Air Filter - Fuel Suction?

[Stoeger]

I'm a little confused about the air filter / gas suction thread. Does the air filter cover NEED to be on (and thus more suction) in order for gas to be sucked into the carb/engine on startup?

I see the vacuum hose on the engine side of the carb, and assume that once the engine is going, we have some vacuum, and that opens the valve, and then gas drops into the carb, and by magic, it mixes with air goes into the engine, and it stays running...

But what about if the motor won't start? With just the starter turning the motor, how is there still enough suction to pull gas into the carb, and then into the engine?

[Jerry]

First of all, YES the cover needs to be on. With the throttle closed, the piston action creates enough vacuum to open the fuel valve. When cold, the choke circuit is allowing a LOT of fuel to enter for a rich gas mixture to start.
With fuel in the bowl (Mikuni style carb) the vacuum is needed to pull the fuel UP from the bowl through the jets and create the correct mixture. If you open the throttle, you are decreasing the vacuum by such an amount that if the fuel filter is not in place, there is insufficient vacuum to pull up the fuel. It runs way too lean.
I hear a lot of people asking why their bike will start and run good for a few minutes and then spit and sputter. After asking them, they have not installed an air filter. (Check some old posts). Once the choke closes off the excess fuel, it dies. Install the filter and problem solved.....
Make sense?

[Kikwear]

That's interesting. Before I disassembled my Spree, it did not have a filter and loved to sputter and run rough. Obviously, I will be installing an air filter when reassemly occurs. Amazing how fine tuned these machines are...lack of filter causing a major running problem.

[Jerry]

The Japanese did a wonderful job of engineering. Unfortunately, the tolerances are so tight that it practiically eliminates any modifications. Bigger piston, exhaust or carb for instance is generally out of the question. Stick to stock and they can't be beat.