Fuel tap questions. - Closed

ProXimaCore · Dougal Joined: 01 May 2003 Karma :

On my bike the fuel tap has 3 settings. Reserve - On - Prime.

If I understand correctly the tap has some sort of vacuum so that no petrol comes out when it's on or reserve without the bike being turned on. Well nothing seems to come out when the tank is disconnected from the hoses.

My question is what is the Prime setting used for? I know when it's set to this a small flow of petrol is released. But what situation would you use this for?
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Honda NSR125R -> Suzuki GSXR400R GK76a -> Kawasaki ZX636 B1H -> Honda CBR400RR NC29 -> Nothing Sad

Robby · Dirty Old Man Joined: 16 May 2002 Karma :

Prime lets fuel through without vacuum being needed. If for any reason your float bowls were dry, you could fill them by putting it to prime, which is easier on the batter than spinning the engine on "on" until enough fuel gets into the carbs to start the engine.

Don't leave it on prime though. Has a habit of filling your engine with petrol.

Guest · Brolly Dolly Joined: 22 Apr 2004 Karma :

What robby said - beat me to it.

On and reserve are via a diaphragm that doesn't open unless it's sucked by a vacuum from one of the carbs, so you don't get flow unless the engine is running.

If you have to disconnect the pipes to get the tank off, mark them carefully so they go back in the correct place. If you get the vacuum pipe on the prime outlet you'll fill the crankcases with petrol when you turn it on.

ProXimaCore · Dougal Joined: 01 May 2003 Karma :

Guest · Brolly Dolly Joined: 22 Apr 2004 Karma :

'Usually' (good disclaimer that Wink

) the tank breather comes from a metal tube somewhere near the top of the underside of the tank. It'll be a long, thin tube that just pokes down the back of the gearbox.

If the petrol tap bolts into the bottom of the tank and there's no fuel pump, you'll have two pipes from it - a thickish one that feeds petrol to (probably) the middle of the carb bank, and a thinner one that operates the diaphragm. This connects to somewhere on the engine side of the carbs - inlet manifold, carb mounting stub or whatever.

I'll have a dig for some pics

Guest · Brolly Dolly Joined: 22 Apr 2004 Karma :

Can't find the right one, any chance you could take a picture or two?

ProXimaCore · Dougal Joined: 01 May 2003 Karma :

Guest · Brolly Dolly Joined: 22 Apr 2004 Karma :

To save trawling through a load of posts and probably missing something, can you give us a list of your troubles?

Bike, mileage, service history, bits fitted/removed (air filters, cans etc), nature of problems, what you've tried so far. The more info the better, let's try and sort it. Thumbs Up

ProXimaCore · Dougal Joined: 01 May 2003 Karma :

Guest · Brolly Dolly Joined: 22 Apr 2004 Karma :

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

As mentioned the noise from the starter relay is normal when the battery is flat.

Also note that the bike needs a reasonable amount of power (either from the battery or from the charging system) to run. If you charging system isn't working then the bike will die (or with some charging systems they only click into charging mode when the engine has been revved past a certain point, but only really had this on cars).

While many bikes do have an engineered flat spot at half peak revs (for the noise tests), your bike is (from memory) a GSXR400 which revs fairly high and so 4000~5000 would be well under the point of the flat spot. With CV carbs the fueling at that point will be controlled by the needle jet.

In addition to the point Guest has mentioned I would also suggest that you check that fuel is getting through to the carbs. If the breather pipe for the fuel tank is blocked / kinked then fuel cannot leave the tank to go to the carbs. Try getting the bike to run then open the fuel cap and see if it then idles. Also try starting the bike with the fuel tap on prime (this way you have eliminated the vacuum pipe and the breather pipe).

If you had the pipes connected the wrong way round then you would have just filled the engine with petrol and it is unlikely the bike would run at all.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

ProXimaCore · Dougal Joined: 01 May 2003 Karma :

Oh I forgot to mention one other thing. Possibly important.

I've had problems with the spark plug leads as they aren't sealed and were leaking in water. Causing the bike to act funny and struggling up through the revs a little like I've described above when it rained. The last time the bike did work properly was before the last outing in a heavy downpour. Since then it hasn't been the same. When this happened before, the bike always righted itself after a day or so when it had dried up. But the next day I went out with the bike I had to bump start it for the first time. Each time I wanted to start the bike from there it took a bigger effort at starting. All the above problems then followed.

Don't know if that's relevant but probably worth sharing. Can't believe I forgot about that bit.

On top of all this the rear shock has lost all dampening too. I have got a spare one but that's all going to have to wait. Not having much luck lately. Everything ran so well for months too.
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Honda NSR125R -> Suzuki GSXR400R GK76a -> Kawasaki ZX636 B1H -> Honda CBR400RR NC29 -> Nothing Sad

Guest · Brolly Dolly Joined: 22 Apr 2004 Karma :

Ah!

Try drying the coils out with a hair dryer and if it works ok. Won't affect the oil usage but it'll eliminate one thing.

Git · Brolly Dolly Joined: 17 Dec 2003 Karma :

ProXimaCore · Dougal Joined: 01 May 2003 Karma :

Right, finally back on to fixing this thing. (cheers for help so far guys.)

I finally got the battery charger back and charged the battery. The battery is very healthy now and has been for a few days so it looks like that's ok.

Just tried bump starting the thing. It seems to start to work then starts to shake the bike with big vibrations as it fails. So I'm stuck once more.
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Honda NSR125R -> Suzuki GSXR400R GK76a -> Kawasaki ZX636 B1H -> Honda CBR400RR NC29 -> Nothing Sad

Guest · Brolly Dolly Joined: 22 Apr 2004 Karma :

Have we eliminated the petrol tap yet? Turn tap to 'on' and remove one of the drain screws from a float bowl - you should get enough to fill the bowl and no more. If that's ok, then turn the tap to 'prime' - petrol should now flow through the bowl again.

If it passes that ok replace the drain screw, I'd say the tap and the pipes are working correctly and the tank is breathing.

ProXimaCore · Dougal Joined: 01 May 2003 Karma :

ProXimaCore · Dougal Joined: 01 May 2003 Karma :

Just to close this off. I got the bike working tonight. As I originally suspected, the vacuum hose had come out of place. The connector had worked it's way loose from the carbs.

Thanks again for everyone that gave advice. Thumbs Up

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Honda NSR125R -> Suzuki GSXR400R GK76a -> Kawasaki ZX636 B1H -> Honda CBR400RR NC29 -> Nothing Sad