Secondary Timing

[sonex1084]

I'm a bit befuddled and am fearful I am missing something. I originally had the secondary timing on my AeroVee set where I was very happy. Recently I had to replace the secondary triggers and trigger cap. After the replacement, the engine started well, sounded good and had good throttle response for a test flight. The RPM on take off was only 2880. The EGT's and CHT's stayed in the green through the climb to 3000 MSL (2500 AGL). Fuel Flow was 6.79. My biggest intent for the flight was to check the secondary timing. Running on the primary ignition I recorded the following CHT's, 395, 349, 333, 315 and the following EGT's 1298. 1165, 1343, 1247. That was after setting the throttle to 3100 RPM and leaning for a fuel flow of 4.5. Running on only the secondary ignition the CHT's rose between 27 degrees and 36 degrees. The EGT's decreased between 78 degrees and 106 degrees. I assumed based on the CHT increase that my secondary timing was advanced so I turned the trigger cap 2 to 3 degrees clockwise (facing from the rear of the engine forward).

The next day I flew and saw an increase of RPM at take off at just over 3100. At 3000 ft I set the engine as before, 3100 RPM, leaned for FF at 4.5. OAT was 68 degrees. Running the primary ignition The CHT's were 370, 360, 346, 328. EGT's were 1285, 1172, 1312, 1231. Running the secondary ignition only, I got a 100 RPM drop, an increase of CHT's between 23 degrees and 32 degrees. Not much better than before. EGT's decreased between 29 to 62 degrees.

I'm assuming I need to retard the secondary ignition a little more but like I said, I already retarded it 2 to 3 degrees. When I set the timing with the original triggers, I only adjusted the trigger cap 1 degree and had the engine timed perfectly. Oh yes, there is only 5 hours on the engine.

Any advice is appreciated. On the bright side, I love my airplane. I couldn't be more happy with it.
Thanks, Matt

[pappas]

With only 5 hours on that thing, you shouldn't stress at all about the numbers you are seeing. Your temps look really good for a 5 hour run time.

We can all get pretty picky about one or two degrees but with 110 hours on mine, I find that you should set up the secondary to where it runs well even if that is not "perfect" (Good luck getting perfect). It likely won't ever be set exactly to the factory specs as far as twisting your timing cap goes.

If you can maintain CHT temps in spec during the climb, are seeing the RPM you are expecting to see at take-off, climb, and cruise, and your EGT's remain under 1400 or so, you don't have anything to worry about except learning how to fly your new bird and checking for oil leaks and anomalies.

You will likely not get your secondary timing finely tuned until you have at least 50 hours on it. If it starts, idles well, and runs nicely in climb and cruise, move on to the next things on your to-do list. Circle back to the timing at 50 or even 100 hours.

[mike.smith]

I agree with Lou. Your numbers are just fine. And you will never get CHTs and EGTs to balance with one another. It's just not possible with such a simple, normally aspirated induction system.

[sonex1084]

I thank the both of you for responding and your advice. I feel so much better having experienced Sonex pilots give their inputs. Last week I made another small adjustment to the trigger cap and am now happy with the timing. The engine is performing better than ever, great throttle response and power output. My CHTs have come down about between 10 and 20 degrees, but like you both said, not perfect. I will take your advice and fly it. I am so happy with my little airplane, it is a joy to fly. Thanks again.

[GraemeSmith]

Might as well keep in in the same thread for continuity...

Ground Check
.....a mag check at 1,600-2,000RPM should reveal little or no change in RPM. If a change of 50 RPM or greater is noted.....

50 RPM. Does that mean (numbers are illustrative - not actual):

A) Mag 1 off - 40 drop
Mag 2 off - 40 drop

The DIFFERENCE is 0

or

B) Mag 1 off - 0 drop
Mag 2 off - 40 drop

The DIFFERENCE is 40 RPM

--

In a "traditional" SLICK Mag type set up - if each dropped 50 and the difference was zero - that would be a good mag check

But we are not comparing identical Mag types here and the manual is not clear where the change in RPM is occurring.

--

And I see the In Flight Check and CHT observation is of more value to operating the engine.

--

Still - I am about to disturb things while changing the crank oil seal - and you KNOW that no matter how perfectly I put it back "as it was" - I'm going to get a different result..... :-)

[WesRagle]

Hi Graeme,

I hate to butt in, but I have a basic question about mag checks that maybe you, or someone else, can answer.

I've been reading, studying, and thinking about fuel injection and electronic ignition lately. It occurs to me that, with our fixed ignition timing, and while running at lower at lower RPMs (say 1500), the ignition is advanced well beyond what would produce maximum torque. When we eliminate one of the ignition sources, and so have only one flame front, that would have the same effect as retarding the ignition slightly. So why, pray tell, don't we get a mag rise? Is the mag drop we experience simply the result of an increased number of miss fires due to eliminating one of the ignition sources?

As a reference, one of the books I've been reading is an old book on ignition systems, "The Doctor's Step-By-Step Guide To Optimizing Your Ignition" (ref: https://www.amazon.com/Doctors-Step-Guide-Optimizing-Ignition/dp/0965085600).

In that book the author states "...a well tuned system gets good reliable ignition on the average of 88 to 94% of the time." I found that a little shocking.

Anyway, just curious.

Wes

[GraemeSmith]

WEs,

My shade tree/kid racer mechanic thoughts:

2 plugs - there for redundancy
But 2 plugs also makes fuel burn well with two sources of ignition starting the flame front.
The carb is tuned to this setup
Stop a plug firing during a mag check - the mixture is now slightly too rich for the cylinder
RPM drops slightly for being too rich.

--

And on occasions when a mag check produced a really rough result - we assume a plug is oil or lead fouled. So one cylinder may not actually be firing.
Back to both mags and run hard for a few moments to get the plugs good and hot and burn off the contamination.

--

At least that was always my reasoning.

[Bryan Cotton]

Is the mixture too rich, or is the timing effectively retarded? Two flame fronts gets you to complete combustion sooner, which is also the case for more advanced timing.

I'm not sure why there is no big change for the Aerovee from single to dual ignition, unless the combustion chamber is so small that it doesn't make a big difference.

[Area 51%]

Graeme....You shouldn't have any worries about resetting the timing as the trigger is attached to a plate that needs to be removed to get at the nut. Just don't remove the trigger from the plate.

Personally, I got tired of chasing CHTs and RPMs and installed a pointer at the flywheel teeth. Adjusted the pointer to align with a tooth that lit-up with the timing light attached to a primary ignition lead (doesn't matter which one), then used that tooth to adjust the secondary trigger.

FYI....there are 88 teeth on the flywheel, so that's a smidge over 4deg per tooth. I was two teeth off using the suggested method in the assembly manual. The real trick, of course, is moving the trigger an RPH.

[pappas]

I didn't install a pointer, (good idea though), but I did paint white lines on the accessory bracket and one flywheel tooth at TDC. It makes it easier to set the timing for me. Of course, once it is set well, you rarely revisit it.