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  #21  
Old 09-16-2017, 04:17 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
 
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I saw something very similar to this on a Champ with a bad exhaust muffler (a plate came loose and partially blocked the exhaust at high power).

Carl
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  #22  
Old 09-16-2017, 08:26 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Raymo View Post
The 18-19 GPH figure sounds very high to me. My O360-A4M, fixed pitch, has never seen anything above 15.5 in flight.

When you pull the mixture back to keep it running above 2400 RPM, what is the fuel flow? I suspect it is somewhere in the 14-16 GPH range.

EDIT: Sending the fuel servo back is the right approach, IMO. Hopefully you sent the divider too.
+1

I looked yesterday and my 320 was drinking 11 GPH at 2400 under high load (climb). Your 400 is about 25% larger in displacement, which would equate to a bit less than 14 GPH at 2400. 18 seems filthy rich at that RPM and not surprising it is coughing. I hope they find the issue in the servo.

Larry
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  #23  
Old 09-17-2017, 05:44 PM
Chris Engler Chris Engler is offline
 
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Thanks to all who have offered suggestions....your collective brain power is much appreciated!

My thoughts as I continue to chase the coughing:

- Fuel flow does seem high at 18-19 gpm. Using the IO-390 charts and adjusting for field elevation of 800 feet ASL, I should be seeing around 15 gpm at 2400 RPM fully rich. Keep in mind everything is new and un-calibrated including the red cube which is using the GRT standard default programming value of 83.

- Fuel pressure....appears to be a gauge/programming issue. We've confirmed the mechanical pump is a high pressure version and the electric pump delivers almost 60 gph to the throttle servo so it doesn't appear to be a fuel delivery problem.

- Restriction in air flow....I'm using the stock Vans snorkel and AN air filter but also ran with cowl off and alternate air open so plenty of air available. My understanding is that the fuel servo uses the air tubes in the front of the inlet to the servo to meter the correct amount of fuel to the injectors. If that's correct, an undersized snorkel would simply result in not reaching peak power but not cause a stumble since the servo would meter based on air available and would not overly richen the mixture.

- Exhaust restriction....this is a very real possibility....going to remove and closely inspect. The link below is a post that sounds very familiar and the root cause was a blocked exhaust.

https://www.pilotsofamerica.com/comm...e-o-360.53888/
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Chris Engler, Greene, NY

RV 8 Under Construction (N184CE Reserved), Showplanes Fastback, Superior IO 400; Dual 10.4 HXr

Kitfox 7; Rotax 914, Built and Sold

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  #24  
Old 09-18-2017, 05:39 PM
Chris Engler Chris Engler is offline
 
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Update on the exhaust inspection....nothing out of order, pipes clear and baffles looked fine using a borescope. Spoke to Clint at Vetterman's and he kindly offer to ship a set of straight pipes to try. If the fuel servo gets a clean bill of health from Precision, going to try a new set of mags and the straight pipes...the saga continues!
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Chris Engler, Greene, NY

RV 8 Under Construction (N184CE Reserved), Showplanes Fastback, Superior IO 400; Dual 10.4 HXr

Kitfox 7; Rotax 914, Built and Sold

VAF donation gladly paid through July 2018
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  #25  
Old 09-18-2017, 10:44 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Chris Engler View Post
Update on the exhaust inspection....nothing out of order, pipes clear and baffles looked fine using a borescope. Spoke to Clint at Vetterman's and he kindly offer to ship a set of straight pipes to try. If the fuel servo gets a clean bill of health from Precision, going to try a new set of mags and the straight pipes...the saga continues!
I don't think an exhaust problem would cause excess richness. You would see a limited ability to increase power, but would not see coughing/sputtering and belching black smoke. That is pretty typical of an overly rich mixture and exhaust restrictions simply restict total flow. mixture is set by the carb or servo and metered by total air flow. I don't see how an exhaust restriction can cause a rich mixture in a mechanically metered (i.e. air flow based) setup, as they are designed to match fuel to the current air flow present.

I suspect they will find something in your servo.

Larry
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Last edited by lr172 : 09-18-2017 at 10:47 PM.
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  #26  
Old 09-26-2017, 08:36 AM
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Default Update?

Chris - any update on your investigation?

Carl
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  #27  
Old 10-14-2017, 05:48 PM
Chris Engler Chris Engler is offline
 
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Wanted to post an update on the XP-400 situation. As I mentioned in previous posts, I noted an engine cough/stumble during a first start on an RV-8 project. The engine will run perfectly up to around 2500 RPM. At that point, with mixture full rich, advancing the throttle will result in a noticeable power loss with RPM to dropping into the 2200-2300 range while the engine coughs and some backfiring occurs. The strange thing is if I lean the engine approximately 1/3 of full travel, (i.e., 1/3 of the way back from full rich) everything is fine and I can reach and hold 2600 RPM which is the max static RPM I should reach per the MT governor manual. Over the past few weeks, I've tried the following to isolate the problem....all made no difference at all:

1. Installed new mags provided by Superior

2. Had the fuel servo bench tested/calibrated at the factory (Silver Hawk EX-5VA1). Per the factory, the servo was spot on in terms of flow volume and required no adjustments to the calibration.

3. Removed the snorkel and performed a test run.

4. Tried a different fuel servo provided by Superior.

5. Removed the mufflers and bore scoped them to verify no obstructions.

6. Replaced the mufflers with straight pipes.

At 2500 RPM and full rich (i.e., just before the trouble starts) the engine parameters are as expected....EGT spread highest to lowest is around 50 degrees, fuel pressure is 30-31 psi, manifold pressure is steady at 26-27 inches, and fuel flow is around 18 gph.


At this point I'm out if ideas other than pull the motor and ship it to Superior....any other thoughts from the collective wisdom of VAF is welcome and appreciated!
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RV 8 Under Construction (N184CE Reserved), Showplanes Fastback, Superior IO 400; Dual 10.4 HXr

Kitfox 7; Rotax 914, Built and Sold

VAF donation gladly paid through July 2018
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  #28  
Old 10-14-2017, 06:09 PM
Jgibson Jgibson is offline
 
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Default One Of First Checks

For me would be valve springs and/or lifters after you've extensively exhausted all other possibilities that you've addressed.
A weak lifter, broken or weak valve spring causing a 'float' issue will display the same symptoms. Cross contamination of either exhaust or intake cycles will make the engine behave as you've described.
Just a thought. Something to investigate and hope it helps.
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  #29  
Old 10-14-2017, 06:47 PM
lr172 lr172 is offline
 
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It seems apparent that the servo is throwing too much fuel at your engine in the higher RPMs. The fact that you are seeing clear signs of an over rich mixture (sputtering / belching black smoke) and pulling the mixture back allows the engine to produce full power (or close to it) would indicate that the engine is healthy and you have a fuel metering issue. A reach, but have you confirmed that you have the servo model appropriate for your engine size? Also, I would pull an injector and get the part number. Have the folks at silverhawk confirm it is the correct part for your engine/configuration. I am not a servo expert, but would have concerns that too large of an injector nozzle might allow too much fuel to flow. I can't remember if the servo meters just flow or also factors in pressure. The servo is expecting several pounds of pressure in the injector lines at that flow rate and too large of an injector would reduce that pressure and allow more fuel flow than the servo is counting on.

Can we assume this ran on some firms test stand or dyno for an hour or two? That should eliminate a lot of variables in the engine, as you have a theoretical confirmation that it is capable of properly producing the power it should.

It's too bad they banned Don from this forum, as I am sure he would have some good input.

Larry
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Last edited by lr172 : 10-14-2017 at 07:05 PM.
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  #30  
Old 10-14-2017, 07:53 PM
Chris Engler Chris Engler is offline
 
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Thanks for the suggestions. Valve float had occurred to me as well but since I can lean and get a solid 2600 RPM, I still tend to think it's a fuel delivery problem as Larry suggested.

Larry - from my discussions with Precision Airmotive (the fuel servo manufacturer) the servo meters the amout of fuel throughout the RPM range via four air inlet tubes in the throat of the servo based on "velocity pressure" acting on a diaphragm valve to meter the correct amount of fuel. What's perplexing is that the metering process is working across the entire RPM range and then something is breaking down over the last 100 RPM. Per the Precision web page (and confirmed through discussions with them) injectors are consistent (i.e., same part number) from the 320s through the 540s so it's unlikely there's a mismatch on the injectors but certainly worth pulling and checking all 8. The other problem with the injector theory is that if the servo is doing its job correctly metering the amount of fuel for a given air volume through the servo throat, it would seem that even injectors that are too large would only have the appropriate amount of fuel volume to inject into the cylinder based on airflow though the atomization my be off.

Also, forgot to mention previously, I've confirmed the plugs are in fact the same plugs listed on the engine build sheet and they're black in color when pulled and inspected suggesting an overly rich mixture as opposed to grey suggesting overly lean.

Regarding the history of the engine before I purchased, it's serial number 004 of the XP-400 run. The first 5 engines were the "test mules" used in program development. My engine log shows 150 hours of bench running between 2007 and 2008 with various combinations of accessories, then in 2008 a teardown for internal inspection (no wear or issues noted) and was subsequently reassembled with new rings, bearings, and gaskets and preserved. Another 5 hours of run time was documented in November 2011 and the engine was again preserved. I purchased the engine in 2013 from Superior during the initial phase of the RV8 build.

On a brighter note, I'm getting a valuable education in the operation of fuel injected airplane engines as a result of all this!
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RV 8 Under Construction (N184CE Reserved), Showplanes Fastback, Superior IO 400; Dual 10.4 HXr

Kitfox 7; Rotax 914, Built and Sold

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