"If you want a good, reliable, classic feeling motorcycle, it's tough to beat a Sportster."
--Colorado Jeff
From the Boutique:
Dual Plugging the 1000 cc Harley Davidson Sportster (ironhead)
by Mark Ferguson
Certainly, the best improvement I have made to my Ironhead in terms of rideablility has been to perform a dual plug conversion. It increased my fuel mileage by about 3-5 mpg. It made the bike run smoother. It improved throttle response. And besides, it looks cool :)
There are several shops around the US that can perform this task. When I did mine a couple of years ago, Lake Shore had about the best deal, $75 (if I remeber correctly.) I actually did the work myself, as at the time I worked at a machine shop and had access to all the tools I needed.
But instead of me trying to explain it all, an XL-Lister, Mark Ferguson, also performed the conversion himself and did a fine job of documenting it. So, I will let him tell you all about it. A quick note, as Mark will explain there are basically two ways to do the extra plug. I used the "conventional" method. This is performed in the same steps as Mark describes, just placed in a different place. -- DougM
I have recently completed installing dual spark plugs in my 1980 Sportster. While researching this modification, I found that it is performed in two ways, One which I will call "conventional" places the second plug more-or-less symmetrically opposed to the original. Another method which I will call "hidden" places the second plug closer to vertical behind the rocker box. Both methods place the plug electrode in roughly the same place.
I found the hidden method attractive because I didn't want to upset the stock appearance of my motor, I don't think this method is superior to the other in anyway besides aesthetic, in fact I would choose the conventional method if you don't mind looking at the plugs and wires. Plug access is also restricted in the hidden method, although not severely (at least not on my bike).
The shown dwgs. and photos. should be self explanatory, this is a machine shop operation and should not be performed by the inexperienced unless you are in need of a set of interesting bookends. The dimensions (see fig. 2)included are taken from one set of heads (mine), I assume yours are close but I can't make that claim. Do not assume all of the features in the dwgs are accurate, those important to the process are, others are estimated. I have included a dwg. of what I believe is the correct way to perform the conventional method, but since I have not verified this dwg it is presented for clarification only, perhaps through collaboration I could come up with a known accurate dwg. in the future.
When machining my heads, I made a fixture plate which included a center drilled (scribed would work) mark representing the point where the plug axis intersects the head gasket surface (see engineering dwg.). When setting up the machine, I set the 7.6 degree angle, then used this center drilled mark to find the plug axis, the 1.39" dimension is also taken from this point. The fixture plate used the head bolt holes to locate the head, allowing accurate removal and replacement of the head during the process, two diagonally opposed head bolt holes held 15/32 dowel pins, while the other two were tapped 7/16-20 for head bolts. I would recommend drilling two more 1/4" holes 3/4" apart near the center of the fixture plate, these are used to locate the chamber molds I'm going to discuss next.
As a reality check, I made Bondo molds of my combustion chambers, you may not need to do this if you trust my dwgs. (I wouldn't if I were you). I used Vaseline as a mold release, use a lot around the valves and plug the spark plug hole with clay or something. fill the chamber with Bondo, then clamp a 1/2" or thicker piece of Plexiglas over the mold area, the idea is to squeeze out the excess Bondo and provide a flat surface coincident with the gasket surface. It will take four "C" clamps, and don't forget to grease up the Plexiglas plate. Go easy with the clamps, as the cast iron fins are fragile (try to clamp to places other than fins). If you don't have the materials I've mentioned, use your imagination. Plexiglas is used over other flat materials so you can see when the bondo has been squeezed flat to the gasket surface. Be sure and mask off the rest of the head to keep bondo out of the fins - a real clean up hassle. After the Bondo has cured, remove the Plexiglas plate, then install head with Bondo still in place to the fixture plate, make sure head orientation is correct (new plug location in head in correct place). Now drill through the two 1/4" holes in the plate into the Bondo mold, drill about 3/8" into the bondo. Disassemble head from plate and remove mold from head ( a light rap to the tip of the valve stem with a rubber mallet will free the mold), and press two 1/4" dowel pins into the mold. You now have a representation of the combustion chamber which you can accurately place on the fixture plate to verify you machine set-up before cutting any expensive metal. I installed a 1/2" endmill in the mill and used it to touch of the Bondo mold to verify the 1.390" dimension on the engineering dwg, you may need to adjust this dimension if your heads vary much from mine. I used 1/2" endmill, because the plug root diameter is close to 1/2". I used the same 1/2" endmill to make the large bore, then used a 1/2" drill to drill for the plug tap, 1/2" is smaller than the recommended bit, but I used it at the advice of Doug Mansfield who suggested it would provide a tighter thread fit, it did, and I can now recommend it as well. Note that the plug is offset from center 1/8" towards the exhaust valve, I did this because I felt the plug was too close to the larger intake valve. You needn't make two fixture plates, just make two plug axis marks, each 1/8" off center.
Once the plug hole has been tapped, the excess threads on the outside wall of
the chamber need to be cleaned off, and the new hole edge inside the chamber needs
to be raduised to eliminate hot spots, work carefully with a carbide burr in a die
grinder or Dremel type tool.
That's it, I would like to thank Doug Mansfield and Mike Hughes for sharing
their experience and input. If you perform this modification using my dwgs.,
please let me know if you find them accurate, or where you had to adjust them.
mferguson@shpi.com
Later Mark added:
I've had the dual plugs on my 80xl on line now for 100
miles or so, it was definitely worth doing. I used to have to
be carefull to avoid detonation when riding two up on hills,
or when lugging the motor on hills solo (I know, don't lug the
motor). It is now very difficult to get any detonation out
of the motor. I found that when idling (especially when cold),
disconnecting the second set of plugs would drop the idle speed,
this seems like a good thing to me, it suggests that the second
plug is burning previously unburned fuel. I found that with
the dual plugs, the motor was now running lean, I attribute this
again to the burning of previously unburned fuel, I bumped the
main jet up a notch, and Voila, it's perfect. I'm still running
dual fire slaved off the stock module, this means I'm evaluating
only the plug conversion, not a change in advance curves, I plan
next to install an aftermarket ignition (spyke/crane/dyna etc.),
at that point I think I'll get Salt Lake HD to dyno tune it for
me.
DUAL PLUGS SET UP, 1967 74cu.in. SPORTSTER
Contributed by Patrick Delli, aka The French Owl, July 2001
E.Mail: patrick@delli.fsnet.co.uk
I was surfing the Web from England when I found this site and all its merry
contributors…
I was particularly interested by the Dual Plug contribution from Mark. I have
spend quite a few years messing with "old" Iron Sportsters, finding them more
interesting to work on than later alloy ones. They are also more prone to
individuality and their scope/possibilities are almost endless.
So, I also went the twin plugs route a few years back on one of my Sportsters, a
1967 magneto XLCH but on a slightly different way than Mark. Something I hate is
spend too much time making things out…I am a very busy Electrician so any spare
time I have, I would rather spend riding !!!
So, when I looked at those Iron Heads, I thought there must be an easy way to
drill and tap a second plug. I looked in my garage and found a piece of square
tubing about 2" by 6", around 10" long and I could see the head sitting nicely
on it. I then welded 4 studs on it so I could bolt the head down on the biggest
flat, slightly on one end of it so the exhaust stub did not interfere with the
gig.
Then, milling time: bolted the gig on the milling table, then screwed a 14mm x
1.25 tap with a longish body in the existing plug hole, used that long body to
set the milling head at the right angle with a micrometer dial fixed in the
miller chuck, clamping everything tight when I was happy with the alignment. Put
a stop on the miller "up" travel, brought the table down, unbolted the head from
the gig, turned it 180 degrees, rebolted it and went up again against the stop,…
hey presto ! spot-facing, drilling and taping to get a new hole exactly
symetrical to the factory one.
The only draw back of this quickish method is that the new spark plug, when
installed interferes slightly with the rocker box and it has to be trimmed a bit
on the bottom. Or you can do what old drag racers use to do: split the rocker
box in two and have an external oil feed between the two halves.
As Mark points out, the twin plug conversion does work and makes the Iron
Sportster a smother bike. Mine was fired at the time by a twin-fire electronic
ignition setup with twin coils between the cylinders where the horn usually is.
Timing wise, those old Sportys do heat a lot so you cannot go as low as you
might think possible…I found my engine most happy around 40 degrees advance from
45 degrees of factory setting.
Sadly this engine died when the front Ditch cylinder broke at its base about 5
years ago…It took me a long time to source a pair of s/h replacements but I have
them now so it is "more mods" time: started a new project with this bike with
rear head mounted in the front and the front one at the back so both exhausts
and carb will be on the LEFT. As I hate carrying extra weight, ie battery for
the ignition, I have been pondering on the installation of two Fairbanks-Morse
magnetos, one "a la XR 750" in place of the dynamo, and the other where it
should be on the timing cover. Not quite finished but it'll be done !!! The
inspiration for that "reverse head" mod was a photo of Larry Darr's Iron XR 750
in Allan Girdler's Harley Racers. It seems Larry was missing his old KR !!!
A friend of mine in Reading , England has done the twin plug setup another way:
the second plug is horizontal between the pushrods tubes, so it is both
accessible and does not interfere with the rocker boxes…
So long,
Patrick
