Anticept Posted March 31, 2016 Report Share Posted March 31, 2016 The air mass moving through the venturi and past those holes at the bottom of the piston create vacuum above the diaphragm. It's not even close to a total vacuum, but it causes Delta P as I said, which is also known as... Differential Pressure! I can see where the confusion is though, I didn't actually mention the diaphragm, so one might think engine vacuum or maybe something else... thus I fixed my post. Anyways, to add: the higher the atmospheric pressure, the more air rushes by the piston at WOT, AND the pressure is higher in chamber #1, creating even more Delta P. When atmospheric pressure is low, both the airmass moving by the piston is slower, AND the pressure in chamber 1 is lower, which means that Delta P is even lower than one might initially think. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted March 31, 2016 Report Share Posted March 31, 2016 Call it vacuum or pressure differential or whatever. So what if the needle doesn't always give up control to the main? I would prefer some leaning at the highest altitude anyhow. I'm not buying that a needle adjustment only works at altitudes higher than where airports exist, how can that work? especially given that it works down to sea level in the real world. Link to comment Share on other sites More sharing options...
Roger Lee Posted March 31, 2016 Author Report Share Posted March 31, 2016 Hi Ed, Since you rarely go to the lowlands I wouldn't bother me to see you change the idle jet and the main jet and needle & nozzle. You always seem to be at 10K-13K most of the time? I would how ever want to make sure for the first few flight you monitor EGT's and engine temps. There is a chart or two for Rotax at high altitude jetting. If you don't want to go that far you could easily run in the #1 clip position on the needle. The #1 slot is at the top of the needle and will lower the needle and give you a little leaner mix for a while, but once you start getting up closer towards WOT then the jet is the main controller and that should be replaced. Give Leading Edge or Lockwood a call and just see what they have to say. It's just a phone call. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted March 31, 2016 Report Share Posted March 31, 2016 Roger, California and surrounds are full of valleys and beaches. I start and end my flights at altitude but low altitude comes into play all the time. When I flew in front of a 503 I used to have a selection of jets and would re-jet en route, with the CT I'm way to fast for that. Since Rotax always defaults to full rich at WOT the only reasonable improvement for me would be a mixture control that worked at WOT ( or a 914) Link to comment Share on other sites More sharing options...
FastEddieB Posted March 31, 2016 Report Share Posted March 31, 2016 I think I've linked to this before, but it may help... Edited to add: Notice how much throttle is used with the "choke" in play! Link to comment Share on other sites More sharing options...
Roger Lee Posted March 31, 2016 Author Report Share Posted March 31, 2016 Hi Ed, You fly at WOT a lot more than most and this is the rich spot for the carbs. Rotax did that on purpose to help stave off detonation. You can run leaner than stock at lower altitudes safely with the stock parts. If you change those parts then you'll have to keep a sharp eye on temps. Rotax leanest rpm is 4000-4600 area. This would be the rpm to stay away from if you lean down. Link to comment Share on other sites More sharing options...
Anticept Posted March 31, 2016 Report Share Posted March 31, 2016 Call it vacuum or pressure differential or whatever. So what if the needle doesn't always give up control to the main? I would prefer some leaning at the highest altitude anyhow. I'm not buying that a needle adjustment only works at altitudes higher than where airports exist, how can that work? especially given that it works down to sea level in the real world. When the piston is fully open, the needle barely has any displacement in the needle jet. At this point, the main jet is the smaller of the two orifices, and thus is the one becoming the limiter. Time for some napkin math. If the carb goes full rich at around 92% throttle (someone fact check me), and if we assume that Rotax determined this during testing at their Gunskirchen facility in Austria, which has an elevation of 1,155 feet, and you are at a city with an elevation of 7,880 feet, that's a difference of 6,825 feet. Applying the rule of thumb of 1 inch per 1000 feet, that's 6.8 inches of mercury difference! For me at my airport, 900 feet, that's a manifold air pressure difference of a whole whopping 20%!!! Now, with you, chamber #1 in your carb is also at a lower pressure, further magnifying the effect of the high elevation and making it even more difficult for you to get the piston to fully open. I would not be surprised if your piston doesn't open completely even at WOT. Now my napkin math doesn't take into account RPM, which *does* matter, but that's still pretty significant above, and is probably outside of margin of error. My suggestion about changing the needle position came from eric tucker 2 years ago. Link to comment Share on other sites More sharing options...
Brucegoose Posted May 31, 2016 Report Share Posted May 31, 2016 Can anyone give any feedback on the HACman mixture control kit? Regards to all Bruce Link to comment Share on other sites More sharing options...
FlyingMonkey Posted May 31, 2016 Report Share Posted May 31, 2016 Can anyone give any feedback on the HACman mixture control kit? Standby for a strong opinion from Roger! Link to comment Share on other sites More sharing options...
Roger Lee Posted June 2, 2016 Author Report Share Posted June 2, 2016 In simple terms all Hacman does is change the air pressure in the carb bowl. So far everyone I know that has used it for a while has taken them out. I have a guy here in Tucson had some serious issues with engine quitting. Took his out and all is well. Why try to over engineer something that works, that few really understand before they mess with it and little understanding of how the new system will really affect their engine in any given circumstance. For me it's better to leave a fully functional and operationally sound system alone and keep your engine reliability. BUT if you just have to mess with it good luck because many others haven't been. Link to comment Share on other sites More sharing options...
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