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jstewart
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Joined: Wed Jun 09, 2021 3:30 pm

New to this site

Post by jstewart »

I am buying a new to me a air head R90S. Always wanted one of these bikes. This bike is somewhat of a mongrel in that it is not a pure original R90s. The frame is a 1977 R100 which is supposedly stronger, the cylinders are from a R90/6, The heads are from a early 1980's R100, Carbs are the later 32mm CV's. The color is Daytona Orange. There are other updated parts such as the top triple tree, Dyna 3 ignition. Pistons are R-90s with updated Hastings rings. Stated compression ratio by current owner is (9.2/1) but my mechanic doubts it is that high with the larger chamber R-100 heads. Plan on buying replacement 38mm Delorto carbs from Moteren Inrail as well as possibly a Seibenrock cylinder and pistons. I would have preferred the silver color but the price is within my budget on this bike.

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schrader7032
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Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX

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Post by schrader7032 »

Welcome to the forum! I guess you're right on "mongrel"! I guess people will recognize the paint color but not much else will speak to the R90S! If the heads are early R100 type then you're probably right, the compression is likely around 8.2:1. What about the bottom end...what is the engine case like? Do you have the 1000cc crank and cam? You will have to work to find the right tune for your marriage of parts...hopefully it will be a strong runner.

I guess the two R90S color schemes are in the eye of the beholder. I prefer DO.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

jstewart
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Joined: Wed Jun 09, 2021 3:30 pm

Re: New to this site

Post by jstewart »

Thanks for the reply. Actually it looks very much like a R90S until you look more closely. The DO paint is new and everything else is fresh including the repainted frame. Based on owner description these are the particulars:
Frame--77 R100S
Engine--78 R100 cases, crank, rods, with flywheel lightened and balanced
Transmission--77-R100 torn down inspected and new shifting components
Carbs--32mm Bings
Heads--R100/7--not sure what year but they have the rocker arm mounting scheme used used until end of production in 1995
Cylinders--R90/6--with R90S pistons and Hastings rings
Final drive ratio--3.0:1 (33/11) same as R90S.
Wiring harness--78-R100RS
Instruments--R100RS ( speedometer, electronic tachometer, volt meter, quartz clock)
LH switchgear--R90S
RH switchgear--R90S
Top starter/engine cover--R90S
Mirrors--short black from 85 from R80
Exhaust--not sure but probably reproduction, looks new
Front brake rotors--Italian made Gramicca
Calipers-- R90S rebuilt
Master cylinder--85 15mm handlebar mounted BMW direct master cylinder
Rear shocks--Works Performance street tracker
Front forks--stock with RS springs and a 2nd anti dive spring in the last 1/3 of travel
Top fork tube support is from San Jose BMW and adjustable Telifix adjustable upper fork slider
Front wheel--Brand new spoked type used on all 1981 bikes
Rear wheel--OEM rear R90S with new spokes, bearings, polished
Tires--New Continental high performance Bias type

That's about all I know right now until I get my hands on the bike. It looks brand new in pictures. My definite plans for the future are new Delorto 38mm carbs and to raise the displacement from 900cc to 1000cc's and the compression to 10/1. Dual plugging the heads is probably also in the future when the Moteren Isrial 1000CC kit is installed. Not sure what else until I get a feel for the bike. :D

jstewart
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Joined: Wed Jun 09, 2021 3:30 pm

Re: New to this site

Post by jstewart »

Found out today this bike was not built by the current owner. He had a friend who worked at San Jose BMW in California. They built the bike to his specification. He then picked it up and rode it back home to the central US. It was built up out of what they considered the best components from the R series 900 & 1000 parts. :D

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schrader7032
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Joined: Fri Oct 27, 2006 3:00 am
Location: San Antonio, TX

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Post by schrader7032 »

A true marriage of parts! That will be quite the "dance" with the parts guy when trying to get replacement items...can't just go with the VIN all the time.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

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srankin
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Joined: Fri Jul 14, 2017 8:45 pm
Location: Spencerport, NY USA

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Post by srankin »

Hello welcome as well.

You have a mix of stuff that for the most part will work well together or not. Before you order parts like the cylinders and such, double check with the parts microfiche the connecting rods on the bike are the proper units for a 1000CC engine. I believe but am not 100% sure the connecting rods for the R90S and the /7 100cc bikes are the same. This will be important as the Siebenrock kit does not have them as part of the kit. Installing the kit will bring you to a better power output due to higher 9.5 to 1 compression which is better than the US tuned bikes at 8.5 to 1. Again I am not certain but this is the same as the R90S compression ratio but, I could be wrong, the R90S may be as high as 10 to 1. 10 to 1 compression with todays gas could be a bigger pain than it is worth, 9.5 to 1 works well as long as premium gas is used and the ignition timing is not too far advanced.

I know the R90S bikes had Delorto carburetors on them for the three years the bikes were built. The only advantage of the Delorto carburetors over the CV Bing carbs is the Delorto carbs have an accelerator pump built in to give a bit of an extra kick on acceleration. In all honesty, from my riding an R90S and a 78RS, the difference in acceleration is minor on the R90S. I guess what I am saying is, there is no real need to spend the money on replacing the CV carbs unless the tiny difference is of vast importance to you. Both carbs work very well and are very reliable. I have a theory the only reason BMW didn't continue to use the Delorto carbs in the higher performance bikes was simply because they cost BMW more to buy than the Bings and the average rider wouldn't notice the difference or care about the slight difference in performance. That is another topic of discussion no for here.
Another change to make depending upon how much you want to spend is to find a set of early year 1000cc heads with the bigger Bing CV carbs. This would be the 77 to perhaps 80 years, a check of the microfiche would confirm this. The bigger carbs breath better than the smaller ones you have on your bike now. The problem with this of course is the bigger heads are NLA so used ones will have to be used. Also, bigger carbs will have to be found.

Anyway I apologize for the long winded note. There are a lot of things you can do to improve performance and such but to what cost? Like Kurt said, it it can become a dance with the parts guys, hopefully a merry dance. Good luck, St.
Owner of a 84, R80RT and 78, R100RS

jstewart
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Joined: Wed Jun 09, 2021 3:30 pm

Re: New to this site

Post by jstewart »

Thanks for the input stankin, not to long at all. I would not have even considered this if I didn't have a really good BMW airhead mechanic in my area. Gunther Wuest has his shop in Fredricksburg Indiana which is only 45 miles from me. I intend to take the bike directly to Gunther for a complete evaluation and service before taking it home. What I do to the bike will lean heavily on Gunther's advice. Gunther has restored many R90S bikes and in fact has 2 in his shop now. The mongrel will be in good hands with Gunther. 8-)

jstewart
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Joined: Wed Jun 09, 2021 3:30 pm

Re: New to this site

Post by jstewart »

Update: I found and bought a new set (2) of 38mm Dellorto carbs PHM38BSI from a trusted seller. Cost for these new carbs was
$499.00 plus tax & shipping. Best price I could find. Gunther had a couple of used sets but I don't want to start out with used carbs. 8-)

jstewart
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Joined: Wed Jun 09, 2021 3:30 pm

Re: New to this site

Post by jstewart »

Thanks guys for the input, I go to pick up the bike the 28th of June. It will go straight to Gunther for a complete checkout from there. I will post my progress with the mongrel. :D

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srankin
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Location: Spencerport, NY USA

Re: New to this site

Post by srankin »

Js, I don't want to be a jerk, but technically your bike is not an R90S. LOL, it doesn't bother me what you call it but some of the die hard R90S people get real touchy about the R90S.
No matter, you have a fine bike, enjoy it and make it what you want. St.
Owner of a 84, R80RT and 78, R100RS

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