Carb sync note
in 912 / 914 Technical Questions
12 years ago
Here is a little carb sync note.
""I have seen and heard of people thinking all they have to do on the carb syncs is a mechanical sync. Nothing could be farther from the truth."" The mechanical sync will only get you to a starting point for an idle sync, but even now that needs some adjustment to make the carbs synced at idle. You must do a pneumatic sync on the idle and the higher rpm set point. I sync at idle and then I use 3500 rpm to set the higher rpm sync. I have seen many use 2500 rpm to set the pneumatic sync point, but that's too low. Try it next time. Sync it at 2500 and advance the throttle on up to 3500+ and see that you are out of sync. Plus we don't fly that close to idle rpm. At 2500 rpm there just isn't enough air and fuel flow to make an "accurate" and meaningful high rpm sync. Usually once you sync up at 3500+ rpm the sync tends to stay together. Our engines run most of the time above that 3500 rpm seems like a much better place to sync. The compensating tube can't fix everything and if it could then you wouldn't have to do a pneumatic sync.
My Opinion:
I have also seen people not sync the carbs at idle and back the idle stop screws completely out so they are non functional and just set the pneumatic sync for 2500. Now your not only not synced at higher rpms, but idle is not synced and you just vibrating to death while you set somewhere. This seems very counter productive.
You should also have an idle stop point on your throttle in the cockpit so you can't pull it back so far and bend the idle stops. The best way is to have the throttle in the cockpit come to a stop at the same time as the carb idle stops come in contact on the carbs.
""I have seen and heard of people thinking all they have to do on the carb syncs is a mechanical sync. Nothing could be farther from the truth."" The mechanical sync will only get you to a starting point for an idle sync, but even now that needs some adjustment to make the carbs synced at idle. You must do a pneumatic sync on the idle and the higher rpm set point. I sync at idle and then I use 3500 rpm to set the higher rpm sync. I have seen many use 2500 rpm to set the pneumatic sync point, but that's too low. Try it next time. Sync it at 2500 and advance the throttle on up to 3500+ and see that you are out of sync. Plus we don't fly that close to idle rpm. At 2500 rpm there just isn't enough air and fuel flow to make an "accurate" and meaningful high rpm sync. Usually once you sync up at 3500+ rpm the sync tends to stay together. Our engines run most of the time above that 3500 rpm seems like a much better place to sync. The compensating tube can't fix everything and if it could then you wouldn't have to do a pneumatic sync.
My Opinion:
I have also seen people not sync the carbs at idle and back the idle stop screws completely out so they are non functional and just set the pneumatic sync for 2500. Now your not only not synced at higher rpms, but idle is not synced and you just vibrating to death while you set somewhere. This seems very counter productive.
You should also have an idle stop point on your throttle in the cockpit so you can't pull it back so far and bend the idle stops. The best way is to have the throttle in the cockpit come to a stop at the same time as the carb idle stops come in contact on the carbs.
Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell
