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  #31  
Old 09-05-2016, 03:16 PM
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I made 3 trips around the pattern at KAVL this morning and didn't die (me or the engine)! Fuel flow on takeoff was slightly less (10.3 GPH) which, I guess, makes sense since there was less air going in through the hole. CHTs were around 375 and oil temps were in the middle of the green. OAT was 80F on the ground before takeoff. The intra-cowling temperature got up to 177 degrees after landing, with no tendency for the engine to die. EGTs were in the 1300-1400 range on climbout, full rich at 3,500'. It's still a mystery...
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  #32  
Old 09-06-2016, 04:07 PM
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It was 87F today in Asheville so I flew once around the pattern to check things out. I was only held for 5 minutes at the hold short line and my oil temp was 185 F. Lower cowling temp got up to 192 F, CHTs were around 350 F. No problems with the engine running rough or wanting to die. This doesn't mean the problem has been solved, but it's going to be real embarrassing if that missing plug was the problem and it's been been missing for over a year and I never noticed.

Just before shutdown I performed an idle mixture test and got no rise at all with the engine hot. I think I'm going to leave it as-is for now.
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  #33  
Old 06-16-2017, 05:01 PM
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Default Still haven't found the problem

The missing plug wasn't the problem because the engine still dies when hot on the ground, like waiting for 20 minutes in a que to take off. I instrumented the entire fuel system with thermocouples and the only area that's getting excessively hot is the carburetor itself. I added more insulation between the exhaust system and the carburetor, and that didn't help. So I'm wondering if the hot oil in the sump is making the carburetor bowl hot enough to boil the fuel inside. I've never heard of that being a problem in a Lycoming. Neither have I ever heard of anyone putting a blast tube on their carburetor, but maybe that's what I need to do. Actually, though, I'm not sure how well that would work on the ground. I'm really running out of ideas here. Anyone?
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  #34  
Old 06-16-2017, 08:18 PM
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Quote:
Originally Posted by snopercod View Post
The missing plug wasn't the problem because the engine still dies when hot on the ground, like waiting for 20 minutes in a que to take off. I instrumented the entire fuel system with thermocouples and the only area that's getting excessively hot is the carburetor itself. I added more insulation between the exhaust system and the carburetor, and that didn't help. So I'm wondering if the hot oil in the sump is making the carburetor bowl hot enough to boil the fuel inside. I've never heard of that being a problem in a Lycoming. Neither have I ever heard of anyone putting a blast tube on their carburetor, but maybe that's what I need to do. Actually, though, I'm not sure how well that would work on the ground. I'm really running out of ideas here. Anyone?
Give me a call tomorrow afternoon, lets talk through this.
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  #35  
Old 06-17-2017, 06:46 AM
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Thanks, Bill, I'll give you a call later. And thanks to all the commenters on this thread - those are all good ideas. To recap:
1. The problem only occurs when idling on the ground when the lower cowling temperature gets above 180F. The probe is mounted just below the outlet of the oil cooler so the temp is a good proxy for oil temperature as well. (I don't have a calibrated oil temp gauge.)

2. All fuel lines are insulated with firesleeve.

3. Neither the gascolator, electric boost pump, or mechanical fuel pump are getting excessively hot. They're around 120F when the problem occurs, although the fuel is maybe 140F at the outlet of the mechanical fuel pump.

4. Turning on the boost pump doesn't help, which leads me to believe that bubbles are not forming anywhere upstream of the carburetor.

5. The initial boiling point of 100LL is around 210 F and it really starts to boil about 250F
My current theory is that the boiling is occurring in the bowl of the carburetor and the source of the heat is probably a combination of the oil sump and exhaust stack. The only thing I could do to cool off the oil on the ground is add another SCAT duct from the baffles to the oil cooler plenum. I already have a provision for that so it would be no big deal, although I would need some kind of shutoff valve on the second line so the oil doesn't get too cool in flight.

I like krw5927's idea of a thermostatically-controlled fan blowing outside air on the carb bowl. I also lr172 's idea of a recirc line although it would mess up my fuel flow readings slightly. I actually already have the plumbing for that via an 1/8" aluminum line to the panel for my fuel pressure gauge. i already have a valve that would work.

Thanks again for all the good ideas here.
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  #36  
Old 06-18-2017, 11:51 AM
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Originally Posted by snopercod View Post
Thanks, Bill, I'll give you a call later. And thanks to all the commenters on this thread - those are all good ideas. To recap:
1. The problem only occurs when idling on the ground when the lower cowling temperature gets above 180F. The probe is mounted just below the outlet of the oil cooler so the temp is a good proxy for oil temperature as well. (I don't have a calibrated oil temp gauge.)

2. All fuel lines are insulated with firesleeve.

3. Neither the gascolator, electric boost pump, or mechanical fuel pump are getting excessively hot. They're around 120F when the problem occurs, although the fuel is maybe 140F at the outlet of the mechanical fuel pump.

4. Turning on the boost pump doesn't help, which leads me to believe that bubbles are not forming anywhere upstream of the carburetor.

5. The initial boiling point of 100LL is around 210 F and it really starts to boil about 250F
My current theory is that the boiling is occurring in the bowl of the carburetor and the source of the heat is probably a combination of the oil sump and exhaust stack. The only thing I could do to cool off the oil on the ground is add another SCAT duct from the baffles to the oil cooler plenum. I already have a provision for that so it would be no big deal, although I would need some kind of shutoff valve on the second line so the oil doesn't get too cool in flight.

I like krw5927's idea of a thermostatically-controlled fan blowing outside air on the carb bowl. I also lr172 's idea of a recirc line although it would mess up my fuel flow readings slightly. I actually already have the plumbing for that via an 1/8" aluminum line to the panel for my fuel pressure gauge. i already have a valve that would work.

Thanks again for all the good ideas here.
I still struggle with a rough running idle when it is hot out with my Bendix FI. I believe that the unit is absorbing heat through the oil pan and without a solid flow of air to cool it off, it is getting hot enough to boil fuel. I can hear it boiling off down there right after shut down. The Oil temp is a bit misleading, as it measures the oil temp AFTER the oil cooler. I believe most of these coolers deliver a 25-30* delta T (i.e.temp drop) with air moving through them. Therefore a reading of 190 likely equates to a sump temp of around 220* Over time, without meaningful air flow, the carb or FI will quickly reach 220* They are made of aluminum and will therefore take the heat pretty quickly. The only insulator is the thin gasket, which I don't believe does much to slow it down.

Just my speculations here. I intend to install a 1/4" phenolic spacer this summer to see if I can slow down the heat transfer.

Larry
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  #37  
Old 06-18-2017, 04:50 PM
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Originally Posted by lr172 View Post
I believe that the unit is absorbing heat through the oil pan and without a solid flow of air to cool it off, it is getting hot enough to boil fuel.
That explanation fits the symptoms. Which do you think I should try first: Adding another SCAT duct to the oil cooler plenum to cool down the oil, or adding a SCAT duct blowing on the carburetor? I could try either one. Either would originate from the back of the baffles behind cylinders 2 & 4.
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  #38  
Old 06-18-2017, 07:04 PM
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You want to be careful pressurizing the lower cowl. If you do that, you will have cooling issues in flight, which you currently do not have an issue with.

Your installation might be a good candidate for the AntiSplat cowl flap. That will help get the heat out on the ground and keep your CHT's happing in cruise.
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  #39  
Old 06-23-2017, 06:01 PM
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Default Don’t touch that jet!

Hi Snopercod,

To start I'd like to tell you I really enjoy your flight trip write-ups. Keep up the good work!

Sorry about your engine woes...It's been really HOT about the country.

I just have a few things for you.

Excerpt from the carburetor link that follows...

""Don’t touch that jet!
A restriction at the base of the discharge nozzle serves as the main jet in MA series carburetors. In HA-6 (horizontal side-draft) series carburetors, the power jet in the base of the bowl performs the same function. Some well-meaning technical writers have instructed mechanics to hone these jets in an effort to cool cylinder head or exhaust gas temperatures. Apparently they believe this to be a “cure-all” for lean running engines. A mechanic would be ill advised to follow such instructions since no process approval exists for opening these jets in the field. Furthermore, tampering with the jet size could mask other serious problems. An induction leak, a missing carburetor body sensor plug... O.K. I added that one., an incorrectly sized economizer jet, an incorrect float setting, or perhaps even the wrong choice of carburetor could all contribute to a “lean” running engine. Another problem with this mode of attack is that the jets vary in design. Some jets are straight, while others are contoured or stepped. A mechanic who takes a drill bit, a ream, or sandpaper to the entrance of a stepped opening may soon regret that decision. Fuel flows could exponentially increase by merely breaking the stepped edge and thereby creating a venturi entrance. By contouring a stepped jet we’ve managed to decrease the pressure and increase the velocity of the fuel through the jet. When you start boring out nozzles, several things can happen. As you begin to see appreciable results – the tendency is to take out more and more material. “If a little bit is good, a lot may be better.” Wrong""...

Please read the rest in this informative article HERE.

Sounds like it touches on a few of the symptoms your experiencing...

Secondly, new jet not in your future?

Go HERE...And look at "Accelerator Pump Adjustment" pages 1-5 and 1-6.
Perhaps the high ambient temperature setting would help a little?

Couple more things... when you did your instrumentation. Did you insulate your carb body (outside of the bowl chamber) temp probe from the ambient air? Armorflex insulated tape works well...and what's the oil sump housing temp? Lastly, (whew) is your carb heat damper shutting off completely?


Respectfully submitted,
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  #40  
Old 06-23-2017, 06:58 PM
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Originally Posted by Tommycat View Post
Don’t touch that jet!
Too late When anguishing over what to do to get my takeoff fuel flow up, I read all the information I could get my hands on (including the excellent link you provided). The usual suspects said "Don't touch that jet" and maybe they were right. But many others have had success with drilling out a jet. After all my research, I decided to use my best "It's experimental" judgement and drill out the jet. Doing so brought the fuel flow into the normal range, and adding the "aeration holes" (AKA "peppermill") solved the uneven fuel distribution problem. There's always the chance, though, that those mods introduced some other problems like I am experiencing.

Years ago I adjusted my accelerator pump linkage to the dreaded "hole 1" - the minimum setting. I did that based upon video evidence showing a puff of black smoke coming out my stacks when I pushed the throttle forward during the takeoff roll. Hole 1 seems to be right for me because I never experience any kind of hesitation when adding throttle.

No, I didn't insulate the carburetor bowl, and no, I didn't insulate the thermocouple from the ambient air. It was just a quick and dirty check to see what was getting hot.

What I did yesterday was to double the air flow to my oil cooler by adding a second 2" SCAT duct to the inlet plenum. I had it that way when I first started flying, but the oil was staying too cool. Well, I think it was too cool...I really need to calibrate my oil temp gauge to be sure; I have the test equipment to do that (Fluke 87V) so I'll put it on my to-do list.

I'm glad you are enjoying the trip writeups. It's fun flying around with my RV buds. We should have an RV-10 join us pretty soon when Jason gets out of Phase 1.
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