Hi,<br /><br />First post to this board which seems to be excellent!<br /><br />A friend and I bought a boat a few weeks ago, a Flipper 620HT, Finnish design so probably only known locally. It's 6.20m by 2.45m (20' about I guess).<br /><br />We both have more of an EE/CS background than mechanics but with the help of some friends and engine diagrams we were able to get the 1979 Johnson (v4, 85hp, 2-stroke) running after it had been unused for several years. <br /><br />The boat runs about 25knots at what seems to be WOT, though we have no RPM gauge yet since the controls and gauges were for a different (and since stolen from the previous owner) engine, so we had to rewire everything by trial-and-error between the new controls and throttle cables and the engine.<br /><br />Within a week though the boat will be taken out and winterized and our list of winter projects (although pretty big already) is not yet long enough 
<br /><br />To cut the story a bit shorter I'll leave out most of the motivations, but we're not completely happy with the engine's performance (mostly starting it!) and are thinking about ways to modify it in a way that would allows us more control of what is going on rather than getting into the black art of finely tuned mechanics (think carbs, jets, reeds, etc).<br /><br />Semi short version of an idea:<br />Take away both carbs and the reed valves. In the now existing space, connect a manifold+gasket (a friend should be able to put something together sealing the crankcase.<br /><br />Now, my understanding of the engine principles is as follows, so please correct me if I'm wrong! Inside the crank case is always an under pressure compared to atmospheric pressure due to the piston actions (when the engine needs more air/fuel). The reed valves open because of this and air is sucked in through the carbs and reed valves, and is throttled by a butterfly valve. This throttling would mean the crank case is never actually pressurized up to atmospheric pressure unless maybe during WOT? So the engine is throttled by the underpressure in the crank case, which is the amount of fuel/air available for the next power stroke.<br /><br />Now, back to the plan <br /><br />The new manifold has an electronically controlled air valve in series with an air flow sensor. We would time this valve to open at the right time (when the reed valves would open in a normal engine), and throttle by varying the duty cycle of this valve. Open-longer means more air. Crankcase underpressure would take care of moving the air. Based on the readings of the air flow sensor a fuel injector also present in this manifold would inject fuel into the crankcase at the 14.67:1 stochio ratio of air-to-fuel. A lamda (oxygen) sensor in the exhaust could help finetune the fuel injection? (does this work the same as for 4-strokes?)<br /><br />Oil would either be pre-mixed (if the fuel injector nozzle can handle the oil) or injected seperately via another injector.<br /><br />If this were to actually work (I'll put up a webpage with lots of pictures and graphs to brag about it ) it should mean smooth operation (for example a healthy idle) over the entire RPM range and better fuel consumption than currently? Choking can be done by increasing fuel injector duty cycle.<br /><br />Did I miss any fundemental problems? Should there be an air pressure sensor inside the crankcase and throttle according to it's readings? <br /><br />Any comments or suggestions or ideas? Note that is not to say we would mind if the engine ran perfectly in stock configuration, it's as much "want" as "need", who doesn't like to experiment?
<br /><br />To cut the story a bit shorter I'll leave out most of the motivations, but we're not completely happy with the engine's performance (mostly starting it!) and are thinking about ways to modify it in a way that would allows us more control of what is going on rather than getting into the black art of finely tuned mechanics (think carbs, jets, reeds, etc).<br /><br />Semi short version of an idea:<br />Take away both carbs and the reed valves. In the now existing space, connect a manifold+gasket (a friend should be able to put something together sealing the crankcase.<br /><br />Now, my understanding of the engine principles is as follows, so please correct me if I'm wrong! Inside the crank case is always an under pressure compared to atmospheric pressure due to the piston actions (when the engine needs more air/fuel). The reed valves open because of this and air is sucked in through the carbs and reed valves, and is throttled by a butterfly valve. This throttling would mean the crank case is never actually pressurized up to atmospheric pressure unless maybe during WOT? So the engine is throttled by the underpressure in the crank case, which is the amount of fuel/air available for the next power stroke.<br /><br />Now, back to the plan <br /><br />The new manifold has an electronically controlled air valve in series with an air flow sensor. We would time this valve to open at the right time (when the reed valves would open in a normal engine), and throttle by varying the duty cycle of this valve. Open-longer means more air. Crankcase underpressure would take care of moving the air. Based on the readings of the air flow sensor a fuel injector also present in this manifold would inject fuel into the crankcase at the 14.67:1 stochio ratio of air-to-fuel. A lamda (oxygen) sensor in the exhaust could help finetune the fuel injection? (does this work the same as for 4-strokes?)<br /><br />Oil would either be pre-mixed (if the fuel injector nozzle can handle the oil) or injected seperately via another injector.<br /><br />If this were to actually work (I'll put up a webpage with lots of pictures and graphs to brag about it ) it should mean smooth operation (for example a healthy idle) over the entire RPM range and better fuel consumption than currently? Choking can be done by increasing fuel injector duty cycle.<br /><br />Did I miss any fundemental problems? Should there be an air pressure sensor inside the crankcase and throttle according to it's readings? <br /><br />Any comments or suggestions or ideas? Note that is not to say we would mind if the engine ran perfectly in stock configuration, it's as much "want" as "need", who doesn't like to experiment?