I've tried starting it up with the (almost) full tank and it's not happening (yes, the tap was on.) Checked the vacuum hose, no visible damage. Went to check the spark plugs but they're in inconvenient places and my spark plug tool is in my other garage which is locked and I'm losing the will to do it now, so I'm about to cook some dinner.
Anyone know of any issues that could cause this that aren't related to fuel? I've been told the reg/recs are common problems on these… could it be that?

If the reg/rec is shot and the battery has gone flat as a result, the bike won't turn over, either because the solenoid won't fire or the starter won't have enough guts behind it. Take the tank off, and check that fuel comes out the tap when it's set to on or res. You'll need to suck on the vac hose to open the diaphragm, as standard the taps don't flow in any position without engine vacuum present (and of course, never when 'off' regardless).

Okay, thanks. sucking on a vacuum hose is better than sucking on the fuel line - I've previously found out that despite how much I love the smell of petrol, it tastes disgusting. haha
What if it's a reg/rec problem and the battery is still working? It turns over and all the lights are as bright as usual.

ant_v4r wrote:What if it's a reg/rec problem and the battery is still working? It turns over and all the lights are as bright as usual.

The bike will start with the reg/rec disconnected. 'All' it does is provide the correct output from the charging system to maintain the battery. If the reg/rec is shot, then either the battery will go flat because it isn't being charged, or if as is often the case the reg/rec fails by not doing the reg part of it's job, you end up feeding 17-18 volts to the battery and ruining it.

The only likely way a reg/rec issue would cause the bike not to start is if the battery had been discharged as a result of the charging system not working correctly, or if the battery has been overcharged for a significant period of time which tends to cause them to lose charge very quickly when they aren't being used.

If your battery has enough charge that the starter motor will engage and spin the motor over, your issue is about as cast iron guaranteed not to be the reg/rec or battery as it can be.

Make sure fuel flows from the tap first, if it does, the next thing to check for is if you have a spark when cranking the engine. If not, there is most likely an electrical issue, or the plugs are knackered.

Ahh, okay. I was just worried because I heard they're common problems on CBR and VFR 400s.

Does the vacuum hose need to be sucked to just let petrol flow through the tap?
I've currently got the fuel tank off - I was getting ready to test the plugs for decent sparks. One thing I prefer my RS for - one cylinder = one spark plug and one carb. Those front two cylinders are awkward as hell to get to haha

ant_v4r wrote:Does the vacuum hose need to be sucked to just let petrol flow through the tap?

Yes, it's a safety feature. As standard, the tap will only flow when in the on or res position, AND the engine is running. This is to prevent the possibility of a sticky float valve allowing the fuel tank to discharge into the engine if the tap is left on, and a rather expensive outcome if you don't notice before hitting the start button.....

speedy231278 wrote:Yes, it's a safety feature.

Wow, on a 20 year old bike? I've not heard of it before and I've only seen/ridden newer bikes! haha well, trust those Japs to come up with something as smart as that!