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Diagnosing engine performance

LettersFromFlyoverCountry

Well Known Member
This is an extension of the thread on getting the paperwork in for an airworthiness inspection, which morphed into an analysis of engine ground testing. Makes sense to start a new thread.

On Alex Peterson's advice -- who said "take Paul Dye's advice" -- I tried to run the engine last night to 2000 until the CHT got close to 300.

I found I could not get the engine past 1900. I have to tach indications on the GRT EIS, one off the mag, and one off the Lightspeed EI.

So, here's the thing. I've gotten this far on the project, going slow and trying to conquer the strange land of things I don't know about, but i want to be VERY careful in approaching this situation because it just doesn't seem like proper place for a "I wonder what turning this screw will do?" method and that's pretty much what I would be doing.

So, I'm looking for resources to read on the step-by-step approach to analyzing what's going on here.

I'm assuming that step #1 is to check the throttle and mixture cable attachments to be sure they're extending the arms on the servo to their full travel.

After that, though, it gets a little murkier. If you're an A&P, what would be the logical progression of steps to take here.

By way of background, this is a TMX IO-360 M1B, it's a fixed-pitch propeller from Sensenich. It's a 72FM8S9-1-85 (standard). Sensenich information says the static RPM range LIMIT is 2050 to 2250.
 
Assuming your tach indication is correct....

Is it a new engine? It may have low compression/power due to un-seated rings or something. Make sure you're not violating any break in instructions.

Assuming it's not a new engine:
Do a mag check to check for a funny drop or roughness on one or the other
Check the mixture and throttle cables
Beg or borrow a MP gauge and note that indication while doing the ground run
Check the compressions
Read the plugs when you pull them
Check mag timing and condtion

Report back
 
You are taking the correct approach to resolve what you think is a problem.

What is unknown is whether you do in fact have a problem. I have never run my RV-6A, Lycoming 0-360 with Catto three blade prop at full throttle on the ground while stationary.

The RPM at LIFTOFF is circa 2200-2250 RPM (full throttle). From that I would expect that not moving it would be less. It gets to around 2700 RPM at cruise speed and full throttle at suitable altitudes.

Thus, 1900 RPM on the ground...not moving...may be expected.
 
See if you can borrow an optical tach Bob - it is possible that youhave something mis-set in the EIS and are getting a false RPM reading (speaking from experience).

Paul
 
Tonight (after the Timberwolves game) I was going to take her out and do the "mercury vapor light" thing with the prop. I suppose that's not a completely accurate method. I'll look around for an optical tach.

(yes, new engine)
 
Do the tach check

The good news is that if you tried to take off - with that RPM, you'd make it.

But, you should chase this down
- check the tach indication to be sure correct
(do this in low wind conditions)
- check the throttle / mixture linkage
(operate from cockpit and see if there is additional travel
possible at the carb.)
(does anything change if you move the mixture?)
- try a mag check

Watch the run times and temps on this as you troubleshoot.

Dan
 
The good news is that if you tried to take off - with that RPM, you'd make it.

But, you should chase this down
- check the tach indication to be sure correct
(do this in low wind conditions)
- check the throttle / mixture linkage
(operate from cockpit and see if there is additional travel
possible at the carb.)
(does anything change if you move the mixture?)
- try a mag check

Watch the run times and temps on this as you troubleshoot.

Dan

Fuel injected. I'll get a second pair of eyes to watch the servo as I move the throttle/mix cables. Up to now, I just pushed full forward and got out to go look and everything seemed fine, but maybe that's not an efficient method.

I shut down the mag -- which kicks on the RPM reading from the Lightspeed -- and documented no change.

I did lean it a bit with mixture to see if there was a change but did not observe any. On the other hand, I was hearing "tic toc" in my head by then so I didn't fully lean it out. I wanted to get the engine shut down before the CHTs went much higher. Total run time was 3 minutes.
 
Bob,

WRT EIS settings, review paragraphs:

7.6 Tach P/R – Setting for Correct Tachometer Readings
7.6.1 Tach – Connections to Magneto P-leads, Ignition Coils or Tach Outputs
7.7 TachSen

Also review Figure 7b. Other Tachometer Connections

However, as recommended, a good optical tach rpm check will eliminate tach output reading errors from your troubleshooting procedures.

Regards,
 
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Bob,

WRT EIS settings, review paragraphs:

7.6 Tach P/R ? Setting for Correct Tachometer Readings
7.6.1 Tach ? Connections to Magneto P-leads, Ignition Coils or Tach Outputs
7.7 TachSen

Oh, yes, that's already been done. Can't remember the Pulse per revolution but I've set those correctly in the EIS.
 
See if you can borrow an optical tach Bob - it is possible that youhave something mis-set in the EIS and are getting a false RPM reading (speaking from experience).

Paul

Go to Hub Hobby on Rice street and purchase an optical tach for an RC airplane. 19.95 and will read a two or three blade prop.
 
I did lean it a bit with mixture to see if there was a change but did not observe any. On the other hand, I was hearing "tic toc" in my head by then so I didn't fully lean it out.

Leave your mixture full rich, except when idling, aggressively lean almost to the point it quits. Only when you go to push the throttle in do you push the mixture in. Your engine will appreciate this.

On a further note, on the rich side of peak, HP is determined by mass airflow, so leaning a little will make very slight changes to power, almost not worth looking at. On the lean side of peak egt fuel flow controls HP. So unless you are up high in the Rockies, leave it full rich at full power.

Optical Tacho will tell the story, and during the ground roll, first 200' you will pick up RPM, and most I see do 2250-2300 then.
 
Go to Hub Hobby on Rice street and purchase an optical tach for an RC airplane. 19.95 and will read a t
wo or three blade prop.

There now. They don't carry them. Drives me crazy how these delays crop up. I'll have to order online and wait. While I'm waiting, make me an offer on this thing will you?
 
No, I haven't done anything. That's why I'm asking about a logical progression and how-to references. As I said, I don't know that much about engines; I'm not going to blindly go in there and start messing around because I really don't know what I'm doing and there's a fair chance I can make things worse.

So what I've gleaned so far is the first step is to confirm the readings of the tach, then a visual confirmation that the control cables are making the arms on the servo hit the stops. Then....?

The only alt air I have is the rinkydink "door" on the bottom of the FAB and that's confirmed "closed."

I will check for intake leaks but first I'll need to learn how to do that.

In the meantime, I'm praying to my little dashboard statue of Tony Bingelis. :)
 
So what I've gleaned so far is the first step is to confirm the readings of the tach, then a visual confirmation that the control cables are making the arms on the servo hit the stops. Then....?
:)


Then....report back! Those two results will eliminate whole portions of the fault tree, and help the experts define the next step.
 
UPDATE #1:

I detected a very slight play in the throttle arm on the fuel servo so it appeared the throttle cable was not fully opening the throttle. I made some adjustments -- loosening the rear cable screw and tightening the forward one at the bracket -- took it out and fired her up. 2000 RPM! That's good, right? I really don't want to do this very much; I'm getting leading edge scratches in the beautiful prop.

Of course, now the problem was I couldn't get the RPM to go below 1000. D'oh! From one full turn of the two nuts? Geez. This is going to be fun.

I did notice something odd, when I shut off the EI (Lightspeed). The RPM went UP by 50. That surprised me.

I haven't timed the Lightspeed per the manual, mostly because I was trying to avoid the idea of having my head inches away from a whirring propellor, but then I remembered when we timed the mag some months ago, we made a mark on the flywheel for TC...even though there was an engraved mark for it already (although I didn't see it).

When I put the ring gear (I think it's called the ring gear) on, I didn't line the timing mark up properly. Is that a deal breaker? Not to properly time the mag, but that I wonder if it would mess up the Lightspeed?

Anyway, my next step is to start over and take the prop off and get the timing marks in the right spot. Re-time the mag (after I learn again how to do it), time the Lightspeed, adjust the throttle cable...but first...I hope to confirm the RPM readings are correct on Sunday (looks like rain tomorrow).

I took some video of the gauges as I was doing all this. If I can get it to work, I'll stick it on the blog later.
 
On throttle travel, are you using a quadrant at the panel, or just a regular "knob" cable? if the latter, you should have plenty of travel in the cable to get full throw. Quadrants can be a little tricky to make sure you can get all the movement you need between idle and wide open.

Ring gears are frequently keyed so that they can only go on one way (to make sure the timing marks are in the right place) - but I have had at least one that was not, and could go on in any of the six possible "rotations" - yes, this raised heck with the timing! Good idea to double-check everything. it's a pain to pull off the prop, but sometimes you just have to start over. The positive thing about that is that you will become quite adept at engine set-up and timing....

Good luck!

Paul
 
Mike Hilger was good enough to stop by today and determined the impulse coupling was firing way too early. The throttle cable travel is way messed up. I shouldn't have been running the engine at all, as it turned out.

I've adjusted and readjusted the throttle length but concluded that the total travel of the cable is just slightly less than the throw of the arm on the throttle. I could, I guess drill another hole in the arm but I don't know if that's a good thing or not.

throttle_cable_throw.jpg


So I've ordered timing equipment and now I'm draining oil and removing fuel and oil lines and secured wiring to get at the magneto, which may have to come off.

Sigh. This is one of those days when it feels like this *$%#&- project will never be done.
 
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Cable throw

Bob,

For a long time I thought I had the short throttle cable throw problem. It turned out that I mounted the control bracket reversed. I had the face of the bracket flush with the face of the panel. On the drawing it was recess from the face of the panel. Since it is hard to take all control cables out then reverse the bracket I added a plate to move the bracket forward (toward the subpanel). It did not take much time to get that done. That solved my problem. If all your parts are stock, most likely you have made a mistake. You may have made same mistake as I did. Before you drill another hole on the control arm check your control bracket.
 
Ha! I heard so many stories like this that I was careful not to repeat it. I also have plenty of slack and length in the cable; it's just that --the at least presently-- the total length of travel on the throttle cable at that end is less than the total throw of the throttle assembly.

For now, I've just had to walk away from the operation for a bit. I think I need to just pay someone to finish this at some point. It's clear I'm out of my league on this and this isn't a part an amateur should be guessing on.
 
I could, I guess drill another hole in the arm but I don't know if that's a good thing or not.

I have done it that way (especially with quadrant controls with minimal travel). If you can solve it at the other end, that would be better - but if not, then a hole closer to the pivot point is not out of the question.

Paul
 
Paul, Maybe you have experienced something I am not thinking of right now, but if you pulled the No1. cylinder through to TDC of the compression stroke and the TC mark lined up, and the 25BTDC mark was about where you would expect it, how could this be wrong?

For Letters benefit, remove the top plug and as you pull through airr will be escaping out the plug/thumb as you reach TDC of the compression stroke.

I did notice something odd, when I shut off the EI (Lightspeed). The RPM went UP by 50. That surprised me.

I haven't timed the Lightspeed per the manual, mostly because I was trying to avoid the idea of having my head inches away from a whirring propellor, but then I remembered when we timed the mag some months ago, we made a mark on the flywheel for TC...even though there was an engraved mark for it already (although I didn't see it).

When I put the ring gear (I think it's called the ring gear) on, I didn't line the timing mark up properly. Is that a deal breaker? Not to properly time the mag, but that I wonder if it would mess up the Lightspeed?

That does not surprise me. If the EI was retarded, the mag would fire on time, and the combustion event would happen about right too. The EI spark would happen much later and be irrellevant to the process.

If the EI was very advanced, the combustion would start much earlier, with a less than fully compressed charge of fuel and air, and your Peak Pressure Pulse may have been much closer to TDC than you would like :eek:

Glad you are getting this sorted now before you did anything further.
 
Paul, Maybe you have experienced something I am not thinking of right now, but if you pulled the No1. cylinder through to TDC of the compression stroke and the TC mark lined up, and the 25BTDC mark was about where you would expect it, how could this be wrong?

Of course....that is OBVIOUS now! ;) I was much younger and less experienced (and in a hurry) back then.....
 
Just to update. Upon further review, I'm pretty sure the the RPM went up when I shut the mag -- rather than the EI -- down. And that would make since *if* we timed the mag wrong back when we figured the TDC position and then marked the ring gear. And, apparently, that's just what we did as the impulse coupler was firing way past that point.

I'll know more about this for sure when I get this equipment I bought from Aircraft Spruce later this week and I can confirm the accuracy of everything. Then I'll time the mag and see where we are.

I did notice the last few times I started that the prop would turn, then stop, then continue. Mike says that was the cusp of a kickback, which I guess is consistent with the current theory. He says if this turns out to be the case, that likely accounts for the reduced power.

I would like to hear from other TMX IO-360 owners who've checked to see if their ring gear is NOT indexed to be sure I'm not missing something.

I am generally flummoxed about the throttle situation but for the next few days, I'm just going to let the project sit and forget I'm even building an airplane.

Paul, Maybe you have experienced something I am not thinking of right now, but if you pulled the No1. cylinder through to TDC of the compression stroke and the TC mark lined up, and the 25BTDC mark was about where you would expect it, how could this be wrong?

For Letters benefit, remove the top plug and as you pull through airr will be escaping out the plug/thumb as you reach TDC of the compression stroke.



That does not surprise me. If the EI was retarded, the mag would fire on time, and the combustion event would happen about right too. The EI spark would happen much later and be irrellevant to the process.

If the EI was very advanced, the combustion would start much earlier, with a less than fully compressed charge of fuel and air, and your Peak Pressure Pulse may have been much closer to TDC than you would like :eek:

Glad you are getting this sorted now before you did anything further.
 
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I am generally flummoxed about the throttle situation but for the next few days, I'm just going to let the project sit and forget I'm even building an airplane.
Bob-In my experience there is very little room for adjustment in the cables supplied for Van's. I think you are on the right track. It's not the overall length of the cable that's the issue, it's the amount of throw. If you can't get enough throw to go both to the full stop and the idle stop, you'll have to (a) find or make a new hole lower on the actuator arm on the carb/FI to effectively give you more throw, or (b) purchase a cable from Spruce that has a bit more throw. You might check (with someone who knows) whether perhaps your idle stop or your full power stop is not set right. If so, you might not have a problem.

This is one of those things that makes me wonder why Van's doesn't just offer a cable with a bit more throw, since so many in the past have commented on this and there's really no disadvantage to having a cable with slightly more throw. FWIW, I have just enough throw for full travel + about 1/16" of "spring back" when I firewall the throttle. Enough, but just barely.
 
Precision Airmotive said today they have two sizes for throttle arms - 2" and 1 5/8" and if I sent the one I have back, they'd send me the one of my choice. The one I took off was 2 1/2".

This should solve the problem of the throttle cable travel being shorter than the throttle throw.
 
the right track

Letters, you are on the right track.
Just read this thread twice (all four pages) and there are some good answers from some experienced folks on here. I hear your frustration, as the learning curve steepens it can get interesting.
As for your plane, it's going to be a good one, these problems can all be solved with a little help from friends, VAF forums and manufacturers. You are well on the way to a happy running engine.

Checking the instrumentation system was the first thing to do and that is right on. Even on the turboprops I work on when faced with a funky indication the first thing to do is check the instruments for errors before swapping components.

Engine control rigging is a fine art that is not fully explained I think. You are not alone in your experience though, I've seen a -7A with the same problem, a lack of travel due to a throttle arm on a FI servo that was too long.

When you get the new throttle arm on make sure as alpinelakespilot2000 said there is a gap at the panel at full throttle. This is necessary on the mixture as well, it ensures the controls are hitting the stops at the engine end (fuel servo) before the knob hits the panel. An eighth of an inch is about right.

Magneto and EI timing is the logical next item to tackle. When the mag is timed correctly you should hear the impulse coupling snap as the prop rotates through TDC by hand (with everything off of course). Don't be afraid to run the engine as much as necessary to determine the problems are solved. I've had to run newly overhauled turbo'd Continentals extensively on the ground to troubleshoot problems with props and governors and turbo setup and after 20 hours in the air the rings seated. Your mileage may vary though.
After it's timed and running make the adjustments to idle speed and mixture. It should pick up about 50 to 75 rpm as the mixture is pulled out. I've seen an overhauled IO-360 engine that picked up 200 rpm as the mixture was slowly turned out!! It was way too rich and smoothed out considerably after a simple adjustment to the link on the fuel servo.
Eliminating problems one by one can be a time consuming process. Think of how much you are learning along the way. One of the most important lessons is one you already know. Sometimes the best maintenance action is to put down your tools and walk away from the plane. That is part of human factors training that I've had to do and an important thing to recognize.
Keep going with the thread as it's a great learning tool to others in the same boat.
Let us all know when you get a smooth running engine and an RV grin I'm sure is soon to follow.
Phil
AME at Large
 
There are evenings when I can't believe I ever thought I was smart enough to build my own airplane; this is one of those nights.

I've read stuff like crazy, and I've talked to people about timing magnetos and they all seem to know what they're doing and I try to do it and I'm absolutely clueless.

I found TDC -- actually confirmed that the mark on the ring gear that I repositioned was repositioned accurately -- and connected the timing light to the mag where the P-lede goes. I kept the P-lede connection on and flipped on the mag switch on the instrument panel. I grounded the mag light as suggested.

I loosened the mag and rotated it until the light went out and the buzzer went off (which wasn't much, which made me wonder whether it was that far off to begin with).

I clamped the mag and then rotated the prop. Look where the impulse coupler fired:

magneto_timing_mark_1.jpg


Not even close.

Meanwhile, oil is spewing all over my nice installation (I think because the oil cooler is higher than the oil filter adapter location and I can't stop it.

I am so ... frustrated (there's also oil now on my laptop).

If anyone within an hour's flight of KSGS who knows what they're doing can come over tomorrow afternoon or evening and show me what I'm doing wrong....I will GIVE you this airplane project as long as you get it out of my life.

That's how stupid I feel.

Oh, and when I put the prop back on and the spinner plate on after taking everything off last weekend, it didn't occur to me to make sure the spinner plate lined up with the holes in the spinner. Whoops...I put it back on wrong so now all those bolts have to come back out and everything needs to be safety wired.

Mr. Provenzani was right. Sigh.

Thank you for letting me cry on your shoulder. I'm now going home to my ridiculously ill wife to open a Leinie and forget this mess. I'll clean the oil up tomorrow. Maybe.
 
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Bob,

Take a deep breath and relax man.....you are far from stupid.

Look at what you have created so far and say to yourself "I built that!"

Man you are . close to the finish line. Don't get to wound up in the little things this close to the end.

I am sure that some local help will be around soon and nudge you back on the right path.
 
I think I know what's going on, Bob (long)

Wish I could be there to help. When setting your timing, rotate the prop in the normal direction until you hear the impulse coupling snap. This is important to be sure that the impulse coupling is NOT retarding the timing while you are trying to set the timing. This MIGHT be what happened to cause the problem you just posted.

Then rotate the prop backwards a little until you are 45 degrees or so past the desired timing mark. Now, rotate the prop in the normal direction until you get to your desired timing mark (mine is 25 degrees before top dead center). This is the position where you want the contact points to be JUST opening as indicated by your timing light buzzer. When the points open, the plug fires. It is important that you approach the contact point opening position from the correct direction. Otherwise, gear lash and hysteresis (fancy word for drag or slop in a system) will result in an error during operation. When the points are just opening, snug up the magneto clamp nut. This is just the first of an iterative process to get it just right.

Now, back up the prop 45 degrees again, and then move it very slowly in the normal direction until the points open (indicated by the light / buzzer sounding off). Read (interpolate) the timing setting on the flywheel. This is the normal operation timing setting you have at this point. If the light / buzzer goes off before your desired timing point, your setting is too far advanced (say 28 degrees for a 25 degree desired setting). If the light/buzzer goes off too late, your timing setting is retarded from the desired set point.

At this point, I would loosen and move the mag VERY slightly in one direction or the other. Snug it up again and make a note of what you did.

Now repeat the process and see where you are. Iterate until you are within a degree of the desired timing setting. You can do better than this, but for a first try, +- one degree is really good.

Now, do the same for the other mag. If you have a timing light/buzzer that hooks to both mags at the same time, you can eventually get both mags to fire at the same time AND at the correct timing setting. Expect this to take a little while depending on how anal you are at the time.

When you are satisfied with your setting, torque the clamp nut to the spec for your engine. I always use a new internal star lock washer when resetting my timing. You don't want this to move or loosen in operation. If you are really picky, check your timing one more time. The torquing process can move things a little.

The role of the impulse coupling is to do two things when starting the engine. First, it retards the timing by about 25 degrees so that the plugs fire near top dead center. This prevents kick back, and facilitates starting. It is also the reason that the impulse coupling should snap at or near TDC (top dead center) or 0 degrees. Second, it snaps the magneto at a higher rpm than the engine is actually turning to increase the power of the spark. Mags produce a weak spark during the start cycle, and the impulse coupling snapping helps generate a stronger spark.

Sorry this is so long. Hope it helps a little,
 
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Whoa dere, eh? You are so close! You just didn't do it right yet. The points and the coupling are on a common shaft so even if you twisted up your distributor you wouldn't get that indication. Okay, disconnect that P-lead so we all know P-leads are out of the equation and not swapped. Pull that stupid mag right off. I suspect that it will be at the wrong angle even if you can twist it far enough. Drop your timing pin in the L hole and gently turn the mag backwards until the pin drops in the lower hole. Be gentle turning because the pin might have to "jump rope" the distributor finger as it comes around. Now your mag is pinned at/near point break.
Back your prop into #1 compression. Pull forward to 25 BTC. Set the mag back into the hole with around a 30 degree up slant. The gear teeth are big and one tooth is a big swing so if you're off a tooth you'll know. Put the upper retainer block & nut on finger snug and PULL THE PIN. Put the bottom block on finger tight. Put just one side of your timing thingy back on. Swing prop back 45 degrees and then advance until coupling snaps. Now BACK the prop to around 30 and come forward to 25. If you back too far you will rehook your coupling and have snap it again. Now twist your mag until light goes off. Snug the mag nuts a little, back prop to 30. Light should be ON. Bump prop forward with heel of your hand and it should go lights out around 25. It will be off a degree or two because of gear slop. Couple tiny twists, checked by bumping forward each time will get you right on.
PRINT AND FOLLOW THESE INSTRUCTIONS AND I GUARANTEE YOU WILL GET IT RIGHT.
 
I know how those days go, but don't give it up when you're so close. One of the remaining items on my to-do list before I can get the inspection and fly my plane is to see your plane fly. To that end, I spoke w/Bryan Flood and he'd be willing to help out (as would I).

What time should we be at your hangar this weekend?
 
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