M25XP "Puffing" Noise from intake after rebuild

[ZacharyWS]

I recently rebuilt my Universal M25XP and had the head rebuilt with Valves, guides, and seals. On initial startup it ran great, warmed up nicely with minimal white smoke (assumed engine oil from assembly burning off). I noticed quite a bit of crank case pressure coming from the top of the valve cover but nothing out of the ordinary of an engine with unseated rings.



After warming it up and changing the oil (10-15 mins @ 1500) I ran it up to 2200RPM. At this point I heard a "pop" noise and the engine became noticeably louder. I immediately shut it down and pulled the valve cover to inspect. I found all of the valve clearances in spec with the exception of the #1 valve. This valve was at least 1-2mm out of spec. I have no idea how the adjustment could be that far out as I had checked valve lash just hours early when assembling the engine. After resetting it to .007" I restarted the engine. It started easily but still has the "chuffing" or compression noise coming from the intake. It seems to blow some white smoke as well. I pulled everything apart again and checked the push rods and they are not noticeably bent.



I need to get out there and do an additional compression test but aside from that where do I start?


Thanks!
-Zak

[DougR]

Sounds like you have bent an intake valve.....
Perhaps something like a small washer went thru the intake tract or the valve stuck in the guide momentarily and was hit by the piston.

Compression test time.

DougR

[a64pilot]

If you can do what is called a leak down test, it may tell you more.
However I’d pull the head not sure I’d bother with a compression test.

[model 10]

Sure, you can tell by turning it over by hand. #1 will have lower compression and you will be able to hear the leak through the intake manifold as you bring it towards TDC.

[RaymondR]

I'd pull the head and check it, if the head drops off the valve it will do a lot of damage. 7 thou increasing to 1-2 mm unless you found the lock nut loose is a disaster in the making.

Large Caterpillar engines had a bad habit of dropping valve heads if they did not get their 10,000 hour overhaul and when it happened it would take out the piston, liner, other valve, and the turbo and sometimes the head as well.

[jimbunyard]

As an outlier, did you check that both valve keepers are still in the #1 valve?

[Gone Bananas]

Had this happen 2x and followed by subsequent replacement of the whole engine 2 x because didn’t follow up as you are doing . By the way Universal wouldn’t help even though both engines were still under warranty

Other suggestions may be right too, but sounds like blow back- one of your cylinders clearances are off and perhaps cylinder wall failure. I would go through getting compression Cks on your cylinders. I left it as thought hmmm just a little noise--- but after running for another 100 hours all the cylinders started failing

[ZacharyWS]

So I went and pulled the valve cover again and inspected the valvetrain before continuing. I checked the lash on everything once more for good measure. As I rotated the engine by hand everything appeared to be correct. I checked over the keepers and the lock nuts on the lash adjustment. Everything seemed to be correct. I decided to just toss it back together and fire it up and see where I was at again. Amazingly, it started right up without the "puffing" noise. I let the engine warm up to operating temp and cycled the thermostat. It has not made the noise since.


The only thing that I can figure is that somewhere in the rebuild of the head performed by the machine shop something may have caused the valve to stick open. When I adjusted it I cycled the valvetrain, freeing the stuck valve?


I am going to keep an eye on it for sure, but I dont see any reason to pull the head as it sits. I have changed the oil twice since the rebuild now as to alleviate the possibility of any contaminants in the oil from the rebuild process.

[Singularity]

Originally Posted by ZacharyWS

I am going to keep an eye on it for sure, but I dont see any reason to pull the head as it sits. I have changed the oil twice since the rebuild now as to alleviate the possibility of any contaminants in the oil from the rebuild process.

The old-timers say listen to your machine, that it will tell you when it's having problems. They teach this in flying school. When you read accident reports, it is common to have a significant anomaly, followed my an interval period of normalcy, followed by cates tropic failure. Regardless of the background details, the script is always the same.

I personally would not want to be sitting on that engine in 11 months, having run 60-some hours, about to head either on an extended passage, or to a destination where I did not want to have to pay for repairs. <--pilot thinking. Always certain, not always right.

[a64pilot]

I’m not saying it is, but for some interest Google “Lycoming engine morning sickness”
Sticking valves, usually exhaust valves until the engine is fully warmed up isn’t uncommon in Lycoming aircraft engines, the fix usually is to ream the valve guide to give you a little more clearance, it can lead to a dropped valve.
I’d fly it and watch it, if it begins to reoccur, it may be worth fixing before anything bad happens.
Often on a cold motor it will stick, but as the head heats up and clearances open up, it stops. That is why it’s called morning sickness.

[Singularity]

Originally Posted by a64pilot

I’m not saying it is, but for some interest Google “Lycoming engine morning sickness”....

Good advice, but forgot to add Marvel Mystery Oil. Just kidding. Not really.

My main concerns would be stresses about the valve while it was sloppy. Sounds like things got loose, sloppy, maybe a little bent, adjusted/tightened, then hammer formed back in to place such that power may be restored, but upper valve components might be subject to early failure.

This is the dark place my imagination would take me, and I don't know if it's mechanically correct or what should be the greater concern. This is otherwise where having a mechanic whom you know and trust like a brother us invaluable...

[jhulmer]

No one has mentioned timing. Are you sure cam timing is not off slightly after reassembly? I know on the Yanmars it's easy to skip a tooth.

[ZacharyWS]

Timing: I am positive that the timing is correct. Everything was marked prior to disassembly, and everything lined up going back in.


As for the valve guides and the morning sickness.... I do know the machine shop had to machine the guides to accept the new valves. There is a chance that they did not clearance them properly.


The head has new Valves, Guides, and Seals.

[NoTies]

Could one of the push rods possibly have not been seated properly on the rocker ball and finally worked its own way into the correct place when running. I have had this happen to me once or twice through not paying enough attention when assembling rocker shaft.

[a64pilot]

Good valve guides are bronze of course and are an interference fit, meaning that the hole in the head is slightly smaller than the guide, so they have to be reamed to proper tolerance, it’s usually only a very little metal you have to take out, and the possibility exists that maybe they missed one.
Of course maybe there was just a burr or possibly a metal shaving or something that is no longer there too.
But something was holding that valve open.
Do they have the little rotator cups on top of the valve stem? Have you looked to see if it’s still there?