Well, here it is! Bought unseen and to be honest, I am very happy with it! As you can see, it has had a few modifications. Only problem is, I can't get it to start! It turns over, but doesn't fire. With the ignition on, I can here the fuel pump whirling, but when I look in the tank I can see the fuel squirting upwards! Is this normal? Also, there is no petcock, but what is the round tap under the tank? (See photo). I don't know why the fairing decals say 748 Superlight, so don't ask
The round knurled object you've taken a picture of is the petrol cock, be careful turning it as often haven't been used for many years, if at all. There's a good chance you have a ruptured fuel line then by the sound of it. As it's new to you it's probably best to work your way through the whole bike checking as you go. More help will be along soon.
Start with the basics - are you getting a good spark at the plugs, are you getting fuel into the cylinders, and is there good compression?
If it’s a carb model and the petrol pump was changed before you bought the bike, check the connections. It is possible to connect them so the pump runs backwards, giving you a lovely little fountain in the tank..
I think on the carby 750/900's the fuel pump runs continuously with the ignition on. Fuel should go through the pump and fuel filter, and out of the tank through the pet cock, down the delivery pipe, which goes to a three way 'Y' connector near the carbs, one outlet goes to the float chambers, and the other is a return to the tank which terminates at an outlet under the 'hump' at the front of the tank (where the external breather pipe fits). With ignition on, the pump constantly supplies low pressure fuel to the Y piece, and supplies the float chambers if there is a demand from the carbs. Any excess LP fuel from the pump not required by the carbs travels back to the tank via the return line, which is why you can hear fuel bubbling in the tank with the ignition on. You can probably see ripples on the fuel if you look down the open filler with the pump running. Not sure how you would see a fuel fountain when looking down the open filler cap unless it was a split pipe in the tank. If the petcock was closed then the pump would still run, but nowhere for the fuel to go so don't think you would hear any fuel returning to the tank. Its a bit of a pain to remove and refit the pump (refitting the filler cap assy mainly....) so might be with checking for fuel flow to the carbs with isn on before stripping the tank. That's my understanding - HTH!
That is very helpful Mr Bimble. As soon as I can feel my fingers again, I'll pop over to the garage to test fuel flow. I do have a big spark, and it did fire up when I squirted fuel down the carb, so definitely a fuel flow problem. Apparently the previous owner had it running a few weeks back, so it might just be a blockage somewhere.
Let's hope it's a simple fix. Mind you I doubt either of mine would be very willing to start today at - 5c
With the low temperatures, could it be carb icing, if the previous owner says it ran a few weeks back? I agree with previous posts, be careful with that petcock, move it gently both ways before unscrewing counterclockwise to open fully, although I never touch mine unless the tanks coming off. It should be all the way out, then I turn mine a half turn back in, so it's not all the way to the stop. It sounds like pump is fine, it's low pressure so if you are seeing it in the tank it could be a block somewhere, maybe worth opening the taps underneath the carbs to check there's fuel in the bowls, but I reckon it could be the jets in the carbs that are blocked, doesn't take much.
The fuel feed line from the filter within the tank has been known to split with the fuel just recirculating inside the tank , robbing you of fuel flow to the carbs. An easy check for fuel feed to the carbs is to drain the float bowls and then switch the ignition on with the drain screws still out and check for fuel flow There’s also a sock style filter factory fitted at the fuel inlet at the carbs that could be blocked
