Solving O-300 idle/power issues

[craigmail361]

All, recently a buddie and I (both experienced airline A&P mechanics) both purchased a clean 1960 C172A with a O-300D out of AZ. Eng T.T. ~1200 hours, airframe T.T. ~2200 hours. Flew back to NV in Oct 2025, put about 100 hours on it (oil changes every 25hrs) with only minor issues of having to cruise climb it, as it likes to get hot due to no factory oil cooler. (running Aeroshell 50, the 100w stuff with ashless dispersant) One day on a preflight, we discovered oil leaking out of both Slick Magento vents and discovered some oily streaks down the left airframe coming from the exhaust. We continued to run it as we did not have any issues discovered (Mag check, Leaning for best power, Etc). After a few subsequent flights, the oil leak out of the magnetos got worse, and more oil down the left side of the airframe.

We removed the engine cowls, pulled the mags. We drained a ton of engine oil out of both of them. We pulled the existing plugs (Champion REM40E) and noticed some oil on almost all of them. Buddie said, "Hey let me see what kind of compression this thing has". Puts his hand over the top plug hole on number 5 with bottom plug in, and all the air comes out of the magneto mounting holes as I spun the prop around, not holding compression. We found this on a few of the cylinders and proceeded its best to pull the Top End apart.

When we purchased the aircraft in Oct 2025, annual passed the month prior and our prebuy passed with high compression numbers. The aircraft ultimately sat more than it was flown, as it flew only 5-10 hours a year.

We pulled the top end, discovered the Piston pin diameters were great, but could not get them to release out of all the pistons at first. After we got them removed, we suspected that the aluminum pistons got hot and that's what caused them to be so difficult to remove. The piston tops and ring lands were baked in deposits and found it more suitable to just do all new rings and pistons; call it a field overhaul.

We bored and measured all top end components IAW the continental overhaul manual, to discover that all the chrome jugs were in great condition, bore gauge measurements inspected well within specification with no chrome damage, valves not stuck, triple valve springs good, rockers and rocker shafts in excellent condition. After measuring the rings, we found the gaps to be okay but only found that they were .020 too thinned out from wear on almost all the pistons, deeming them unserviceable. We received all new pistons from airpower, and all new rings. Remeasured everything prior to installation which includes grinding the rings down to spec (specifically the top ring). Reassembled with all new base gaskets, all new intake tubes (6), intake manifold gaskets, push rod seals, and valve cover gaskets. Got the mags back from a 500hr overhaul, everything checked out well and timed to 28/26 degrees per what the continental Overhaul manual calls for.

We did our runs and it made about 2300 static on the ground, no leaks, mag checks within 50 rpm, and flew it around for about 3-4 hours which included the addition of some break in oil and did fantastic, felt like twice the power, until...

On take off one day after great power checks, my partner did a 50nm XC trip and came back to do some pattern before the sunset, and on his last take off, engine rpm momentarily dropped from 2500 to 1200 rpm approx. 600 AGL for 3-4 seconds, then regained. Power was full in, mixture set, mags both, carb heat off, set to both tanks, half tanks, trimmed for VY. The engine somehow regained its power on his crosswind turn while he maintained the same pitch attitude. He continued the pattern and landed safely.

After thorough inspection, we could not find anything. We performed an additional compression test, and everything was 75/80 to 79/80 HOT. Re- checked mag timing, nothing found. We thought it could be a fuel issue.

We insisted on rebuilding the carburetor. We found nothing out of the ordinary besides a worn accelerator pump that still worked. We proceeded with a rebuild kit, hot tanked the carb, and reassembled with all new components but kept the factory brass float in (inspected and found no discrepancies or holes, and also verified correct float level). Never had to readjust the A/F mixture so we reconnected our carb controls and were done. Checked all the strainers installed on both the aircraft and the carb and no debris was found.

I proceeded to fly it this time and found that it made a ton of power than it did before the rebuild. I flew about 0.5 hours, and on my 3rd take off in the pattern, the rpm dropped about 200 rpm just before my crosswind turn at 400 AGL. I leveled off, and the engine regained, then dropped, regained, then dropped, until I pulled some power back, even in level flight.

After this attempt, we replaced all 12 plugs with Tempest REM38E plugs. We thought this would fix it. (Plugs and wires had 700hrs on it). We proceed to fly it and flew it 2 times and it did great until our 3rd flight, because it didn't pass the mag check. By this point, frustration got to the best of us. We proceeded to get 2 new ignition harnesses and sent the mags back out to be certain to complete the potential ignition issue. The mags came back just fine, and we put it all back together. All new ignition system. By the way, the engine temps maintained a way lower temperature than it ever has because of the cooler plugs. (180 on ground, 200 in flight).

On our ground runs, we now notice it runs like a Harley. Engine runs rough at idle and doesn't smooth out until after 1000rpm but still makes 2300 rpm static. Feeling a little worried to take back up after not finding a smoking gun after all of this work, and we feel out like were out of options. We're going to test the primer, play with the timing and A/F mixture for now.

Really looking for some advice or some tests we can do from anyone who knows O-300's to help regain our confidence in this beautiful aircraft. Below I have attached all the repairs we have made. Hey, at least it's not leaking in the hangar anymore.

6 pistons, rings, top end inspected and reassembled (good compressions)
Carb rebuilt
Pressurized smoke to the entire intake and exhaust system, no leaks found.
12 new spark plugs, 2 new ignition harnesses
500 hr mag overhaul on both mags (slick 6300's)
no evidence of primer leaking, makes power and passes mag check now.
prop is balanced, never pulled off.
fresh air filter
verified carb heat mechanism is completely operational.
fuel lines and strainers are all in phenomenal shape.
wiring to mags are great
ignition switch in good shape
all new engine cowling baffles
noticed fuel shut off leaked by in the OFF pos. when we inspected the strainer. other than that, its operational.

 

[Domenick]

VERY, VERY long first post.
BTW, I didn't read it.
Welcome to the forum.

 

[Gary Ward]

Sounds like carb ice to me?

 

[bnt83]

What's your field elevation and density altitude? Is it running sloppy rich?

Dry tappet clearance?

Weak valve springs?

Timing wrong? Are you guys using a piston stop and protractor kit or marks on a crankshaft or similar? I never use the timing marks on the engine after being bit by an O470 once. Are both mags firing the same plugs at the same time? Broken impulse coupling spring? Either magneto internal timing set incorrectly?

Firing order and harness leads locations verified?

 

[455 Bravo Uniform]

This all sounds like fuel, not spark, not hard parts (compression).

I was going to suggest carb ice as well.

The rough ground running…gascolator cleaned?

Any chance of fuel starvation from tanks to gascolator? Vapor lock after combo of ground running and take-off heat?

 

[Lama1125]

Check the butterfly in the carb. Had a similar issue on a C172C with the O-300D. Butterfly was slightly loose and worn causing it to 'flutter' randomly. I'd sent it to Mike's Fuel Metering and have them fully update it. That fixed my issue completely.

 

[Bell206]

the rpm dropped about 200 rpm just before my crosswind turn at 400 AGL. I leveled off, and the engine regained, then dropped, regained, then dropped, until I pulled some power back, even in level flight.

we now notice it runs like a Harley. Engine runs rough at idle and doesn't smooth out until after 1000rpm but still makes 2300 rpm static.

So, these are the two issues above you are dealing with now?
As mentioned above, have you calculated if you were operating in conditions conducive to carb icing?
What fuel loads were onboard when the engine RPM dropped vs when the engine ran good?
Have you checked any of the fuel delivery system like tank vents, caps, etc?

 

[Dan Thomas]

One day on a preflight, we discovered oil leaking out of both Slick Magento vents and discovered some oily streaks down the left airframe coming from the exhaust. We continued to run it as we did not have any issues discovered (Mag check, Leaning for best power, Etc).

That was the first mistake. Just asking for trouble if you fly with oily mags.

Carb ice is possible, but the O-300 has its carb mounted on the sump like the Lycoming, making the ice a little less likely. Ice can accumulate on the ground, then break off the carb's bore walls and throttle plate and obstruct air briefly. if it gets into the secondary venturi it really will run rough, but the rough idle means something else is haywire, like maybe mistakes made during the magneto work. If the points slip a little, the timing goes way off and the spark gets weak. It would be rough at idle, too.

Nobody has suggested loose baffling inside one of the mufflers. It can block the outlet and really slow things down. Or a defective SCAT hose, or, more likely, a defective SCEET hose. That stuff has an inner liner that can pull loose and choke off the airflow.

 

[craigmail361]

temp that day was 85 degrees and 6 percent humidity for the 1200 rpm drop, the 200 rpm was drop just the same. it's the dry Nevada desert with no clouds. @Bell206- we went back up after the 200rpm drop, but worried it might come back because we never found a smoking gun with this whole teardown. We had no condition that would create a carb icing condition. It sounds like 2 different issues, but ultimately the real issue still stands with the poor idling and fear it might come back at any time due to nothing obvious being found. Fuel loads were always either 3/4 or full. the day of the 1200 rpm drop was half tanks.

@Lama1125- checked the butterfly set screws, every was very secure. both of us assembled it per that component manual breakdown.

@455bravouniform- not sure how to check for vacuum lock, but that could be it. we might idle using single tank operation and try to isolate a leak. there is a really faint fuel smell in the cockpit when it sits, but hard to say. the gascolator had zero debris, and really clean. we're running 100ll, the old owner ran mostly mogas prior to us purchasing it.

@bnt83- elevation at most these airports is 2000-2500ft msl. DA can be over 5000 on a 100-degree day. Verified all wiring, and the new kit helps label each plug to prevent mismatch. We are using the markings on the crank to align with the case to time at 28/26 or vise versa, cant think off the top of my head. We are considering borrowing a protractor if we can get our hands on one. When we had the mags sent out, he double checked the primary coil by testing resistance while heating it up, he said he couldn't find anything. impulse couplings were great, I believe he just changed all the components in the mags that are required for the 500hr inspection.

 

[WDD]

Can you replicate this problem on the ground by...

1) Activating or deactivating carb heat?
2) Shutting off a mag?
3) Activating / deactivating the cabin heat?

And.....
Have you checked the P leads?
As mentioned above, have you checked the air intake from the filter all the way to the carb?
If this acts up on the ground, do you get different temps on the cylinders? That is, what cylinder(s) are droppping out to make it sound like a Harley?

 

[Magman]

I’ll second Dan’s comment on muffler baffles. I’ve had a couple A’s that

had nice looking baffle seals. Issue was they were not wide enough to

actually provide a good seal and the engine would run hot. After peeking

inside the installed cowling I added additional seal material. Kind of

ugly but very effective.

 

[Dan Thomas]

I’ll second Dan’s comment on muffler baffles. I’ve had a couple A’s that

had nice looking baffle seals. Issue was they were not wide enough to

actually provide a good seal and the engine would run hot. After peeking

inside the installed cowling I added additional seal material. Kind of

ugly but very effective.

The baffles I was referring to are those inside the muffler. They help muffle the exhaust. They get fed hot when the engine is pulling hard, and suffer distortion and erosion. They can break loose or fold up and block the outlet, and now the engine can't breathe.

 

[Magman]

Engine Cooling baffles and Muffler baffles are 2 different things.

Hope I didn’t cause confusion.

Muffler baffle pieces bounce around so you can have intermittent

power loss.

 

[AV8R_87]

In other words, check the muffler bearings.

 

[craigmail361]

@WDD- carb heat doesn’t replicate the issue in flight, neither does cabin heat or a mag coming off. Like I was saying, we haven’t experienced the rpm loss since, but we never found any smoking gun to ensure that either. Additionally, the p leads and wiring to the mags look good, all the way to the firewall. We used a temp gun to check the engine running and all cylinders are firing and they are all the same temps as well, that’s why I feel as if we have a fuel issue more than any other issue.

@Magman- is there a better way to look at the mufflers? We can pull them back off to look inside of them, but is just looking inside the muffler enough to determine something’s wrong in there?

Attached is the video of it running. We start with a mag drop test, then we try to stabilize it at 1000 rpm. You can see it jumps around 50rpm or so. Same at idle, it runs pretty rough. In the video it’s running on both tanks, 85 degree day and no humidity, DA around 3000msl. Next time I get over there I’ll do single tank isolation to see if that clears it up. The only way it clears up is when your at higher rpm like a mag check and etc. possibly running way to rich?

 

[40YearDream]

That was the first mistake. Just asking for trouble if you fly with oily mags.

Carb ice is possible, but the O-300 has its carb mounted on the sump like the Lycoming, making the ice a little less likely. Ice can accumulate on the ground, then break off the carb's bore walls and throttle plate and obstruct air briefly. if it gets into the secondary venturi it really will run rough, but the rough idle means something else is haywire, like maybe mistakes made during the magneto work. If the points slip a little, the timing goes way off and the spark gets weak. It would be rough at idle, too.

Nobody has suggested loose baffling inside one of the mufflers. It can block the outlet and really slow things down. Or a defective SCAT hose, or, more likely, a defective SCEET hose. That stuff has an inner liner that can pull loose and choke off the airflow.

A local flight school has an old 172 with the O-300 engine. It made the local news about a year ago from a landing (and subsequent rural road takeoff) in a field, due to carb ice (training flight with a student at the time). Carb ice could be the issue here.
That said, I've had a couple hours in that bird - it generally runs smoother and vibrates noticeably less than the typical flat-4. If you're getting a rough idle at 1000 rpm, something must be off.

 

[baboss]

Vapor lock? Prolonged ground run/taxi times?
I'd expect the engine to fall flat on its face if so, not run rough though.

I see you run 100LL so I'll not go into the issues I see when running winter blend mogas in the spring as temperatures rise in my O300-D.

 

[Dan Thomas]

@WDD- carb heat doesn’t replicate the issue in flight, neither does cabin heat or a mag coming off. Like I was saying, we haven’t experienced the rpm loss since, but we never found any smoking gun to ensure that either. Additionally, the p leads and wiring to the mags look good, all the way to the firewall. We used a temp gun to check the engine running and all cylinders are firing and they are all the same temps as well, that’s why I feel as if we have a fuel issue more than any other issue.

@Magman- is there a better way to look at the mufflers? We can pull them back off to look inside of them, but is just looking inside the muffler enough to determine something’s wrong in there?

Attached is the video of it running. We start with a mag drop test, then we try to stabilize it at 1000 rpm. You can see it jumps around 50rpm or so. Same at idle, it runs pretty rough. In the video it’s running on both tanks, 85 degree day and no humidity, DA around 3000msl. Next time I get over there I’ll do single tank isolation to see if that clears it up. The only way it clears up is when your at higher rpm like a mag check and etc. possibly running way to rich?

That RPM variability at 700 RPM is a spark plug cutting out. At low speeds the magneto output is low, and a plug with a failing resistor will do that. You can hear it cut in and out.

If the plugs are older Champions, there's at least part of the problem. They had terrible issues with resistors until they started using the monolithic resistors a few years ago. You might also have a failing plug harness.

 

[WDD]

They had terrible issues with resistors until they started using the monolithic resistors a few years ago. You might also have a failing plug harness.

That rang a bell. Had the Cessna guy at a conference talked about bad champion spark plugs. Do you have fine wire, or regular?

BTW - I'm assuming you've sumped the tanks and haven't found any debris, etc. No one else that has used your fuel pump has had issues?

 

[Bell206]

we never found a smoking gun with this whole teardown.

So you haven't checked the entire airframe fuel system yet?

 

[Domenick]

...If the plugs are older Champions, there's at least part of the problem. They had terrible issues with resistors until they started using the monolithic resistors a few years ago. You might also have a failing plug harness.

This is what was inside those Crapion "replaceable" resistor pugs:

When you look inside you'll see a screw. Removing the screw reveals the resistor and spring.

 

[Dan Thomas]

90% of engine performance issues are electrical. Ignition. In modern cars, electricity is almost everything. Electronic ignition. Electronic fuel injection. When a car is seen at the side of the highway, there's an excellent chance that it's an electrical problem.

In airplanes, magnetos rule. There are two of them, partly because they tend to fail. Sparkplugs fail. Harnesses fail.

But fuel? If that airplane can take off and climb, it's getting plenty of fuel. If something blocks the fuel, that engine quits instantly. The fuel doesn't get metered gradually like you do with the mixture control. That engine needs a fuel-air ratio of between 8:1 and 18:1, and everything outside of that will not burn.

Two magnetos, because they cause problems. Two plugs per cylinder. One carb or one fuel injection system, one fuel line, because they seldom are at fault. If the filters are clean and the tanks aren't empty or full of water, the engine will get enough fuel.

But, people start messing with the fuel system, carb, and everything else instead of ruling out ignition problems first. A lot of time and money is spent chasing imaginary fuel system problems.

 

[moparrob66]

Can you remove the muffler and shake it? The idea of broken muffler chunks flapping around in there sound like it would cause the intermittent power reduction. If exhaust cant go out, air cant go in.