carb needle clip

[chanik]

The bing carbureators have 4 possible settings for the jet needle that sets baseline mixture. Rotax recommends #3 clip setting but I can't find any reference to how much this affects mixture. I seem to fly with EGT of 1280 which is a bit rich but not too bad. Would the #2 setting more like 1400 or 1600? has anyone tried different clip positions and recorded the difference in EGT and/or fuel consumption? Should it really be #2 in coller clime and #3 for better cooling in summer? In general with 100LL you would want a slightly leaner mixture since it is less detonation prone and has a bit more energy than E10 mogas. I hate the "just use #3" with no explanation.

[Roger Lee]

Hi Kurt,

 

Changing the needle clip one position is approximately an 80F change. I have used number 4 position this last summer. It did help all my temps. Approx. 80-90F EGT,

15-20F oil temp, 15-20F CHT. Fuel only changes a few tenths of a gal (.3 -.5).

 

These are not hard and true, but should be some what representative. You can try the #2 position and take a flight and watch all the temps. The air is cool and damp where you live. If you fly somewhere where it is cold and dry beware. Cold and dry will make the mixture leaner.

 

I am back in the #3 position now for the winter with 2" tape over the top part of the radiator and may go back to #4 this summer.

[chanik]

Hi Kurt,

 

Changing the needle clip one position is approximately an 80F change. I have used number 4 position this last summer. It did help all my temps. Approx. 80-90F EGT,

15-20F oil temp, 15-20F CHT. Fuel only changes a few tenths of a gal (.3 -.5).

 

These are not hard and true, but should be some what representative. You can try the #2 position and take a flight and watch all the temps. The air is cool and damp where you live. If you fly somewhere where it is cold and dry beware. Cold and dry will make the mixture leaner.

 

I am back in the #3 position now for the winter with 2" tape over the top part of the radiator and may go back to #4 this summer.

Cold and dry will run the engine leaner but it seems for you the dominant effect desired was engine cooling with the much richer setting since your air was quite warm. I guess #2 setting should be fine so long as one stayed under the 1650°F max EGT specification for 912s as well as under 150°C for CHT since that would also rise slightly with leaner mixture.

[FastEddieB]

No real input on needle position, but thought I'd point to a couple of interesting articles by Mike Busch:

 

http://www.jpitech.com/press_releases/Bush_Article.pdf

 

http://www.avweb.com/news/savvyaviator/savvy_aviator_59_egt_cht_and_leaning_198162-1.html

[Roger Lee]

Hi Guys,

 

When you all look at max temps for the 912 in the manuals make sure you are looking at the proper engine. Some of the stats are for the 912UL 80 hp and some for the

912ULS 100 HP.

 

There is a difference.

The max EGT for the ULS is 1616F and nominal is 1470F. I have seen several EGT probes in the wrong place. They are suppose to be 100mm or 3.93" from the exhaust flange. If they are closer your EGT will show hotter and if farther then the EGT's will be cooler. The max CHT for the ULS is 275F, but with a 50/50 mix of coolant that is 248F for the coolant so that means the CHT is there, too. If you had Evans coolant then it would be 275F, but in our tight cooling the Evans makes the engine run too hot and you loose any benefit from the Evans. We don't have separate coolant gauges, but that's ok since the CHT and coolant are very close. The max oil temp is 266F. These temps can be influenced by OAT, prop/engine load and gauge inaccuracies.

 

Kurt,

 

If you use the #2 clip position then so long as you stayed under 1500F for your normal operations you will be fine. I'm never a believer of taking engines to a max temp because nothing good will come of it over time and more wear will ensue over time and if one little thing changes while your at max then you may loose the engine. In my young years, I've been there done that.

 

Engine dependability and longevity is king.

[chanik]

I think 1500 sounds like a good compromise too. Never want to run close to the edge with anything WRT airplane operation. Now, when is the fuel injected 912ULS coming?

[chanik]

I've been running with the carbs clipped to #2 setting for awhile now. On a 85deg day, I get up to about 1420 EGT. 195 CHT and 215 oil so all very reasonable. Roger was right, it did add about 80F to the EGT. However the fuel consumptiuon is much lower. I used to burn 5.1Gal/hr at 5200RPM now down to 4.5. AT 5000RPM it is 4.0 down from ~4.7. So now I can fly 8hr legs instead of my usual 7hr ones

[chanik]

More flying data... Since I am almost always right around 1400F EGT after many more flight hours, I'm starting to wonder if the #1 needle position isn't the correct one to use. That should put me at ~1480F and I see in the ROTAX installation manual, section 11.2 that they recommend 1470F as the nominal desired EGT, with 1580 max cruise and 1616 takeoff/ 5 minutes. I'm not sure this is a good idea though. With Lycont engines, optimal mixture was desirable for engine reliability but I think this was more due to lead buildup issues if one ran rich. So running 100LL, I would definitely use #1 because the EGT runs cooler with the higher octane anyway and Lead is an acute problem with the 912. But I seldom run 100LL. Rotax doesn't say anything about engine life except stay under the max temperatures. But suggesting a nominal 1470F and also suggesting the carb needles at #3 is inconsistant. Maybe they are just paranoid about old or poor quality mogas so they tell people to run extra rich. So my real question is whether it is better for the engine to run optimal mixture at the higher EGT or is it harder on the engine or does it not matter? The extra fuel savings and slight extra power doesn't justify jeopordizing the engine but maybe it is better for it. Any Rotax gurus out there know the answer?

[Roger Lee]

Hi Kurt,

 

I wouldn't go to the #1 position. There are other things at play to consider than just EGT temp. Leaning the engine out to this point could easily shorten the engine life if it doesn't seize or score some metal parts. You don't fly just under one condition and fly in a multitude of scenarios plus you may have a poor EGT setup, which by the way I have seen a a few CT's. If the temps are not expertly verified and or the EGT system is off a little you'll toast a $20K engine. #3 is factory and #2 could be used under the right flight characteristics. You want to raise your temps more add a little pitch in the prop. There is nothing wrong with running 1400F EGT. You have to remember all of Rotax's comments are usually based on a constant speed prop use and not a fixed pitch and they mix the 912UL and 912ULS data together. If you don't know how to separate or interpret this data you can do more harm than good.

Leaning more than #2 at sea level where you live could be asking for engine issues. Leaning should be considered at higher operating altitudes and not sea level. If you run close to a lean situation and some thing changes out of your control during flight your toast. When winter comes the colder air will lean the mixture even more. If your running cool at #2 at sea level I would be extremely suspicious of the temp readings themselves.

 

 

My quote on the Rotax forum is:

"The worst thing that ever happens to a Rotax engine is it's owner".

[chanik]

I want to understand what's going on here in some depth though. Here's one of the better primers on leaning I found:

http://www.huygens.org/sape/pilotage/Engines/index.html

The only diff is that we don't have to worry about softening metal due to CHT since we have nice liquid cooling, but the rest is pretty representative of IC engine leaning.

Basically, I'm moving up from the left on the EGT graph in Figure 2. I doubt my thermocouples are off by much as they track each other well and both rose the expected ~80F with the needle change. The fuel consumption and CHT, oil tmep rise also dovetail perfectly with what I expected. 1470F makes good sense as a target as it is still ~7% Rich of Peak and pretty close to maximum power. So one thing I would conclude from this study is that I want to stay consistently below 1500F. 1616 may be the ROTAX max but you would be running on the edge of detonation to see those temperatures. One other big warning sign to watch out for is engine roughness. In a 4-cyl, one cyclinder inevitably gets close to detonation first and the engine runs noticeably rougher.

 

You make a good point about the variable pitch derived rotax numbers though. THAT has a much more critical impact on detonation onset than maybe being a couple points low on octane with poor gas. Given the number of people I've seen on this forum running much too coarse a pitch on their props (including my plane when I bought it), I think that is the reason ROTAX wants to be far rich with their default mixture recommendation.

[207WF]

This is an interesting question in view of the tradeoff between oil temperatures and mixture at altitude. If I went one notch richer on the clip, Roger's experience suggests I could get 15-20 degrees cooler on the oil. That would be a big plus at lower altitudes and high OATs. But, I think that my plane runs relatively too rich at altitude. For example, fuel burns this summer on the trip to OSH were higher at DA 14,000 and 5200 rpm (about WOT) than at 9000 feet DA and the same rpm (about 70% throttle). I estimate a 0.4 gph difference. At the higher altitude true airspeed was maybe 4 knots slower. I suspect if I had been leaner at the higher altitude I could have better speed and lower fuel burn, although higher temps. What I want is a mixture control! WF

[Roger Lee]

Hi All,

 

This is why you can not take things at face value without factoring in your personal flying characteristics. You must factor things in like engine rpm at WOT (prop/engine loading), you must factor in your normal flying altitude (this is a big one), Time of year (hot vs cold) aircraft weight during the average flight and few other minor items. If you don't know how this figure and how to determine what all the info on your EMS tells you then it is better to leave the needle clip alone as most of the world Rotax users do. You have to know if your EMS temps are correct. If you fly under 5K-6K most of the time leave the needle in the #3 spot. If you fly out of a higher elevation airport or fly 7k or higher most of the time then you could move it and keep the temps up a little and run a little leaner for better mixture control. The down side of that is when you decide to fly at lower altitudes or take off at low elevations. The OAT temp will factor into this too.

Rotax sets the carbs to run at 1K from the factory.

The higher you fly the richer the mixture and the lower in altitude the leaner. The colder the OAT the leaner and the hotter the air it runs a little richer. The easiest way to drop engine temps is to unload the engine with a little flatter prop pitch. This can make a 15F oil temp decrease on climb.

[chanik]

Some more info.

 

I flew today, OAT 80F. CHTs stayed under 205 and oil under 215F. EGT only 1370F at 5000RPM. Roger suggested I look at the EGT probe position as my temps seem suspiciously low for the #2 clip position. If the probe is further down the exhaust stack, the temperatures will read lower. In fact, my probes are closer than they are supposed to be. More like 60mm away from the flange when 100mm is the suggested distance. I think I may be so low because my prop is pitched a bit shallow. I did several careful runs at 2000' AGL. The prop slowly winds all the way up to 5750 after 20 seconds of level flight with WOT.

[Roger Lee]

Hi Kurt,

 

5750 at sea level may not be all that bad if you usually fly at 7500' + all the time from your sea level field. If you don't then adding back about 1.25 degrees should get you down to approx. 5550 WOT.

You are right that if those probes are only 60mm out then the EGT's should be running hotter. Adding pitch will raise all the temps and net you some better cruise speed.

The key here is to run WOT for your test at your every day normal or average altitude. When you fly out of sea level or high elevation airports then of course those numbers will change if you test it's WOT again.

[sandpiper]

Repitching my prop earlier this summer from max 5400 to 5550 seems to have eliminated my high oil temp problem. Before, I had to reduce power or step climb or else I would hit redline. Now I can maintain full throttle and climb from 175msl to 9500 staying out of the yellow or maybe 10-15 degrees into the yellow starting with 90K and decreasing to 80K by the time I level off.

[chanik]

Having flown for 18 months with the #2 clip with cool temps throughout winter to summer and high to low altitudes, I decided to try the #1 position. It is less of an EGT boost. I was expecting another 80F rise in EGT, up to something just shy of 1470 (which should be max EGT) but only saw about 40F rise from the #2 setting. 65Deg day and 40% humidity and I was showing 1420 max EGT. Will have to put some more time on it but it seems ROTAX is very much on the rich side with their recommendations.

[Safety Officer]

Hi Kurt,

 

I would be careful. Given the right circumstances combined with the right outside temps you could cause damage before your EGT's register a problem. That's the way it happens to most. The cost of such damage would only start at $4500 and go up. Not to mention long term slow damage you may not see for a while until it is too late. No one recommends the #1 position any where. EGT's are only a single tool to look at not a complete picture. My question would be: Why run on the edge when it would take only a few seconds to cause serious damage when you know the other clip positions are safe?

[FastEddieB]

One thing I don't recall seeing mentioned is spark plug condition.

 

Here are mine at my recent annual (click for larger image):

 

 

IMG_4857 by fasteddieb, on Flickr

 

I would say they appear to show a mixture that's a tad rich, being a bit "sooty" if powdery brown/tan is the goal.

 

I've never messed with my carb clips, and I'm not sure what position they're in. Looks like I might be able to get away with just a tad leaner.

 

One caveat - I had one of those clips break on a BMW motorcycle. If it does, you'll lose power to that side completely. Might be worthwhile having some spares around and maybe replacing them proactively if you're going to be in there futzing with them anyway.

 

edited to add: here's a thread containing an image of a broken "e-clip":

 

http://pilotodyssey.com/PO/viewtopic.php?f=2&t=743&view=next

 

That's exactly what mine did.

[Roger Lee]

Hi Eddie,

 

Did you run your engine up at 4K+ rpm for at least 10 minutes before checking the plug color? If the engine had idled at all then the plug color would be skewed toward the darker side. When the plug check is done properly I haven't seen any with dark to black plugs. You must run the engine up completely out of the idle circuit.

 

Charlie Tango could probably get away with that at is elevation and altitudes he flies, but your at sea level.

[FastEddieB]

Good point.

 

I'm actually aware of the need for that, having shut off bikes at highway speeds and coasted to a stop to check the plugs.

 

There probably was some idling involved. Next time I'm scheduled to take the cowling off I may do what you suggest, shutting down right after a 4,000 rpm. 10 minute runup.

 

BTW, these plugs had most recently done that round trip to Page/Tucson, running mostly 100LL with Decalon. I went ahead and replaced them, but no observable lead, which is a good thing.

[chanik]

I am being extra careful and have read aboiut everything there is out there on leaning and EGTs, but like I said I have 18months of fine running on #2 clips including hot summer trips to AZ and Nevada. The real thing to watch carefully is the CHTs. Most technical documents, including those from Lycoming and COntinental, don't even care what the max EGT is. They recommend finding the max and then using EGT shift to figure out how to run a certain amount rich or lean of peak. And if CHTs get too high, the 'black-belt' technique is to run very LOP Running 50F ROP is about where CHTs max out for engine stoichiometry and that's where I'm set now but they are only maybe 3F higher than with #2 and 12F higher than when I was on #3 clips.

http://www.lycoming.com/support/publications/service-instructions/pdfs/SI1094D.pdf

 

Why mess with it? Well saving $3 an hour in MOGAS isn't the motivation. But range and duration is, which for me is now 15% to 20% greater. On 100LL, leaner helps with less fouling. The Bings effectively stop compensating above 7000ft so for all my high altitude flying, I have more power since otherwise I would be extremely ROP over the Sierras. I think ROTAX is just trying to allow for an extreme worst case with the #3 recommendation: steep climb out of Tuscon, mid summer, dry with 3 month old MOGAS and a coarse pitched prop. You want super rich for that scenario.

[Roger Lee]

Hi Kurt,

 

Do not apply any Continental or Lycoming rational to the Rotax. That has already caused tens of not hundreds of thousands of dollars damage over the last 20 years from people doing that. These carbs are not a standard slide and cable operated carbs. They are air pressure differential slides. CHT's mean almost nothing to you as the heads are water cooled. The OAT has little to do with EGT's. Fuel is the main cooling agent in the engine wait air and coolant behind that. Changing from #2 to #1 clip position won't get you 15% - 20% better fuel economy. Plus what tiny amount you might save in fuel may be the tip of the iceberg in damage. Don't forget 91 oct. may play a detonation part in this if something is off a little with that. There are too many variables to take such a gamble and flying out of sea level.

I have seen the disastrous results of owners playing with this from the ruined engines in the Rotax classes. The last guy was last Feb. It cost him $6K to fix his detonation damage. He said it happened so fast no gauge caught or showed it until the damage was done.

Some things just aren't worth the throw of the dice.

 

Like I said, if you were flying up at Ed's elevation and regular altitude it may be different, but you don't live there.

[chanik]

Hey Roger,

I'm being very cautious, but I think the numbers and data from ROTAX and elsewhere back me up.

 

Of course the CHT values are different (lower) with liquid cooling but the combustion chemistry and graphs are really the same. I have indeed measured about 17% lower fuel consumption with the leaner setting and using fuel to air-condition the cylinders is a poor practice. Anyway I am STILL well ROP, hard to say how much becuase I can't hunt for the peak. Now I know this goes against many pilots' training but fundamentally: leaning does not cause detonation. Bad/contaminated fuel followed by heavy engine loading are the riskiest things. Carbon and lead deopsits are bad due to the tendency to cause preigniton but depostis come from running too rich. High CHT also is a big contributing factor and running a bit ROP yeilds the highest CHTs due to max power combined with lack of fuel vapor cooling as you mentioned (but it is a tiny effect, fuel vapor is certainly not the main cooling agent in an IC engine) I'm skeptical that the last guy got detonation by leaning with with carb clips, but would like to talk to him about what happened if you can arrange it. Sounds like bad fuel, maybe some JET-A or diesel to be so quick or maybe some predetonation due to lead deposit.

 

The thing that still puzzles me is why my engine is biased to run so rich. Rotax states that nominal EGT should be 1470F but I can't get close to that even on #1 needles. Maybe that is just with their airbox. What do other people with EMS systems see for EGT readings? Anyway I plan to measure this for awhile making sure the CHT especially stays within comfortable bounds, else I'll set them back to #2. I am still well within all operational limits for the powerplant according to ROTAX so feel pretty comfortable with this test configuration. I doubt ROTAX would even have the leaner clip positions if they were such a risk of instant engine damage.

 

On compensation, I have tracked EGTs on slow, constant RPM climbs and descents many times and the EGT stays flat up to about 7K then steadily drop with altitude indicating richer operation. I think Roger or Jeremy mentioned that ROtax confirmed the Bings only compensate up to about there. I like to test and verify things myself, as you can tell. It's that engineering thing.

[Roger Lee]

Hi Kurt,

 

Things to consider. First you may be trusting your gauges too much. The EGT probes need to be 100MM from the exhaust port. Having the probes closer or farther will cause the temps to be higher or lower. The EGT's need to be checked for accuracy and if a Dynon is used the probe type number in the program should be checked. These carbs are not a Rotax carb, but supplied by Bing. The other problem is any damage you can see on a gauge will be after it has already occurred.

[FredG]

Steady state cruise, 5240 rpm, 113 kts IAS, egt L 1367 F, R 1324 F, oil temp 205 F. 3200 ft altitude. Carb needle in stock position.

FYI.

fg