Nothing complicated under there really, even if it looks intimidating These days, they have bunged a few fancy gadgets on, but essentially the old mantra still applies. For your bike to run properly you need; air, fuel, and a spark. You need these in very specific quantities.
You might imagine that petrol and air mixed together with a spark will always ignite, but that’s not the case.
This is why the endlessly knowledgable Art is posting pictures of the plugs, it’s the equivalent of sticking a thermometer in the patienrt’s mouth. If it’s been starved of air, fuel or spark, then the state of the plugs will reflect this. It’s known as running ‘lean’ or ‘rich’, where lean refers to an engine that is receiving too little fuel (or too much air air), and rich refers to an engine that’s receiving too much fuel (or too little air).
You can’t directly measure the ratio of air/fuel that’s reaching your cylinders, but things like plug condition gives a good indication.
Air flow: Check for kinked or split hoses. Remember that the af is critical, so the smallest split can cause a huge problem, especially when the engine is under load and the air flow is high. One trick is to spray some carb cleaner or similar around the general area. If there is a leak, the bike will suck a small quantity of the carb cleaner through and because it’s flammable, it will alter the note of the engine. You also have a bundle of vacuum hoses under there, they need checking too, they’re the hoses about the width of a pencil that connect to your carbs. On the subject of air, a really badly blocked airfilter would cause the problem you describe, but it’s generally a problem that happens gradually (unless a seagull has inadvertently flown up your intake).
Fuel: Ideally, you would be checking that your carbs are functioning properly, but in lieu of that, you can at least check everything around them. For example, your fuel tank isn’t a sealed unit. As the engine draws fuel down it creates a vacuum in the tank, so clearly you need to let air into the tank at the top or the bike won’t get fuel from the bottom. There are a couple of breather hoses that lead from your tank, one is to safely divert the fuel you accidentally spill around the lip of the filler cap, the other is to vent the tank. Make sure these are freely routed down the side of the bike, and not kinked.
Spark: You’ve clearly got a spark, because the bike is running. However, that doesn’t guarantee a healthy spark under load. Clearly producing a spark requires a sufficient amount of electrickery, and more so when you’re producing those sparks in quick succession. A rough and ready check on your electrical system is to buy a 10 quid multimeter from B and Q, set it to 20v DC, and put the probes across your battery.
You test: The voltage before you switch anything on, then the voltage with the bike running. You are looking for a rise of at least 1 volt.
The ‘sudden’ nature of your problem suggests to me a fairly obvious problem. It’s the intermittent faults that are hard to pin down. (well it was doing it on the way here mate!)
My money is on a disconnected/split/perished/blocked bit of rubber causing your issue.