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Fixed Pitch - Surging at Cruise

10builder

Well Known Member
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).

20zvi1u.jpg
 
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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.
 
Gary,

Agree - definitely this engine is running on the rich side and my issue is resolved with aggressive leaning.
 
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
 
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
 
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
 
Drilled to what size?

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.
 
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.
 
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....?
 
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.

jet size depends upon whether the carb has an enrichment circuit. Mine does not and I used either a #39 (.995) or #40 (.980) drill bit. I think it was the former. I need all of that jet, just not most of the time (FP). I pull the mixture out about 1/2" for T/O and have to feed some back in as my RPMs come up after stabilized climb. I have a target EGT that I shoot for.

Larry
 
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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....?

Good point Dan - I should have documented the leaning better. When the weather clears up here in middle TN next week I'll document the fuel flows and temps.
 
A sinking carb float could be another cause of your symptoms. Do you know if your carb has the blue composite float? There have been issues of the previous floats sinking and a mandatory replacement of old floats.

Bill
 
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

Ed,

Determined my carb is model MA-4SPA: 10-3678-32, and according to the Kelly Aerospace application guide there are three possible p/n's for the O-320-B2B:

10-3678-32
10-4910
10-5009

Thanks,
 
A sinking carb float could be another cause of your symptoms. Do you know if your carb has the blue composite float? There have been issues of the previous floats sinking and a mandatory replacement of old floats.

Bill

Bill - that is an unknown. The only history i have is that the carb has approximately 300 hours since rebuild, which occurred in early 2014. My reading indicates I need to look for "MF" stamped on the data plate.
 
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