Timing dual plug motors

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Panshovevo
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Timing dual plug motors

#1

Post by Panshovevo » Thu Mar 10, 2011 6:11 pm

I have read somewhere that Harley motors with dual plugs need to have the advanced timing retarded from the factory specs because the dual flame fronts cause peak cylinder pressure prior to TDC when set at 35 degrees BTDC. All of the aircraft engines (dual plug with large bore) I have worked on had timing specified in the neighborhood of 20-25 degrees BTDC, and I seem to recall hearing that a Pan or Shovel with dual plugs should be timed at 20 degrees.
Any thoughts?

This brings up another question. Does anyone make advance weights that would allow the retarded timing to stay at 5 degrees BTDC, yet limit the advance to 20 degrees?
If not, any suggestions on how to accomplish this?

(I'm working on my cone Shovel, but figure these questions would apply to automatic advance Pans as well.)

Regards,
John



Cotten
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Re: Timing dual plug motors

#2

Post by Cotten » Thu Mar 10, 2011 7:21 pm

John!

Double-plugged Pans are extremely rare,
and aftermarket flyweights for them are even more so!

....Cotten

Panshovevo
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Re: Timing dual plug motors

#3

Post by Panshovevo » Fri Mar 11, 2011 1:25 am

I went back through some S&S tech literature, and was reminded that they recommend that the advanced timing be retarded only 5-8 degrees on dual plug motors. Anyone heard anything different?

Regards,
John

Cotten
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Re: Timing dual plug motors

#4

Post by Cotten » Fri Mar 11, 2011 2:57 am

John!

I confess that I only paid attention in the late 80s when double-plugging Shovels became a fad because of the drop in fuel octane.

The whole buzz at the time was that it would allow Shovels to avoid predetonation and excessive heat, without retarding beyond the Factory timing mark.

Apparently it doesn't?

....Cotten

pan620
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Re: Timing dual plug motors

#5

Post by pan620 » Fri Mar 11, 2011 2:46 pm

back in the days we would set the timing 4-5 degrees retarded (mark in the front of the hole) and add some compression with dual plugs.

Panshovevo
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Re: Timing dual plug motors

#6

Post by Panshovevo » Fri Mar 11, 2011 3:15 pm

Cotten
As I understand the situation, factory timing is decided by when peak pressure happens in the cylinder, which, ideally, is about 10 degrees ATDC.
Dual plugs cause the mixture to burn faster by creating flame fronts on both sides of the piston. As a result, peak pressure (and temperature) happen sooner. This can create excessive heat in the cylinder, and can lead to detonation and pre-ignition.
Unless there is something very unorthodox about this situation, or something I'm not considering, or not aware of, anyone that did the mod for the reason you stated was seriously uninformed and probably disappointed.

My Shovel was dual plugged, with single fire electronic ignition when I got it. No idea why. Near as I can tell, it's basically stock, except for the carb, exhaust, and ignition.
I bought it from a German roofer whose idea of a good time is to blow by the Porsches on the autobahn at 150+ mph, on a Jap crotch rocket. He only bought the Shovel because a friend needed money, and he was flush. Later on, he wasn't, neglected the bike until it quit running, and I bought it.


Regards,
John

Hawg
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Re: Timing dual plug motors

#7

Post by Hawg » Fri Mar 11, 2011 3:57 pm

Best Suggestion Here Is To Dump The Points and Go To a True Single Fire Ignition. I Had Built a 96 Inch Shovel Motor In 1988 Dual Plugged I Ran An MC Power Arc Ignition System Because It ONLY Fired The Cylinder The Was On a Compression Stroke. It Also Fired The Plugs In That Cylinder At The Advance Point You Dialed It In At and Again At TDC. ( 4 Sparks With 2 Plugs Per Cylinder) This Increased Power and Fuel Mileage I Had Put a N2O (Nitrous Oxide) System On This Bike and That Ignition System Was Capable Of Retarding The Spark (By Being Wired Into The N2O Button) When You Hit The Nitrous Button. I'm Sure Their Systems Have Changed To Keep Up With The Newer Twin Cam Motors And Computer Controls This Is Their Web Addy http://www.powerarc.com/hdproducts.htm" onclick="window.open(this.href);return false;

Panshovevo
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Re: Timing dual plug motors

#8

Post by Panshovevo » Wed Mar 16, 2011 4:46 am

Hawg wrote:Best Suggestion Here Is To Dump The Points and Go To a True Single Fire Ignition.
Hawg,
As I said above, my Shovel has single fire electronic ignition.
How much timing advance did you run, and how much did you retard for nitrous mode?

Incidentally, I pulled the primary cover the other day, put a degree wheel on the sprocket nut, and marked the rotor and inner primary for TDC and 30 degrees advance for both cylinders. Using an advance timing light, I was able to determine that there is only a 20 degree spread between retard and full advance with the current set up. I wonder if that is by design, or a flyweight malfunction. I haven't pulled the ignition unit to check the advance mechanism yet.

Regards,
John

Cotten
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Re: Timing dual plug motors

#9

Post by Cotten » Wed Mar 16, 2011 2:34 pm

John!

The sad aspect is that even a single-plugged machine benefits from timing retarded slightly from spec, due to lower octane fuels from when the machine was designed.

Performance is lost, but in the interest of longevity.

It is probably one reason (among others) why the fad faded, athough I have a set of fossil Knuck heads that I have always planned it for, as the fins have been knocked off anyway!

....Cotten
PS: I love single-ignitions too! But you don't need over-engineered can't-fix-it-except-to-replace-it elecktrawnics to do it.
The MOCO dual point circuitbreaker was more than adequate.
With a little ingenuity, it could be made to be a mechanical auto advance as well.
Last edited by Cotten on Wed Mar 16, 2011 2:43 pm, edited 2 times in total.

Panshovevo
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Re: Timing dual plug motors

#10

Post by Panshovevo » Wed Mar 16, 2011 2:41 pm

I would be willing to bet that many were converted for bragging rights among the TT riders (tavern to tavern).

Regards,
John

FelthamW
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Re: Timing dual plug motors

#11

Post by FelthamW » Thu Mar 17, 2011 12:04 am

Ahhhhhh, this conversation reminds me of days of old! TT bikes? You betcha. Bragging rights too.
I ran a pan bottom, Mackie shovel top, 98" Sidewinder with the MoCo dual point timer. Dual plugged. Twin Accel coils. How I ended up with the dual plugs was the compression ended up higher than I wanted (Axtel pop ups) and it pinged. BAD! Once I dual plugged it I could retard it by as much as 5 degrees and it wouldn't ping - but it just wouldn't go right. It ran stronger at full advance. Yep, had a degree wheel on it. Tried a Crane 316 adjustable timing cam and didn't like it much. Lifted the front jug off the case too - Hello Delkron!
All in all, the "hype" of the day was to retard it a bit and don't hole a piston. I did anyways. I ended up with forged S&S pistons at 9.5:1 and now it runs fully advanced. Doesn't ping. Much.....
Love that damn bike. My son is learning the ropes. I can't kick it like I used to. I am thinking knee replacement in my old age. That kills me considering I still want to ride my pan.
Ride safe, ride hard, run like the devil. :shock:
WF

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