Hovertrekker
New Member
Hi all. New guy to the forum here and looking for some advice please.
I want to get your opinion on a project of mine. I took a Rotax 493 twin (2002 Ski doo MXZ 500 snowmobile engine) and mounted it upside down in a hovercraft. Yeah I know this is aviation forum, but we fly too, just 8 inches off the ground
I should have asked some experts before I went to the trouble because I'm having some problems with it not running properly. It ran great in the previous machine mounted the right way up, but this craft requires it be plugs down.
Do you think such an engine like the 493, 593, 793 etc could run upside down reliably? Have you ever heard of anyone doing this? I know the 582, 583, 503 series of engines are often run upside down in ultralight aircraft and hovercraft, but they don't have reed valves in the intake and RAVE valves in the exhaust.
My concern is that the reed valves and rave valves are angled downward when mounted plugs up, but when you turn them upside down they are now pointed upward against gravity, This causes the fuel to have to travel upward to the reed valves. I can't keep a steady throttle rpm while cruising, it cycles up and down all the time. Just looking for some confirmation to scrap this project or not.
I'm afraid I may be the only person to have ever actually mounted this type of engine upside down. So far I've never heard of anyone else doing it. The engine runs very smoothly throughout the rpm range and is mounted properly, so not a vibration issue at all. (It's a belt drive reduction setup and the fan is properly balanced.) Also I took great precaution to ensure the oil pump/water pump gear is properly/permanently lubricated. The carb intake boots have the "balance bottle" thing, which of course can't be used in a plugs down configuration otherwise fuel would puddle and fill the thing up! So I removed it and plugged the holes. The carbs are flipped 180 degrees, but the boots can't be turned. It's unknown to me that this has anything to do with my problems.
So the problem is the engine rpm cycles up and down by about 500rpm constantly, it feels like fuel is building up, rushing in, burning off, and then repeating. When the engine is cold it starts up beautifully and runs perfectly with the choke on at about 4000rpm, and it stays right there perfectly, it's after it warms up and the choke is removed that I have this problem at around 4500 - 6000 rpm. It just won't run at a steady rpm, always going up and down. Sometimes it reacts well to a little bit of choke, making me think I'm running lean, but my plugs are very dark and the EGT are quite low around 900-1000 deg. making me think I'm running rich!! I'm normally cruising this thing between 5000 - 6000 rpm, usually not more than that although max rpm is around 8000. Also it doesn't like to run in between this rpm range, if I give it a bit of throttle, it want's to jump right up to 6000 plus, and If I back off a bit, it want's to drop down to 5000 or so. If I'm running rich overall, why does a little choke give me some needed rpm when it starts to bog down? I've cleaned the carbs several times, changed plugs several times, tried so many things to figure this out. (I'm running mikuni 500 main jets which are the stock engine jets). I'm going to try a smaller main jet and see if it makes any difference.
Thanks,
Dave in Indiana
I want to get your opinion on a project of mine. I took a Rotax 493 twin (2002 Ski doo MXZ 500 snowmobile engine) and mounted it upside down in a hovercraft. Yeah I know this is aviation forum, but we fly too, just 8 inches off the ground
I should have asked some experts before I went to the trouble because I'm having some problems with it not running properly. It ran great in the previous machine mounted the right way up, but this craft requires it be plugs down.Do you think such an engine like the 493, 593, 793 etc could run upside down reliably? Have you ever heard of anyone doing this? I know the 582, 583, 503 series of engines are often run upside down in ultralight aircraft and hovercraft, but they don't have reed valves in the intake and RAVE valves in the exhaust.
My concern is that the reed valves and rave valves are angled downward when mounted plugs up, but when you turn them upside down they are now pointed upward against gravity, This causes the fuel to have to travel upward to the reed valves. I can't keep a steady throttle rpm while cruising, it cycles up and down all the time. Just looking for some confirmation to scrap this project or not.
I'm afraid I may be the only person to have ever actually mounted this type of engine upside down. So far I've never heard of anyone else doing it. The engine runs very smoothly throughout the rpm range and is mounted properly, so not a vibration issue at all. (It's a belt drive reduction setup and the fan is properly balanced.) Also I took great precaution to ensure the oil pump/water pump gear is properly/permanently lubricated. The carb intake boots have the "balance bottle" thing, which of course can't be used in a plugs down configuration otherwise fuel would puddle and fill the thing up! So I removed it and plugged the holes. The carbs are flipped 180 degrees, but the boots can't be turned. It's unknown to me that this has anything to do with my problems.
So the problem is the engine rpm cycles up and down by about 500rpm constantly, it feels like fuel is building up, rushing in, burning off, and then repeating. When the engine is cold it starts up beautifully and runs perfectly with the choke on at about 4000rpm, and it stays right there perfectly, it's after it warms up and the choke is removed that I have this problem at around 4500 - 6000 rpm. It just won't run at a steady rpm, always going up and down. Sometimes it reacts well to a little bit of choke, making me think I'm running lean, but my plugs are very dark and the EGT are quite low around 900-1000 deg. making me think I'm running rich!! I'm normally cruising this thing between 5000 - 6000 rpm, usually not more than that although max rpm is around 8000. Also it doesn't like to run in between this rpm range, if I give it a bit of throttle, it want's to jump right up to 6000 plus, and If I back off a bit, it want's to drop down to 5000 or so. If I'm running rich overall, why does a little choke give me some needed rpm when it starts to bog down? I've cleaned the carbs several times, changed plugs several times, tried so many things to figure this out. (I'm running mikuni 500 main jets which are the stock engine jets). I'm going to try a smaller main jet and see if it makes any difference.
Thanks,
Dave in Indiana
