Carb Ice Questions

[sonex1374]

Sosthène,

Gary Motley went thru a similar problem on his first aerocarb after several hundred hous of operation. The symptoms he described sound similar to yours. He had a really frustrating time trying to tune his carb and just couldn’t seem to get it to run properly again. He ultimately replaced the carb with a new one and the engine ran perfectly again.

We’re still not entirely sure what the problem was, but wear in the orifice or the needle was suspected. Although it’s not pleasant to think that the carb might require work or even replacement, the total cost of doing so is still relatively low. That might be the next diagnostic step, and if it proves to have no change with the new carb you could always resell the replacement carb as a nearly-new used model and get most of your money back.

Jeff

[berger]

Hi Jeff,

Nice to hear from you again ;-)

Thanks for your comment which makes me feel a little less alone with this "unhappiness" of my engine and/or AeroCarb. Indeed it is very frustrating to think that such a simple element as the AeroCarb might "break down" somehow, especially so because it took me quite a long time to make it run right in the first place, with a customized #4 needle.

Indeed the inner wall of the orifice are scratched, but the needle is in a great shape.

What else could go wrong... I thought, back in 2016 that the body and the cover plate might have had a leak. I tightened the nuts a little more, but this did not cure the "unhappiness". Still I think that a leak around there might show on the engine behavior and also that the assembly is a little "light" to be air-proof. Since this happens after many hours, this might be related to elements changing over time like the orifice wearing or the Delrin plate sandwiched between the body and the cover plate which might give-up enough to let some air in there.

So, the experience of Gary Motley would mean that he was not facing an icing issue, rather some un-harmonized hardware. Perhaps Gary will post some first hand data...

Thank you for your comment.

[Onex107]

I have experienced carb ice in a Cessna and what you are describing does not sound like carb ice. If ice is building on some part of the AeroInjuctor you would see a change in mixture or rpm before it breaks loose and is injested into the engine. I really doubt that the fuel is vaporizing near the needle or slide. When I have seen frost on the intake manifold it's been higher up, above the "Y" joint. And with a hot engine/manifold that doesn't happen.
The problem I had with a rough running engine, an hour after taking off, was a case of latent burps, and it took getting rid of the gascolator to stop it. It took an hour of flight, and leaner fuel consumption, for the temp. in the gascolator to become critical. It was too difficult to insulate. A downward sloping fuel line from the tank to the carb lets bubbles migrate to the top where it is cooler. I imagine the size of the vapor bubbles makes the difference between a rough engine and one that stops every 15 seconds. Vapor in the gascolator could effect fuel flow and a rough running engine could just be super lean. That should show an increase in EGT if it lasts long enough to register. On top of that, in this area of the U.S. we have winter car gas and summer car gas. The winter gas has a higher vapor pressure and is more prone to vapor problems.
I advise you to not jump to conclusions and change carbs until the problem is identified. You are close to solving your problem, keep on keeping on.

[berger]

Hi OneX 107,

Thank you for your input, especially about the sequence of the carb icing event. This rang a bell in me and I came up with the following thoughts:

This thread leaned toward carb icing a few days ago, and now, after a finer analysis of carb icing symptoms, this seems unlikely because the initial burp that I sometimes experience, if it was a piece of ice being ingested, should come after the rising of some EGT and slight roughness, as OneX 107 mentioned. What I notice is reverse, whereas the initial burp is not always present.

Putting the parts flat on the table:

1/ The unhappiness does appear sporadically (every 3 to 5 flights), especially when the OAT is mild, not when it is near (or below) freezing. Perhaps when it is humid, but lately, I flew for several minutes in a light rain with no symptoms.

2/ The phenomenon has (at least) once disappeared after I performed some aerobatics.

3/ The AeroCarb seems to be in a good shape except for the metering orifice which is scored and the scoring seems to have increased since the fall of 2016. The comparison of the (poor quality) pictures I took back then and now seem to show so. The orifice is no longer circular but has two shallow lateral ears ground by the sharp edges of the wobbling needle. The fuel flow through the orifice is tested as abundant.

4/ Even though the symptoms are similar, the sequence of the events do not speak for carb icing. Also, it seems very seldom with the typical Sonex-AeroCarb installations. Kerry is very formalized about this point.

5/ Gary Motley went thru a similar problem on his first AeroCarb after several hundred hours of operation and the issue disappeared when he replaced his carb by a new one. It seems that the orifice also had some wear. Lacking info here, somehow.

6/ Kai mentioned that what came out of the needle controlled orifice, was not gas vapor but a solid stream of gasoline which needs the warmth of the sump to eventually vaporize.

I see now three different scenarios:

A/ There is some vapor lock. The initial burp might speak for it, as well as the mild OAT required to see the unhappiness. Why would this happen now that the engine is running cooler than within the first 125 h (prior to top end and conversion) + 155 h (from top end to beginning of Unhappiness) remains a mystery. Aerobatics might evacuate the bubbles.

B/ The worn orifice is making a lousy job at spreading the fuel evenly, producing the symptoms. But the mild OAT speaks against this scenario, whereas my recent experiments with a razor blade dragged on a similar piece of Delrin might show that the blade bites into the Delrin at an angle which is less pronounced when the Delrin is warm than cold. Aerobatics might shake the needle in a more centered location.

C/ A combination of both A/ and B/

Any criticisms or comments on the above is welcome to eventually find the way out.
Or, on a humorous note, take your pick!

Thanks to all for your contributions.


Sosthène

[GWMotley]

Hello group,
When I had to rebuild my aerovee After about 330 hours I used the carb just as I had removed it before the build. However, I was unable to tune the carb except at one point only. It just would not tune. I tried several different needles, took it apart but just could not really find anything obvious. I did have a suspicion that the metering orifice was somehow too large. So after about two weeks of frustration I finally just ordered a new aeroinjector. To my surprise, I noticed a new modification right out of the box. Sonex was now shipping carbs with a nylon bushing in the mixture metering orifice. Sure enough, I was able to install and tune it within thirty minutes. It is my belief that this issue may be the reason so many seem to be unable to tune one of these carbs. There may be just enough variances in manufacturing that some are a lost cause. I also noted at the time that this new bushing was not on the parts catalogue either. Thus if you have been able to tune but are unable now, check to see if the bushing is there or not. I would check with Sonex to see if the will modify the carb and install the new bushing.

[berger]

Hello Gary,

Thank you very much for sharing the details of your mishap with your AeroCarb.
Interestingly, my AeroCarb, received around 2007 is fitted with a white Delrin bushing as a metering orifice. Is it so that you received one AeroCarb which would not be fitted with a plastic bushing as a metering orifice, or is it that the bushing material was different?

On my side, it took me quite some time to make my AeroCarb work fine, back in 2008-2009. I also noticed that there were several quality issues with the carb and the needles of which I've tested four, before I've found the right settings not without tinkering some bit. This eventually worked fine for about 280 hours when the "unhappiness" of my engine appeared without any significant modification. Now it has 355 hours on it. However, I thought that the manufacturing quality of the AeroCarb/AeroInjector improved since then, as the "AeroInjector tuning" subject is much less present on the forums.

Did you fly your aircraft while trying to tune the AeroCarb for the second time or did you do it on the ground only?
If you flew the airplane, what were the symptoms you noted?

Thank you for sharing and helping.

[GWMotley]

I was unable to tune the carb well enough on the ground to even attempt to fly it. The bushings were not part of the original injectors. Even in the tuning manual you can see just plain metal at the orifice. I would still encourage you to think about replacing the bushing. It’s probably cheap and may at least aid in trouble shooting.

[Onex107]

My AeroInjector is mounted on my Aerovee in a Onex. In this case the vapor problem was overcome by insulating the fuel system, completely, with a foam pipe insulation, and removing the gascolator. An important part of that was the wrapping of the exhaust pipes. I think the gascolator and the radiant heat from the pipes was the last change necessary.
During the "vapor problem" period, I went through several changes to "tune" the carb. No. 1, the needle holder had too much play in the ball joint. Nearly 1/2 turn of adjustment, making the needle setting unstable. I had a new one made by a friend that uses a disk instead of a ball with "0" play. This can be accomplished other ways by tightening up the ball. No. 2, after making several redesigned needles in an attempt to get a WOT setting and an idle setting without having to make a mixture change, home made needle #7 turned out to be almost and exact copy of the #3 supplied with the AeroInjector. If you haven't tried #3 I'd advise you to do it. The only time I have to change the mixture is during climb out, a little richer to cool one high EGT and then at cruise to raise the EGT's to a normal level. The mixture isn't changed until the next flight. With the #2 and #2 1/2 the mixture has to be leaned almost to cutoff, during taxi, to keep it from stalling. In the first 100 hours I probably had the cowling off 100 times. That was 200 hours ago with no more adjustments. Now I just change oil every 25 hours and keep on flying.

[berger]

Hi Gary,

Thank your for your reply.

You are perfectly correct, the AeroInjector manual do display a picture of the orifice being not a Delrin bushing but seems to be part of the body. Strange because the AeroInjector is subsequent to the AeroCarb and my AeroCarb has the bushing already.

Indeed, I'm now considering replacing either the bushing --I have two blanks here from the time I had many troubles trying to tune the carb for the first time-- or the whole body. It seems that the blank of the bushing has to be pressed and then drilled in place. This is what Jeremy told me back in 2009.

Thank your.

[berger]

Hi OneX 107,

Just agree with you about the very excessive slack in the ball joint holding the needle carrier.

Also, I tried 4 different needles back then an eventually I took a #4 which was close to correct and had to file it to better fit the whole range of RPM. The tip was way too thick and the root was too thin, requiring extreme leaning on the ground. I thought back then that the orifice was drilled too large and/or possibly not reamed to any precise diameter. Now, the middle section is slightly too thin and I need to lean it about 3/4 inch in cruise.

Next move of mine is to thermally insulate the gascolator, with some bubble sheet having both sides covered with an aluminum film and see if this improves the "unhappiness". If not, I'll replace the orifice (or body) and/or install a thermal sensor to check the AeroCarb air flow temperature.

Happy you now can just enjoy flying ;-)
Thank your for your help.