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byoungplumbing(at)gmail.c
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 Posted: Sat Jan 21, 2023 10:13 am Post subject: Carbs - enrichener - cold start. |
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"However, I don't quite understand when you said the enricher does notwork unless the throttle is closed."
Maybe I should have been more clear...
When starting a Cold engine, and the starter is turning 300 rpm or there about,,, very little air is going through the carb. Compaired to an engine idleing at 1800 rpm or running at 5000 rpm in cruise.
With the small amount of air going through the carb while being cranked and the main throat of the carb open.... Only a fraction of the air will go through the enrichener circuit at a rather slow rate of speed. If it can't create enough vacuum on the fuel suction port to lift the fuel you don't get fuel or the full benefit. And if you get some fuel through the enrichener, but most of the air is bypassing the enrichener the mixed air is not as rich as it could be.
On a cold engine, if only half the fuel is vaporising, the air fuel air mix exposed to the spark plug will not fire.
If you close the main carburetor and force all the air through the enrichenor, it will pull all the fuel in the enrichener reservoir very quickly, if half is vaporized, the hope is that the fuel air mix at the spark plug will be in the range where it will burn.
When the engine is hot,,, hopefully all of the fuel will vaporize. At that point the engine will start running rough. That is the time to close the enrichener and run on the main jet, and or the needle jet and needle.
If your engine is constantly running too hot you can open the enrichenor richen up the mixture and cool the engine down. If that happens you should rejet the engine.
If the engine does not start right away, let the starter rest for several seconds to allow the enrichenor reservoir in the float bowl to refill.
Quoting from the operator's manual for all versions of rotax 912 page 10 - 19
quote
cold start:
with throttle closed and choke activated (open throttle renders the starting carb ineffective.) close quote
If it makes more sense substitute the word enrichenor for the word "choke" and also the words "starting carb"
Boyd Young
Mkiii 988 hours
Northern Utah
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stuart(at)harnerfarm.net
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| Posted: Sat Jan 21, 2023 10:58 am Post subject: Carbs - enrichener - cold start. |
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Interesting. I did not know they used the 54 on the 912 engine.
In my case it is a 447 with a recoil starter. It starts easily and runs quite well as set up from the factory with the exception that I had to lower the needle clip (raise the needle) one notch. Most of my flying is done at 2000 to 3000 MSL. As long as the prop is loaded everything works as expected but if I should un-load the prop for more than about 30 seconds it will cause the EGT's to climb into the red zone. I don't remember the exact temp that I have set the alarm for but it is below what Rotax says should be the max.
This is usually not a problem at the upper end of the throttle but can be when reducing power for descent and nose down unloads the prop and there you go. This is usually in an RPM band around 4000. It seemed to me that this is caused by the engine running lean and the enricher lever was the closest thing to mixture I had available. Opening the enricher adds just enough fuel so that the EGT stays within specs and seems to work fine.
The only time I had a problem with it was after landing I forgot to close the enricher and as soon as I closed the throttle all the way, the engine quit. Enricher off, throttle closed and a quick pull of the rope had me going again.
Not knowing the details of the design and just looking at it, it appears that the enricher sucks up fuel from the small chamber in the bowl through the dip tube and dumps it into the throat of the carb. My guess was that the vaporization was taking place in the intake and crankcase as the fuel and air mixed and tumbled around.
I haven't had the carb apart in years, so the next time I open it up I will be studying it more thoroughly.
Thanks for all the info.
Stuart
On 1/21/23 12:13, B wrote:
| Quote: | "However, I don't quite understand when you said the enricher does not work unless the throttle is closed."
Maybe I should have been more clear...
When starting a Cold engine, and the starter is turning 300 rpm or there about,,, very little air is going through the carb. Compaired to an engine idleing at 1800 rpm or running at 5000 rpm in cruise.
With the small amount of air going through the carb while being cranked and the main throat of the carb open.... Only a fraction of the air will go through the enrichener circuit at a rather slow rate of speed. If it can't create enough vacuum on the fuel suction port to lift the fuel you don't get fuel or the full benefit. And if you get some fuel through the enrichener, but most of the air is bypassing the enrichener the mixed air is not as rich as it could be.
On a cold engine, if only half the fuel is vaporising, the air fuel air mix exposed to the spark plug will not fire.
If you close the main carburetor and force all the air through the enrichenor, it will pull all the fuel in the enrichener reservoir very quickly, if half is vaporized, the hope is that the fuel air mix at the spark plug will be in the range where it will burn.
When the engine is hot,,, hopefully all of the fuel will vaporize. At that point the engine will start running rough. That is the time to close the enrichener and run on the main jet, and or the needle jet and needle.
If your engine is constantly running too hot you can open the enrichenor richen up the mixture and cool the engine down. If that happens you should rejet the engine.
If the engine does not start right away, let the starter rest for several seconds to allow the enrichenor reservoir in the float bowl to refill.
Quoting from the operator's manual for all versions of rotax 912 page 10 - 19
quote
cold start:
with throttle closed and choke activated (open throttle renders the starting carb ineffective.) close quote
If it makes more sense substitute the word enrichenor for the word "choke" and also the words "starting carb"
Boyd Young
Mkiii 988 hours
Northern Utah
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Ducati SS
Joined: 15 Oct 2009
Posts: 93
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| Posted: Tue Jan 24, 2023 12:22 pm Post subject: Re: Carbs - enrichener - cold start. |
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OK, a bit of background. I am a retired mechanic with eight ASE certs, started my career as a motorcycle snowmobile mechanic, have a top technician award from US Suzuki. Here is the basic carb sequence, 0 to 1/8 throttle= pilot air or pilot fuel, 1/8 to 1/4= pilot jet, 1/4 to 3/4= needle jet and jet needle, 3/4 to full throttle=main jet. If you look at the air intake side of the carb you will see a small hole which feeds air into the enricher circuit. The entire purpose of the venturi is to create a pressure drop which allows atmospheric pressure to push fuel up thru the jets. The main jet has little to no function at start up or idle. The enricher circuit relies on high vacuum ahead of the slide to pull fuel up the dedicated enricher circuit. If the throttle is opened the vacuum drops and the enricher becomes ineffective. This has been going on for at least forty years and is still grossly misunderstood. If your low speed screw is behind the slide it is a pilot air screw, if in front of the slide it is a pilot fuel screw. They adjust the same circuit but must be turned in opposite directions for the desired outcome. Pilot air.. turn out for lean, pilot fuel, turn in for lean.
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stuart(at)harnerfarm.net
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| Posted: Tue Jan 24, 2023 2:11 pm Post subject: Carbs - enrichener - cold start. |
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Yes, this all makes sense and follows what others told me when I first
started working on the 447 setup. Even the air screw being in front of
the slide and the confusion it caused when I first adjusted it. Didn't
figure it out until it dawned on me that I was opening up the air
passage, not the fuel.
My confusion about the enricher being able to cool the EGT remains. If
the enricher becomes ineffective due to loss of vacuum, why does it
still work at 4000 RPM?
Please understand I am not trying to argumentative, I am just trying to
understand why what I have experienced does not fit with what has been
said. Unless there is some other factor at play that I don't know about.
I learned to fly in a 1941 Aeronca Chief and the only temp gauge was for
oil. Also no mixture. Then I had a Cherokee 180 with oil temp and
mixture, nothing else. I was always taught that with mixture keep it
rich at high power settings to keep the engine cool. Without EGT the
procedure was to lean until it stumbled then go a bit richer until it
smoothed out. I also know about leaning either side of peak, but without
the gauge running more rich was the safe bet.
Moving on to the 447 with all the warnings about EGT and frying the
exhaust ports, I put in probes on both cylinders. In flight everything
is fine until I unload the prop or power back to the 4000 range for
descent. The EGT goes up, I open the enricher and it cools back down so
I my mind it must be adding fuel.
Is it possible at that RPM range there is still enough pressure drop to
let some fuel flow through the enricher, enough to cool the EGT?
Also, would it be better to raise the needle (lower the clip) one more
notch or to be looking at a different needle/jet set? I am reluctant to
try something different as it performs perfectly as long as the prop
stays loaded and the fuel economy is pretty good at a little over 4
gallons per hour.
Thanks for the info,
Stuart
On 1/24/23 14:22, Ducati SS wrote:
| Quote: |
OK, a bit of background. I am a retired mechanic with eight ASE certs, started my career as a motorcycle snowmobile mechanic, have a top technician award from US Suzuki. Here is the basic carb sequence, 0 to 1/8 throttle= pilot air or pilot fuel, 1/8 to 1/4= pilot jet, 1/4 to 3/4= needle jet and jet needle, 3/4 to full throttle=main jet. If you look at the air intake side of the carb you will see a small hole which feeds air into the enricher circuit. The entire purpose of the venturi is to create a pressure drop which allows atmospheric pressure to push fuel up thru the jets. The main jet has little to no function at start up or idle. The enricher circuit relies on high vacuum ahead of the slide to pull fuel up the dedicated enricher circuit. If the throttle is opened the vacuum drops and the enricher becomes ineffective. This has been going on for at least forty years and is still grossly misunderstood. If your low speed screw is behind the slide it is a pilot air screw, i!
f in front of the slide it is a pilot fuel screw. They adjust the same circuit but must be turned in opposite directions for the desired outcome. Pilot air.. turn out for lean, pilot fuel, turn in for lean.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=510021#510021
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Ducati SS
Joined: 15 Oct 2009
Posts: 93
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| Posted: Tue Jan 24, 2023 2:48 pm Post subject: Re: Carbs - enrichener - cold start. |
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Because at starting RPM there is precious little flow thru the venturi so if you raise the slide you have next to nothing left in the way of air flow to reduce pressure. At higher operational speeds there is significant airflow and drop in venturi pressure.
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lcottrell
Joined: 29 May 2006
Posts: 1494
Location: Jordan Valley, Or
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| Posted: Tue Jan 24, 2023 3:37 pm Post subject: Carbs - enrichener - cold start. |
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Stuart, I am not a mechanic , but I have flown several 447's. If you
need to cool your egt's when you chop the throttle, then your needle
jet is set wrong for your altitude.
Larry
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Airknocker
Joined: 26 Apr 2014
Posts: 6
Location: ND
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| Posted: Tue Jan 24, 2023 6:11 pm Post subject: Re: Carbs - enrichener - cold start. |
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Ok, now I get it. When Boyd originally said that "the enricher does not work unless the throttle is closed" I took it too literally.
What he was actually saying is "at cranking speeds there is not enough vacuum to make the enricher work if the throttle is not closed"
That makes total sense. It also makes sense that the enricher has some effectiveness with the throttle open once the engine is running.
Some days I feel dumb. Thanks for explaining it to me.
Larry, I don't remember the jet and needle numbers off the top of my head but they are the correct ones for sea level and up to my elevation based on the charts from Bing. It was my understanding that changing up the chart would make the engine run more lean because they are for less dense air at altitude.
I'll have to delve into this a little more once I get back to flying.
Thanks everyone!
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Richard Pike
Joined: 09 Jan 2006
Posts: 1671
Location: Blountville, Tennessee
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| Posted: Tue Jan 24, 2023 8:19 pm Post subject: Re: Carbs - enrichener - cold start. |
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Copied & pasted from the CPS tech articles page:
This is why - when the gasket at the bottom of the plunger that sits on the bypass was damaged - we could not get the carb on the 277 properly jetted.
Yes, the throttle slide has to be all the way down to generate enough vacuum for the enrichner to work when starting.
However, once the engine is running, then there is enough vacuum throughout the carb to constantly suck fuel through the enrichner bypass if the plunger is not sealing it off for whatever reason.
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Kolb MKIII N420P (420ldPoops)
Kingsport, TN 3TN0
Forgiving is tough, being forgiven is wonderful, and God's grace really is amazing. |
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aoldman(at)xtra.co.nz
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| Posted: Tue Jan 24, 2023 11:37 pm Post subject: Carbs - enrichener - cold start. |
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As I understand it as you increase altitude the air thins out so you also need to reduce the amount of fuel going into the engine ,other wise the mix becomes rich, less air = less fuel to keep the ratio the same .I have about 500 hours flying behind both 447 and 503, 447 arronca and 503 MK3 kolb, I know they both started very well with the throttle closed and not well with it opened even a little, had some time convincing those that were used to flying lycommings and such to change their starting habits
Regards
Downunder
| Quote: | On 25/01/2023 15:11 NZDT Airknocker <airknocker(at)harnerfarm.net> wrote:
Ok, now I get it. When Boyd originally said that "the enricher does not work unless the throttle is closed" I took it too literally.
What he was actually saying is "at cranking speeds there is not enough vacuum to make the enricher work if the throttle is not closed"
That makes total sense. It also makes sense that the enricher has some effectiveness with the throttle open once the engine is running.
Some days I feel dumb. Thanks for explaining it to me.
Larry, I don't remember the jet and needle numbers off the top of my head but they are the correct ones for sea level and up to my elevation based on the charts from Bing. It was my understanding that changing up the chart would make the engine run more lean because they are for less dense air at altitude.
I'll have to delve into this a little more once I get back to flying.
Thanks everyone!
--------
"I don't care, I'm still free. You can't take the sky from me" - Josh Wedon
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http://forums.matronics.com/viewtopic.php?p=510027#510027
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