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R90s

arptra

Member
R90S Timing issues

Just picked up a 74 R90S it is nice but it doesn't seam as responsive as my 76 R90S. Does anyone here know if the cams ETC are the same for both years?
 
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There was a part number change for the cam in '75, don't know what the mechanics involved were, if any. Kent or Paul probably do.

Have you been through it yet? Valves, points, carbs, timing, etc.
 
+1 to the above. Plus be sure you have a clean air filter. Basically, do a good tune up and see where you stand.
 
Any standard airhead from the period needs some upgrades, unless you are collecting pristine stock collector bikes. I got a '74 R90S which is hardly recognizable from stock. In my mind every modification has been an improvement.

It's possible a previous owner switched cams on the '76. In fact, it's possible a lot of swaps have been done which wouldn't visible to the eye.

Just a few general comments, remember, an R90S is no big deal from the standard 900 except for few trim items, a 3 to 1 rear gear, 9.5 compression ratio, couple of gauges, that's about it. Trannies were weak on the '74s, but you can always update that item. However, if you treat the tranny easy, always clutch it no power shifting, it'll goo along time.

I've owned mine since '86, it's my favorite bike, and I've owned a few. Just working on it this afternoon, for fun. Good luck, enjoy. Cheers. Thank you BMW.
 
just removed bowls and carbs look to be recent rebuild, yeah I will go through it I was just wondering about the cam if they are different. My 76 would run circles around this one at this point. I see the old push Rod seals will need fixed as usual.
What about those dellorto's , was thinking pumps may need adjusting. Tranni feels good and tight.
 
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How many miles on the OD, that could be a clue to the condition of the engine and it's performance.
 
Odometer may have flipped over 100K miles, thus, 118K. 18K is very exceptionally low mileage for a '74 anything.
 
I really don't think so, tach , speedo and volt work perfectly I have a few and my other R90S is on up there on miles and you can tell a difference. I have a 84 R100 with 34K pd with 20k and another PD with 49k so I have a few to compare it to.
 
you are right though, it would be a long shot to find one with that low of miles. I done a vin check and they said not enough digits
 
Any standard airhead from the period needs some upgrades, unless you are collecting pristine stock collector bikes. I got a '74 R90S which is hardly recognizable from stock. In my mind every modification has been an improvement.

It's possible a previous owner switched cams on the '76. In fact, it's possible a lot of swaps have been done which wouldn't visible to the eye.

Just a few general comments, remember, an R90S is no big deal from the standard 900 except for few trim items, a 3 to 1 rear gear, 9.5 compression ratio, couple of gauges, that's about it. Trannies were weak on the '74s, but you can always update that item. However, if you treat the tranny easy, always clutch it no power shifting, it'll goo along time.

I've owned mine since '86, it's my favorite bike, and I've owned a few. Just working on it this afternoon, for fun. Good luck, enjoy. Cheers. Thank you BMW.

wasn't it just the kick start were weak?
 
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The first year 5-speed was also suspect...hence the treat it easy suggestion. Parts in this transmission are NLA, so it if needs to be rebuilt at some point, it will require more research.
 
'74 Cluster Gear

One of the major suspects of the '74 transmision was the dogs on the layshaft/cluster gear. That the the biggest gear cluster in the tranny. The gear slides back and forth by the shift fork. Those dogs were casted and forged but not machined to an exact fit. The design was for all four dogs to be engaged equally to distribute the load rotationally. Some did, others not. You could have just one engaged and the other three, free spinning. The one dog would shear off by fatique. Then, the process starts over. You coud have 1,2,3 of the remaining dogs engaged, or not. A ticking bomb, crapshoot. My guess if you lost just one it would work, OK; then maybe not. On dissasembly of my '74 back in '90 I had clanging in the tranny. Removed the drainplug and out fell one dog. Took the tranny apart and found the 2nd broken dog. No telling how long the bike was ridden like that. I bought a used cluster gear and replaced; it was expensive. Some of the early '75's probably got this same weak set up. Hope this helps. The upgraded trannies have a pronunced rib running for/aft about mid way down. The dogs in those trannies were machined to an exact fit. The other weak point was the shifter spring. Another story.
 
I think that you're a little off base with the camshaft thing. In 76 BMW went to a cam with a widet front end that required a different front cam seal with a larger id. The cam profile didn't change.
I know that the other 76 models came with different rear drive ratios so that, for example, a 75 R90 came with a 3.09:1 ratio. The 76 model had a 3.20 ratio. Might want to check that on your bikes.
 
thanks for the info,,

OK, checked timing and it was set on the F dot, tried to get it to S and not enough room on point plate? points were set at .16 I moved to .19 and then .10 I get closer with 10 but not there yet, Any idea what that will be?
 
Going smaller on the gap should retard the timing. I'm not sure I'd go much below 0.010". While it shouldn't be necessary, some take off the points backing plate and extend the holes for more rotation. What I would do first is try a brand new set of points if you haven't done that yet.
 
Going smaller on the gap should retard the timing. I'm not sure I'd go much below 0.010". While it shouldn't be necessary, some take off the points backing plate and extend the holes for more rotation. What I would do first is try a brand new set of points if you haven't done that yet.


that would make sense on point gap because that is the direction I need to go, problem is it doesn't help it run any better, seams to run best on F at idle but not as it should and is still inconsistent.
yeah I set it at .10 because I read where one guy had to set his R90S there to get it in time.
 
Strange that the bike runs best with F at idle. Have you confirmed that the TDC mark is in the window when the piston has moved all the way out?
 
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