O360 high power, intermittent stumble…

[Jakes Simmons]

     Two steps back for every one step forward. Was making progress on the AeroCruze issue and now I have an engine stumper. 76 Ranger with the O360 and PowerFlow exhaust has started an intermittent stumble during climb out at MP’s above 25”. It’s not audible, no backfiring, but I can feel a vibration come and go, almost like when there is a little water in the fuel. Mag checks at run up and cruise power are perfect, no change from last 100 hours. Single point CHT and EGT needles are right where they always are. Adjusting mixture mildly doesn’t affect it. Once I’ve either climbed past 25” of MP or reduced throttle below that number, it either becomes imperceptible or goes away. Slightly reducing throttle to back off the enrichment circuit doesn’t seem to affect it either. A mechanic who used to maintain the aircraft years ago felt it was an exhaust spring issue, broken or weak. There is also some play in the throttle bushing where it goes to the enrichment circuit. Bunch of avenues to pursue but I don’t have an open checkbook to follow them all. Looking to see if anyone here has had similar issues or more trouble shooting ideas. I’ve checked carb heat, mags, prop balance, temps, no water in fuel, spark plugs, everything nominal. Except for this one issue, Ranger runs like a champ as always. Appreciate the help…

[Jakes Simmons]

Also, approximately 500 hours on engine since O/H, 25 years ago. Mags have 60 hours since overhaul and were also adjusted two months ago during annual. Prop is five years old and 200 hours since new. It’s been dynamically balanced twice, including once last year.

[Ragsf15e]

15 minutes ago, Jakes Simmons said:

Also, approximately 500 hours on engine since O/H, 25 years ago. Mags have 60 hours since overhaul and were also adjusted two months ago during annual. Prop is five years old and 200 hours since new. It’s been dynamically balanced twice, including once last year.

From the symptoms, I would start at Ignition.  When you say your plugs were “checked” what do you mean?  Did you put them all on a dvm and check the resistance?  What about the harnesses?  Checked thoroughly for any tiny defects or fittings that aren’t snug (on mags or plugs)?  How old is the harness?

Mag checks are looking for gross failure (dead plug, dead mag), whereas something less than a complete failure might only show up at high power.  Also, checking the resistance of all the plugs and security of the harness is basically free.  

[Igor_U]

I agree with Rags15e above.

I had similar symptoms on my F that turned out to be high resistance Champion Spark plugs. Since then I was buying Tempest. Of course, new engine couple of years ago came with Champs... 

[Bartman]

In my IO-360 powered J this would be a good time to do a high-powered LOP mag check.  This helped me pull the trigger on a new ignition harness and solved my intermittent stumble.

[bigmo]

See if you can borrow a full set of plugs and swap them all and do a quick test flight to verify. That’s a simple 1 hour check of a high culprit item. 

[Jakes Simmons]

That’s a good idea, swapping plugs outright. The airplane came out of annual in October, plugs were rotated, cleaned and gapped then. Mech said everything looked good,  it I wasn’t having issues then. Currently the covers are getting pulled to check for broken valve springs. I’m way outta town. Once mech’s are back from Thanksgiving, I’ll see if they can swap plugs while cowls are off. Will also let them know to examine harness more closely. 

[A64Pilot]

If you can’t get anywhere with ignition, my next suspect would be a sticking exh valve.

Having said that high manifold pressures do require higher voltage to fire and very often a weak ignition works fine until manifold pressure is high then it begins to miss.

 

[EricJ]

I'm amazed that some people have engines that run smoothly enough to detect such subtle issues, let alone worry about them.