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Thread: Siebenrock kit for R90/6; High Comp. or Low, dual plug or single?

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  1. #1
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    Siebenrock kit for R90/6; High Comp. or Low, dual plug or single?

    I am just about ready to buy the Siebenrock big bore kit for my R90/6 (Code # 1100072). I was looking at Ted Porter's Beemer Shop website for the top end gasket kit also (Code: TopEndKit70-75Siebenrock) and in bright red letters they say; " Recommend dual plugging heads if using standard base gaskets giving 9.5:1 compression. Low compression base gaskets lower compression to 8.8:1."

    I really do not want to go through the expense and the tuning complexity of using dual plugs. Although I would love to have the power of high compression pistons, this bike is going to be used for touring and normal street use. I have always used premium gas in this bike and will continue to do so. My question(s) is/are; does anyone think that dual plugs are REQUIRED if I use the 9.5:1 compression ratio? If I can get away with using a 9.5:1 compression ratio and single plugs, what sort of precautions or tunings should I use? Can I double up on base gaskets (or variable thickness material) giving me a compression ratio in between? Is 9.5:1 going to simply create too many complexities and variables to where I should just save myself a headache and go 8.8:1?

    I had a similar situation with a Honda CB750/850 I built last year. I used high compression pistons and although it is now ride-able it took months of aggravation to tune and it still pings somewhat during low rpms/high load situations. I don't mind some fine tuning here and there but I do not want to spend a boat load of money just to have to take it apart again and again to get it right.

  2. #2
    Administrator 20774's Avatar
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    I can't tell you if you would be OK, but I was in the same situation with my overhaul on my R100/7. I bought the kit from Ted. I was going from 9.0:1-ish to the stated 9.5:1. I wasn't comfortable with doing that and didn't like the extra effort of dual plugging. So, I opted for the metal base gasket per Ted's recommendation. This base gasket is actually an R60/5/6 part used to lower compression for those bikes. It's about 0.67mm thick. I'm happy with what I did and the way the bike now runs.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    In process again.

    Freshend up my '74 R90 back in '90. Had over a 100,000 miles in the od. Did an economy rebuild. Pulled off the heads, pistons, cylinders. Did a fresh grind on the valves and seats. New guides. Just at the point of needing new pistons. Didn't have the bucks for that, so I beadblasted the top of the piston and installed new rings and a low compression cylinder base gasket. Honed the cast iron cylinders with some nice cross hatching. Reassembled and rode it hard for 30,000 great, fun, happy, smiling, no problem miles. Parked it and started riding a K100RT and R1100RS. A few years ago started rebuilding the R90 engine. Pulled the heads, cylinders, pistons. again. Bored and honed the cylinders to fit first over Kolbensmitt stock compression pistons with new rings. Built a fixture and dual plugged the heads. New guides, new valves that were back cut, fresh cut seats. Back to stock base cylinder gaskets to up the compression back to stock 9.0:1. Reassembled all that stuff. Will report on results when I get the bike all back together. The reason I dual plugged was to imitate what the guys did back in the '70's to slightly hop up their bikes. I drilled the airbox like the R90S for the same reason. Back cut valves all old school hotrod tips. No, it wont run like an R90S, but I'm hoping it will have a little more umph in the fun factor.

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    There is another thread that I started a little over a month ago about the heads and valves, etc. One factor is that the time to do the machining for a dual plug head has come and gone. As of today, my local shop has all of the new valves and guides in it. I don't want to add complexity to what is now finished work. If there is anybody who has tried the high compression setup without the dual plugs I sure would like to hear from them and how they did (do) it.

    If all I have to do is retard a little timing or richen up a pilot jet (or something similar) without worrying about hand-grenading the engine by pulling some long hill this summer then I would go with the high compression setup. If the low compression setup is more reliable by far then I'd be inclined to go that route. I figure that I am increasing my engine displacement by 15%. Even by erring on the side of caution I'm probably going to realize substantial torque gains.

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    Registered User toooldtocare's Avatar
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    My 1974 R90S was rebuilt from a basket case of spare parts. It was put back together stock, but someone had already installed dual plugs in the heads. During the rebuild it was decided to use only one plug per head. The bike always pinged a bit, no matter where the timing was set, and it was hard to start. After I sold the bike the new owner connected the second plug, installed a Dyna III, and retarded the timing per instructions from Dyna. The bike ran so much better, no pinging, and even started better. I cannot say the improvement was due to the dual plugs, or the Dyna; maybe both.

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    Yea, I'm not trying to build a super autobahn monster, just trying to get a little more gityupgo with what's there. No super high compression, exotic carbs, max dispacement etc.. Mostly trying to have a nice old school looking bike that runs really well and put a smile on my face.

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