16-05-2023, 02:17 PM
Hi Hugh
I wouldn't worry about the height of the carb above the mechanical pump, from memory the delivery pressure is more than 1 PSI which corresponds to a head of more than 3 feet of petrol. Worth checking that the unions are tight and not drawing in air on the input (tank) side.
The angle of your carb is effectively the reverse of a transverse setup like a mini, and with the float bowl to the side there should be little or no effect from facing up or down hill. The standard fuel level in the SU is 3/8 inch below the top of the metering jet and is not especially critical, as long as it doesn't sit high enough to cause leakage when switched off and parked. The "suction" on the jet when running is many times more than the amount required to raise the level by 3/8 inch to the top of the jet and allow discharge. Setting fuel level is normally done indirectly by measuring the float valve clearance, as it's easier than taking out the metering needle and peering down the metering jet.
If pulling the choke out makes things better, it suggests to me that the mixture is leaning out. This could be caused by the liquid fuel feed to the metering jet being replaced by fuel with a lot of micro bubbles of fuel vapour, the "froth" being less dense than normal. I think the combination of heat from a hard working engine and reduced airflow from lower speed could be making things just a bit too hot in the float chamber. Given the thermal mass of the float chamber, I would estimate that is takes a few minutes to heat up and cool down.
You don't say what fuel you are using, but you could try E5 rather than E10 in case the ethanol content is the culprit. The Esso brand is supposedly actually ethanol free (E0), but it is hard to tell. Unfortunately, moden fuel is blended for use in systems which continuously circulate it back to the tank to cool it, and I suspect it contains a greater percentage of lower boiling point components.
You might like to try measuring the temperature of the float bowl with one of those IR thermometers when and if things are playing up. A water soaked flannel would be one way of cooling the bowl down fairly quickly as a diagnosis.
I wouldn't worry about the height of the carb above the mechanical pump, from memory the delivery pressure is more than 1 PSI which corresponds to a head of more than 3 feet of petrol. Worth checking that the unions are tight and not drawing in air on the input (tank) side.
The angle of your carb is effectively the reverse of a transverse setup like a mini, and with the float bowl to the side there should be little or no effect from facing up or down hill. The standard fuel level in the SU is 3/8 inch below the top of the metering jet and is not especially critical, as long as it doesn't sit high enough to cause leakage when switched off and parked. The "suction" on the jet when running is many times more than the amount required to raise the level by 3/8 inch to the top of the jet and allow discharge. Setting fuel level is normally done indirectly by measuring the float valve clearance, as it's easier than taking out the metering needle and peering down the metering jet.
If pulling the choke out makes things better, it suggests to me that the mixture is leaning out. This could be caused by the liquid fuel feed to the metering jet being replaced by fuel with a lot of micro bubbles of fuel vapour, the "froth" being less dense than normal. I think the combination of heat from a hard working engine and reduced airflow from lower speed could be making things just a bit too hot in the float chamber. Given the thermal mass of the float chamber, I would estimate that is takes a few minutes to heat up and cool down.
You don't say what fuel you are using, but you could try E5 rather than E10 in case the ethanol content is the culprit. The Esso brand is supposedly actually ethanol free (E0), but it is hard to tell. Unfortunately, moden fuel is blended for use in systems which continuously circulate it back to the tank to cool it, and I suspect it contains a greater percentage of lower boiling point components.
You might like to try measuring the temperature of the float bowl with one of those IR thermometers when and if things are playing up. A water soaked flannel would be one way of cooling the bowl down fairly quickly as a diagnosis.