RPM/Performance decrease after cylinder replacement & carburetor rebuild

[Sjensen]

During my plane's recent annual inspection, I had to replace the #4 cylinder and rebuild the carburetor. This was my first time dealing with both of these issues, so I wasn't sure what to expect. However, I've noticed a significant loss of power, especially during climb-out, and a decrease in RPM performance. Previously, I could achieve around 2700 RPM at full throttle in the air. Now, it struggles to reach even 2500 RPM and typically hovers around 2300 RPM at full throttle.

My question is simple: Is this normal when breaking in a new cylinder? Is it typical to experience a 400 RPM loss during this break-in period, and will the power return once the cylinder is fully broken in? Or could there be an issue with how the carburetor was re-installed? For reference, my plane is a 1966 Cessna 172G with an O-300 engine, and the #4 cylinder was replaced with a Superior Millennium cylinder (SA10200-A20P).

Any help or advice would be greatly appreciated!

Thanks,
Spencer

 

[Bell206]

Is this normal when breaking in a new cylinder?

No. I'd discuss with your mechanic before further flight.
Did this happen on your initial post- maintenance flight or after several flights?

 

[Clip4]

You need to take it back to the mechanic.

 

[SebIp]

Or a different mechanic if they don’t know why. We have found not all mechanics really understand engines.

 

[flyingbrit]

Or could there be an issue with how the carburetor was re-installed?

Some one needs to check whether the throttle is opening all the way. There should be about 1/8" gap between the throttle knob and the panel when the throttle is pushed in all the way. If no gap, then there is a good chance it was not rigged properly when the carb was re-installed.

 

[455 Bravo Uniform]

Nope, not normal. Take it back.

If it was safe to do so, I’d have aborted the take-off if max RPM were not achieved pretty soon after full-throttle. It should be part of your scan. “Full power. Making full power (tach). Instruments green.”

Where you located in Indiana? I’m KLAF based.

 

[Lndwarrior]

I don't mean this to be snarky or condescending - but wondering what your RPM was on the roll? Did you not verify you had takeoff power before lifting off? If so, was it 300 RPM below normal?

Just wondering how you found this out in the air and not on the runway?

 

[Sjensen]

No. I'd discuss with your mechanic before further flight.
Did this happen on your initial post- maintenance flight or after several flights?

I have been discussing this issue with my mechanic, but I wanted to seek additional opinions here as well. Yes the problem arose during my initial post-maintenance flight so I aborted my first attempt and terminated the flight because something felt off. After consulting with my mechanic, he assured me that all the RPMs were correct according to the POH during a full power ground check. Based on his reassurance, I decided to try again. However, during the second attempt, the aircraft still felt sluggish and seemed to be lacking power.

 

[Sjensen]

You need to take it back to the mechanic.

Roger that!

 

[Sjensen]

Or a different mechanic if they don’t know why. We have found not all mechanics really understand engines.

Yes well I'm starting to have the same feelings...

 

[Sjensen]

Some one needs to check whether the throttle is opening all the way. There should be about 1/8" gap between the throttle knob and the panel when the throttle is pushed in all the way. If no gap, then there is a good chance it was not rigged properly when the carb was re-installed.

Thanks for the info, I'm going to pass this along to my mechanic! And I had suspicions that it may have something to do with the throttle cable and I mentioned this to my mechanic but since I'm not a mechanic I really didn't know but I'm glad you have reassured my thoughts on this.

 

[Tools]

Same carb, or replacement? Just curious. My tiny pilot brain is thinking something went wrong with carb.

Cylinder is also suspect, would want leak down data on that.

In terms of reinstall errors… scat tubes done wrong, carb heat not even hooked up, maybe rigging, not likely but possible.

Like already said, something definitely wrong.

Ultimately, an engine’s ability to produce correct rpm is the litmus test its ability to produce the power it’s supposed to. If it hasn’t been part of your initial takeoff roll scan, it likely will be now! No judgement… guess how that little tidbit worked its way into my scan?!! Yep…

 

[Sjensen]

Nope, not normal. Take it back.

If it was safe to do so, I’d have aborted the take-off if max RPM were not achieved pretty soon after full-throttle. It should be part of your scan. “Full power. Making full power (tach). Instruments green.”

Where you located in Indiana? I’m KLAF based.

I actually did initially aborted the takeoff and decided to terminate the flight for the day. After discussing the issue with my mechanic, he had me convinced it was safe to fly because according to the POH, the engine was achieving the required RPMs during a full power ground check.

The next day, I attempted to fly again, but both on the runway and in the air, the plane still didn't feel right. As you mentioned, I couldn't get the engine "in the green." I informed my mechanic that I don't feel safe flying until the issue is fixed. However, my concern now is that he might not know how to fix it, which is what led me to post on this forum.

Yes I'm in Indiana @ KMQJ

 

[Sjensen]

I don't mean this to be snarky or condescending - but wondering what your RPM was on the roll? Did you not verify you had takeoff power before lifting off? If so, was it 300 RPM below normal?

Just wondering how you found this out in the air and not on the runway?

I actually did initially aborted the takeoff and decided to terminate the flight for the day because I wasn't achieving what I thought was a proper RPM on the roll but after discussing the issue with my mechanic, he had me convinced it was safe to fly because according to the POH, the engine was achieving the required RPMs during a full power ground check.

But then the next day, I attempted to fly again because my mechanic was saying everything was likely ok but both on the runway and in the air, the plane still didn't feel right. So I have informed my mechanic that I don't feel safe flying until the issue is fixed. However, my concern now is that he might not know how to fix it, which is what led me to post on this forum.

 

[Sjensen]

Same carb, or replacement? Just curious. My tiny pilot brain is thinking something went wrong with carb.

Cylinder is also suspect, would want leak down data on that.

In terms of reinstall errors… scat tubes done wrong, carb heat not even hooked up, maybe rigging, not likely but possible.

Like already said, something definitely wrong.

Ultimately, an engine’s ability to produce correct rpm is the litmus test its ability to produce the power it’s supposed to. If it hasn’t been part of your initial takeoff roll scan, it likely will be now! No judgement… guess how that little tidbit worked its way into my scan?!! Yep…

Same carb to my knowledge, I was told it was just "rebuilt".

When you say leak down data, do you mean like running a compression test?

The only thing I can confirm from reinstall is that the carb heat does work because I tested that as part of my run up tests.

As mentioned in previous conversations, I did notice something was wrong during my takeoff roll and aborted the flight. However, my mechanic assured me it was safe to fly because, according to the POH, the engine was achieving the required RPMs during a full power ground check. The lesson I've learned from this experience is that, even if your mechanic says everything is fine, you should trust your instincts. If something doesn't feel right, don't put the bird in the sky! I appreciate everyone's support in reinforcing this important reminder.

 

[SebIp]

On our engine we had a vibration problem and it turns out to be three of the rods are wrong length. We think we hr previous owners when they had cylinders replaced their old shop did not change the rods. You’d think dry tappet clearance would be part of the work but it seems like not anyone’s doing it when they should.

 

[Sjensen]

On our engine we had a vibration problem and it turns out to be three of the rods are wrong length. We think we hr previous owners when they had cylinders replaced their old shop did not change the rods. You’d think dry tappet clearance would be part of the work but it seems like not anyone’s doing it when they should.

Great feedback! This is also something to consider and discuss with my mechanic! Thanks!

 

[Gary Ward]

How many mechanics fly the plane after work this extensive?

 

[bnt83]

On our engine we had a vibration problem and it turns out to be three of the rods are wrong length. We think we hr previous owners when they had cylinders replaced their old shop did not change the rods. You’d think dry tappet clearance would be part of the work but it seems like not anyone’s doing it when they should.

I just finished a field overhaul for myself, using brand new cylinder kits, brand new camshaft and new tappets kit, 5 of the existing pushrods were too short when I checked dry tappet clearance.

 

[Salty]

On our engine we had a vibration problem and it turns out to be three of the rods are wrong length. We think we hr previous owners when they had cylinders replaced their old shop did not change the rods. You’d think dry tappet clearance would be part of the work but it seems like not anyone’s doing it when they should.

Push rods. Initially I thought you meant connecting rods, and I was like

 

[Mikey52]

Airplane model and engine model?

 

[Tools]

Yes. Compression tests. It’s actually a leak down test, not a compression test… nomenclature.

And I agree, 2500 wouldn’t scare me, but if it was 2700… something is wrong. You are correct to apply your own common sense and intuition.

His attitude of it passed some esoteric metric that is at odds with other metrics, isn’t great. If you look enough, you WILL find the issue. It may be a big deal, it may not. But SOMETHING ain’t quite right. It’s good to know.

 

[flyingbrit]

Airplane model and engine model?

Listed in post #1

 

[Sjensen]

Airplane model and engine model?

1966 Cessna 172G with an O-300 engine

 

[Bell206]

according to the POH, the engine was achieving the required RPMs during a full power ground check.

Technically the POH is not the defining reference when performing a static RPM check. While it is part of the maintenance function checks the mx manual or TCDS should have the required limits. Below are what your static limits should be. Did your aircraft meet those limits with the indicated prop installed?

 

[Sjensen]

Yes. Compression tests. It’s actually a leak down test, not a compression test… nomenclature.

And I agree, 2500 wouldn’t scare me, but if it was 2700… something is wrong. You are correct to apply your own common sense and intuition.

His attitude of it passed some esoteric metric that is at odds with other metrics, isn’t great. If you look enough, you WILL find the issue. It may be a big deal, it may not. But SOMETHING ain’t quite right. It’s good to know.

Yes I agree, I also just feel like things should be the same as they were before the annual if not better after replacing and fixing things and like you said just simply saying it passes because some metics doesn't really put me at ease.

 

[Sjensen]

Technically the POH is not the defining reference when performing a static RPM check. While it is part of the maintenance function checks the mx manual or TCDS should have the required limits. Below are what your static limits should be. Did your aircraft meet those limits with the indicated prop installed?
View attachment 128989

I am not 100% certain that is the prop I have but I do know I have a McCauley prop so I am just going to assume it is the same prop I have for this scenario and to answer your question no, it hovers more around 2100-2150 during a full power static run up and rolling down the runway I'm hovering around 2300 RPMs which I'm almost 100% sure I used to be at 2500 or more rolling down the runway.

 

[Dan Thomas]

As Bell says above, the TCDS is paramount. You can do the check yourself. Park the airplane with nothing behind it and at 90° to any wind, preferably no wind, clamp the brakes on hard and go to full throttle and see what the tach says. Remember that the tach may be old and under-reading, so a tach checker is a good thing to have. Do this over clean pavement, and hold the elevator full up to avoid sucking up grit.

You won't get your 2700 in the climb. The prop is pitched to allow redline in level flight, full throttle, near sea level, but even at altitude (like 6000 feet) it should still work that way. Air density is lower so HP is down, but so is drag on the prop and airplane. It always worked for me.

As other have noted, incorrect pushrod lengths can cause problems, especially pushrods that are too long so that the valves aren't closing. The carb might be a factor, with the wrong jet or nozzle installed during the rebuild, or maybe the intake runner to the replaced cylinder has a loose connector hose and it's sucking air. Bad deal, really lean operation on that new cylinder. Throttle or mixture rigging would have to be way off to cause a 400-RPM shortfall.

More than a little likely: The mags might have been removed and reinstalled and mistimed. If the ignition timing is retarded some, the power will be way down.

 

[Sjensen]

As Bell says above, the TCDS is paramount. You can do the check yourself. Park the airplane with nothing behind it and at 90° to any wind, preferably no wind, clamp the brakes on hard and go to full throttle and see what the tach says. Remember that the tach may be old and under-reading, so a tach checker is a good thing to have. Do this over clean pavement, and hold the elevator full up to avoid sucking up grit.

You won't get your 2700 in the climb. The prop is pitched to allow redline in level flight, full throttle, near sea level, but even at altitude (like 6000 feet) it should still work that way. Air density is lower so HP is down, but so is drag on the prop and airplane. It always worked for me.

As other have noted, incorrect pushrod lengths can cause problems, especially pushrods that are too long so that the valves aren't closing. The carb might be a factor, with the wrong jet or nozzle installed during the rebuild, or maybe the intake runner to the replaced cylinder has a loose connector hose and it's sucking air. Bad deal, really lean operation on that new cylinder. Throttle or mixture rigging would have to be way off to cause a 400-RPM shortfall.

More than a little likely: The mags might have been removed and reinstalled and mistimed. If the ignition timing is retarded some, the power will be way down.

Dan,
Thank you so much for all the information! Your ideas and suggestions are fantastic along with everyone else that has chimed in! I'll be sharing these scenarios directly with my mechanic and asking them to check each one specifically. This is exactly the kind of insight I was hoping for by posting here!

 

[Bell206]

it hovers more around 2100-2150 during a full power static run up

Well in detective work they call that a clue. Verify the prop number then discuss with your mechanic about the TCDS figures. But as with any story there's always to sides. Have your mechanic explain why the difference in the static RPM figures if he said they're good but the TCDS shows different ones. Once you get those static numbers back to specs you should have your climb performance back.

 

[Domenick]

Might want to check your tachometer too.

This works from the cockpit:

https://www.amazon.com/dp/B0006N72U2/

 

[frfly172]

Find a mechanic who understands the O-300 it’s a simple engine. On static check if your getting over 2600 you shouldn’t get 2300 at full power when straight and level.

 

[Dan Thomas]

Find a mechanic who understands the O-300 it’s a simple engine. On static check if your getting over 2600 you shouldn’t get 2300 at full power when straight and level.

If he got 2600 static he'd be past redline before he even left the ground.