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Old 07-01-2019, 07:00 PM
msaltzman14 msaltzman14 is offline
Join Date: Mar 2017
Location: New York, NY
Posts: 57
Default O-320 Fuel Imbalance Causing Hot CHTs- Advice Plz

Hi All-

I have an RV-9A with an O-320 in it. I've been trying to get my CHTs down for a few months now and just did a real leaning test at cruise with a knowledge friend and may have found indications of the problem. Instead of a baffling issue (I checked everything and while I'm sure it could be improved, it's all working right, the seals are good etc), I think I have a significant fuel flow imbalance in my cylinders that is leading to very different leaning across the cylinders.

Here is data I collected today at the following parameters:
20.7" MP
11c OAT
2500 RPM

Cylinder 1: Peak EGT @ 5.5 GPH, 1410 F
Cylinder 2: Peak EGT @ 6.0 GPH, 1400 F
Cylinder 3: Peak EGT @ 7.8 GPH, 1360 F
Cylinder 4: Peak EGT @ 7.8 GPH, 1380 F

As we can see, the front two cylinders peak at about 70% the GPH of the rear two, which means they are getting way more fuel than the rear two, given a certain amount total flowing into the engine. I also basically can't get the rear two more than 80degrees rich of peak, even at full rich at 6500'. I'm essentially running my two rear cylinders near peak EGT all the time, causing high CHT temps in cruise (I'm right around 400 for the rear two and 340 for the front two in cruise). The EGT at about 50 degrees ROP for Cylinder 3&4 results in EGTs of around 1150 for Cyl 1&2 and 1300 for 3&4.

The next interesting tid-bit, potentially unrelated but maybe another complicating factor came from today's mag check. I have one electronic mag and one magneto.

Going from Both to Electric Mag Only:
Cylinder 1: +85 EGT
Cylinder 2: +120 EGT
Cylinder 3: +75 EGT
Cylinder 4: + 60 EGT

Going from Both to Magneto Only:
Cylinder 1: +50 EGT
Cylinder 2: +45 EGT
Cylinder 3: +25 EGT
Cylinder 4: +45 EGT

I think these are unrelated problems (With the electric mag looks like cylinder 2 is a real outlier, maybe the spark is too delayed, maybe the plugs are fouled, etc).

Any ideas here? Primarily concerned with getting more equal fuel distribution, and then tackling the magneto/p-mag imbalance. I'm not an A&P, so I'm hoping to get very knowledgeable so I can bring my mechanic very actionable things to do and go through it with them.

Thanks all!
Mike Saltzman
Flying -9A N7KR
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Old 07-02-2019, 01:59 PM
pjc's Avatar
pjc pjc is offline
Join Date: Apr 2012
Location: Tampa, FL
Posts: 200

Next step for me would be to rule out induction leaks and verify timing (including consistent timing to each cylinder).

If that is not contributing (and assuming the valve train and compressions are sound), I would investigate the ?pepperbox? spray bar mod for the 0-320. That is purported to improve fuel distribution.

Searching these topics on VAF will supply excellent procedures and experience.

Good luck.
Vans RV6 flying
SZD 48-2 sold
2018, 19, 20 Dues paid
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Old 07-04-2019, 11:02 AM
petehowell's Avatar
petehowell petehowell is offline
Join Date: Jan 2005
Location: MN
Posts: 2,290
Default Carb Heat

Try some carb heat. My O-320 behaves like yours, but I can use carb heat to "steer" the mixture.

At 7500 feet, WOT, I can get the cylinders to peak at just about the same time with the carb heat pulled on about 1/4. Play around and I will bet you will find a solution.

Good luck -send me a PM if you would like to discuss on the phone.

Amateur Plane - RV-9A N789PH - 2400+ Hrs
Amateur Radio - KD0CVN
Doggies Delivered - 25+
St. Paul, MN
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