Hopefully you figure it out soon. Some good info being thrown out in this thread too. Good reading. My initial thought was an air/fuel ratio problem... when thinking about the issues with the engine you found before this, along with it's current issue.. it just kind of sounded like maybe there was something going on causing it to run lean/detonation etc.. But the valve spring problem sounds promising. Good luck I'm rooting for you!

onefitty wrote:

PAMR MX wrote:I would bet ya a beer both magneto condensers are bad.

Yup, my money's on condensers and / or points

how fast can something like that fail? They were changed with the magneto overhaul last January, and only 15 or so hours have been put on it since. I won't rule it out though.

Most make TBO and beyond, but occasionally they fail... We've had condensers fail in less than 10 hours and points misaligned, burnt and occasionally not installed correctly. We used to do mags in house but now send them out due cost, maybe not a good thing...

If cost is an issue, can you borrow a set of mags to try? Only a couple hours to swap over

Oh, and you might be surprised to find how common it is to mis-diagnose ignition faults as carb / fuel issues. They can give very similar symptoms. I usually go plugs and mags first. I may be way off, but I doubt your valve springs are the problem

Flap in hose may not be observable , only show up when it has suction . Try replacing hose

onefitty wrote:Most make TBO and beyond, but occasionally they fail... We've had condensers fail in less than 10 hours and points misaligned, burnt and occasionally not installed correctly. We used to do mags in house but now send them out due cost, maybe not a good thing...

If cost is an issue, can you borrow a set of mags to try? Only a couple hours to swap over

That's good to know. The new valvesprings should be here tomorrow and hopefully with enough time left in the day to get them put in and try running it again. I'm hopeful but not holding my breath that will fix it, if not, I'll pull the mags and check the internal timing, points, cam and change the condensers. I really only want to do one thing at a time so when it's fixed I know exactly what the problem was, otherwise I would have dug into the mags today. I've gotten some advice that the plugs, even though they ohm out OK may still be faulty, I think pulling and inspecting the mags even though more time consuming will be less expensive and I'd like to try and knock out the cheaper stuff first. If mags are good then I'll try new plugs. At some point I'd still like to pull out the economizer screw/needle and clean that up as well.

Thanks again everyone for all the input, I've been an A&P for nearly 10 years and I'm learning a lot going through this whole process, working on these airplanes is a lot of fun and rewarding. Not like the mundane recurring tasks/inspections that keep jets flying.

BlackWater wrote:Wow, that sounds like detonation! ...

My vote as well, especially since it had two cracked pistons and rings before. Initially reaches full rpm then as the detonation starts rpm drops because of premature/uneven firing. Sure sounds like detonation to me.

So today I put all new valve springs in it and took it out for a run......No more vibration or big RPM dips at full power. I am still getting about 50ish rpm swings on the tach but I cant feel or hear it, if I sat there with my eyes closed I wouldn't know the tach was swinging, the bouncing of the needle seems to coincide with the movement of the airplane in the wind so I really think this problem might be solved!

I ran around town this afternoon looking for a digital tach of some sort so I could confirm the needle movement on the tach is just the tach itself but my search was unfruitful. Either way I'm going to cowl it up and take it for a spin, this thing was bucking like a bronco before on takeoff, you could feel/hear it very obviously so I'll know right away that the problem is fixed for sure. I'm going to try and get it up tomorrow, the weather is supposed to be better. I'll post my results.

Barnstormer wrote:

BlackWater wrote:Wow, that sounds like detonation! ...

My vote as well, especially since it had two cracked pistons and rings before. Initially reaches full rpm then as the detonation starts rpm drops because of premature/uneven firing. Sure sounds like detonation to me.

I would like to add, there were actually 4 cracked pistons, but the cracks were radiating from the holes in the oil control ring (below the pin) down to the bottom of the skirt, one crack on each of the 4 pistons in exactly the same spot... The cylinder with the broken rings had a bit of surface rust inside it when I bought the plane. Before I purchased it it had maybe 20-30 hours put on it since 2011, and before that it had been sitting since 1989ish. There was no indication on the tops of the pistons or anywhere else of stress/melting/cracks that I would associate with detonation.

Newbizor

Maybe a small light on the horizon.

I have owned and flown old rentals where the tach. consistently "shook" 50 rpm. +/- 25.
Usually fixed in those days by taking it to a automotive speedometer shop for over haul.

Had a mobile tach. man come by the shop one day with enough spare time to do a show and tell to educate me on how all the pieces worked. Unfortunately I am a slow learner so the folowing may be suspect.

Learned that the tach. time will match the clock time at or near only one RPM setting.
Believe that RPM value can be magnetically "moved" - re-calibrated a bit.

Nuff for now

I've run digital electronic tachs in several airplanes, and my experience is that most engines cycle a little bit in RPM. Most don't notice that because the mechanical tachs don't display it as well....just small movements of the needle, whereas the digital tachs change the numbers in the display.

MTV

Congrats on finding the issue...let's hope that's it.

Meanwhile, everyone has a few more tricks in their bag.

Awww the old weak valve springs. I will put a beer in the mail.


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Hope you were able to make it run right. Just a side note. I had the exact same symptoms on a customers 170 with a c-145 after the one piece venturi was installed. After talking with Precision Fuel Systems they said that the one piece made the airflow across the venturi too sensitive on some engine applications. The AD was revised to allow the 2 piece venturi to be reinstalled, but would also require a 100 hr. inspection to make sure the primary wasn't getting loose. I reinstalled the 2 piece venturi and the engine ran like a swiss watch again. I had tried everything to make it run right with the one piece including sending the carb to Precision, but in the end I had to go back to the original configuration. I also had this same thing happen with an O-200. Ran great until the very last portion of full throttle was applied, then junk happened again. Sounds like maybe you've got it fixed.

PAMR MX wrote:Awww the old weak valve springs. I will put a beer in the mail.

I'm not celebrating just yet....I'll know for sure when I fly it tomorrow. I pulled a plug back out of cyl #2 after the run and it was sooty again, still confused as to why that's the only cylinder with the sooty plugs....

Newbizor wrote:So today I put all new valve springs in it and took it out for a run......No more vibration or big RPM dips at full power. I am still getting about 50ish rpm swings on the tach but I cant feel or hear it, if I sat there with my eyes closed I wouldn't know the tach was swinging, the bouncing of the needle seems to coincide with the movement of the airplane in the wind so I really think this problem might be solved!

I ran around town this afternoon looking for a digital tach of some sort so I could confirm the needle movement on the tach is just the tach itself but my search was unfruitful. Either way I'm going to cowl it up and take it for a spin, this thing was bucking like a bronco before on takeoff, you could feel/hear it very obviously so I'll know right away that the problem is fixed for sure. I'm going to try and get it up tomorrow, the weather is supposed to be better. I'll post my results.

Barnstormer wrote:

BlackWater wrote:Wow, that sounds like detonation! ...

My vote as well, especially since it had two cracked pistons and rings before. Initially reaches full rpm then as the detonation starts rpm drops because of premature/uneven firing. Sure sounds like detonation to me.

I would like to add, there were actually 4 cracked pistons, but the cracks were radiating from the holes in the oil control ring (below the pin) down to the bottom of the skirt, one crack on each of the 4 pistons in exactly the same spot... The cylinder with the broken rings had a bit of surface rust inside it when I bought the plane. Before I purchased it it had maybe 20-30 hours put on it since 2011, and before that it had been sitting since 1989ish. There was no indication on the tops of the pistons or anywhere else of stress/melting/cracks that I would associate with detonation.

Well how about that! Well done fixing the fault

Your engine didn't happen to be bought from a guy in PA on Barnstormers? If so I know a little bit about the engine. I am just wondering because the years of non use and low hours sound the same.

Congrats on solving the problem, glad it wasn't detonating. Great comments from everyone. Boy this old stuff can be a pain in the ass.

Wow...thanks for the update. I've never run into a spring issue before. It's an education for me for sure.

Thanks!

IMHO this is where a good engine monitor / multi-point EGT / CHT gauge would really earns it's keep, isolating problems to a certain cylinder(s) for diagnosis purposes.

Barnstormer wrote:

BlackWater wrote:Wow, that sounds like detonation! ...

My vote as well, especially since it had two cracked pistons and rings before. Initially reaches full rpm then as the detonation starts rpm drops because of premature/uneven firing. Sure sounds like detonation to me.

I'm curious what might be causing detonation?
1) too-low octane fuel
2) timing too advanced
3) ??

So I flew it today and it ran great, made full power and loved it. I climbed up to about 4,000ft (800ft field elevation) and the engine didn't miss a beat! I cant explain how happy I am, what a huge relief. I can't thank you all enough, actually last night I made my donation to the site.

I flew for about 45 mins and landed to do a quick oil level check/general visual inspection under the cowl (maybe 5 mins) before starting it back up and taxiing to take it back up, I noticed it took a few blades to start which was a little unusual but brushed it off to being fully warm. I got to the end of the runway and did another runup and when I selected my right mag I got NO rpm drop, selected the left mag and DEAD. Shit.... I taxied back to the ramp and started to investigate. Right away I stuck my head under the panel to inspect the wiring to the switch ( I just replaced all the wiring in this whole plane, the switch is only a few years old) and it was all intact, terminals tight and no rubbing on anything. Next I popped the panel on the cowl and looked at the mags/harnesses (still there) and the p-leads back to the firewall. All appeared well, I noticed the right mag P-lead was movable by hand when I rotated it so I tightened the nut back down (not that that had anything to do with it I know). I started it back up and this time I got a drop when selecting the right mag, but when I selected the left mag I got a very big drop, rough running and backfiring (LOUD!) out of the exhaust. I shut back down and went inside the FBO and drank a soda while I mentally planned my attack. About 20 mins later for shits and giggles I thought I would just try it again, started the engine (started right up) good mag drop on both mags.... whatever, I blasted off and flew back home (just a short 10 min hop) and when I landed before I shut down I did another mag check and the Left mag was dead again....

Why would the mag work cool but die once its warmed up? Bad Coil? I guess I'll be pulling the mag (might as well do both) anyway......

cliff wrote:Your engine didn't happen to be bought from a guy in PA on Barnstormers? If so I know a little bit about the engine. I am just wondering because the years of non use and low hours sound the same.

Nope, this plane has lived its whole life between Minnesota and Wisconsin as far as the logbooks/records show.

lesuther wrote:Wow...thanks for the update. I've never run into a spring issue before. It's an education for me for sure.

Thanks!

Yeah some local mechanics told me I should at least have the springs checked, since the engine had sat for 20 years without moving. It was kind of a long shot in my mind but I'm thrilled it ended up being the culprit.

hotrod180 wrote:IMHO this is where a good engine monitor / multi-point EGT / CHT gauge would really earns it's keep, isolating problems to a certain cylinder(s) for diagnosis purposes.

I know, I'd love a CGR-30P but the thing is damn expensive.....I've been on the fence now on it for some time. I've installed JPI monitors in friends planes and they are awesome tools. I think that might be my next purchase for this plane, hopefully they will have a deal at the EAA booth or something (just dreaming).