Hopefully the brain trust here can unravel a PPonk puzzle?

Newly overhauled O-470R with new Superior Millennium steel cylinders, 7.5:1
New Hartzell 8068-82” prop
McCauley prop governor with fresh overall set to 2700 rpm max

Symptoms: engine runs great except, on takeoff. As the throttle is opened beyond green arc, with prop full, rpm increases to 2500 rpm, then retards to 2450. Stays there to WOT.
When throttled back toward top of green, rpm suddenly increases to 2700.
All other regimes are normal, including exact same sequence if done static, or in level flight.
This only occurs on takeoff, happens every time.

One other noted item is that oil pressure reads 75lbs. Reduces slightly as oil warms up.
#1 and #3 cylinders run the leanest based on EGT readings.
Yes I have a video that I will upload.

Theories?

This is going to sound dumb, but was the carburetor modified in accordance with the XP470 STC?

Have you double checked rpm with an optical proptach?

Sounds like prop or governor to me.

I have about 9 hrs on my new Pponk install and thank goodness it doesn't have those symptoms. I sent my McCauley governor that was new in 1999 up to Maxwell in Minneapolis to be overhauled and reset to 2700. I think they bench test it for rpm's. I didn't touch my MT 2 blade prop. Was your governor overhauled? Could the prop have something goofy letting it change pitch before it gets to 2700 rpm

Following up on this, the carb was overhauled and updated per the STC, at least according to the paperwork.
Best guess at the moment is that it is a tach issue. We tested the tach for accuracy against an optical one. Then we tried the optical tech on takeoff. Pretty sure the tach jumps only on takeoff is an instrumentation issue. “Pretty sure” because it is a bit of a challenge to take off in busy airspace while trying to hold an optical and a cell phone camera. Even for 2 pilots.

So, given it is a tach issue, I can see 3 possible culprits. The tach, which is a yellow tag unit that has the 2700 rpm limit. The cable, which was presumably greased and checked just a few hours back at installation. I am double checking that. And the cable mounting, I am guessing it gets pinched when the engine flexes on the mounts during takeoff.

Any thoughts on exactly where to look for a pinch point?

PNW Flyer wrote:Following up on this, the carb was overhauled and updated per the STC, at least according to the paperwork.
Best guess at the moment is that it is a tach issue. We tested the tach for accuracy against an optical one. Then we tried the optical tech on takeoff. Pretty sure the tach jumps only on takeoff is an instrumentation issue. “Pretty sure” because it is a bit of a challenge to take off in busy airspace while trying to hold an optical and a cell phone camera. Even for 2 pilots.

So, given it is a tach issue, I can see 3 possible culprits. The tach, which is a yellow tag unit that has the 2700 rpm limit. The cable, which was presumably greased and checked just a few hours back at installation. I am double checking that. And the cable mounting, I am guessing it gets pinched when the engine flexes on the mounts during takeoff.

Any thoughts on exactly where to look for a pinch point?

Frankly, mechanical tachs leave a lot to be desired. A long time ago, I went with an electronic tachometer and they allow me to be CERTAIN of what my engine is doing. And, they are super easy to install. I wouldn't waste a lot of time and effort on that tach and drive.

MTV

I put my Horizon P1000 in and love it because it will capture peak RPM too. Wish I could get the people in Calif to respond to my requests though.........

180Marty wrote:I put my Horizon P1000 in and love it because it will capture peak RPM too. Wish I could get the people in Calif to respond to my requests though.........

This. I like them but am having issues with 3 of them. They lock up in flight and require the master switch cycled to reset them, and one is extremely bouncy with the rpm readings. I would recommend an EI over the horizon. I like the bigger display of the horizon, but I haven't been able to get a hold of them for over a year. The lack of customer service pretty much ensures that I'll never install one again, unless they start answering the phone or emails.

The trouble I see with this logic is it puts one on the slippery slope. Why not go with a full engine monitor since my G1 doesn’t record or have G-logging?
But seriously if this isn’t fixable with a slight lengthening of the tach cable I will probably replace with a UMA
https://www.aircraftspruce.com/catalog/ ... ometer.php

Still probably an AMU all in.

We installed an EDM 830 the same time we put the new 0-470-50 in. The stock tac was 125 rpm off.

Update on this issue: Took the cowling off to try to find the tach issue.
The photo shows the tach drive tach cable and carb heat cable. On initial look it would appear the carb heat cable is pushing the tach cable against the oil filter. Surprisingly, while the carb heat cable is touching, and should be routed over instead of under the tach cable, pressing with a finger shows it is not moving the tach cable.
The other surprise is that a finger cannot move the tach cable at all. It is hard against the oil filter. It appears the tach cable rotated when the nut was torqued to the tach adapter. And the effect was to put a twist in the cable shroud which made it rigid against the oil cooler. The problem was then worsened when the engine was pressed back into place in the engine mount.



Whether this will solve the original tach problem will have to wait until we can test after this issue is fixed.
Has anyone else had an issue with the tach cable shroud twisting when the nut was torqued?

I would guess the tach cable sheath was already compromised and the torque finished it off. The sheath should seat against the input nipple and the nut shouldn’t really be able to twist the sheath.