Massive engine problem

This is 4 months old but I just saw it. It's a fascinating problem. Here are some of the clues that I picked up:

1. Came out of a non-IRMT overhaul and then started having problems

2. Estimated rough vibration is 30-40 Hz going to full power

3. From the manifold pressure gauge in the video it appears it is not making full power

4. Only happens when it's cold

5. Fuel pressure drops at full power. He asked if that was normal and I don't think anyone responded. I don't know

6. Same results occur when the mixture is leaned.

I hear cold and I think seals, o-rings, rubber diaphragms. Remember when Feynman dropped the Challenger O-ring in the glass of ice water. When things heat up everything expands, everything gets more pliable and air leaks become less likely. Non-IRMT  rebuild makes seals an increased possibility. 4000 rpm is 66.6 rev/sec and every cylinder fires on every other rotation so that's 33.3 missed firings/sec. 5000 rpm gives 41.6 missed firings/sec. Those numbers line up pretty well with the perceived vibration rate so it might be a single cylinder misfiring. 

If you get the same result by leaning the mixture then this is a lean mixture problem. That screams air leak since changing carbs didn't help. Although the fact that it usually happens at high power settings throws a wrench into it because you actually have less vacuum, less suction trying to pull in leaking air when the throttle is wide open. Unless the idle mixture is rich enough to mask the leak at lower power settings.

First thing I want to know is whether or not it is just one cylinder and if so, is it the same cylinder each time. An easy way to check is to watch all the EGTs during a vibration event. If one EGT instantly drops then that's it. If so, it's probably intake O-ring. Head gaskets are less likely to be affected by OAT than an O-ring or diaphragm somewhere. Definitely not valves either given the OAT clue.

If there is no EGT anomaly during the roughness then I haven't got a clue.

Hi Folks

The problem is (almost) solved.
The cause of the vibration was knocking combustion in cylinder 3, triggered by carbon deposits on the spark plug and cylinder head.
It's not yet 100% clear where the oil was coming from. The valve stem seals and piston rings appear to be in perfect condition, as well the cylinder wall. The piston clearance is 7/100 mm, so it's ok. as well. The cylinder head was examined for cracks (pressurized), but no findings were found.

I've replaced the oil scraper ring and am monitoring the situation. So far, it looks good, but I need to monitor the situation for a while longer. If oil continues to be sucked into the cylinder, I'll gradually replace:
- the valve stem seal
- the complete cylinder head assembly
- piston
until the problem is resolved.

Thanks for your support so far :)

Chris

Thanks for the update!!

I'm trying to fit cold temperatures into this result and am coming up short. Would a marginal valve stem seal allow oil to seep in during cold temps? Although once the deposits are established I would think cold temperatures would not matter unless there is also a cold weather air leak that leans the mixture enough to cause the carbon knock. In other words, why doesn't it knock in warm weather?

If you find anything further, please let us know!

Cold temperatures have the same effect on the mixture as low altitude: The MAP goes up and it becomes leaner.
Interestingly, the use of AVGAS had a significant impact on this vibrations; The higher knock resistance significantly reduced the vibrations, it was much harder to provoke them.

Here another video about how massive the vibrations was...

https://youtube.com/shorts/2kEfYmFxCHk

Chris