High idle speed

by Dominic Pecheur in 912 / 914 Technical Questions 12 years ago

I know this can be a very contentious issue. My local inspector is not happy with the idle speed on a 912ul, it is 2000 revs.
When balancing the carbs the engine is so rough at around 1500 revs that it almost cuts out and is impossible to get the needles equal on the pressure tester. To get the needles equal i altered one of the throttle cable lengths and it all quietened down with the needles now equal. Great- not really because now the idle is at 2000 revs and i cant get it to drop back down.
Both mixture screws were set at 1 1/2 turns.
If the main jet and needle are worn will this create a rich mixture - thus more revs - by allowing fuel to seep past the needle even at idle? These items have not been renewed for some time.
All advice much appreciated as always.

Re: High idle speed

by MARK JACKSON » 12 years ago

MARK JACKSON

12 332 11

Hi Dominic

You need to watch the video on carb balancing so you understand what you are doing does to the carb balance and engine speed.

If this is your first time, you might be better getting someone to do it for you and watch what they do.

Cheers Mark

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Re: High idle speed

by Roger Lee » 12 years ago

Roger Lee

128 5.28k 4.98k

Hi Dominic,

I'm with Mark. The carb sync at idle should not be a problem to set.You need to get a better handle on the cable length and idle set. That UL should idle smooth down at 1650-1750ish. You should not need 2000 rpm or any thing even close. With the cockpit throttle all the way back at idle unscrew the idle stop screws so they don't tough the idle stop plate. The carb's throttle arm should be all the way back and should not travel any further.If it isn't like this then the carbs aren't set up equally and you'll have issues trying to balance. The carb should be fully closed. If the carb throttle arm still moves backward then adjust the cables to get the carb arm all the way back on each carb. Now you can follow procedure and engage the idle stop screw and do the mechanical balance. Then follow it with the pneumatic balance. I would sync the carbs at 3500 rpm and not the book 2500 rpm. That's too low and close to idle and doesn't flow enough air/fuel to get a good sync at flight rpms. I just demonstrated this to a group of people on an inspection class. I use a set of gauges and when set at 2500 rpm for sync the carbs become un-synced as the rpm is raised to
3500- 4000 rpm. Setting it at 2500 rpm many times gives people the vibration in the 3000 range when throttling back to land. You need to flow enough air/fuel to be more representative of the fuel/air flow at flight rpms not lower idle rpms. When carbs are set up properly it usually takes me 5-20 min. to sync the carbs. Even the tough ones any more only take 20-30 min. If you turn the engine on and off for each adjustment it will take longer. I use gauges and not the electronic devices because it's so much easier to see what each carb is doing and which carb I want to adjust.Plus I can diagnose with gauges and you can't with the electronic ones. Carb setup is everything. Do it right and carb sync is easy. That said some throttle setups in some planes are a PITA because of the system. Heavy duty thick cables can be a bigger issue to sync than the small cables that move through the cable sheath smoothly and return back to the last position each time. Some throttle systems have too much friction inside the cable systems or slop in the throttle mechanism which makes some syncs frustrating. Recognition of these systems right up front will alert you that the sync may never be perfect or exact time after time. If you have any problems or questions you can give me a call.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: High idle speed

by Dominic Pecheur » 12 years ago

Dominic Pecheur

3 3 0

Thanks for the input. I must say though that the choke is fully off and the throttle cables are both slack(ish) with the arms not touching any stops, with no further movement available on either throttle arm. In this position i can get about 1500rpm but it is VERY rough. as soon as i move the right hand cable via the adjuster like i say the engine smooths out and the needles on the balancer (not electronic)come almost level but this increases the revs to 2000. I will watch the video as advised and have another go.

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Re: High idle speed

by Dominic Pecheur » 12 years ago

Dominic Pecheur

3 3 0

Right then videos watched and understood. It's a bit late to go have another crack at it. Videos are well worth the cash, thanks again.

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High idle speed

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