So my #3 cylinder EGT is generally just around 1300 in cruise and the #4 runs at peak 1460. If I continue to lean #4 EGT goes down and #3 comes up. Many times there is a 200 degree spread. CHTs are normal. Should I be concerned? Is an issue causing such a large spread? …in descent #3 comes up and is similar to the others. I thought maybe an induction leak, but I am not seeing one.
Very uneven Fuel flow NA O-540
- Thread starter Jakenned
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Yup induction leak is suspect. I'm having the same trouble and it started right after replacing 2 cylinders. I think I created a leak on the new cylinders.
Try blowing air into the air intake, blocking the exhaust, spraying soapy water on intake tube ends and looking for bubbles.
Try blowing air into the air intake, blocking the exhaust, spraying soapy water on intake tube ends and looking for bubbles.
Dan Thomas
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Need more info. What model of O-540 is it? Is the carburetor on the bottom or the back of the engine? The O-540-J3C5D and -L3C5D engines use an intake manifold, cast into the oil sump, that looks like this:
Viewed from the right rear. The HA-6 carb bolts onto those four studs on the back. The air/fuel mixture goes horizontally into that duct running forward, where the mixture hits a wall and goes left and right into the six induction tubes. The numbers on those tubes are parts catalog reference numbers, not cylinder numbers. The cylinders are numbered 1-3-5, from front to back, down the right (copilot's) side, the side nearest us in the picture. 2-4-6 run down the left side. So your #3 is the middle one on the right, and it runs cool. The #4 is the middle one on the left, and it runs hot. Now you'll see that the #3 is taken from the bottom of the manifold, while #4 comes off the top.
That air/fuel mixture, in striking the wall, leaves some fuel on it that runs down and gets into the bottom tubes, #3 and #6. They run rich, and therefore cool. The top ones, #4 and #5, come off the top and are leaner and so hotter.
It's unfortunate. We had a factory-overhauled J3C5D (in an R182) that had a real problem with this. Those two bottom-fed cylinders would actually flood and quit at idle. The carb was brand-new, and had a defective fuel nozzle in it that was not atomizing the fuel properly. Lyc sent me another carb that fixed the problem, along with an invoice for $8800.00 for the carb in case I didn't send the defective one back. You really don't want to have to buy one of those things. And that price was 15 years ago.
Viewed from the right rear. The HA-6 carb bolts onto those four studs on the back. The air/fuel mixture goes horizontally into that duct running forward, where the mixture hits a wall and goes left and right into the six induction tubes. The numbers on those tubes are parts catalog reference numbers, not cylinder numbers. The cylinders are numbered 1-3-5, from front to back, down the right (copilot's) side, the side nearest us in the picture. 2-4-6 run down the left side. So your #3 is the middle one on the right, and it runs cool. The #4 is the middle one on the left, and it runs hot. Now you'll see that the #3 is taken from the bottom of the manifold, while #4 comes off the top.
That air/fuel mixture, in striking the wall, leaves some fuel on it that runs down and gets into the bottom tubes, #3 and #6. They run rich, and therefore cool. The top ones, #4 and #5, come off the top and are leaner and so hotter.
It's unfortunate. We had a factory-overhauled J3C5D (in an R182) that had a real problem with this. Those two bottom-fed cylinders would actually flood and quit at idle. The carb was brand-new, and had a defective fuel nozzle in it that was not atomizing the fuel properly. Lyc sent me another carb that fixed the problem, along with an invoice for $8800.00 for the carb in case I didn't send the defective one back. You really don't want to have to buy one of those things. And that price was 15 years ago.
This is good feedback. I don’t have a flooding problem with #3 but this describes the situation well. My carb is in the rear. So your saying, lean it on the leanest cylinders and just let 3 run cooler. This engine has 360 hrs on it. My last engine seemed to only be about a 100 spread. I’ll do a better search for induction leaks to make sure it’s seal up well.
Yes, where the intake is. Connect the outflow of a shopvac to the intake, and open the throttle. With the exhaust obstructed the intake tubes should be at positive pressure relative to outside. Soapy water on induction tubes should bubble if there's a leak. But, I'm not an A&P. This is just a technique I have read as advocated for finding induction leaks. I have also read on an in-flight technique for identifying induction leaks. Perhaps @Dan Thomas can opine on these techniques or suggest an alternate for locating induction leaks.
Absolute value of EGT is largely irrelevant. It can be greatly affected by small differences in probe placement. The question is when do they peak with respect to the one another as you lean back? Getting a carbed engine balanced is a bit tricky anyhow.
Dan Thomas
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That's the technique. I would add only one thing: take that shopvac hose, and the shopvac, and clean them thoroughly first. Any grit in the outlet or the hose will get blown into the intake, and the cylinders will get scored. Bad deal.