Hard starting engine?
This was originally posted by Bc-Dan as a reply to a question in another section but was too good not to place here.
On a Mikuni carb, there really isn't a "choke". What there is instead is an enrichment circuit. The circuit adds fuel when you flip the lever. The amount of fuel being delivered is dependant on the velocity of the airflow across the opening in the carb. When you give it the gas when trying to start, the throttle lever lifts the slide, making the area where the volume of air is flowing a lot greater. Because the area is greater for the same amount of air, there is a lot less air velocity across the enrichment circuit opening and the amount of fuel being added is not great. Keeping the slide on the bottom (hands off the throttle) maximizes the velocity of the air and creates the greatest amount of fuel being delivered.
These carbs also have three main parts; the pilot jet, the needle jet/jet needle, and the main jet. When the engine is at idle to about 1/8 throttle, the pilot jet is delivering fuel. At 1/8 to about 3/4 throttle, the needle jet/jet needle is metering the fuel (the fuel that the needle jet/jet needle meters comes from the main jet). After 3/4 throttle, the slide lifts the needle so that it no longer is a player in metering fuel, so the main jet is what controls fuel ratio.
That said, if you look at where you are having problems, you can work on that system in the carb. If you have hard starting issues, the enrichment circuit or the pilot jet is where to start looking. It does not take much varnish at all before the air and fuel passages to the pilot jet get clogged, and that jet is VERY small. Disassembling the carb, removing the pilot jet and "flossing" it with a small wire (I use one strand of telephone wire with the insulation stripped, or sometimes monofilament fishing line). Cleaning the passages with the wire is sometimes helpful. Those jets have to be pretty much spotless to work properly. Aerosol carb cleaner in all the passages helps, just be careful when spraying, sometimes it sprays back at you!
You may also want to make sure the enrichment plungers are not stuck in the carb, if they are not coming up, no additional fuel is delivered when you "choke" the engine.
When I start my carb sled cold, I put the lever on full, hands off the throttle, it starts first or second pull, even in the summer. If it won't continue to run, you know where to look! And pumping doesn't help, as there are no accelerator pumps at all in sled carbs; all that will do is build up your thumb muscles