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  #1  
Old 05-08-2019, 04:18 PM
10builder's Avatar
10builder 10builder is offline
 
Join Date: Jul 2006
Location: Nashville, TN
Posts: 147
Default Fixed Pitch - Surging at Cruise

I'm encountering a subtle, pulsing engine surge at cruise and unable to determine the cause. Here are the specifics:

O-320-B2B (160HP w/950TT) with only 12 on this new airframe. (Engine came from a 1974 PA-12 Super Cruiser)
Superior Millennium Cylinders
MA-4SPA carb (~300 hours since rebuild, p/n 10-3678-32)
Twin 4373 Slick mags (SB's current - just back from Champion)
Performance Propeller USA FIXED PITCH wood prop (68x68 picked up used here on VAF)
New engine driven fuel pump with a backup Facet provided by Van's
Red cube cabin side with no bends within 6" of inlet or outlet (fuel flow calibration in progress).
Van's FAB purchased from and installed per Van's

During cruise at around 2500 MSL, 2400RPM and 19.8 inches I can feel the subtle surging and able to make it completely subside (go away) by leaning the mixture. In contrast i did an in-flight mag check and it really only magnified the problem. I've seen references to re-jetting the carb but can't resolve in my mind how that relates here. (Photo attached of cruise parameters. BTW, the left fuel tank isn't empty, but rather the level occasionally drops out due to bad connection somewhere).

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Arrington, TN
SPA Panther N87XP
RV-10 N104BS (Sold)
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Last edited by 10builder : 05-10-2019 at 06:18 AM. Reason: more details
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  #2  
Old 05-08-2019, 06:43 PM
RViter RViter is offline
 
Join Date: Jun 2009
Location: Mesa, AZ
Posts: 244
Default Just a hunch ...

I think you'll find a number of replies quickly but first impressions;
* 8.9 gph for that cruise with O-320 seems higher than normal
* CHT's seem very low, (consistent with rich burn)
* Leaning seems to 'help' (do more of that)
* EGT's are also low, should watch/learn the effect when you lean
* Your GRT (has?) data logging, would encourage you to capture the data and upload to SavvyAnalysis, and do some diagnosis on the trends
* Read Busch's ENGINE book
* You didn't mention plugs but I'd pull, clean, and re-gap to tight side of normal range (note if there is any fouling now
* Be precise with mag timing, and sync

This has been my experience with an O-360 (7 gph at 2500 rpm), FP, CHT's in the 350-370*F range, EGT's in the lower 1400*F range. Fuel Injected.
I'm new at this with only 140 hrs on my RV-6A, some real experts may have different views.
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RV-6A QB Tip-Up, 1st Flight 3/18/17
O-360-J2A (modified & F/I), Sensenich FP (cruise)
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  #3  
Old 05-08-2019, 07:03 PM
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10builder 10builder is offline
 
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Location: Nashville, TN
Posts: 147
Default

Gary,

Agree - definitely this engine is running on the rich side and my issue is resolved with aggressive leaning.
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  #4  
Old 05-08-2019, 08:57 PM
brad walton brad walton is offline
 
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Location: Cypress, TX
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Default

I'm guessing underpitched, too, based on rpm vs mp at low altitude.
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  #5  
Old 05-08-2019, 09:41 PM
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Ed_Wischmeyer Ed_Wischmeyer is offline
 
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Location: Savannah, GA
Posts: 1,573
Default

Your carburetor specification is incomplete. For example, the carb on my RV-9A with an O-320 is MA-4SPA 10-3678-32. I don't know what all the numbers mean other than not all MA-4SPAs are the same.

My engine had high EGTs, so I replaced the MA-4SPA that came with the plane with this one that runs richer, and the factory calibrated it at the rich end of the spectrum.

All I know about carbs is that I really don't know very much as I found out when I thought the solution to my problems was to re-jet the carb...

Ed
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  #6  
Old 05-08-2019, 10:18 PM
YellowJacket RV9 YellowJacket RV9 is offline
 
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Location: Clearwater, FL KCLW
Posts: 1,330
Default

Definitely too rich if those numbers are correct. Have you confirmed the fuel flow accuracy? My o-320 would be at about 6.5gph at that power setting. The GRT has a lean assist mode that can tell you how rich of peak you actually are.

Chris
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  #7  
Old 05-08-2019, 11:45 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,681
Default

I believe you solved your own issue here. You were overly rich and leaning it resolved the issue. I wouldn't expect surging from that level of richness, but not really surprising with the low load.

My main jet is drilled and actively manage the mixture in all flight regimes. While I don't get surging when too rich, it does reduce engine performance.

Larry
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  #8  
Old 05-09-2019, 12:28 AM
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CAVU Mark CAVU Mark is offline
 
Join Date: Dec 2014
Location: San Diego, CA
Posts: 140
Default Drilled to what size?

Quote:
Originally Posted by lr172 View Post
I believe you solved your own issue here. You were overly rich and leaning it resolved the issue. I wouldn't expect surging from that level of richness, but not really surprising with the low load.

My main jet is drilled and actively manage the mixture in all flight regimes. While I don't get surging when too rich, it does reduce engine performance.

Larry
Just wondering what size you drilled the jet to? I run that cool as well O-320 10-5009 carb. Fuel nozzle checked against new and my inlet is .096 and outlet is .128. I never get a temp rise when trying to lean.
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  #9  
Old 05-09-2019, 06:12 AM
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10builder 10builder is offline
 
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Location: Nashville, TN
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Quote:
Originally Posted by brad walton View Post
I'm guessing underpitched, too, based on rpm vs mp at low altitude.
Yes indeed; good observation. This prop is pitched 68x68 and was a bargain I picked up here on VAF. If not careful I can see 2800 RPM at full throttle straight and level flight. I will most likely switch in the future.
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Arrington, TN
SPA Panther N87XP
RV-10 N104BS (Sold)
2021 VAF donation cheerfully submitted!
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  #10  
Old 05-09-2019, 06:43 AM
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DanH DanH is offline
 
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Set up in cruise, allow temps to stabilize at full rich. Record the EGT's, then do a very slow pull to find the peak EGT values. Difference is....?
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