Hi,<br /><br />I have a 2000 Johnson 30hp, which had a rather disappointing performance in the beginning. <br />It lacked power, was a lame duck.<br /><br />So I looked for possibilities of tuning or tweaking it's power.<br /><br />The easiest way was tuning the intake first.<br /><br />The carb's orifice has serially sharp edges so the incoming air would be strongly constricted.<br /><br />So I took a file and rounded it generously and widened the channel from the venturi to the orifice in a way that it may resemble the orifice of a trumpet. <br /><br />Further I widened the venturi by 2 mm, be careful not to damage the small brass tube crossing the area.<br /><br />Much more air now could get into the crankcase but with reduced vacuum pressure in the venturi, so less gasoline would be sucked out of the main jet.<br /><br />A wider jet would solve the problem, so I bored it in steps and did trips with my boat controlling the colour of the plugs after each step.<br /><br />After 3 bores I had found the right bore with the proper air/fuel mixture.<br /><br />Series jet is 67d => 1,7 mm, the optimum bore is now 1,8-1,85 mm (1 inch = 25,4 mm).<br /><br />Further I shortened the butterfly's screws and widened the flange bore a bit (1 mm).<br /><br />Finally the whole assembly was polished.<br />That concluded the works on the carburetor.<br /><br />The Johnson seemed a new engine now, it had lost much of its lameness, acceleration of the boat was greatly improved.<br /><br />But ' appetite comes from eating' we say in Germany.<br /><br />So further tweaking needs doing.<br /><br />The bore in the manifold was matched with the wider bore in the carb flange and the inner casting of the two stay bolts was filed off a bit, so the air would get easier to the outer parts of the leaf valves.<br /><br />The 8 holes in the leaf plate were bevelled from the carb side so the air might easier pass through.<br /><br />Afterwards all edges of the crankcase and the bypass ports were deburred.<br />A time stealing job which only brings little improvement.<br /><br />A much more important tuning step was increasing compression by milling off the cylinder head.<br /><br />I carefully took off 0,7 mm leaving a clearance of 0,5 mm, so the hot pistons couldn't hit against the heads.<br /><br />You feel the increased compression when handstarting the engine but you make it.<br /><br />Finally I changed the Champion plugs for NGK Iridium types and enlarged the air inlet behind the 30-frame in the cover by placing washers onto the bolts.<br /><br />That's all.<br /><br />The performance of my Zodiac boat is much better then before.<br /><br />It now easily gets into gliding with 3 adults and 1100 lbs max weight.<br /><br />Top speed has improved too, but acceleration is best.<br /><br />I can use a 10 x 15 prop pitch instead of serial 10 x 13 which brings speed and reduces consumption by dropping revolution.<br /><br />My average consumption is only 6-7 l/h with 35 km/h speed though air/fuel-mixture is rather fat.<br /><br />So enough posting.<br /><br />Greeting Nordy
