BMW R90S upgrade

[otto_kretschmer]

I bought my R90S 33 odd years ago and I'm finally got the time to do a restoration back to its original Daytona orange and silver. When I bought the bike, it was crashed several times by the previous owners and didn't have the original paint or fairing. It had a nasty looking Luftmeister fairing and was painted black. I ditched the Luftmeister a long time ago and made it a standard motorcycle.

The bike is generally in good mechanical shape except it needs the 2nd oversize pistons. I had a KN airfilter and it eventually got hard and stiff and didn't seal and dirt got thru and wiped out the cylinders cross hatching and it now burns oil and smokes.

I rebuilt the motor with new mains a decade ago. I had the tranny open when I was in college and put in a shift kit and new tranny bearings. I had Brown Motorworks in Pamona rebuild the final drive. I had the flywheel lightened when I did the engine.

Years ago I rode a friend's R80GS and noticed his bike runs significantly smoother. Like glass and I'd like to make my R90S run as smooth as the R80 if possible.

There really isn't much I can do except to buy lighter parts and there is a place that sells them.

Motoren Israel is a shop in Germany that specializes in airheads and I already need pistons, I'm considering adding connecting rods to my list.

They sell Wossner pistons and claim they are 190g lighter than stock (516g/712g).

They also had H beam rods and say they are 450g and stocks are 600g.



https://www.motoren-israel.com/

anyone think the rods are worth the extra $800?

If anyone has rode a R80GS or RT or ST should know how smooth those engines are.

Its about 1.5 pounds of reciprocating mass reduction. It almost seems too good to be true but the original parts were designed 50 years ago.

[speeder]

I ride an old BMW airhead boxer and honestly, it's pretty smooth in the rev ranges where it lives. The redline is ~7k and it only hits that going through the gears once in a while. I'd personally concentrate on getting the tune balanced as well as humanly possible and maybe micro-balancing the reciprocating components.

Don't get me wrong, those are beautiful parts but I don't think that you will get a noticeable difference in feeling or performance from them.

[otto_kretschmer]

thanks Denis,

I have 3 balance tools. I bought mercury sticks when I was in college and I made a home made oil filled manometer and a year ago I bought a (Grok) Harmonizer from Kat-Dash. I still havn't used the Harmonizer but I had to buy one when I saw they were available again.

I agree with you that I probably wouldn't notice any difference with the lighter rods but $800 isn't a huge number.

I have a R50/5 that I turned into a 600 with Wossner pistons. I still think the R80 still runs smoother but its been a while since I've rode one.

When I rebuilt the motor I had the rods checked for straightness so I know the rods are still good.

[Nostril Cheese]

I think youll feel more from the flywheel than anything else. These engines were built to be smooth. Seems like a waste of $800 to me.

[flatbutt]

Otto, you may find a deeper knowledge base on this page:

BMW R1100S / R1200S Tech Forum - Pelican Parts Forums

It may say R1100S but there are alot of knowledgeable riders there. Plus the R90S is the progenitor of the R11S.

[otto_kretschmer]

Originally Posted by flatbutt

Otto, you may find a deeper knowledge base on this page:

BMW R1100S / R1200S Tech Forum - Pelican Parts Forums

It may say R1100S but there are alot of knowledgeable riders there. Plus the R90S is the progenitor of the R11S.

The first generation of oil heads sounded like sewing machines. They are good bikes but I was never a fan of them. The best bikes BMW made were the mid 70s /6 bikes.

[flatbutt]

Originally Posted by otto_kretschmer

The first generation of oil heads sounded like sewing machines. They are good bikes but I was never a fan of them. The best bikes BMW made were the mid 70s /6 bikes.

I did luv my ol' R80RT.

[otto_kretschmer]

Originally Posted by flatbutt

I did luv my ol' R80RT.

I could have bought one about 15 years ago for $1500. 2nd owner. I'm still kicking myself for not getting it.

[carrera84]

Originally Posted by otto_kretschmer

I bought my R90S 33 odd years ago and I'm finally got the time to do a restoration back to its original Daytona orange and silver. When I bought the bike, it was crashed several times by the previous owners and didn't have the original paint or fairing. It had a nasty looking Luftmeister fairing and was painted black. I ditched the Luftmeister a long time ago and made it a standard motorcycle.

The bike is generally in good mechanical shape except it needs the 2nd oversize pistons. I had a KN airfilter and it eventually got hard and stiff and didn't seal and dirt got thru and wiped out the cylinders cross hatching and it now burns oil and smokes.

I rebuilt the motor with new mains a decade ago. I had the tranny open when I was in college and put in a shift kit and new tranny bearings. I had Brown Motorworks in Pamona rebuild the final drive. I had the flywheel lightened when I did the engine.

Years ago I rode a friend's R80GS and noticed his bike runs significantly smoother. Like glass and I'd like to make my R90S run as smooth as the R80 if possible.

There really isn't much I can do except to buy lighter parts and there is a place that sells them.

Motoren Israel is a shop in Germany that specializes in airheads and I already need pistons, I'm considering adding connecting rods to my list.

They sell Wossner pistons and claim they are 190g lighter than stock (516g/712g).

They also had H beam rods and say they are 450g and stocks are 600g.



https://www.motoren-israel.com/

anyone think the rods are worth the extra $800?

If anyone has rode a R80GS or RT or ST should know how smooth those engines are.

Its about 1.5 pounds of reciprocating mass reduction. It almost seems too good to be true but the original parts were designed 50 years ago.

They doing BMW old-school tuning for a very long time. So I think they know what they do.

Had a R90S, first series, as well, but sold (and badly regret)

[pavulon]

Get 'em or live with the regret.

[speeder]

Originally Posted by otto_kretschmer

thanks Denis,

I have 3 balance tools. I bought mercury sticks when I was in college and I made a home made oil filled manometer and a year ago I bought a (Grok) Harmonizer from Kat-Dash. I still havn't used the Harmonizer but I had to buy one when I saw they were available again.

That's a nice tool but my analog version works just as well. I tried to watch the video but found myself yelling at the monitor...what's wrong w this guy? Ok, I should not single him out but do people get paid by the minute for posting YT videos?

He babbles for 10 minutes before even hooking it up to his bike and the whole ~20 minute video has a grand total of about 2 minutes worth of information in it. I just lose all patience w these types of offerings...it isn't even like they are going slow to explain more for idiots...they just babble. And the guy isn't good at tuning motorcycles, he should not be spreading stupid.

Oh yeah, GTFO of my lawn!

[Rot 911]

As the owner of several airheads, presently a 1965 R60/2, the higher you go up in displacement, the more vibration you’re going to feel. The 750s, in my opinion, are the smoothest of all the airheads. If you feel better spending $800 on those rods, go ahead, But I don’t think you’re going to notice any reduction in vibration. And it’s not like they vibrate a lot to start with. As for synchronizing carburetors, I have found the Carbtune, to be the easiest synchronizer to use. Comes out of Ireland, but they ship quickly. https://www.carbtune.com/

[304065]

Kurt's point is a valid one- because the crank throws are offset in an airhead, there is a rocking couple that is significant the larger the displacement. The new R18 does this on purpose, for example. That will contribute to felt vibration

Seems to me that the Motoren-Israel gear is all focused on 1000CC bikes, the theory being that the greatest bang for your buck is to increase displacement to what the Factory did and then take it from there.

Anyway if it's smoothness you want I would consider an equal expenditure on balancing. Here's a good thread on the topic.

https://advrider.com/f/threads/balancing-a-bmw-engine.420346/

[otto_kretschmer]

everyone, thanks for the replies

I'm not going to remove the crank to balance it. Its not worth the hassle.

I may buy a new stock cam and lifters. When I rebuilt the motor I thought I'd hot rod it with a 336 cam and lighten flywheel. The 336 cam had more power but made it unpleasant to ride with a shudder/flat spot at 3k to 4k rpm. I decided that I didn't want a science project so I put the old, original cam and lifters back in and I'm much happier for it.

This time I may buy a new, stock, cam and new lifters. The old cam and lifters were worn but these bikes will RUN and RUN and RUN... even with old, worn out parts. They are really understressed. Just keep changing the oil on time and make sure the valves are adjusted properly.

as an FYI, BMW modded the crank to address the couple on the vertical axis created by the offset pistons. On one counterweight web (I don't remember which one) they drilled two holes in the web, about 1" dia and filled the holes with a slug of tungsten. The tungsten weights are staked so they are not to be removed.

I woudn't try to balance the crankshaft and if some machine shop claimed they could do it I would have a decently long conversation about the tungsten inserts before I handed over my parts.

[carrera84]

Originally Posted by pavulon

Get 'em or live with the regret.

Agree

Bought one yet again and can sleep better now.

[Chocaholic]

A great bike and a wise investment.

[otto_kretschmer]

I havn't touched the bike yet. I keep getting sidetracked on other projects. The most current sidetrack is my Austin Healey Sprite.

[Dixie]

Could you please post a photo of the bike? I think we'd all like to see it. I know I would.

[speeder]

Originally Posted by Rot 911

As the owner of several airheads, presently a 1965 R60/2, the higher you go up in displacement, the more vibration you’re going to feel. The 750s, in my opinion, are the smoothest of all the airheads. If you feel better spending $800 on those rods, go ahead, But I don’t think you’re going to notice any reduction in vibration. And it’s not like they vibrate a lot to start with. As for synchronizing carburetors, I have found the Carbtune, to be the easiest synchronizer to use. Comes out of Ireland, but they ship quickly. https://www.carbtune.com/

This right here. ^^^

I think that the smoothest old airhead was the R60/5. My neighbor got a new one in about 1969 and was showing off by balancing a coin on fuel cap while it was idling. That memory sticks with me.

[otto_kretschmer]

So as I wait for the machine shop to finish the Sprite head I started putting the motor back in the 911.

so the bike..



so the order is going..

get the 911 running

get the Sprite running

paint the Sprite and do all the cosmetics

restore the beemer