George:
I was wondering where you were! Anyway, this info about Cessna stating the the #2 cylinder is the leanest, kind of puts the seal on where we were headed. Thanks...mystery solved.
Curt
High EGT, One Cylinder Only, Full Power Climb
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
EGT
I have exactly the same problem on my C-170B, C-145 engine. Installed a UBG-16 EI on a firewall forward new engine with 70 hours on it now. The #2 EGT runs around 1435-1460 or so on climb out. If I pull the power back to 2400 RPM or so the EGT settles around 1350. Ideally the cylinders should all be running somewhere in the 1200's due to the rich takeoff mixture. But #2 is still about 100-150 degrees hotter than the rest of the cylinders and they are running in the 12's. In cruise and leaned out it still reads a bit higher than the rest but is well within limits. We have checked everything...fuel injector plug, manifold gaskets, rubber hose between the two part intake manifold, plugs, probe position, etc.. I switched EGT probes from the #4 cylinder and got the same results...so it isn't the probes. The CHT is running around 380 at max climb on a hot day. It comes back to run around #340 or so at cruise. Well within limits. Baffling. But, its a C-145 and the intake manifold is not the best and is probably the culprit. Can't change that design so I'm living with it.
JT
JT
Semper Fi
'54 C-170B N2782C
'54 C-170B N2782C
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robert.p.bowen
- Posts: 51
- Joined: Thu Dec 05, 2002 6:39 pm
Curt:
First, I do not believe this is a preignition event, nor is it detonation-related. It is VERY difficult to get a conforming, normally-aspirated opposed engine to enter detonation. Preignition usually takes an "event" such as a helicoil installed too deeply so that it becomes a glow plug, or a cracked insulator on a plug, to trigger preignition. When that happens, your CHT's go way up, quickly. That's not happening in your engine.
John Barrett and the others are likely correct--since you've checked and concluded you have a tight intake system, it's likely an imbalanced fuel flow, aggravated by a poorly designed baffle behind cyl. #2. Bonanza's and SR22's both have this issue with that cylinder. It's correctable with a redesigned baffle, but that's beyond this discussion.
As to in-flight mag checks--the best way to test your ignition system is NOT on the ground. There, incipient failures are rarely discovered. The best way is at cruise (not necessary to do at full power) and lean of peak (not full rich). In this condition your ignition system is under maximum stress. Because CHT's rise slowly, you can do this without risk of damage to your engine. Switch to one mag for about 20 seconds. You should see all EGT's rise and stay up. CHT's will remain constant or gradually decline. Then switch to the other mag (no need to go back to both). If the same result is noted, your mags, harness and plugs are performing normally. If, however, the engine stops running, you have a problem. DON'T go back to "Both." Pull the mixture to idle cut-off, then switch back to both, and ease the mixture back in until the engine restarts. Were you to go back to "both" before pulling the mixture, the accumulated fuel in the exhaust and mufflers wil ignite with a BANG!, possible damaging the exhaust system. If one cylinder EGT drops on this LOP in-flight mag check, you have (probably) a bad plug. You can tell where by remembering the left mag is supposed to fire the lower plugs, and the right mag the upper ones. Replace that plug and retest on the next flight.
The best way to handle the mag switch is as if it were red hot. Switch to one mag, and let go of the switch. That's because it's hard not to go back to Both, involuntarily, if the engine stops. Then put you hand on the switch and go to the other mag, and again quickly remove your hand.
If you do an in-flight mag check just before letdown, the only thing you need do before the next flight (in my opinion) is cycle thru the two mags to be sure a condenser or coil hasn't failed (i.e., a dead mag).
Your experience highlights how important a six-cylinder EGT/CHT analyzer is. I prefer the JPI, but Insight is a whole lot better than a single cylinder CHT gauge!
Hope this helps...
First, I do not believe this is a preignition event, nor is it detonation-related. It is VERY difficult to get a conforming, normally-aspirated opposed engine to enter detonation. Preignition usually takes an "event" such as a helicoil installed too deeply so that it becomes a glow plug, or a cracked insulator on a plug, to trigger preignition. When that happens, your CHT's go way up, quickly. That's not happening in your engine.
John Barrett and the others are likely correct--since you've checked and concluded you have a tight intake system, it's likely an imbalanced fuel flow, aggravated by a poorly designed baffle behind cyl. #2. Bonanza's and SR22's both have this issue with that cylinder. It's correctable with a redesigned baffle, but that's beyond this discussion.
As to in-flight mag checks--the best way to test your ignition system is NOT on the ground. There, incipient failures are rarely discovered. The best way is at cruise (not necessary to do at full power) and lean of peak (not full rich). In this condition your ignition system is under maximum stress. Because CHT's rise slowly, you can do this without risk of damage to your engine. Switch to one mag for about 20 seconds. You should see all EGT's rise and stay up. CHT's will remain constant or gradually decline. Then switch to the other mag (no need to go back to both). If the same result is noted, your mags, harness and plugs are performing normally. If, however, the engine stops running, you have a problem. DON'T go back to "Both." Pull the mixture to idle cut-off, then switch back to both, and ease the mixture back in until the engine restarts. Were you to go back to "both" before pulling the mixture, the accumulated fuel in the exhaust and mufflers wil ignite with a BANG!, possible damaging the exhaust system. If one cylinder EGT drops on this LOP in-flight mag check, you have (probably) a bad plug. You can tell where by remembering the left mag is supposed to fire the lower plugs, and the right mag the upper ones. Replace that plug and retest on the next flight.
The best way to handle the mag switch is as if it were red hot. Switch to one mag, and let go of the switch. That's because it's hard not to go back to Both, involuntarily, if the engine stops. Then put you hand on the switch and go to the other mag, and again quickly remove your hand.
If you do an in-flight mag check just before letdown, the only thing you need do before the next flight (in my opinion) is cycle thru the two mags to be sure a condenser or coil hasn't failed (i.e., a dead mag).
Your experience highlights how important a six-cylinder EGT/CHT analyzer is. I prefer the JPI, but Insight is a whole lot better than a single cylinder CHT gauge!
Hope this helps...
Last edited by robert.p.bowen on Tue Jun 12, 2007 8:55 pm, edited 1 time in total.
Bob-
Bob - Thanks
Bob:
Great information. Thanks. Curt
Great information. Thanks. Curt
Bob,
Be advised that on C-145/O-300 engines the left mag fires the bottom plugs and the right mag fires the top plugs. I believe this is because of the two degree difference in the timing of the two mags. This is the only engine that I know of that is ignition timed and has the mags firing the plugs in this manner.
Be advised that on C-145/O-300 engines the left mag fires the bottom plugs and the right mag fires the top plugs. I believe this is because of the two degree difference in the timing of the two mags. This is the only engine that I know of that is ignition timed and has the mags firing the plugs in this manner.
BL
On my 170 (O300A) each mag fires the top plugs on its own side, and the bottom plugs on the opposite side. A friend's '53 B-model is the same way.
On the other hand, my Swift (same engine) is set up with one mag firing the top plugs and the other firing the bottom. I've replaced the wiring harnesses, and the new ones came labeled that way.
The TCM O300 manual shows the left mag firing the bottom plugs at 28 degrees and the right mag firing the top at 26 degrees, just as Dick said.
I wonder how these 170s came to be different?
John
On the other hand, my Swift (same engine) is set up with one mag firing the top plugs and the other firing the bottom. I've replaced the wiring harnesses, and the new ones came labeled that way.
The TCM O300 manual shows the left mag firing the bottom plugs at 28 degrees and the right mag firing the top at 26 degrees, just as Dick said.
I wonder how these 170s came to be different?
John
John Renwick
Minneapolis, MN
Former owner, '55 C-170B, N4401B
'42 J-3 Cub, N62088
'50 Swift GC-1B, N2431B, Oshkosh 2009 Outstanding Swift Award, 2016 Best Continuously Maintained Swift
Minneapolis, MN
Former owner, '55 C-170B, N4401B
'42 J-3 Cub, N62088
'50 Swift GC-1B, N2431B, Oshkosh 2009 Outstanding Swift Award, 2016 Best Continuously Maintained Swift
The mag timing gets checked on every annual, and set to L28/R26. The thing is, mine isn't the only 170 I've seen this way, so I'm wondering if somebody somewhere had a good reason for doing it differently from the TCM book, or if TCM has published two different versions, or a service bulletin, or something.
Anybody else out there have a C145 or O300 with each mag firing the top plugs on one side and bottom plugs on the other side?
John
Anybody else out there have a C145 or O300 with each mag firing the top plugs on one side and bottom plugs on the other side?
John
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cessna170bdriver
- Posts: 4068
- Joined: Mon Apr 22, 2002 5:13 pm
The TCM overhaul manual for the C125, C-145, and O-300 series engines specifically states and shows that all bottom plugs are fired by the left mag and all top plugs are fired by the right mag. I've never seen a manual for these engines that calls it out otherwise. Maybe whoever miswired your engine did so by indadvertently referring to an O-470 manual?
Miles
Miles
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robert.p.bowen
- Posts: 51
- Joined: Thu Dec 05, 2002 6:39 pm
BL and Miles,
I knew I could count on you guys to keep me straight! I went back to the C-125, 145 and 0-300 Overhaul Manual, and it specifies the left mag fires all lower plugs, and the right the uppers. Apparently there is some "range dispersion" in the C-170 fleet as to how they're actually wired. Curious why our firing pattern is different from the 520 and 550 TCM engines; but you must be certain of the wiring pattern to trouble shoot using an engine analyzer.
In any event, I'll go back and edit my post to correct it.
Thanks,
I knew I could count on you guys to keep me straight! I went back to the C-125, 145 and 0-300 Overhaul Manual, and it specifies the left mag fires all lower plugs, and the right the uppers. Apparently there is some "range dispersion" in the C-170 fleet as to how they're actually wired. Curious why our firing pattern is different from the 520 and 550 TCM engines; but you must be certain of the wiring pattern to trouble shoot using an engine analyzer.
In any event, I'll go back and edit my post to correct it.
Thanks,
Last edited by robert.p.bowen on Wed Jun 13, 2007 12:09 am, edited 1 time in total.
Bob-
Due to the airflow within the cylinder the lower plugs flame-propagation is different than the uppers....and THAT is why the Left mag fires earlier than the Right. (This according to the now-retired TCM guy in my area.) If you do not have your engine wired In accordance with the left mag-lower, right mag-upper.... then the ignition timing for your engine is wrong due to the mags not lighting the flame at the appropriate time.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
Another twist on the story of the top/bottom mag timing. Originally TCM set these engines up so that one mag fired the top plugs and the other mag the bottom plugs. To keep the mag drop in tolerance they adjusted the timing with the 2 deg lag. On newer engines they have one mag firing the top plugs on one side and the bottom plugs on the other. With these engines both mags are timed the same and the mag drop is tolerable.
If someone decided to improve engine performance by setting the timing of both mags the same, they would have to go with one mag firing the top plugs on one side and the bottom plugs on the other side. The extra 2-degs of mag timing gets one a little more hp, better fuel economy, better combustion, less tendency to stick valves, etc In principle one could set both mags at 29-degrees (28 + or minus 1 degree) and go with the cross over plug firing to get the best performance out of the C145.
I suspect if you talk to your DER you would find this is nothing more than a log entry associated with a new harness and mag timing check - but who knows it may not be to another DER.
If someone decided to improve engine performance by setting the timing of both mags the same, they would have to go with one mag firing the top plugs on one side and the bottom plugs on the other side. The extra 2-degs of mag timing gets one a little more hp, better fuel economy, better combustion, less tendency to stick valves, etc In principle one could set both mags at 29-degrees (28 + or minus 1 degree) and go with the cross over plug firing to get the best performance out of the C145.
I suspect if you talk to your DER you would find this is nothing more than a log entry associated with a new harness and mag timing check - but who knows it may not be to another DER.
Have been reading this post with interest as I have noticed fuel mixture unbalance in my engine. Just finished my ground up on a 53B including major. Took it up for 2 hrs last night to break in the motor. Noticed that the right exhaust looks lean, the left rich. Have checked the intake system for leaks, Even pressure tested by pressurizing a cylinder with the intake valve open, throttle closed, and spraying with soapy water. no bubbles. No leaks through the primer system either. Thoughts? Normal?
George
George
On those airplanes that had the ignition harness firing "half and half", I would strongly suspect that someone installed a harness designed for one of the larger Continental engines. such as the O-470. New harnesses generally have each wire marked for the proper plug position. Harnesses for the C-145/O-300 are marked differently than those for the big bore engines. Since either size engine may use the same mags, It's possible someone installed the wrong harnesses.
BL