Priming too much means there’s too much fuel in the cylinders and not enough air, so the fuel can’t ignite. From the manual:
Do not overprime! Once you have over primed an engine you cannot start it and it will have to be de-primed. Do so by shutting down and open the throttle fully so fuel fumes can escape. Depending on how much you over primed, this may take up to 10 minutes. Check the priming level in the Tablet’s Live Data page or the prime switch mouse-over label.
From the changelog:
Engines
• Retuned take-off, climb and cruise performance
• Start-up process is now authentic, meaning you have to hold the START switch first and listen for the flywheel to spool up (about 5 seconds), after which you may hold the ENGAGE switch to couple the propeller.
• A 2 to 4 second prime pre-start-up is now required. When over primed, you need to open the throttle to let fumes escape and wait a few minutes.
• Advanced thermodynamics with custom modelling of oil, coolant, and intercooler systems
• All cooling systems have working automatic shutters, or can be controlled manually as per the original aircraft
• Engine stresses, which build up over time when pushed too much
• Carburettor icing can occur when you fly through a cloud and low MP and are within a 4C (+/- 7C) temperature range
• Priming now requires at least 4Psi of fuel pressure (check gauges)
• The turbo’s steel structure will start glowing when hot (visible in low light conditions)
There are several factors in there that make starting more difficult. Priming is the main one. It’s hard to get the timing right. Tooltips will give you the priming status - it’s a useful assist. The tablet also gives you priming status. You can also refer to these timing and temperature guidelines (also from the manual). Keep in mind that you can always do more, you can never do less - meaning, if you overprime, it takes longer to resolve that.
Pilots should respect the guide below in order to achieve a successful engine start:
<0 °C - 5 seconds
0 – 10 °C - 4 seconds
10 – 20 °C - 3 seconds
20 – 30 °C - 2 seconds
30+ °C - 1 second
Engines of this period were notoriously balky - as the guidelines suggest, they’re dependent on temperature and correct priming, and failed starts were common. Radial engines are even more ill-tempered (wait for the Flying Iron Hellcat to give this a try). If you search YouTube for airshow videos, you’ll find a lot of examples of warbirds not wanting to start. It’s part of their character. In critical situations - for example, in the Battle of Britain, when scrambles had to happen instantly - ground crew would start the engines at least once an hour to keep them warm, supplied with fuel, and ready to start as needed.
But… there’s one major respect in which starting the P-38 is harder in MSFS than in real life. That’s because the sim doesn’t let you hold down two switches at once. In the real aircraft, you’d hold the start switch in place and then use another finger to depress the engage switch while keeping the starter held down. Changing from one to the other is unrealistic and also hard to do with the mouse. The only way I’ve found to work around the limitation is to do what I mentioned in my post above (which is what I learned from a post higher up in the thread) - if you have two available hardware switches, or two unassigned keys on your keyboard, assign one to “Starter 1 (hold)” and the other to “Starter 2 (hold).” Those operate the “engage” command. With hardware assignments, you can get closer to the real-world technique - hold down “start” until the flywheel spins up, keep it held down, then use your hardware switch or keyboard key to engage the driveshaft and start the engine. Much easier than the mouse-only method.
Hope this helps!
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