?Sticking Valve or Carb Ice?

[marathonrunner]

It could also be intake springs not just exhaust. As long as you have them off you can test them easily or , have them tested if you do not have the tool to do it. does not take long to do. Like was mentioned, they are probably among the least expensive parts to have to replace.

[fangzz]

If a spring or springs were 'weak' to the extent it showed in engine performance, wouldn't it be much more prevalent and evident? Would it come and go? Would the engine performance go from excellent to shuddering and then resolve? Or would the engine performance just sort of be poor more-or-less all the time? I'm having a hard time with an intermittent float scenario, with such a shocking difference in performance on the shoulder periods of the float.

[Bruce Fenstermacher]

Drew I'm nearly sold on the theory this is a single cylinder issue and a valve issue is about the only system that is not redundant for each cylinder.

I'm not sold on any one valve issue so I would check and clean the valves and guides and check at least the exhaust springs of course after all other things are checked and eliminated such as plugs. It is a pretty simple process to check the springs. In my mind a calibrated measuring device is not needed. If the springs are close to limit with any reasonably close device, replace them.

Can't wait till you get into this so we have more info cause we've about beat this horse all we can.

[fangzz]

Yep!!! And I'm really greatful for you guys.

[GAHorn]

Yep!!! And I'm really greatful for you guys.

yes, and WE are grateful YOU volunteered to have this problem in lieu of all US!

[FredMa]

Fanz, a spring issue can come and go The reason is as follows. There are multiple springs for a reason. each spring has it own resonant frequency (frequency at which it vibrates) when a springs frequency is matched by the frequency of the engine or a harmonic of that frequency, it becomes suddenly weak. That is the reason for multiple springs. At any given frequency there is another to back up the one that is weak due to a matched resonant frequency. If one is out of the allowable limits for compression strenghth it can not function as a back up to the other springs when they are at resonant frequency and the valve will float at that frequency but work perfectly at another rpm setting. Hope that makes sense. Does your problem usually occur at the same rpm? If not then a weak spring will not be your problem.

[flat country pilot]

fangzz wrote

Compression test.

Seems like an easy place to start. Why not do a compression test and either isolate the problem or eliminate the possibility?

[marathonrunner]

Can also be a broken spring...lots of possibilities

[marathonrunner]

Did you ever get one to be leak free

[N2625U]

I've got a Triumph Spitfire. A drip under the engine, transmission and rear end means they've got oil and good to go! Thank God Lucas didn't get into aviation...

[fangzz]

Update.

I owed you guys an update after the Friday look-see.

The IA and I inspected the engine and exterior closely. I noticed heat indications on #2 exhaust fins and valve cover (an exhaust gas leak around the top valve cover bolt, similar to the other cylinders but slightly more evident) that was slightly more than on any of the other cylinders. The baffling back there behind #2 is poor, imo. IA agrees. That might account for some coking in that cylinder, if I’m not mistaken. Other than that, we found nothing. Pulled all plugs and examined to find only one with a good sized chunk of lead inside near the base of the insulator. The rest were uniform and clean. Plug wires all good. Carb air box fine. Compression and leak tested all cylinders and found them all healthy with NO leaking. Removed valve covers and pulled the engine through, closely watching the valve action. Using a wooden dowel, peened the exhaust valves. All returned to their original positions.

Buttoned up and flew with IA on ground watching and listening. FCF completed without a hitch or sign of this problem. Not surprising, since it has never happened except 3-10 hours apart. So, what we did, really, was take a good look around and RULE OUT a bunch of stuff. (Could this POSSIBLY be trash in the fuel line somewhere??)

Where do we go from here?

Will install a 6 probe cht/egt analyzer this week. Haven’t ordered yet and would like a recommendation for and against any units you have experienced. Looking at the Westech K28PDX. Looked at the Electronics International UBG-16 you mentioned Aryana, but that’s more than I think I want. I will seal off the baffling and openings there in the back of #2. Took a look at the engine overhaul entry in the logbook (SEP ’09) and the cylinders were sent off to Harrison Engine Services of Laporte, Indiana, for chroming, new valve seats and valve guides. VALVE SPRINGS WERE NOT REPLACED. While unbuttoned to install the above, will replace the exhaust valve springs (see below, thanks to Bruce), clean the #2 exhaust valve stem and guide (watching closely for the symptom George mentioned about a ‘broken’ guide, visible when the valve was open, but not when closed (but, they’re new, soo..)) using guidance found in Textron Lycoming Service Instruction 1425A, aka, the rope trick. No kidding. It actually calls out ‘dental floss!’ But, Bruce, it didn’t say anything about ¼” soft copper tubing. ☹ . Maybe next time. This is a standard O-300A, ostensibly reamed to ‘new’ dimensions; any of you chief mechanics happen to know the correct reamer size? Or, do you recommend first mic’ing the valve stem and going .0035 over? Or, what?


Superior AEC TCM
PN24031 5.13 4.83 9.85
PN24029 4.40 4.55 6.59
PN625958 2.70 3.08 6.29
Per valve 12.23 12.46 22.73

Fred, to answer your question about the harmonics, the condition has always occurred after cruise flight duration of enough time that the engine was at full operating temperature, and immediately after a small reduction in power/rpm to, for example, begin descent. So, the way I read your post, that implicates the springs. Does that answer the question you posed?

Thanks for staying with me on this, guys, and stand by for more, later. Drew

[GAHorn]

Update.... using guidance found in Textron Lycoming Service Instruction 1425A, aka, the rope trick. No kidding. It actually calls out ‘dental floss!’ But, Bruce, it didn’t say anything about ¼” soft copper tubing. ☹ . Maybe next time. This is a standard O-300A, ostensibly reamed to ‘new’ dimensions; any of you chief mechanics happen to know the correct reamer size? Or, do you recommend first mic’ing the valve stem and going .0035 over? ...

Airplanes are not backyard, muscle-car projects. Don't start reaming guides willy-nilly...and certainly without specifications in-hand, and the proper tools for the job.

[blueldr]

If you're thinking about a six cylinder CHT/EGT instrument, when I installed my Continental IO-360 engine I elected to install the K&E Electronics "HEXAD" instrument. It has potentiometers to allign the EGT bars during operation so that any deviation will be quickly indicated. As a long time flight engineer on reciprocating engines, I was very pleased with the performance of that instrument.

[fangzz]

Bluldr...thanks. that was helpful. I'll take a look.

[Bruce Fenstermacher]

Dental floss is used to help pull the valve back into the guide after it is dropped into the cylinder. I've tried it but use other easily made tools to do the job.

Well know you know the condition of lots of stuff on the engine. Here is what I'd do because you really didn't find anything that would fix the problem you have.

I'd either remove and measure the exhaust springs or just replace them. While I had the springs off I'd simply do a unscientific wiggle test. I'd also to an unscientific slide test. A good wiggle test would have an ever so slight wiggle. Just enought slop really to percieve the valve doesn't fit perfectly in the guide. No slop is suspect but not disqualifying. A slide test is simply that. Slide the valve in and out by have and feel how it moves. It shoudl move easily with no drag. Any drag is a disqualifier specially if there is no wiggle.

I'd drop the valve into the cylinder and clean the stem and guide of any valve that I'd previosly disquailfied and once you do it and feel comfortable with the proceedure you probably will to all the valves with no wiggle just for good measure. And in the end you might say what the heck this is fun and easy and just do them all.

I'd use the soft copper pipe that Lycoming hasn't discovered yet. But you can use any other mentioned method. I wouldn't ream the valve guides at this time unless I found one that was unusually tight after cleaning.

What your really doing here is getting intimintly familier with your engine. The more you know about it first hand the better your will feel about flying behind it. (or may be not )

I'd make a note of what you find. If you think the guides might be a little tight and the problem persists after knowing the valve spring condition, that is where I'd go next. Of course you may find more info to use with the gauge you intend to install.