Final answer:
The instructions likely refer to a dual-fuel engine system, warning against the use of the accelerator when the more powerful second engine that uses a solid fuel mixture is running.
Step-by-step explanation:
The question is asking about the operation of a vehicle or machinery that has a dual-engined setup using different types of fuel. The blank spaces in the sentence should be filled as follows: Do not use the accelerator if the second engine is on as this could provide too much fuel to the engine.
This advice is particularly relevant in a dual-fuel vehicle or mechanical system where the first engine (which uses a liquid fuel/oxidant mixture and produces a green flame when ignited at 3 seconds) operates differently from the second, more powerful engine (which uses a solid fuel mixture and produces a yellow flame when fired at 4 seconds). Using the accelerator could cause an excessive amount of fuel to the already-running second engine, potentially leading to engine damage or other hazardous situations.