Changing From
The Rotary Valve Engine

The Non-Rotary Valve Engine
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In the previous pages, I experimented with a rotary valve design.  Although the engine could be coaxed to run and, at times, ran really well, it was very troublesome to keep running.  After the last run, the rotary valves were removed and the seats were found to have galled, making them unusable.  At that point, I decided that I'd proved the point that rotary valves could be done but were not practical.  

Since I really want my engines to be able to run continuously and with little fiddling, I decided to design and build a new head and use poppet valves and a face cam.

Here's the story.
3 February 2017:
This is the day I decided to abandon the rotary valves.
8 February 2017:
After partially completing the new head design, I took a little time to make up an image of roughly what the new arrangement will look like.

Here's the layout.
Here's a look at what I am working on.  Using one of the rotary valve gears, I will machine the hub into a face cam.  There will be two followers, located rotationally 90 degrees apart which share the 100 degree cam.  This means that both valves will have the same duration and will have ten degrees of overlap.  If necessary, I can always remove the gear and take some off of the cam.  Lowering the duration will automatically reduce the overlap.

Short pushrods will connect the followers to the rocker arms which will operate some valves salvaged out of an unknown small engine.  The compression ratio will be a bit higher due to the lack of the rotary valve volumes and the shorter spark plug path.  If this proves problematic, I can always make up a spacer to go between the head and the liner.
While working on the new head design, I decided to find out why there was slight scoring on the cylinder wall.  After removing the bottom plate and piston, I discovered that I had not machined enough clearance in the ring area of the piston.  I un-hung the piston and turned another 0.015" off of this area and re-assembled the rod and piston into the engine.

Galled spot on top land of piston before turning.
9 February 2017:
Yesterday afternoon, I lost all excuses to start on the new head.

There's a cylinder head in there somewhere.
I think the UPS guy is a really good sport.  This thing is HEAVY!  It is a hunk of malleable iron that is five inches in diameter and six inches long.  We are getting ready to have fun!
10 February 2017:
Today was my day for fighting chatter on my little lathe.   At least the turning part of the cylinder head machining is done.  Those little 0.003" cuts are murder!

Here's the head so far.
The next thing to do is to drill and tap the mounting holes for the top bearing/cam plate in the circumference of the head.  After that, I will work on the top bearing/cam plate and get it mounted to the head.  With the plate mounted, I can accurately square up the two parts and do the rest of the machining on the head and plate while in the one setup.  This should insure that everything will fit.  (says here in the fine print).
11 February 2017:
I've now got the head and the top sideshaft bearing/cam plate married together.
11 February 2017:     
Using the boring head to match the plate to the head.                                                           Shecking the fit.                                                                               Bolted together.                       

Ready for more machining.
If you noticed the notch in the aluminum plate, that's where it was machined before it was scrapped (you use what you've got).  There is also a filled bolt hole on the top surface that is just about invisible.  I made a threaded plug, bottomed it out in the plate then sawed the excess off and milled it flat.

I will be making a gear guard out of sheet metal that will wrap around the plate.  This will cover up the bolts and the notch.  I know you will never tell!

The plate will remain on the head and all of the machining will be done with it in place to insure all the parts will fit.
12 February 2017:
The head is almost done.  All I need to do is to make the valve guides and lap the valves.
Just finished cutting the seats.                                                                                              Boring out the ports.
Everything today was just the normal machine shop stuff.  Note in the above left photo the tool I use to cut the seats.  This is something I found in a junk pile when I was a kid.  It was originally a hand operated seat cutter but I turned the shamk to fit one of my collets and it makes really nice seats in the mill.  

You will also note that, when I bored the ports (right photo above), I mostly guessed the positions of the ports.  The main thing that I worked into the design is that, when the flat on the top bearing plate is vertical, the angle is correct for the ports.  Using the Type A-1 Eyeball got the ports to align with the bores below the valves.  Also making it easy is the fact that I have the ports coming out exactly half way up the head.

Test fit of gears.
I may have a problem with the spark plug thread.  Somehow, when I worked up the drawing, I didn't get the tap drill size right for the plug so the threads are shallow.  Since there is about 3/4" of thread on the plugs, it should be all right.  If not, I can always go back and make a bushing to press into an oversize bored hole, then stake it on the inside of the combustion chamber.  I'll do that if I need to.

I'm going to go ahead and make the brass valve guides and get the valve keepers made and the valves lapped and in.  Then, all that is left besides the manifolding, is the rocker arms, the cam followers and the cam.
13 February 2017:
The valve guides and keepers are done and the valves are lapped-in.  I may go ahead and make a gasket and bolt the head down to check for valve leaks.  If they are all right; the head can be considered permanently mounted (Unless that plug thread fails).

We have valves!
The shaft for the cam gear is in place but still must be cut to length and the end drilled and tapped for the cam follower bracket mounting bolt.
14 February 2017:
I got the head on the engine.  The spark plug tightens up all right even with the loose thread fit and the valves seem to be holding pressure fine, although the seat on the intake is a bit thin.  By rights, I should have put the valve in the lathe and used the Dremel to change the angle of the face until it matched the seat in the head.  It should work fine, though.  
     Cam roughed-out.                                                                                         Milling cam profile.
You will notice that I filled the precious valve port in the cam by pressing in a plug.  The plug disappeared (mostly) after finishing the cam.  When milling the cam, I spent a lot of time turning the crank on the rotary table.  While doing so, I realized that the geometry of the cam profile is more complicated than I first thought.  For the cam ramps to line up properly with the center of the cam, theworkpiece should have been shifted half the diameter of the cutter on each end of the cam rise in order to have the ramp square to the center of the cam.  Then, there's the complication of having to compensate for the diameter of the cam follower and the valve lash.  

I ended up making it to the design specification and will see how the duration works out.

The sideshaft will need to be replaced, as the length and position of the components mounted on it have changed.  Right now, I have a blank piece of shafting just to check clearances.

Finished cam.
The geasr tooth clearances are just right and they run smoothly.  I've changed from a bronze thrust under the cam to a PTFE (Teflon) thrust.  I had the material and, since it's self lubricating, I won't worry about oiling it.

It looks like the cam followers and rocker arm assembly are next.
15 February 2017:
Spent the whole day on the cam shaft and the cam followers.  There are a lot of fiddly bits to those parts.

We have cam followers!
Everything fits and I think it will work.  The only thing I have to correct is the overlap.  It's about fifteen degrees now.  This ain't a racing engine and with that amount of time both valves are open, it won't want to run slow.  I'll pull the cam off and set it back in the rotary table and correct the landing ramps so they are perpendicular to the centerline of the cam and that will possibley reduce the overlap to where it will work out.  I'd like to see about five degrees or so..  

Tomorrow, once I get that sorted out, I will make the rocker arm assembly.

Gettin' close!
18 February 2017:
The sideshaft is made and in place.

Had a sort of slow, klutzy day today.  First, when drilling a slanted hole in the small timing gear for oiling the top bearing, I broke off a bit.  That took about an hour to fix.  Then, when making the rocker stands, I snapped a 6-32 tap in one of the uprights.  After fiddling for about a half hour, trying to save the part, I finally gave-in, tossed it into the trash and made another one from scratch.  I need to buy some new taps!

In addition, I made a couple of degree tapes for the flywheel so I could check the valve timing.  

Here it is with the rocker stand.
I had hoped to have the rocker arms done today but, stuff happened.  

A rudimentary check of the cam timing was done and I found that the duration is about 220 degrees, much too much considering that is the duration for both valves.  That will make for about 40 degrees of overlap!  Kind of radical, I think.  I will put the cam back in the mill and whittle on it to get the duration down to about 190 degrees.  That will have the exhaust opening at about 5 degrees before BDC and closing at about 5 degrees after TDC.  Likewise, the intake will open at about 5 degrees before TDC and close about 5 degrees after BDC. That ought to be fine for a low performance slow running engine.  I can always shift this a bit by changing the overall valve timing.

When I get the rockers and pushrods done, I can tweak the cam.
19 February 2017:
So close yet so far!  The rockers and pushrods are done, the cam is modified for about 190 degrees duration (with valve lash of 0.010"), the exhaust adapter is done and the mixer adapter nearly done.

Almost there!
I suppose I should have bored the ports out to 1" instead of leaving them at 7/8".  I just didn't think about it while the head was in the mill.  Anyway, I made some adapters to increase the ports from 7/8" to 1.048" which is what the ports the rotary valve arrangement.  The exhaust is a press fit for both the adapter and the pipe.  The intake adapter will be a press fit in the head with a slip fit and setscrew to mount the mixer from the rotary valve version.

I've also got to move the fuel tank down so the filler clears the mixer.

It just may work out that I can hook the governor to the throttle of the mixer and get it to work.  That arrangement will keep my hands free to make adjustments without having to chase the throttle setting.

Now for the suspense moment.  I have to be out of the shop with a cardiac monitor until probably Wednesday so the final push and start-up will be delayed at least until then.  
21 February 2017:
Today, I snuk out into the shop and got the engine ready for a try at starting.   Here's how it looks, ready for the attemptt.
All that's left is to turn on the ignition and crank.                                                                                      All the shrouds are on  and the governor is hooked-up.                                          
The governor is connected to the throttle arm via a length of music wire.  Although the alignment is off and the setup will probably require some revision, it should work after a fashion.

I did crank it through a couple of intake strokes to make sure the mixer was drawing fuel and I was tempted to turn on the ignition and see if it would make smoke but, since lunch was ready
and because of the cardiac monitor I'm wearing, I'm not supposed to get sweaty and knowing me, I will shed a puddle whilst cranking.

I've made the executive decision to take the rest of the day off.  Tomorrow, if I get back from town in time, I will give it a crank (or fifty) and see if it will cooperate.  I will, of course, make a movie of it, which will ensure that it will not cooperate.
23 February 2017:
The last thing to do before starting the engine is to respond to a numerous request by the Australian Head Office of Hoyt-Clagwell & Company.  Denis Basson, President of the office, obstreperously threatened to have a snit fit if I didn't make a finger guard for the top gears.  Asceding to his wishes, here it is.
The gear guard.
After completing the ordered task, I rolled the engine out into the driveway and spent some quality time with it.  In the end, it ran but still needs a bit of tweaking.  One of the things I did to make it run stronger was to increase the size of the venturi.  This allowed it to breathe better and make more power.  I did try it with the venturi completely removed and the engine really took off although it wouldn't idle at all.  Maybe some fiddling with the flutter choke would get it to do better with a wide-open mixer throat.  

It runs!
The movie.
There are some things I think I can do to make it run better.  One is to put an O ring on the mixer body where it goes into the adapter.  I notice that it blows a little fuel out of the approximately 0.001" clearance between it and the adapter. I think this small leak makes the engine do a lot of 8-cycling.  Adjustment of the fuel mixture and flutter choke doesn't fully stop this.  Valve overlap could also cause it.  In addition, I think the governor is too sensitive.  This is easily remedied by simply moving the music wire from the hole nearest the throttle shaft to one farther away.

Another thing that may be making it misbehave is the fuel.  I'm still running what is left of the 32:1 two cycle mix and this probably doesn't make the engine happy.

I've also noticed what may sound like a leaking head gasket although it could also be a squeak in one of the casters.  With my hearing, it's hard to tell where it's coming from.  

I've got to get The Mighty Hoyt-Clagwell 54-75 ready for the Zolfo Springs show so I may not be spending much time with the engine until I return.
27 February 2017:
I got some time yesterday and made some improvements.  First of all, I cut a groove in the O.D. of the mixer tube into which to put a small O ring to seal it agaist the I.D. of the adapter.  This eliminated the slight air leak that was enough to mess up the idle mixture.  I also set the wire pivot point on the throttle arm it's least sensitive position for the governor.  Both of these things helped but the governor is still too "twitchy".

As soon as I get some more time, I will re-design the governor so it is not as sensitive.  

Speeding up the fan and fully enclosing the fan in the shroud should aid the cooling.  In the last run of about a half hour, the engine temperature looked like it was stabilizing at about 230 degrees but I won't know for sure how well it will do until I get the governor settled down so the engine will run steadily.

6 March 2017:
Back to the project.  Spent most of the day working out a new governor using the bones of the old one.  This one should be a little less sensitive and may allow the engine to run steadily.

The new governor.
It's kinda obvious I'm a lazy Geezer.  I didn't remove any hidden lines but you should be able to make out how it works.  The total amount of vertical movement between slow and fast is about 0.500", a little more than the movement of the throttle arm on the mixer when set at the minimum sensitivity setting (the farthest out hole on the arm).

Because the weights don't stick out as much as before, it will require a spring with less force.  The spring is inserted between the top arm of the governor (that the weights pivot on) and the vertically moving lower portion (where the links connect from the weights.  With less stickout, there is less chance of getting whacked by a governor weight.  I'll start building it in the next week or so.
7 March 2017
The work today went better than I thought it would.  The new governor is made and installed.

New governor.
While making the new governor, I figured out why the other one was so sensitive.  The farther out from the center of rotation the weights are, the more centrifugal force is generated.  The amount of force increases too fast to be manageable.  With the swing weights, the angle they swing to as the engine speeds up changes slower as the weights move out.  In any case, the engine runs better now but there are still some stability issues.

One thing I noticed is that the engine occasionally "jerks" while it is running.  I think this is caused by the wasted spark setup (crankshaft speed timer) I have on the engine now.  With the new valve setup, it will be easier to go to a single spark (cam speed timer).  I think what is happening is that the wasted spark is somehow causing a disruption in the timing making the engine fire way early.  It could be something else like some kind of glitch getting into the ignition module but I really don't think that is it.

It ran for a bit over a half hour at a little over 700 RPM and the temperature at the top of the cylinder was about 225F when I stopped the test run.

Anyhoo, here is the latest flick.
Engine running with new governor.
11 March 2017:
Today, I moved the ignition up to the cam gear so it is no longer a "wasted spark" ignition system.

New ignition timer.
I'm using a Hall-Effect transistor that senses the little magnet you can see on the side of the cam.  It seems to work better than the other system, I think, because the magnet is on a longer radius so the magnet passes the sensor faster, allowing less "jitter".

The mixer got a going-over.  I made a new arm on the side opposite the throttle arm and made an idle stop.

With the ignition timing set to 10 degress BTDC, the engine started easily and ran tolerably well although it but still 8-cycles more than I'd like.  Tomorrow, I will fiddle with the flutter choke spring.  Maybe a lighter one will allow the idle mixture to be leaner while allowing enough fuel for acceleration.

Today, I ran through a full tank of fuel (naphtha) with only a few short stops for fiddling.  The RPM was mostly in the 650 range.  At the end of the run, the temperature was getting close to 250F with no signs of trouble.

I think that, with improved carburetion, slower running, a little more cooling fan speed and more shrouding, the engine temperature will stabilize below 250F.
13 March 2017:
Because of the noise, I used most of the day making a muffler.  It works okay, quieting the barking some.
Muffler before assembly.                                                                                                 Muffler on engine.
It was made from stuff sitting around.  The inner tubes are cut from a length of heavy wall tubing.  The outer shell is part of a deceased satellite dish mount.  The size and number of holes in the inlet and outlet pipes was figured to have slightly more combined area than the exhaust port on the engine.  It was a matter of picking a drill size, figuring the area of the holes it made then dividing that number into the area of the port.  24 number 15 drill holes was about right so that's what I went with.  Once I've run it long enough to cook the paint, I will wire-off what is left and repaint with grill paint.

Oh, yes - in the right-hand photo above, you can see my variable speed modification just below the governor.  The lever pulls on a spring that takes the place of the concentric spring that used to be on the governor shaft.  I still have to pick a prpoer spring because, with what is on there now, there isn't a lot of speed change, from about 600 RPM to a little over 800 RPM.

Incidentally, the engine seemed to run happiest today at about 800 RPM.  After running another tank of fuel through it, the temperature was about 240F.
I still haven't figured out one problem the engine has developed.  After it's warmed-up, it occasionally "jerks" like it's suffering from detonation.  I changed over to pump gasoline from the naphtha, figuring the higher octane would solve the problem.  Not so.  It's the same with either fuel and is independent of ignition timing setting.  It also pops back through the mixer occasionally.  I hope there's not a hot spot in the combustion area because it will be a P.I.T.A. to pull the head but if I have to, I will.
14 March 2017:
I stripped and painted the muffler.  While waiting for the paint to dry and using air, I checked the cam timing.  The overlap is less than I thought, only about five degrees, which is fine for this engine.  The timing was about five degrees advanced, with the intake valve opening at about 10 degrees BTDC and the exhaust closing at about TDC.  I made a small change so the overlap is just about centered on TDC.  The engine runs better now but still spits back occasionally.  I will retard the cam timing about another five degrees so the intakc begind to open at TDC and the exhaust closes about 5 degrees later.

My theory about the spitting back is that there is lingering heat from combustion when the intake opens and that fires the charge that is entering the combustion chamber.  I'll keep fiddling but it's now running almost respectably.  Here's the latest video.
19 March 2017:
Over the last few days, I've been fiddling with the engine.  I made an addition to the fan and cylinder shrouding and made a new and larger diameter drive pulley for the fan.  It appears to be cooling better now.

I also pulled the head and checked for burrs and head gasket fuzz that could have been causing the detonation "jerking" of the engine.  Nothing jumped out at me but I carefully radiused anything that looked like it had a sharp angle.  While I was at it, I re-lapped the valves but they probably didn't need it.  Before putting the head back on, I cut the gasket back so it doesn't protrude past the inner diameter of the liner to make sure there were no tiny hot spots.

After putting it all back together this afternoon, I did a test run.  It still spits back and takes spells of not wanting to run smoothly.  I didn't notice any "jerks" this time.  It ran for over an hour on a tank of gasoline, better "economy" than I've seen so far.

With a little over an hour of continuous running at about 500 RPM with no load, the temperature at the top of the cylinder stabilized at about 250F so it may be cooling all right.

Next is to mount one of my 90 Volt permanent magnet DC motors on the skid and belt it to the engine to see how many light bulbs I can light with it.  That will also be an acid test of how well it cools.
29 March 2017:
After a haitus to take care of some buisiness, I got some time in on the engine.  I haven't changed anything with the engine but have added the permanent magnet DC motor.  Today, was the first test of the engine under any kind of load.  I was running three 150 Watt bulbs and one 40 Watt bulb.  The voltage with the lights on was about 80 volts so a wild guess as to the power the engine was producing was around 3/4 horsepower.  The engine wasn't nearly loadedup and I will have to do some adjusting on the governor to get it up to anything near the full load of 500 Watts.

Chugging aweay making a little power.
At the end of about a half-hour run under load, the temperature was around 250F, which isn't too bad.
1 April 2017:
Not much got done today.  Still trying to figure out what the carb backfires are all about as well as the 8-cycling.  Today, I removed the venturi and sleeved it down to see if that made a difference.

The bushed and re-sized venturi.
Before re-sizing the venturi, it had been reamed out to 0,406".  I made a press-in aluminum insert and reamed it out to 0.265".  The engine started and ran about the same with -maybe- a little improvement.  The power output seems to be only slightly less.

I'll run it like this for a while like this and keep working on the backfires and 8-cycling.
2 April 2017:
Well!  No wonder!  I did a really semi-careful valve timing check and here's what I found.  The exhaust opened at 158 degrees After Top Dead Center and closed at TDC for a duration of 202 degrees.  The intake opened at 24 degrees Before Top Dead Center and closed at 5 degrees Before Bottom Dead Center for a duration of 195 degrees.  Right there, I show my inaccuracy in taking measurements because, with the face cam operating both valves, the durations should be the same.  In any case, with an overlap of 24 degrees, it's no wonder it was spitting back.  That amount of duration may work fine for an engine running at 5,000 RPM but not a thumper like this.

To fix the overlap problem, I removed the cam and put it in the mill, removing some of the lobe.  When I got it back together it still wasn't right but was closer.  With the new cam duration, here's what I got.  The exhaust opened at 170 degrees BBDC and closed at TDC for a duration of 190 degrees.  The intake opened at 10 degrees BTDC and closed at 5 degrees BBDC for a duration of 185 degrees.  The overlap was 10 degrees.  My method of checking the timing was to carefully set the lash at 0.010" then, using degree strips on the flywheel, I turned the engine until an 0.002 feeler gauge became tight under the valve stem.  This probably 'splains why there are differing durations for the intake and exhaust valves.

A test run showed some improvement with less 8 cycling but there was still the occasional carb backfire so I suspect the overlap is still a bit much.  I loosened the lash for the intake valve (made no timing measurements) and it ran a little better.  Tomorrow, I will again take the cam off and whittle some more off of the lobe, trying tor either zero or a couple of degrees of overlap. This time, I will use a dial indicator on the valves to determine when they begin to open.  I can't get any more accurate than that.

My theory for the carb backfires is that the intake valve opens while the hot exhaust stream is still flowing and a little flows back into the intake, igniting the mixture.  It's only a theory but it makes sense to me.  the 8 cycling is most likely due to exhaust dilution of the mixture.  Adjusting the mixture and flutter choke do not eliminate it.

3 April 2017:
The cam timing is now as follows:
Checked with 0.010" lash and using dial indicator on spring retainer to determine movement.
Open - 178  ATDC
Close - 2     BTDC
Open - 2     BTDC
Close - 178 ATDC

This gives both valves having a duration of  180 degrees and zero overlap.  The 2 degree figure is due to that being as close as I could get it with a setscrew on the sideshaft.

I ran the engine and it now doesn't 8-cycle if I hold the throttle in position.  At low speeds, it 8-cycles due to the governor responding to engine firing impulses.  Retarding the spark eliminates this but for running slow, the spark needs to be about 25 degrees ATDC, which makes the engine run hot.  After a fairly long run, the temperature was close to 280F.  I don't consider this to be excessive but would stop if it reached 300 degrees.

As for the spitting back and jerking, it still does it occasionally so now, I think I need to work on carburetion.
19 April 2017:
In the last few days, I've modified the design (Version 4) and built a couple of my "Cheap Ignition Circuits".  This one has an LED to indicate firing.
The ignition module is the white box below the fuel tank.                                     The coil is one for a Toyota or Suzuki.          
I've been searching for cheap non-high energy ignition coils and am experimenting with a "dry" (non-oil filled) coil that is a replacvement for Toyota and Suzuki (among others).  They come with a bracket and a ballast resistor.  I dispense with the ballasts and removed the half of the bracket that holds it.  You can see the coil mounted below the fuel tank with the ignition module.  So far, it is working well.

I'm still using a magnet on the cam for timing but am now trying a magnetic reed switch that is mounted inside a piece of copper tubing.  So far, it is working well.  I will post the revised schematic and details of the sensor on my "Cheap Ignition Circuit" page.

After some thinking and a bit of fiddling, I may have come up with one of the reasons for the engine spitting back and jerking.  After the plug fouled this morning, I decided after cleaning it, to open up the gap from 0.025" to 0.032".  It seems to run better with less spitting and bucking.  This afternoon, I will enlarge the gap to 0.040 and see if it is better yet.

6 May 2017:
It's been a while.  First, the increase of the plug gap to 0.040 didn't improve things so, today, I tried something else.  I had made a 10mm standard reach to 14mm standard reach adaptor a while back so I took a long reach 14mm plug I found on the roadside and made a sleeve so it would fit in the adaptor.  Since the head is threaded for a long reach 10mm plug, I figured the combustion chamber volume gained from the "unused" 10mm thread plus the volume of the adaptor plus the volume of the larger spark plug would lower the compression ratio enough to be able to see if lower compression would stop some or all the foolishness of spitting and bucking.
    Spark plugs and adaptors.                                                               Spark plugs after 45 minute runs.                                                   Spark plug on adaptor on engine.
In the left-hand photo at the top is the long-reach 10mm plug I've been using.  In the vertical center to the left is the adaptor, in the middle is the sleeve to have the 14mm long-reach "road find" plug fit into the adaptor and to the right is the long reach 14mm plug.  People have been saying that lash-ups like this cannot work very well to ignite the charge because the charge has to flow up through the hole in the adaptor to the plug to fire.  In operation, it seems to work fine and the engine runs steadily.  I also tried a standard reach "junkpile" 14mm plug as shown on the bottom of the left-hand photo and the engine didn't seem to like it quite as much, probably due to the smaller volume of the smaller plug.

The center photo shows the original 10mm plug and the "new" 14mm plug side-by-side after running about 45 minutes.  You can actually see a little color on the 14mm plug while the 10mm plug is oily.  In the photo on the right, you can see the larger plug on the engine.

The engine was started and it seems that it can stand a bit more spark advance, doesn't spit when warmed-up and hasn't bucked once, even after the engine temperature stabilized at 260F while running at between 500 and 600 RPM.  Actually, the engine is stabilizing at a slightly lower temperature most likely due to being able to run the timing a bit more advanced.

If, after thinking about it for a while, I decide that the compression should be lowered a bit more (and permanently), a thicker head gasket or the plug hole can be bored out and threaded for a standard reach 14mm plug.  Either way, the added volume may tame this beast.

7 May 2017:
I've done a bit more spark plug/compression ratio experimentation.  Today, a longer 10mm/14mm adaptor was made.
New adaptor made using threads from 10mm plug.                                                        New adaptor.                                                                        Plug with adaptor on engine.            
The new adaptor was made using the threaded portion of one of the 10mm long reach plugs.  The 14mm female thread in the adaptor was made the hard way using a 14mm thread chaser which wasn't designed for threading.  It took a while and a bit of  elbow grease but it did make good threads.  The adaptor has a little more volume than the short one and allows more thread engagement in the head and full engagement for the plug.

The engine started readily and ran pretty well but, not being satisfied, I stopped it and removed the 14mm plug and used the sleeve that I made yesterday to give more volume in the adaptor.  This time, the engine didn't run as well.  I think the problem is that I finally got enough volume between the plug and the combustion chamber to make the charge at the plug iffy.  The engine would accelerate fine but when it was allowed to idle, it 8-cycled no matter how the fuel mixture was set.  I think that, with the denser charge on acceleration, enough mixture got to the plug so it could ignite it more reliably but was too rarified at low throttle settings.  I went back to the new adaptor and the engine ran a lot better.

Then, while I was sitting "supervision", it began to occasionally run rough, hunting and misfiring.   It would eventually correct itself.  The engine skid was sitting on concrete and I could see it teetering a little.  The teetering got worse when the engine started acting up so, on a whim, I put my foot on the skid to hold it steady.  The engine straightened out and ran right.  I figured (correctly) that the problem was a bouncing check valve.

The check valve was removed and the 0.130" ball was replaced with a 0.200" ball (the biggest that would fit in the housing).  After that, everything was hunky-dory.  I refilled the gas tank and ran it dry, having to occasionally open the needle valve as the fuel level fell.  Just before it ran out of gas, the temperature was just barely 250F

I tweaked on it until it would run well at 400 RPM if I kept an eye on it.  If I revved it up to around 1,000 RPM and advanced the timing, it ran really well but I don't want to run it that fast.  I set the speed to between 450 and 500 RPM for the run,

I've got to see what the ignition timing is when it's running around 500 RPM.  I think it is firing after TDC for best running.

8 May 2017:
Today, I made a new needle valve assembly.  The needle has an 8 degree taper and seats against a #65 (0.035") jet.  This gives a much finer adjustment.  

When tested on the engine, it worked a lot better than the larger one and the engine is nearly ready for prime time.  After running a full tank of gasoline through it at between 400 and 500 RPM with the timing retarded so it ran well, the temperature was just a slight amount above 250F.  I think if I can get a handle on the timing issue and get it running with spark advanced a few degrees, it should run substantially cooler.

Running at 400 RPM, in order to keep it from 8-cycling, I have to run the spark timing retarded 30 degrees after top dead center.  I don't know where this oddball timing is coming from.  Too little flywheel mass?  To high compression?  Even at 700 RPM the timing is running about ten degrees retarded.  This is a mystery to me.  Anyone got answers?
BOY!  This is fun!
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