fuel flow

[Draco87]

Hey guys

I own a yam sr 125 which will ride very nice, from time to time the bike will surge in power digesting a fuelling issue. Cleaned up carbs and been cleaned with a parts washer too, but still did afterwards, so I checked the fuel pipe, with it draining into a jug I noticed that fuel was flowing nicely but was only coming out at half the diameter of the pipe, like someone had a finger on the end of a hose. So cleaned the tap and filter and it is still doing this but from time to time it will flow fully but then half again. The tap uses a 6mm pipe (according to Haynes) could an oversized pipe cause this or could the pipe be too long and fuel sitting in a bend, i forgot to mention i had bought new pipe due to a crack.

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Did you have the tap off? Could crap in the tank be blocking the filter in the tank, on the tank side of the tap? Some carbs also have a small gauze filter on the fuel inlet.

Surely the fuel pipe you've used is the correct internal bore otherwise it'd be leaking fuel everywhere. It's not getting linked is it? If the pipe is too long it may not help flow either, especially if it bends uphill again.
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[Draco87]

This is the second tap I put on, ive stripped both of them apart and cleaned them I replaced it as it was leaking slighlty like a drip every 4 hours and on/off yap was becoming stiff but it also had the same flow rate on that one there are no leaks now but my pipe is very easy to get on and off a little too easy. I'm using the same pipe I bought from the store on the current tap, assuming it is the correct size

[salem1987]

check the tank breather if you have not already too. Easiest way is turn the tap on with the fuel cap open

[Draco87]

[salem1987]

It might breathe actually through the tank cap. I have worked on a SR before but cant remember!

I dont know if im misunderstanding you, but do you mean the fuel pipe from the tank to the carb may have a crack in it?

If so, just try it without the pipe on at all.

It also might be normal for the fuel to flow like that from the tank, have you checked the inlet manifold for leaks?
Spray some WD40 or carb cleaner type stuff at it while running and see if the engine revs are affected.

Check your float height is at the haynes manual spec too

Hope you get it sorted

[Draco87]

[Robby]

You don't need a lot of fuel flow on that bike, a dribble would be enough. It only needs to fill the float bowl on the carb and keep it full. The maximum flow rate for the tap is about 10 times what the bike could ever need.

Think of it like this. If you fill the tank, put the output pipe over a bucket, and turn the fuel tap on, it will take about 10 minutes to empty the tank.
If you were to fill the tank and ride flat out until the tank was empty, it would take about two hours.

Your surging problem is not going to be down to your fuel flow rate at the tap. I would be looking at the electrics, starting with the simple job of charging up the battery and then cleaning up any wiring connectors you can find, paying particular attention to the ones on the ignition coil.