it can then the cable to linkage needs adjusting.
SU HS4 carbs arrow point to choke linkage. Note the air filter bolt is temp fitted to hold the choke cable anchor to allow choke operation.
A lesser possible issue that is usually associated with an engine that has wear is when being cranked, it isn't able to operate efficient enough air pumping when really cold, with the bore to ring clearances at their widest. This can then see the draw the engine creates being unable to lift sufficient fuel from the main jets to start the engine.
If you have a can of 'Easy Start' spray, then usually a quick spray into each air filter tube and then immediately trying to start with choke will often see an immediate start that can be a pointer to poor engine pumping capacity, or indeed too weak a mixture. If the engine doesn’t continue to run after it has fired up using Easy Start spray then do consider how old the fuel is in the tank as fuel can go off over a period of more than six months and some fresh fuel may be needed.
The engines pumping efficiency is easily checked by doing a compression test cold, and then later when the engine is fully warm. A significant difference between cold and hot, say 30psi, is indicating that there is possibly significant slack in the rings to bores when cold. Note that with the variation of compression tester calibration you can see some variation in readings when using different testers on the same engine at the same time. A very good engine will have all readings within 10psi for all cylinders and a serviceable engine sees all within 20psi. Once over 30psi range of readings indicates some wear of developing issues such as, a single cylinder much lower indicating possible valve or piston ring seal issues, and a wet compression test will indicate which one. Similarly low readings between cylinders next to each other may point to a failed head gasket.
Compression testing an MGB engine. 160psi here is a good reading.
Z009 - Gunson Hi Gauge Compression Tester
However, I am not expecting a worn engine from what you describe but more likely to be the choke operation or possibly the float heights. In respect of the floats, if the float heights are even, but also lower than the standard setting, this will mean that the fuel height in the main jets will also be lower and that demands more energy from the inflowing air through the carbs to lift that fuel out of the main jets.
Lastly the performance of the starter motor can impact as when the engine is colder as the oil is thicker and it takes more electrical energy for the starter to turn the engine; meaning the starter also draws more energy from the battery and often turns slower as a result, and importantly leaves less power for the ignition. This demands that the battery fitted has a sufficient reserve as well as being well charged. The MGB has a very heavy engine to turn over, a long cable run between battery and starter to promote a voltage drop, and a very power hungry starter. To do that when compared to a modern gear reduction type starter that has another benefit as it leaves more energy for the coil to deliver a stronger spark.
Insert image of GXE4441
Of course elements of all these which on their own will not create a problem can cumulatively create the non-start in cold conditions.
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Please Note: The information provided in this blog is intended as a general guide only. While we aim to ensure accuracy, classic vehicles can vary, and procedures may differ depending on model and condition. Always consult your vehicle’s official workshop manual before carrying out any work. If in doubt, seek the services of a qualified professional.
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