TRIUMPH STAG HOT STARTING PROBLEM
QI
have an automatic Triumph Stag, a 1975 car with the original engine and SU carbs. The fuel pump and starter motor have been upgraded, and electronic ignition has also been fitted. The car starts fine when cold, but when hot it refuses to start. Leave it for a few hours and it will then start. I understand that this is a fairly common problem, but no- one can tell me the common solution. Can you help?
Peter Ferguson A
There are two reasons that can cause starting problems when hot, and the first step is to determine which of these reasons is applicable – it will be either an ignition problem or a fuel problem.
To first eliminate the possibility of an ignition problem, the spark needs to be checked when the engine is hot and will not fire. The simplest method is to use a spare spark plug and then, disconnecting one of the HT leads and plugging it into the spare spark plug, that can be rested on a suitable earth point at the top of the engine and by spinning the engine on the key a spark should be seen at the plug. If there is a spark and it does appear to be a good blue one, then the ignition system can be discounted as the problem. If there is not a spark, then the possibility is that either the coil or electronic ignition unit is failing when hot. You would then need to determine which of these components is at fault, and it does depend on which electronic ignition system you have fitted.
On the basis that there is a good spark, the problem will be in the fuel system. This is often due to the fuel in the carbs evaporating when warm, but this will normally only delay the start and not prevent it totally as seems to be happening in your case. With this in mind, the other possibility is that either the float’s needle valve is getting jammed up when hot, or the carb is actually flooding the engine when hot, soaking the plugs and preventing the engine from firing. This can be checked by removing one or two of the plugs when the engine is hot to determine if they are wet or dry.
Thinking further afield, it has been noted by some owners that an uprated pump will supply a line pressure higher than the carburettor needle valves can cope with, and this can allow the carb to flood the engine when turned off. Another reason for this problem is that when the engine is hot and standing still, the heat from the engine will percolate up through the block and increase the temperature of the carbs. The higher temperature causes the fuel to evaporate, but with nowhere to go this finds itself condensing in the inlet manifold. Then when the engine is spun over, the fuel enters the cylinders and soaks the spark plugs, preventing the engine from starting.
If this is the cause of your problem, one solution used by a few owners is to put a fuel cut off switch in the circuit and turn off the fuel pump before turning off the engine. This lowers the level of fuel in the carbs and the pressure in the fuel line, solving the flooding problem. The switch can be easily fitted by cutting into the wiring at the inertia cut off switch next to the fuse box.