The Motor Has An Ignition Misfire Around 1,000 RPM

 I went out yesterday to go flying, got to the end of the runway to do my run-up check and found a noticeable misfire around 1,000 rpm.  It wasn't noticeable at idle nor at 1,700 rpm for the magneto check.  Obviously I decided not to go flying.

Today I went back out with my infrared thermometer.  My plan was to run the motor to determine which magneto was missing by running the motor at 1,000 rpm then switch mags to see if there was a noticeable difference.  The motor started fine and there is a pronounced misfire when running on the right magneto.  The left magneto seems fine.  The right magneto was the one I had problems with earlier in the year.

I kept it running on the right magneto until the cylinder head temperature reached about 200 degrees F.  Then I shut the motor down and quickly opened each cowl access panel.  Using the infrared thermometer I checked temperature of each cylinder.  The expectation was that if the problem were a misfiring plug that cylinder would be cooler than the good cylinders.  No such luck.  The 2 front cylinders were about 20 degrees cooler then the 2 rear  The front and rear cylinders were almost the same left and right.  Clearly not a single misfiring spark plug.

I went ahead and pulled the spark plugs for the right magneto, the easy ones on top of the cylinders.  The first thing was a visual inspection to see if they looked different.  The color was the same normal tan color.  Next I used a 10x glass and checked the plugs for any lead fowling.  None was visible and since they are new plugs they really looked good.

The last thing I did was to very carefully check the electrode gap and make minor adjusts to assure each electrode was precisely between 0.015" - 0.018".

The plugs were reinstalled and torqued.  The motor was restarted.  The problem is still the same.  Clearly something has changed in the magneto.  Before pulling it I wanted to re-read the service manual to see if checking timing, etc. might give some insight as to what to look for in the magneto.  I also need to look up what parts I've already replaced.

Hopefully tomorrow I'll get back out, do any checks, and remove the magneto.  we'll see what I learn once it's off

Even Larger Cowl Outlet Opening

 I've taken advantage of some rainy days to enlarge the cowl outlet.  I've increased the area to 111.4 sq. inches.  It was originally 61.4  sq. in. so that's an 81.4% increase.  I added 25.5 sq. in. the first go around, which made a big difference.  This is almost as big of an increase, another 24.5 sq. in.

I also made a new skirt, which goes the full width of the cowl.

Before taking the cowl off I wanted to fit the new skirt to the cowl and drill all the rivet holes.  Once that is done, remove the cowl, remove the old skirt, and then cut out the new opening.   

I started with a pattern from a piece of heavy card stock.  The idea was to move the leading edge corner forward about another inch.  To get that spacing I clamped the card stock to the old skirt with some pieces of 3/4" thick wood.  Because of the angle this moved the leading edge forward about 1".

The straight edge of the card stock needed to curve down about 5/8" to get a nice fit to the cowl.

The ends got trimmed to match the little skirts around each exhaust pipe.  

The plan was to use the existing rivet holes at the ends, then new holes in the center to attach the skirt .

With a pattern it was easy to make a new skirt from some 0.025" 2024-T3 aluminum.  The small exhaust skirts had a large tab at the outer end, with 3 rivets.

The rivet tabs between them are spaced about 1" on centers.  The holes along the bend line are 1/4" to help prevent cracking.

I took the blank skirt back to the plane to form the curved ends and to bend the angle on the tabs, so they would fit.

I removed the little exhaust skirts and started drilling rivet hole at the outer ends.  The existing rivet holes were 1/8", but I drilled any new holes 3/32" because most of the new tab actually go inside the cowl, even though I drilled the holes with them on the outside.  My idea was that using a smaller starter hole would give me a little flexibility if the holes didn't perfectly align when the tabs were on the inside.  A few of the tabs had to remain on the outside because I didn't cut away all the cowling behind the skirt.

I left part of the cowl beside the exhaust pipes so I could put support brackets where the old center skirt ended.

They're triangle shaped supports made from 0.032" 2024-T3 alunimum.

Before riveting everything together I put the cowl back on the plane to make sure everything fit.  I'm glad I did.  The triangular support brackets didn't fit quite right the first time.  They were easy to re-make and now it fits perfect.

The supports make the skirt much stronger and stiffer.  They also allow the cowl to mount to the firewall under the plane.

I enlarged the openings around the pipes, which adds a few more inches of outlet area.  I'm not sure much air will come out of them so they're not included in my area calculations.

There is good clearance around the pipes

With everything fitted up we're back in the attic to rivet it all together.  First I Alodined the aluminum parts to reduce corrosion and allow paint to adhere better.

The process work best if you immerse the part in the Alodine solution, after acid etch.  Brushing it  on never gets as even a color as immersing the part.  The trick to do this without gallons of expensive Alodine and a big tray is to use a plastic bag.  For the skirt I have a long skinny bag and only 2 cups of Alodine.  You just keep sloshing it around for about 3-5 minutes and you're done.  For the supports I used a 1 gallon zip-lock back.  Double bag it, they leak.  Strong roasting bags work very good and are reusable.

I keep the used Alodine in a plastic bowl with a secure lid.

While riveting I held the cowl in a curved shape with a bungee cord.  

The supports were riveted on first, then the ends of the skirt up to the supports.  Once that was done the center section was riveted.  I was concerned nothing move so I only removed 2 Clecos at any time.  Usually I remove every other Cleco, fill in the rivets, then remove the remaining Clecos and finish riveting.  I think it was worth the slower process.  I also only drilled holes with the #30 drill when I was ready to install that rivet.

I like it.  I'll paint it in the spring once I'm sure it works as needed.

From the back you can see which tabs were on the inside of the cowl.

I decided to cover the 1/4" holes between tabs with some aluminum duct tape.  The adhesive is way way better than regular duct tape.  In the Air Force we called it High Speed Tape.  It really stays on well.

Back on the plane.  I like it.  Now I need the weather to cooperate.  Looks windy for tomorrow, then nice for several days.

Nice Fall Afternoon of Short Sleeve Shirt Flying

 I moved the outlet skirt back to the shallow position, 135 degrees.  That didn't make much difference but it lowered the CHT 3-5 degrees.  

A nice afternoon in the mid 70s.  By 3:30 the thermals were almost done so I was to be able to fly in short sleeves, probably for the last time this year.

We had a very gentile breeze out of the SSW, so 23 was the active runway.  This view is looking the opposite direction (NW) from the downwind.  The town of Orange is just out of view at 10 o'clock and Orange airport at 11 o'clock.  The mountain by the fuel gauge wire is a nice reference when flying here.  It has an old microwave tower at the top.

The view past the right wings is to the east.  Our house is over there about 8 miles among all those trees.

Until I solve the temperature problem I'm staying within gliding distance of the runway.  Here I'm headed back to the SW in the Upwind.  The runway and my hangar is below the spreader bar, looking south.

I've turned Crosswind.  Looking south is the town of Gordonsville, maybe 2 miles of the south end of the runway.  

Overall a nice fall afternoon for flying round and round the airport for over 1/2 hour.

We have a few rainy days coming which I will use to enlarge the outlet opening some more.

Seal Around Air Filter and Nose Cowl

 I moved the outlet skirt to the the full forward, 125 degree, position but decided not to fly the plane until I made another fix.  

I realized there was no seal around the top of the inlet air filter, big gap.  I had installed seals on the sheet metal around the bottom half of the filter but had never installed seals around the upper, nose bowl portion.  I checked the old nose bowl and we had seals on it.

I didn't want to modify the Taylorcraft nose bowl more than I had, in case someone ever wants to use it on a Taylorcraft.  Instead I made a bracket which attaches to the front of the carburetor heat box, behind the inlet filter.  I made it out of 0.032" 2024-T3 aluminum.  I also made a new thinner gasket for the air filter.  The old one is about 1/16" thick so I needed the total thickness to stay the same.  The 2 bolt holes are for the heat box mounting holes.

The felt seal is glued and stapled to the flange at the top of the bracket.  The top of the seal is thinned like I did for the new seals on the main baffle.

It mounted to the heat box just as planned.  I did a little adjusting of the bolt hole locations until the filter screws fit perfect.

There are 2 little gaps where I wanted to make sure the bracket and cowl didn't collide, but there were some small gaps with the old cowl.

It was a warm 75 degree afternoon, nice shirt sleeve flying weather, so I had high hopes.  It turns out none of this made any difference.  I'm going to move the outlet skirt back to the shallow, 135 degree, position before I do anything more.  I'm also going to check the temperature of all cylinders after warm up to make sure I don't have one cylinder running a little lean.  If all that fails I'll move the skirt forward another inch.  We still have 2 more warm sunny days.

Changes To Motor Cowling Outlet - To Improve Cylinder Head Temperature

 Fixing seal leaks didn't solve the Cylinder Head Temperature (CHT) problem.  I have nice big inlets with the Taylorcraft nose bowl, so my next area to work on is the outlet opening.  When I made the new sheet metal I tried to duplicate all this as carefully as possible so it surprises me to be having cooling problems.

The outlet has a skirt, like a cowl flap, which is about 4" tall and hasn't changed since shortly after we built the plane.

I started by checking the angle it's tipped back.  From old photos it was tipped back at 125 degrees.  I have this nice antique folding ruler.  The center joint is stiff enough that it works nice for measuring angles.  Using my best protractor the new skirt measures 130 degrees.  I'm not sure that's enough difference to matter, but my friend Fred suggested a way to adjust this angle.

The idea was to cut off the flange on the ends of the skirt and replace them with brackets to allow the skirt to be bolted at different angles.

I worked out a pattern of 4 bolt holes which allow the shirt to be tipped in 3 different positions with 2 3/16" bolts holding it together in each position.  In the center position the 2 outer holes are used.  In the other 2 positions an outer bolt and it's inner neighbor are used.  This allows the skirt to be at 125, 130 or 135 degrees.  We'll see if it matters.

Once I had this done I realized I'm more likely to get better cooling with a larger outlet opening than the angle of the skirt.

Some quick measurements showed I could move the ends of the skirts outboard 1" and still clear the exhaust pipes.  That would add about 8 square inches to the outlet opening.

I decided to also move it forward 1".  I took the cut out piece, scanned it, and brought it up in TurboCAD.  This allowed me to draw the outlines of the old and new openings to measure their areas.  The old opening was 61.4 Sq. In.  The new one is 86.9 Sq. In., that's an increase of 25.5 Sq. In. or 41.5%.  That seems like a good change.

Rather than make a whole new skirt I cut it in half and put the cowl back on the plane.  Then I fitted up a piece to fill the gap, with a backing plate to tie it all together.  It worked fine.

The next thing was to prevent air going in the openings around the exhaust pipes.  I increased the openings about 1/4" all the way around to give the pipe some more clearance.  It looked like the pipe had rubbed on the cowl at some point in the past.  Because of the angle on the bottom of the cowl, that extra gap may be letting air in, reducing the pressure drop across the cylinders.

My research on line shows that the air pressure on top (inlet side) of the cylinders should be about 5-8 inches of water higher than on the bottom (outlet side).  I think I've figured out how to mount a U-tube manometer to measure this if needed.

For now my plan was just to make some skirts around the pipe openings to prevent air entering.

The skirts have tabs for riveting them to the cowling.  I cleco'd the long side flange in place and used stacks of popsicle sticks to wedge the skirts in place to mark the other tab holes.

Back to the attic to rivet it all together.

It was a bit of a pain to hold and rivet by my self.  I eventually pulled a bungee cord across it which made it easy to hold.

Everything is done and back on the plane.  I'll pretty it up once it all works.

I've made 2 flights with all this.  On the first flight I had the skirt in the center, 130 degree, position.  The problem is not solved but it was a big change.  In level flight the CHT dropped from 460 degrees F to 415-420.  I'd like it in the range of 360-390 degrees F, but this is going in the right direction.  The oil temp after 25 minutes had slowly climbed to 185 degrees F.  It was 210 degrees after 15 minutes before so the motor is running cooler.

For the second flight I moved the skirt to the shallower 135 degree position, No change.  

I did have two nice flights just before sunset and the plane flies great.  I can feel a difference in how the plane turns when well coordinated and when 1 ball width off, so I should be able to learn to feel when I'm making good turns.  

Last night I also tried the cabin heat.  It works great and creates a nice bubble of warm air in the whole cockpit, very good.  I should be able to fly most of the winter here in Virginia.

I'll try the 125 degree position next flight.  After that I'll work on the manometer and consider moving the skirt forward another inch.  I also need to look at possible air leaks by the carburetor.

I'm having fun with all this.