I should pump the throttle several times before attempting to crank the engine. While pumping I should see gas squirt in the carb. However, gas does not squirt while the engine is cranking over.
Does that sound correct?
Fuel systems 101...
For younger people who have never driven a carbureted car, boat engines with carbs can be a bit baffling. Here's some basics.
Injected engines. While cranking, the ECU can see the revs and the engine temp and actively squirts (through the injectors) the right amount of fuel into the intake to be drawn into the cylinders. The fuel pump is electric and it's producing flow as soon as the power is applied. The injectors are also fired electrically. If the engine is cold the ECU programming will enrichen the mixture, regardless of throttle setting. Nothing is relying on airflow or mechanical actuation.
Carbureted engines. These engines rely on airflow through the carb to draw fuel through the passages inside the carb and then out through the venturi in the throttle barrel. The slower the airflow (cranking speed) the harder it is to get the fuel to the intake. For the most part these engines also have a mechanical fuel pump, which needs the engine turning before they can get fuel up to the carb. If the throttle is fully closed (idle position) when the engine was last shutdown (how it usually is) then the choke plate and linkages are 'locked' in the open position, so the mixture won't be enriched unless the throttle is moved and that 'lock' is released. And as there is no airflow initially, there's no fuel in the intake, and without a couple of 'pumps' of the throttle, there won't be for a while, so the engine won't fire. Each engine is different and so it'll take a bit of experimenting to find out if your engine wants 1 pump or 3 (or 7!).
So, what are these 'throttle pumps' all about? Within a carb is a circuit called the 'accelerator pump'. As air is less dense than petrol, when the throttle is opened quickly it takes longer for the flow of fuel to increase than the air, resulting in a very lean mixture for a short time (until the flow of petrol can 'catch up' with the air), that will cause the engine to 'stumble' or bog on acceleration. To alleviate this lag in petrol flow there is a (usually) diaphragm operated pump on/in the carb. As the throttle is opened this pump squirts raw fuel through a discharge nozzle in the throttle barrel air stream (one discharge nozzle in each primary throttle barrel). This is how a couple or 3 pumps on the throttle handle squirts fuel into the intake, and why they don't squirt fuel while cranking (no throttle lever movement, no pumpy pumpy).
Hope this helps.
Chris..........
