Petrol starvation

[gpobantam]

Afternoon folks.

Something strange and puzzling, to me, happened yesterday. I was riding along on my MarkIV Sports when it started missing, lost power uphill and came to a halt, I saw that there was fuel in the tank but none seemed to be coming into the carbs when I tickled them. I disonnected the petrol pipes both sides and no petrol came out of the open taps( reserve and normal).
When I got home I took the taps off the tank and about 3 pints of fuel came out of the nearside and about quarter of a pint from the offside. I checked the flow on each tap and they were both O.K. I replaced the taps and put about 1 gallon of fuel in the tank. Everything was back to normal. Why did the petrol not come out of the open taps when there was about 3 pints left in one side and quarter pint in the other. There was no blockage at the fuel cap( I took it off) and no sign of rusty flakes on the tap filters.?
Thanks for any suggestions

[graeme]

Could it be an air lock in the fuel lines? I have had this happen a couple of times in my riding career - no apparent fuel flow, undo the petrol line at the carby and fuel flows through, reconnect it and all is fine. Very mysterious!

[Chris]

Hi
     The circumstances of this occurrence are very strange and I can offer no explanation other than that offered by Graeme. I have experienced two different situations where no petrol is available at the carb, both on old machines. I have experienced the situation several times where power has dropped off and eventually the engine has died and each time this has been because the fuel cap vent is blocked and eventually a vacuum is created. Fuel then ceases to flow to the carb. However, releasing the fuel cap immediately restores flow. In your case you checked that petrol was present in the tank presumably by undoing the cap and looking inside. Petrol still refused to flow so it cannot have been due to vacuum. The other situation which almost always occurs in fuelling up a machine after the tank has been previously drained. This is caused by airlock in the petrol supply piping and is overcome by blowing into the petrol tank filler orifice with the tap open and the tickler pressed down on the carb. Chris.

[gpobantam]

Thanks Chris.

As you say it could not have been a vacuum because I had the cap off. It was not an airlock - I took the feed pipes off . When I took the taps off at home, as expected petrol gushed out into my waiting container.
I expected the amount of petrol would have been sufficient to flow through the reserve pipe. My thoughts are that the petrol reserve pipe sticks up too far into the tank so when fuel gets as low as it did the reserve pipe if sticking above the level.

I will just have to remember to keep more fuel in the tank.

Thanks again.

Cheers Mick

[phil_h]

I have had problems with the petrol taps with gauze filters in them.
Sometimes the gauze seems to prevent flow, and I can only guess that this is after not being used for a while so it is dry.
I now have a regime of dismantling the tap before using the bike to flush out sediment, and this allows me to wash the filter too, and have not had problems since doing this.

hth
phil