Like all carbs, Amals need to be fitted with an air filter to keep out dust, rocks, dropped nuts and small children. The air horn is fitted with a thread (which differs depending on
Amal model). A pancake-type air filter assembly can be screwed on (similar to an SU), or a ram tube screwed on and a filter sock slipped over the tube. One thing to be wary of is that the factory grey motor Strommie vents the fuel bowl into the air horn (red circle on the diagram below), sucking in air DOWNSTREAM of the air filter. The
Amal vents the fuel bowl to atmosphere via the tickler (green circle on the diagram below), sucking in air UPSTREAM of the air filter.
- float bowl venting.png (110.92 KiB) Viewed 996 times
Does this really matter? It does when you are trying to keep the carb in tune. The jets in both carburettors work because of a difference in pressure. The venturi in the carb throat reduces pressure, giving “suck” on the jets. The jets then flow because the “suck” is lower in pressure than the float bowl.
The diagram below shows how the Strommie pressures work.
- Strommie fuel bowl venting.png (68.96 KiB) Viewed 996 times
Air flows in (from atmospheric pressure), and some pressure is lost in the dirty air filter. More pressure is lost in the venturi (to make the “suck”). Because the Strommie vents the fuel bowl into the air horn, the float bowl sees air filter pressure loss, just like the carb throat. The pressure difference across the jet is thus (atmospheric pressure minus dirty air filter pressure loss) in the float bowl, and (atmospheric pressure minus dirty air filter pressure loss minus venturi loss) in the carb throat. When you subtract those two, the jet really only sees the pressure loss in the venturi i.e. no matter how dirty a grey motor air filter gets, the jets see the same "suck" and the mixture remains the same… just the airflow is blocked, and power reduced.
Now imagine that the Strommie was set up like an
Amal, with the fuel bowl vented to atmosphere. This is shown in the diagram below, with the green lines acting like the tickler:
- Strommie fuel bowl venting as per Amal.png (69 KiB) Viewed 996 times
Now the pressure difference across the jet is (atmospheric pressure) in the float bowl, and (atmospheric pressure minus dirty air filter pressure loss minus venturi loss) in the carb throat. When you subtract those two, the jet now sees the pressure loss in the venturi
AND the pressure loss in the air filter. As the air filter gets dirtier, the jet sees more and more “suck”. This means that for our
Amal, a dirty air filter will make the carb run rich (as well as losing power from low air flow).
The upshot of this is that when tuning Amals it is critical that the air filter is clean, whereas with a Strommie it does not matter so much.
Cheers,
Harv327 Chev EK wagon, original EK ute for Number 1 Daughter, an FB sedan meth monster project and a BB/MD grey motored FED.
. The only drama with an atmospheric tank in the engine bay is that it needs to be vented. Some fuel hose tapped off the tank lid will do the trick, run down the firewall or inner guard to vent under the car. Need to be careful that the hose end is not in direct airflow, or it will vacuum out the tank contents (like the grey motor walking stick)... cut it off a few inches clear of free air. 
