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Old 07-27-2012, 08:08 AM   #1
Sniper X OP
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Why would my 85 R80T have a dual plug head?

The original owner whom I never met did the dual plug head deal on this bike and the guy I got it from said he thought it was probably for mileage but is that even a factor in a DP set up? Why would someone do this?
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Old 07-27-2012, 08:15 AM   #2
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Oak says that two plugs provide a better flame front, thus reduces or eliminates pinging. I don't think it increases maileage much, if at all, but someone else may know better.
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Old 07-27-2012, 08:19 AM   #3
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I always run premimum in my airheads anyway so would probably never get pinging but since it is done and done right it is pretty cool. Dunno of any possible benies from it on a stock bike. My R80 sure isn't as fast as my RS was.....that thing would cruise at 95 all day no problem. this thing feels a tad busy at 95. Feels best at 70~75.
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Old 07-27-2012, 08:26 AM   #4
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70 - 75 cruise is plenty good, at least around here. It might be possible that the drive ratios are different between the two bikes also. I don't have enough coffee in me to look it up though.
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Old 07-27-2012, 08:26 AM   #5
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The later Airheads with low compression are notorious for poor MPG. Dual plugging does improve combustion efficientcy and provides a boost to MPG.
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Old 07-27-2012, 10:07 AM   #6
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If nothing else, it takes more fouling of the plugs to keep the bike from actually running
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Old 07-27-2012, 10:13 AM   #7
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Quote:
Originally Posted by Hedge36 View Post
If nothing else, it takes more fouling of the plugs to keep the bike from actually running
Yeah, I guess someday I'll have to actually inspect the plugs!
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Old 07-27-2012, 10:18 AM   #8
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Quote:
Originally Posted by DoktorT View Post
The later Airheads with low compression are notorious for poor MPG. Dual plugging does improve combustion efficientcy and provides a boost to MPG.
Possibly some of the R80s which were jetted pretty rich.

Certainly no mpg problems on the R100RS I ran for 28 years.

Dual plugging is a solution looking for a problem.

It originated in the late '70s and early '80s when premium fuels were temporarily unavailable in the USA during the changeover from leaded to unleaded fuel. BMWs of the era were all high compression, and dual plugging allowed use of low-test fuel in these bikes. BMW SOLVED that problem by reducing compression in all bikes beginning the 1981 model year. And, the problem was SOLVED by the return of premium fuel.

Were I somehow going to get in the used Airhead market, I'd reject any with dual plugging.

No question high compression = more efficiency and better fuel mileage, but NEVER EVER were the low compression models considered NOTORIOUS for poor economy. That said, I have an '84 R80G/S that is jetted so rich I hardly ever need the choke. By the last year for the R80G/S jetting had been leaned out. And it's true a stock R100RT of '83-'84 returns better fuel economy than a stock R80RT from these same years ... mostly because the engine always spins 1000 rpm faster but also because of the R80's richer jetting. This jetting problem never happened on the R100s.
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Old 07-27-2012, 11:41 AM   #9
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One of these days when I actually run the gas outta mine I'll have to check the mileage. Might take a month! Too much rain...wanted to ride all week.
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Old 07-27-2012, 12:07 PM   #10
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If it was dual plugged for a reason the compression has been raised. Chances are the ignition curve has not been remapped and the stock curve simply retarded is a huge detriment to dual plugging.

Octane ratings went down and stayed down in the mid/late seventies. For the vast majority, 100 octane premium never returned to this country.

For the most part, BMW compensated poorer mileage associated with lower CR by leaning out the main jets. Those jets needed leaning out on a lot of models regardless of lowering the CR. What R80's are jetted richer than R100's? What jetting problem never happened with R100's?
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Old 07-27-2012, 12:09 PM   #11
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Someday I'll have to check the timing....
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Old 07-27-2012, 10:29 PM   #12
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My dual plugged R100RS got 47 MPG. I am curious as to what my timing is.
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