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Went for a flight the other day and before going out I manually rotated the prop.
I noticed considerably less compression on 2 (#4 and #3 I believe) of the cylinders, also, I can hear a hissing in the exhausts during compression.
Leaky exhaust valves? Certainly showing all the symptoms…

I took the plane out for a flight hoping the problem would go away on a hot engine and while it was much better when the engine was hot, the difference in compression was still noticeable and the hissing noise was still there.

I’ll have to remove the heads and take them to my VW guy for a service/valve job, hopefully they can be saved.
In any case, it looks like I’m grounded for a while :-(

FYI, my heads are Mofoco 042’s, machined for 94mm and dual ignition.

If you haven't already, check your valve adjustment first before pulling the heads. I have had similar issues and it almost always was due to the engine needing the valves adjusted back to .006. It doesn't take being off by much for the exhaust side to be too tight and cause the valve to open too early.

Keith
#554

I would agree with check the valve adjustment first. I would go with the .008 on the exhaust valve and .006 on the intake. As everything heats up and expands the exhaust valve could use the extra clearance for good valve seal. Just my 2 cents.

Guys, if your airplane is not 100%, please don't fly it! The notion of flying an engine that is not 100% to see if the issue will correct itself is a recipe for a disaster! Leaky exhaust valves result in the valve overheating and burning, and can also result in the valve failing. Which happened to me with a 1969 VW Bug I bought new, and kept the valves adjusted properly, but at 39,000 miles it swallowed an exhaust valve and seized, the head of the valve broke off (#3 cylinder that gets a little less cooling air due to the oil cooler in the plenum). TCM Continental allows no leakage past an exhaust valve, if leakage is detected in a compression test, the engine is not airworthy, the cause must be corrected. I repeat, please do not fly if your entire airplane is not 100% operational. We have got to keep our safety record as good as we can!

Thanks guys,

I will check the valve adjustment first.
It’s now been about 15 hours since the last one.
If that does not fix the issue I’ll remove the head(s).

N190YX, I hear what you’re saying.
The wise thing to do would have been to just go for a run-up to get the engine warm.

Calm down N190YX, I didn't see anyone offer anything but advise on valve adjustments. Next course of action would be a good warmup and full power run up then and a leak down compression test. Just before you decide to push it off a cliff :)

So far the only problem that ever fixed itself is our government,if u can believe that I have some health insurance I would like to sell you.

N190YX wrote:Guys, if your airplane is not 100%, please don't fly it! The notion of flying an engine that is not 100% to see if the issue will correct itself is a recipe for a disaster! ... TCM Continental allows no leakage past an exhaust valve, if leakage is detected in a compression test, the engine is not airworthy, the cause must be corrected. I repeat, please do not fly if your entire airplane is not 100% operational. We have got to keep our safety record as good as we can!

TCM Continental no longer has the strict "no leakage" rule for valves--they replaced it almost 15 years ago with SB03-3. They have a calibrated orifice that seems to correspond to around 45/80 on most testers. They recommend you do a borescope inspection, fly, and retest if you fail it. I think they got smart and realized the previous rule was unrealistic.

https://www.aircraftspruce.com/catalog/ ... lletin.pdf

Hi, has anybody in the Sonex community checked out the new cylinder heads from cbperformance, "Panchito 044"
they look like the right design for the Sonex, much better cooling features than the regular VW heads.

rizzz wrote:Went for a flight the other day and before going out I manually rotated the prop.
I noticed considerably less compression on 2 (#4 and #3 I believe) of the cylinders, also, I can hear a hissing in the exhausts during compression.
Leaky exhaust valves? Certainly showing all the symptoms…

You don't have the Nickasil cylinders, do you? If you say yes then I'll tell you what may be up. If you say no then disregard :-)

mike.smith wrote:

rizzz wrote:Went for a flight the other day and before going out I manually rotated the prop.
I noticed considerably less compression on 2 (#4 and #3 I believe) of the cylinders, also, I can hear a hissing in the exhausts during compression.
Leaky exhaust valves? Certainly showing all the symptoms…

You don't have the Nickasil cylinders, do you? If you say yes then I'll tell you what may be up. If you say no then disregard :-)

I have the Nicom cylinders from Hummel, they are essentially Nikasils (nickel silicon carbide) but I believe the NiComs have chrome in them as well or so I've read somewhere.
I did ask Scott about them when the whole AeroVee Nikasyl issue happened and he said he never had issues with his brand, he still puts them on his engines as far as I can see now.

But yes, the thought has crossed my mind... you had problems with yours didn't you?