Carb bowl obstruction - Don't be stranded
in General Tech Discussion
12 years ago
Hi Guys,
Your flying along and the engine looses power to about 3800-4000 rpm and runs rough. It could be a couple of things, but first suspect a carb bowl main jet obstruction. Here is a simple tip that may keep you flying and get your rpm back and if that fails how to do a self rescue and not be stranded and all you need is a screwdriver. You can do this fix and be back in the air in 30 minutes. I have done this and know it works.
The debris may never be detected during a run up on the ground. There usually isn't enough fuel flow for a good main jet suction and no sloshing or turbulence of the fuel in the carb bowl. In cruise flight you now have a good fuel flow, turbulence within the bowl and plenty of upward suction to draw that piece of debris right up against the main jet.
Here is an in air fix you can try, but there is no guarantee. While you are doing this next maneuver you should be looking for an appropriate landing spot. Either an airfield or a decent road.
Your first instinct is to go full throttle to get more rpm.That only holds the debris harder against the main jet, but it is human nature to try for more rpm. Once that fails and providing you have a little altitude pull the throttle back to idle and glide for about 15 seconds with a little side to side "S" turns. When the throttle is back at idle the main jet is not in play and the suction is gone allowing the debris to hopefully fall away and settle to a lower point. The idle jet is separate from the main jet. After your 15 second glide slowly advance the throttle back to cruise rpm and head for an airport.
If this in air idle rpm fix doesn't work then continue to a safe landing.
Once on the ground then take off the upper cowl. Pop the carb retention spring off, then loosen the carb flange screw and push the carb back out of the rubber flange socket. It's snug so push hard. If you have the older stainless steel fuel supply lines you can now lift the carb upward enough to clear the drip tray to take the carb bowl off. If you have the red Teflon fuel lines all the better because you can lift the carbs and rotate them out towards you to have a little better access to the bowl and bale. If you don't have a drip tray then this is real easy.
Now push or pry with your screwdriver the carb bowl bale back towards the firewall. Drop the carb bowl and look for your debris and toss the bad fuel out. Put the carb bowl back in place making sure it fits into its mounting outside edge grove and pull the bale back in place. Be mindful of the bowl gasket. Push the carb back into the rubber flange until you hear and feel it kind of seat in place. You will feel it snap in place and the carb will look like it is in far enough that it can't go any further. Tighten the rubber flange retention ring screw and do the same procedure to the other side. Once both are done put the cowl on and test run at full throttle for about 5 minutes.
If you have the stock Rotax airbox then you will need to loosen the airbox side rubber flange and slide it out of the way to push the carb back out of the rubber flange. So both sides of the carb gets the rubber flange loosened.
You should now be ready to take off and head for home. This has helped me rescue others a few times so hopefully it can help you if the need ever arises.
It works.
http://www.youtube.com/watch?feature=player_embedded&v=JdS7OtbcxwY
Your flying along and the engine looses power to about 3800-4000 rpm and runs rough. It could be a couple of things, but first suspect a carb bowl main jet obstruction. Here is a simple tip that may keep you flying and get your rpm back and if that fails how to do a self rescue and not be stranded and all you need is a screwdriver. You can do this fix and be back in the air in 30 minutes. I have done this and know it works.
The debris may never be detected during a run up on the ground. There usually isn't enough fuel flow for a good main jet suction and no sloshing or turbulence of the fuel in the carb bowl. In cruise flight you now have a good fuel flow, turbulence within the bowl and plenty of upward suction to draw that piece of debris right up against the main jet.
Here is an in air fix you can try, but there is no guarantee. While you are doing this next maneuver you should be looking for an appropriate landing spot. Either an airfield or a decent road.
Your first instinct is to go full throttle to get more rpm.That only holds the debris harder against the main jet, but it is human nature to try for more rpm. Once that fails and providing you have a little altitude pull the throttle back to idle and glide for about 15 seconds with a little side to side "S" turns. When the throttle is back at idle the main jet is not in play and the suction is gone allowing the debris to hopefully fall away and settle to a lower point. The idle jet is separate from the main jet. After your 15 second glide slowly advance the throttle back to cruise rpm and head for an airport.
If this in air idle rpm fix doesn't work then continue to a safe landing.
Once on the ground then take off the upper cowl. Pop the carb retention spring off, then loosen the carb flange screw and push the carb back out of the rubber flange socket. It's snug so push hard. If you have the older stainless steel fuel supply lines you can now lift the carb upward enough to clear the drip tray to take the carb bowl off. If you have the red Teflon fuel lines all the better because you can lift the carbs and rotate them out towards you to have a little better access to the bowl and bale. If you don't have a drip tray then this is real easy.
Now push or pry with your screwdriver the carb bowl bale back towards the firewall. Drop the carb bowl and look for your debris and toss the bad fuel out. Put the carb bowl back in place making sure it fits into its mounting outside edge grove and pull the bale back in place. Be mindful of the bowl gasket. Push the carb back into the rubber flange until you hear and feel it kind of seat in place. You will feel it snap in place and the carb will look like it is in far enough that it can't go any further. Tighten the rubber flange retention ring screw and do the same procedure to the other side. Once both are done put the cowl on and test run at full throttle for about 5 minutes.
If you have the stock Rotax airbox then you will need to loosen the airbox side rubber flange and slide it out of the way to push the carb back out of the rubber flange. So both sides of the carb gets the rubber flange loosened.
You should now be ready to take off and head for home. This has helped me rescue others a few times so hopefully it can help you if the need ever arises.
It works.
http://www.youtube.com/watch?feature=player_embedded&v=JdS7OtbcxwY
Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell
