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Thread: R90/6 Seibenrock Kit?

  1. #1
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    R90/6 Seibenrock Kit?

    If you have done this would you weigh in with comments. Was it worth the cost? etc. Thank you.

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    http://forums.bmwmoa.org/showthread....ock-1000cc-kit

    My response to an earlier post:

    I choose the Siebenrock kit for the smaller 97mm cylinder opening. Mine was an "early" '76 so the case holes are smaller. It bumps the displacement up to 1000cc. It comes standard with a 9.5:1 compression ratio. However, there is a thicker base gasket option that reduces the compression ratio to 8.8:1. I choose this route.

    The reason is that I was wanting the engine to be under-stressed. From what I've heard on these forums, the higher compression leads to detonation problems. I also just finished a Honda CB750 build with a high compression 850cc piston kit and was not completely happy with it. Tuning was very difficult in order get the spark knock under control. Among the many carb and ignition mods and adjustments, I wound up putting a longer duration, hotter cam in order to bring down the dynamic compression ratio. It is brutally fast but it still rattles at lower rpm's under heavy load, not very streetable.

    My R90/6 was kinda tired. It smoked heavily from the left side if started after sitting on the side stand for any length of time. A cold compression test revealed 155psi on the right and 130psi on the left. It was time. I bought the kit from Ted Porter, Beemershop. My only complaint was that it was backordered for at least 6 to 8 weeks. All told, with all the bells and whistles, it cost about $1300. While everything was taking its time getting here I also had the heads re-done. I replaced the exhaust valves and guides, had the seats re-ground, etc. Installation was pretty straightforward, nothing that I couldn't handle. I have no problem asking stupid questions so if I came to an area that I wasn't sure about, I called Ted Porter (about 2 or 3 times that day). He was completely and totally helpful and patient. Most of the questions that I had, I sort of already knew the answer but I just needed him to confirm. He is MORE than willing to help. Buy your kit from him.

    So how does it run? As you can tell from this thread I have been running around on it for a year with a bit too rich of a mixture. However the performance is still the same. With the low compression option it wasn't a massive shift in power, it was a modest increase. Instead, it moved the power band down the rpm range. It used to be that power came on at about 4000rpm or so. Now, it seems like it pulls strongly from idle. Another advantage is that it used to have a rather annoying resonant frequency vibration at 2500-3000 rpm; that is now gone. Overall the engine vibrates FAR less and is MUCH smoother.

    I have an 11/32 final drive on this that I swapped from a R100/7 (stock is 11/34) With the Siebenrock kit providing more usable power in the lower rpm range this setup is somewhat ideal. However I am going to swap it back to 11/34 for a while this summer just to see how the kit works with a stock setup.

    I can only imagine that if I had opted for the high compression setup it would be a fire breather. I think the low compression version gives real improvement for real world touring applications.

  3. #3
    Administrator 20774's Avatar
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    I got the Siebenrock kit as a replacement for my worn out top end on my /7. The BMW replacement parts were pretty costly...the kit was a savings. Lighter components...I weighed the old and new parts. Nikasil-like coating on the cylinder walls, so should last a long time. This arrangement helps to transfer heat easier away from the cylinder...the old steel liner just traps the heat.

    I could tell the difference right away, although that might be just going from a tired out cylinder/piston/head to new everything (head had new unleaded valves installed). But it sure felt great.
    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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    Cost for someone to do it for you would vary widely. Shop time is usually $65-$80 hr. I would NOT let someone do it for $25 hr and a couple of cases of beer. Once all of the parts arrived I was able to assemble it in one easy day. If you do not possess mechanical aptitude it is of course, best to find someone to do it for you. However the trickiest part of the reassembly would probably be compressing/installing the rings or reassembling the valve train correctly.

    Perhaps the best thing that an experienced mechanic would do for you other than putting it all together would be to evaluate the big picture. Do you need head work? Most tired bike engines need a valve job before they need pistons or rings. What is the condition of the clutch/transmission? Putting an 15%-25% increase in power through the rest of the drive train doesn't always turn out well if those components are also tired and weak.

  5. #5
    P Monk
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    running rich?

    are you still running the stock .150 jets and needle in second position? After my rebuild with original jugs I tried running with the needle in the first position, but since I live at sea level I went back to stock settings. Installing an Alpha ignition to mine was the biggest improvement to my R90. It now wants to run faster and smoother than ever.
    P. Monk
    74 R90/6 (also know by bride as the Black Hole). 09 R1200 GS. My wife, 1953 model who has survived aplastic anemia and a bone marrow transplant.

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    Right after I installed the kit I bumped the main jets up to 155. Gas mileage was poor, 32mph. Plug readings showed normal. After a year of crappy mileage I bought all new brass for the carbs including stock #150 main jets. I put the needle in the stock 2nd position but it felt too lean and gave a sort of surging throttle response. I raised it one more notch. Since then the mileage is back to where it is suppose to be, 42mpg. Plug color is still good. Performance is much livelier and crisp.

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    Ooops, also forgot to add that I installed a Dyna 3 ignition. It worked great.......that is until it left me stranded in the middle of the Kansas prairie in the middle of February. ALSO left me stranded on the shoulder of the main outer belt interstate in Kansas City during rush hour. Analog ignition points are cheap and easy. Plus, they are more prone to resist the electromagnetic pulse of a nuclear blast therefore rendering me post-apocalyptically viable.

  8. #8
    P Monk
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    emp proof points

    exactly why I will travel with a set of points to put back in. I got stranded once when a Dyna set screw slipped and again when an Omega went out. With the Omega I still had the points and advance mounted and it was just a matter of changing wires to coils. The alpha mounts to the crank but at least the brains are not under the engine cover and will be much cooler. I also put in a set of .155 main jets, but mileage has gone way down. I am going back to the .150 set. Interesting that you went richer on the needle position. Several of the more intellegent ones on the forum say that going from 900 to 1000cc causes richer running.
    P. Monk
    74 R90/6 (also know by bride as the Black Hole). 09 R1200 GS. My wife, 1953 model who has survived aplastic anemia and a bone marrow transplant.

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    Quote Originally Posted by dpmonk View Post
    exactly why I will travel with a set of points to put back in. I got stranded once when a Dyna set screw slipped and again when an Omega went out. With the Omega I still had the points and advance mounted and it was just a matter of changing wires to coils. The alpha mounts to the crank but at least the brains are not under the engine cover and will be much cooler. I also put in a set of .155 main jets, but mileage has gone way down. I am going back to the .150 set. Interesting that you went richer on the needle position. Several of the more intellegent ones on the forum say that going from 900 to 1000cc causes richer running.
    Actually I read the blurb that Kevin Cameron wrote in Cycle World a couple of months ago about that. A smaller intake charge equals less vacuum pulling fuel through the jets...larger displacement pulls more air and therefore fuel hence smaller jet size is required. In real time practice, it seems to work for me. As far as raising the needles, I tried getting used to it at the stock setting. At first I thought that I needed to let the bike warm up better. After about 1/2 hour of operation the lean surging simply was not improving. At 1/4 throttle settings (or less) on surface streets it was just too finicky. Raising the needles solved the problem.

    I have had a couple of bikes that I converted from contact points to electronic ignition. For the most part it is usually FAR more convenient to set the ignition once and never have to do it again. However I have noticed that a couple of Honda CB750's that I have built have a little less "roar" with CDI units. You can kind of tune it out of them but still....... I think also on the SOHC4.net there is an article that I read once that states analog ignition point produce a hotter spark when tuned correctly. The problem is, you're having to tune every 5k miles.

    I think the next step for me is the ignition amplifier. It flows FAR less current through ther point therefore not wearing them out as quick. Also if it fails, you can simply unplug it and go about your way.

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    I have a good mechanic, Peter Bombar of Bombar Beemers here in Durham, NC that will get the carbs right, but I want to hear more from those who have actually upgraded a R90/6 with Seibenrocks kits about the performance. If it is only a few HP and ft/lbs of torque, than I am not going to do it? On the the other hand, if it is really a significant difference, like going from a R90/6 to a R90S than please weigh in. Has anyone actually dyna-tested before and after the R90 to R100 Seibenrock kit? Seibenrock claims 10 HP and 30% increase in torque. That seems optimistic by only increasing the CC's by 100. Please advise.

  11. #11
    Registered User Bob_M's Avatar
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    My understanding is that the only mechanical difference between the R90/6 and the R90S is the carb. The top speed was virtually the same, but the S could accelerate a bit stronger. Don't forget that putting significantly more power into a R90/6 will reveal weaknesses in the frame, the suspension and the brakes.

  12. #12
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    Quote Originally Posted by Bob_M View Post
    Don't forget that putting significantly more power into a R90/6 will reveal weaknesses in the frame, the suspension and the brakes.
    And rider...
    Jim
    '78 R80/7 and '84 R100RS (Blues Brothers), '86 K75C (Icy Hot)
    '90 and '93 Mazda Miatas (Jelly Bean and Red Hot), '02 325ci (Blue Streak)
    '96 Giant Upland (big Kendas & freshly greased bearings!)

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    The difference between 90/S and non S is, 33/11 rear gear, 9.5 CR, accelerator pumps on the carbies, and I think a slightly different trans ratios. If held WFO an S should out pull a standard on the top end.

    It's all in the head of the rider/owner. And what they compare it to. A well tuned S with a few tweeks could likely outrun a later RS, IMHO. It's about what the bike will finally pull against the wind.

    It's all relative CR being the biggest, but then you gotta think about lousy gas and pre ignition! Old bikes, what would we do without them? The faster it went, the older we're bent.

    Oh, and don't forget to get the speedo re calibrated. With a three to one rear gear, 4K rpm yields about 65 mph. A 3.09, 3.20 and so on down the line yields less. This could all be wrong as I'm not a guru. FWIW, 2cents.
    Last edited by 8ninety8; 05-19-2015 at 04:26 PM.

  14. #14
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    Seibenrock kit for the R90/6, $1300. Stock BMW parts, oversized pistons, rings, wrist pins, etc, maybe as high as $500 or more....NOT including machine shop work such as boring or machine honing ($100-$200). With the Seibenrock kit you get modern metallurgy, Nikasil cylinders as opposed to plain old cast iron.

    If you're concerned about real world performance consider that steadfast reliability is perhaps the best performance attribute to have. Nikasil cylinders are pretty impervious to wear. Most people that I have heard talk about it say that it basically brings your airhead into the 21st century (maybe beyond?) and makes it a "forever bike."

  15. #15
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    I did this on my R110S

    by Bombars about 2 weeks ago. Seems to run much smoother and stronger. Of course I only have about 350 miles on the rebuild

    sdc
    Enjoying the ride, but always on the alert for a rally.......

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