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Old 09-03-2014, 05:08 PM   #1
Hollow Road Rider OP
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Dual Plugs Fantasy R100 GS

I recently had a ride on a dual plugged R90S. It was so unbelievably smooth, I was smitten. She comfortably idled at 500 rpm.

I have the name of a machinist who is known for his excellent work on these.

My big question is, what else will I have to do in terms of component replacement, to make this work?

Also, is there a known link to a step-by-step guide for this?
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Old 09-03-2014, 05:20 PM   #2
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Not meaning to be a smartass at all, but there could very well be other reasons that it was so smooth.
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Old 09-03-2014, 05:27 PM   #3
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Originally Posted by chollo9 View Post
Not meaning to be a smartass at all, but there could very well be other reasons that it was so smooth.
As my dear mother used to say, "Don't be such a smartass."
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Old 09-03-2014, 05:44 PM   #4
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I'd guess the different feeling of the R90S engine had a lot more to do with its heavier fly wheel and slide carbs than it did with dual plugging.
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Old 09-03-2014, 05:50 PM   #5
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Sorry, but I meant only to be helpful. I've ridden a number of 'em and can't say there's a consistent feeling that I've gotten from dual-plugged bikes. Many agree and many disagree. I just know that there are a lot of factors that add up to a smooth running engine and I'm not sure that dual-plugging is worth what many people think it's worth. It does do a very specific thing, but outside of that, I'm not so sure that it does anything else.

My dear mother would have never used the word "ass", I was known as an "aleck"! Now Dad, he was a sailor . . .
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Old 09-03-2014, 08:12 PM   #6
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Quote:
Originally Posted by Hollow Road Rider View Post
Also, is there a known link to a step-by-step guide for this?
Sure: http://bmwmotorcycletech.info/

:)

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Old 09-03-2014, 09:54 PM   #7
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I have dual plugged a lot of airheads. Contrary to many reports, I don't think dual plugging in itself makes them run smoother, idle better, start better, or get them better mileage. And yet my own bike is dualed plugged. Why? I raised my compression. I am very conservative about compression ratio and pinging but I have been around so many airheads that had their CR lowered for pinging. BMW went too low in the early 80's with 8.2:1 CR. IMO, they got it perfect in '85 with 8.5:1. I wouldn't run any higher single plugged. Dual plugged? I am running 10.0:1.

Many dual plugged bikes are not timed for best performance per Snobum and Oak who for years advocated timing them to TDC at idle which just kills performance. After decades of resisting, they have finally started recommending what many tuners have known for decades: They run best completely altering the advance curve and keeping timing at idle stock and retarding the timing further up the rev range. The last time I read snobum, he was still recommending both ways at once but . . . . at least that was the way I read it. He is notoriously difficult reading.
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Old 09-03-2014, 10:48 PM   #8
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I also own and have ridden several dual plugged bikes and can't say that dual plugging smooths things out. It does seem like they do start easier. I think you can get better mileage depending on jetting. I didn't dual plug any of mine, they came that way.

I'll second the comment that the smoothness you felt on the 90S was either the heavy flywheel or careful balancing of pistons and rods, not due to the dual plugging.
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Old 09-04-2014, 03:48 AM   #9
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In the late '70's leaded Premium gas became nonexistent and my R60/5 developed a worser ping problem. After juggling ignition timing for a while, I finally ended up taking the CR from 9.6:1 down to 8.5:1 with Low Test base gaskets. Worked like a charm for 30+ years. A couple of years ago I did a new top endon the bike and one "performance modification" I made was going back to the stock base gaskets and high(er) compression. And so to alleviate the ping problem I dual-plugged the heads. No ping on that ping-prone engine (even with Regular grade gas, much to my surprise!), with the engine a little peppier (particularly noticable in the low rpm, light throttle range) and it picked up a couple of MPG on the fuel mileage.

From that standpoint, dual plugging was positive, but it was simply one link in the chain.

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Old 09-04-2014, 05:19 AM   #10
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Hmmm....

...fantasy fading now.
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Old 09-04-2014, 09:39 AM   #11
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As SS said, the main advantage in dual plugging your bike is to enable you to run a half decent compression ratio on today's crappy gas without pinging.

I also find that my dual plug engines start well and run smooth, but then they both have balanced engines! The higher compression ratio makes more energy, so your fuel consumption should decrease and as the engine no longer needs a rich mixture to damp down pinging, you should be able to lean out the jetting a bit.

So dual plugging combined with a higher compression ratio and maybe some porting will start you on your way to ending up with an engines that runs much better than stock. It's not a quick fix for all airhead ills but as part of a systemic upgrade it will help. OEM static timing, around 28 degrees at full advance. This is my advice to you.
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Old 09-04-2014, 10:12 AM   #12
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I need to back up.

What is a "balanced engine"?
Sounds to me like you tear it down to parts, balance the parts on some sort of special mill, and then reassemble everything. Am I close?
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Old 09-04-2014, 05:17 PM   #13
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Quote:
Originally Posted by Hollow Road Rider View Post
What is a "balanced engine"?
Sounds to me like you tear it down to parts, balance the parts on some sort of special mill, and then reassemble everything. Am I close?
You're close, everything that is in pairs is matched in weight, then rotating items are balanced in terms of reciprocating mass, e.g., crank counterweights vs. pistons/rings/wristpins/rods. It makes for a much smoother running engine and, when desired, can allow running at higher rpm. It's one of the best racing "trickle-downs" that has actual benefits in the real world. Expensive though.
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Old 09-04-2014, 05:26 PM   #14
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Quote:
Originally Posted by Wirespokes View Post
I also own and have ridden several dual plugged bikes and can't say that dual plugging smooths things out. It does seem like they do start easier. I think you can get better mileage depending on jetting. I didn't dual plug any of mine, they came that way.

I'll second the comment that the smoothness you felt on the 90S was either the heavy flywheel or careful balancing of pistons and rods, not due to the dual plugging.
+1 .. true or false, scuttlebutt is that early R90S were put together with extra care. my 74 R90S is butter smooth same as my R80G/S with light flywheel that was balanced to a 1/10 gram .. both equally smooth .. G/S spins up faster due to lighter flywheel and lower gearing. R90S doesn't start cooking until you are pass 80 mph or so ... neither one is dual plugged.

_cy_ screwed with this post 09-04-2014 at 05:32 PM
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Old 09-05-2014, 07:16 PM   #15
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I'd agree, its the 100s that just aren't that smooth. At least none of the ones I've owned/ridden. Still love them though, its imperfection that makes airheads so enjoyable, so a little un-smoothness doesn't bug me much.
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