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  #11  
Old 12-30-2020, 07:37 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by 00Dan View Post
I did some testing today. At 20" MAP and approximately 2250 RPM at 2200', I got the following results by leaning until roughness (I didn't pay attention to RPM).

#1: 53 dF EGT rise
#3: 21 dF EGT rise
#4: 58 dF EGT rise

CHT on #4 was stable at approximately 310 dF the whole time.
I tested ICO rise at full idle, which is set to approximately 650 RPM right now. There may have been a small (~20 RPM) rise but it was hard to tell.

Thanks for the link.

If you are only getting a 20-50* rise at 20" from full rich, you have some major issues going on. I would expect at least a 150-200* rise from full rich to peak EGT at 20" Maybe less at 28", but at 20" it should be very rich even at 2200 MSL with a carb.

Larry
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Last edited by lr172 : 12-30-2020 at 07:42 AM.
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  #12  
Old 12-30-2020, 07:43 AM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
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Originally Posted by lr172 View Post
If you are only getting a 20-50* rise at 20" from full rich, you have some major issues going on. I would expect at least a 200* rise from full rich to peak EGT at 20" Maybe less at 28", but at 20" it should be very rich even at 2200 MSL with a carb.

Larry
Those numbers aren’t necessarily peak, rather it’s how far I got before the first cylinder to peak went lean enough that I could feel it. I’m not prepared to rule out one cylinder running particularly lean. Just my luck that it may be the one that I don’t have EGT hooked up on. One piece of potential supporting evidence for this is my #4 CHT - I suspect it would be running hotter if the entire engine was running very lean.

Last edited by 00Dan : 12-30-2020 at 07:50 AM.
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  #13  
Old 12-30-2020, 12:19 PM
lr172 lr172 is offline
 
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Originally Posted by 00Dan View Post
Those numbers arenít necessarily peak, rather itís how far I got before the first cylinder to peak went lean enough that I could feel it. Iím not prepared to rule out one cylinder running particularly lean. Just my luck that it may be the one that I donít have EGT hooked up on. One piece of potential supporting evidence for this is my #4 CHT - I suspect it would be running hotter if the entire engine was running very lean.
Hottest CHT will be at 80-100* ROP. They cool a bit as you lean to peak and then cool rapidly when leaner than that. CHTs also drop when richer than 80-100* ROP.
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  #14  
Old 12-30-2020, 01:01 PM
00Dan 00Dan is offline
 
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Originally Posted by lr172 View Post
Hottest CHT will be at 80-100* ROP. They cool a bit as you lean to peak and then cool rapidly when leaner than that. CHTs also drop when richer than 80-100* ROP.
Correct, which leads me to believe at least #4 is running sufficient rich to keep it at that temperature.
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  #15  
Old 12-30-2020, 04:13 PM
00Dan 00Dan is offline
 
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To clarify some questions Iíve been asked, some additional details.

The engine runs smooth and starts easily. I did some testing with airspeed which the EGT does respond to, although it rises faster in a dive than it decreases in a descent (Test done at constant throttle).

As far as Iím aware itís been running like this for years. The other cylinders without CHT probably havenít cooked themselves as oil consumption is good (approximately 2-3 quarts per 50 hours).

Basically, it all comes down to the fact I canít lean it out very far. The EGT readings I posted above sort of back this up. I hope to have a JPI installed within the next month and hopefully it will provide some additional diagnostic data before I start pulling things apart.
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  #16  
Old 12-31-2020, 12:51 AM
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rv8ch rv8ch is offline
 
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Quote:
Originally Posted by 00Dan View Post
...
Basically, it all comes down to the fact I canít lean it out very far. The EGT readings I posted above sort of back this up. I hope to have a JPI installed within the next month and hopefully it will provide some additional diagnostic data before I start pulling things apart.
I don't have any knowledge of the JPI but have you considered an engine monitor like the GRT EIS 4000 with a small EFIS? Very inexpensive, and will get lots of info. http://grtavionics.com/vaf.html
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  #17  
Old 01-30-2021, 11:58 AM
00Dan 00Dan is offline
 
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Location: Atlanta, GA
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Default Nozzle PN

Resurrecting this thread because I have some new info.

I was going through the engine log the other day and I discovered the builder at one point swapped the nozzle in the carb. It originally had a 47-773 and he swapped it for a 47-828. What sort of difference would this be expected to make?
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  #18  
Old 01-31-2021, 07:57 AM
lr172 lr172 is offline
 
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Originally Posted by 00Dan View Post
Resurrecting this thread because I have some new info.

I was going through the engine log the other day and I discovered the builder at one point swapped the nozzle in the carb. It originally had a 47-773 and he swapped it for a 47-828. What sort of difference would this be expected to make?
The nozzle is the main jet and it is the biggest contributor to how the carb matches fuel flow to air flow through the venturi. I can't say whether that swap was an increase or decrease in the jet size, but it likely moved the fuel flow rate, at full rich, up or down.

I recommend that you keep leaning, even if one cyl gets rough, to determine the EGT Delta from full rich to peak on the three cylinders. This will help pin point if you have a carb issue or an issue with the one cylinder that is lean. Recommend doing this at altitude (3-5K) and WOT to help eliminate intake leaks and becuase this is where you really want to know if the fuel flow is metering correctly.

Larry
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Last edited by lr172 : 01-31-2021 at 08:00 AM.
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  #19  
Old 04-02-2021, 08:36 PM
00Dan 00Dan is offline
 
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I finally got the chance to fly my plane fully instrumented. I only flew for a little bit today since it was getting dark but I noted fuel flow and RPM in a couple full throttle conditions. It was cool here today, only 45-50 F on the ground.

In the takeoff run and climb I saw approximately 10 GPH at 2150 RPM. In cruise at 7500, I saw about the same fuel flow at 2650 RPM. When leaning, the engine became rough at about 9.5 GPH.

I did install the SDS intake flanges to preempt any potential intake leaks, and that appears to give me normal leaning authority on the ground.

My uneducated perception here is that I’m simply in need of a rejet (assuming I choose to live with the airspeed induced leaning found in my model carb). Any advice before I bust out the drill?

Last edited by 00Dan : 04-02-2021 at 09:52 PM.
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  #20  
Old 04-03-2021, 08:21 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by 00Dan View Post
I finally got the chance to fly my plane fully instrumented. I only flew for a little bit today since it was getting dark but I noted fuel flow and RPM in a couple full throttle conditions. It was cool here today, only 45-50 F on the ground.

In the takeoff run and climb I saw approximately 10 GPH at 2150 RPM. In cruise at 7500, I saw about the same fuel flow at 2650 RPM. When leaning, the engine became rough at about 9.5 GPH.

I did install the SDS intake flanges to preempt any potential intake leaks, and that appears to give me normal leaning authority on the ground.

My uneducated perception here is that I’m simply in need of a rejet (assuming I choose to live with the airspeed induced leaning found in my model carb). Any advice before I bust out the drill?
9.5 GPH at 7500' is right around or a bit higher than max power (approx 75-100 ROP). I routinely run 8 GPH at 8000', which is around 20-40 LOP. Rejetting is not your answer, at least at cruise. You should remember that many carb'ed engine won't be happy LOP. If WOT, intake leaks have a small impact on mixture. Trying pulling back the throttle just a bit at cruise as well as adding carb heat to see if it helps to get leaner. Many seem to have some success with this. Have you checked your ignition timing and insured that each plug is firing?

Shortly after T/O, nose down and accelerate to 2500 RPM, still WOT. What is the fuel flow then? You're looking for around 13.5, give or take, at low altitudes. If you're getting that, no need to rejet.

Down low and WOT or max prop RPM, compare full rich EGT to peak EGT. If you are getting a delta of 200*, your jetting is fine. DO this quickly, as you want to avoid time at peak EGT under high power settings down low.
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Last edited by lr172 : 04-03-2021 at 08:33 AM.
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