Vibration on a Cessna 150...
Hello all,
My problem Is regarding a vibration issue that I have been fighting for over a year. I purchased a Cessna 150 just over a year ago. I bought it because in 2010 the engine cylinders were replaced as the then purchaser flew the airplane but at annual the engine had a 20/80 compressions (flew fine amazingly). So he decided to due extensive work to the engine/airplane. The engine had roughly 400 SMOH and he installed 4 new Millenium cylinders, overhauled carb, new mags, spark plug leads. About $20K were spent in all (so I’m assuming no shortcuts were taken).
I purchased the plane early 2013 and all seemed fine. As part of the annual (one year ago, June) I had the prop dynamically balanced (CopaAir Services in Indiantown FL). At initial reading the vibration was very bad (off the chart and no reading could be taken) and they had to take the reading at a lower RPM setting (1.2 ips). They finally balanced the engine with the resultant numbers at .2 ips. After the annual I sensed the vibration was worse. At full power on takeoff was fine, but once I retarded the throttle to 2300-2400 RPM the floor seemed to shake, the instruments shook as well and the pitot tube. Speaking with CopaAir they stated that the vibration was difficult to minimize and possibly I should check the prop. I went ahead and had the prop overhauled.
The prop was installed and we removed the weights that were previously used to dynamically balance the engine. The engine seemed better. But I had them try to again dynamically balance the engine/prop again. When they went to measure the initial reading again it was very high. Using reduced RPMs we read ( 1.0 ips). We rotated the prop and took a new reading ( 1.1 ips).
The mechanic then flew the airplane to "feel" the vibration. He leveled off but kept full power (something I never do, I'm very easy on the engine). Vibration seemed fine but once the throttle was retarded for cruise it was bad. We landed and found a cracked spinner bulkhead (which I firmly believe was a recent occurance. The bulkhead was replaced and we tried to again dynamically balance the engine/prop. The readings at the front of the engine are 1.22 IPS @ 296deg. Vibration at the rear of the engine is .67 IPS @ 358deg
CopaAir took readings of the engine cylinders with the cowl off to question the combustions causing vibration.
At 1500RPM all cylinder temps were close but EGT on cylinder #2 was 140 deg cooler.
At WOT with RPMs at 2550, the temps were all in 350f except for #2 cylinder which was at 400f. EGTs were all 600 to 650 degs as measured at the exhaust using a laser infrared but #2 cylinder was 175degrees cooler at 475 degrees. They inquired with Don from Don’s Dream Machines and he stated that the temp difference would not cause such a big vibration. To be safe regarding #2 cylinder, they ruled out hydraulic lifters, induction leaks, bad spark plugs and compression was 77/80. Since engine makes full power the cylinder was deemed to be okay. I read on Harry Fenton's website that a .030 over pushrod could fix this issue.
So, the vibration, they said is most likely due to one piston that is heavy (or lighter, I guess). I have a hard time swallowing this as these were 4 new pistons that were all installed at the same time and there did not "seem" vibration before. I say seem cause possibly I have become more aware of it as I have owned it, flew it and generally became more familiar with it. But I’m no expert and it could be possible.
Any guidance would be GREATLY appreciated!