Folks, quick question, does the fuel pump stay running at all times when the engine is running? I presume that it does, but need to check. The 1984 K100RS I’m trying to get running again after a fire had the little harness melted when a fuel hose split and sprayed fuel onto the head. I’ve replaced all the fuel hoses and injector harness, injectors etc, but I couldn’t save or replace the short 4 wire harness from the main harness to the fuel tank. I’ve cut off the melted wires and am planning to run a hotwire with a fuse and a switch direct to the battery. Any reason why I shouldn’t? Thanks in advance.

Better to run the wire to fuse six I think, which is already switched power and includes a fuse, that's where it got power from originally. 

Let's say you have a crash and your brick stalls. The computer denies power to the fuel pump, so it stops. If you wire it to the battery with a switch, fuel might continue pumping until the switch is turned off, that's potentially dangerous. Anyway it would probably get annoying after a while to always have to turn the pump on and off separately.

Thanks mate, unfortunately a previous owner has done a lot of additional wiring on this bike, so I think I’ll just wire in the pump to a separate switch until I can get hold of another harness and take all the wiring back to original. I think there’s a remaining accessories plug that’s only on when the ignition is on, so might wire it into that one.

Do you really want your bike to have another fire?

Why do you think that will happen? The fire happened because the old fuel hoses had deteriorated over 40 years. Having a separate switch to power a low pressure fuel pump isn’t going to cause a fire.

Daveyson just explained this . The pump must stop automatically as soon as the engine stalls. If not, il will increase pressure in the fuel hoses, as the injectors will remain closed, until a jonction pops up and it leaks somewhere... Knowing the flow of the pump, this will take a few seconds.
Wire the pump to a fuse that is only powered when the engine is running, not to a fuse powered when it's switched on.

Well I wired in a separate switch and probably averted another fire. While testing the pump (with the ignition off) I discovered that one of the new injectors isn’t seating correctly and fuel sprayed over the head. Had it been wired into the ignition I could have had a fire. I’m happy that I caught it.

Another option is to wire it to the fuel relay. Not sure if you have a brick workshop manual, but there are electrical diagrams in the tech section of this forum.

daveyson wrote:Another option is to wire it to the fuel relay. Not sure if you have a brick workshop manual, but there are electrical diagrams in the tech section of this forum.

If you connect directly to the fuel relay then that bypasses the fuse. It would be better to find the GRN/WHT output wire from the #6 fuse that used to go to the fuel pump.

Yeah, that's a given. Probably a lot of changes to the wiring is why the fuse six option wasn't chosen.

The crash example I was thinking of was a bit different. I thought about say a barb breaking on a drop, then you could have fuel at low pressure, but high volume, pumping out on the crash site, not good, but pretty unlikely anyway.

Wiring to switched power probably not a good permanent solution (which I think Terry has already said) as there are times when you want ignition on, but not the pump, so just making the battery work harder than it needs to.

Thanks guys, I hesitated in reading your replies in case someone roasted me, (pun intended) so I’m glad that you’re a helpful, friendly bunch.

Good news is that the bike is now running for the first time since the PO had the fire that burned off the injection harness and the wires from the main harness to the plug in the tank for the fuel pump. It initially ran really poorly and I was gutted, so I did the usual, made sure all the power caps on the injectors were tight, pulled the plugs and checked for spark, all good, but it still ran like a pig.
I’d replaced the injectors with some cheap Ebay injectors because the old ones were pretty crispy after the fire, but I hadn’t thrown them out, so swapped them in, hit the button, and it ran as smooth as a K100 should. While I had it running I checked the alternator output and it was fine, excellent.
The big red light in the dash is on, not sure why as my other K100RS SE ABS1 has that light on, as well as the ABS lights until I take off and tap both front and back brakes, and it goes off.
I will eventually wire the switch into a switched wire (I’ll look for that green and white wire previously mentioned) but for the moment I want to put it all back together and go for a ride, before I strip all the painted parts off and repaint it. It’s currently silver, but I’ve always wanted a black K100RS. If the engine feels strong I might have to install the Luftmeister turbo kit I’ve had sitting in a tub for the past few years.

Terry in Oz wrote:
The big red light in the dash is on, not sure why ...

That's ok and normal, it's a brilliant little safety feature. If both brake light circuits are working, and the tail light, the bulb monitor unit is happy and turns off the warning light. If, for example you're riding down the road and the brake light filament breaks, the bulb monitor unit detects an open circuit, and the warning light comes on to warn you that the brake light isn't working. How nifty is that!

I have a renewed respect for the importance of brake lights and rear mirrors since getting back to bike riding after a few decades of cages.

Thanks mate, yep, I took it for a lap around the block and the red light went off. All good now, just need to keep cleaning it up and replacing the scorched items.

Well I took the “Hindenburg” for a shakedown ride today, and of course, it became a shakedown/breakdown ride. The bike was going great, until it stopped, and luckily I was able to roll to a stop clear of traffic. It was the fuel pump, so I checked the fuse and it looked fine, but it turned out that the new switch was faulty.
Luckily I’d packed lots of tools, zip ties and insulation tape, so I pulled the switch out, disconnected the wires, zip tied the terminals together then wrapped them in insulation tape and zip tied them to the inner fairing.
I started the bike and turned for home, but the freeway heading south closed for maintenance and the nearest road that runs parallel was choked with traffic so I ended up doing 50 miles, the last 35 or so, trouble free. Got home, pulled the fuse I’d wired in, and switched the ignition off.
Later in the week I’ll wire the pump into the green/white at the back of fuse #6, and hopefully it’ll all be good. Apart from that, the bike runs great, and it was quite pleasant apart from almost being taken out by a Wallaby right on dusk.

Terry in Oz wrote:Well I took the “Hindenburg” for a shakedown ride today, and of course,  it became a shakedown/breakdown ride. The bike was going great, until it stopped, and luckily I was able to roll to a stop clear of traffic. It was the fuel pump, so I checked the fuse and it looked fine, but it turned out that the new switch was faulty.
Luckily I’d packed lots of tools, zip ties and insulation tape, so I pulled the switch out, disconnected the wires, zip tied the terminals together then wrapped them in insulation tape and zip tied them to the inner fairing.
I started the bike and turned for home, but the freeway heading south closed for maintenance and the nearest road that runs parallel was choked with traffic so I ended up doing 50 miles, the last 35 or so, trouble free. Got home, pulled the fuse I’d wired in, and switched the ignition off.
Later in the week I’ll wire the pump into the green/white at the back of fuse #6, and hopefully it’ll all be good. Apart from that, the bike runs great, and it was quite pleasant apart from almost being taken out by a Wallaby right on dusk.

Impressed with the on the hoof repair and wallaby avoidance skills!