Re: lean -rich setting, 15 hp evinrude?
joaklay,<br /> It seems that a large part of the bog situation is an issue with the diameter of the venturi. It is relatively large for the volume that is drawn through it at low rpm. When you open the throttle quickly, the velocity through the venturi is drasticly reduced, thus reducing the pressure differential between the low pressure side of the venturi and the float chamber causing a "lean stumble" as the fuel flow into the throat of the carb is reduced at this point. It is quite basic physics. Richening the low speed side reduces this situation somewhat, but also causes it to run "fat" at idle. Another operational work around is to partially open the throttle initially to allow engine rpm to rise to a "fast idle", thus maintaining a reasonable velocity over the venturi to support good fuel mixing in the carb throat. Basicly, open the throttle in a smooth fashion allowing rpm to rise as you do so, the higher the rpm gets, the faster you can roll open the throttle. If the area of the venturi was reduced, you would have better fuel atomization at lower rpm with the throttle plate in the open position, but not enough air flow to support the engine at high rpm ( one of the critical differences between the 9.9 and 15hp) and end up with a lower WOT rpm, the lower net HP being developed. A richer low speed mixture will make for easier starting, particularily when cold, but will be detrimental for extended extreme low rpm operation. Some of the bog can be eliminated with careful setup of the corelation of timing advance and throttle actuation/initiation. This throttle "stumble" or "bog" is what led motorcycle manufacturers to utilize "variable venturi" or "constant velocity" carbs on modern motorcycles.<br /><br />THE SHORT VERSION:<br /><br /> Use careful setup to eliminate some of it, and your wrist to get around the rest.<br /><br />Or......<br /><br /> Get a 9.9 which has a smaller carb.<br /><br /><br /> Cheers,<br /> Snow.