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Very Hot Topic (More than 25 Replies) That good old hanging rpm topic again (Read 1539 times)
Barry
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Re: That good old hanging rpm topic again
Reply #45 - 10/09/17 at 06:18:54
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Hard to know what to do with the new springs. A delayed maximum advance is not best for power but if it cures the problem. Maybe they'll improve with age. 

Was there any slack in the way the old springs fitted ?
  

Barry Cheshire, England 79 R45
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wilcom
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Re: That good old hanging rpm topic again
Reply #46 - 10/09/17 at 06:43:01
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tunnelrider wrote on 10/09/17 at 01:25:42:
full advance is not achieved until now 4000rpm,


In your previous post you stated that your advance was spot on.
" once timing has been set at idle?  If so mine is ok"

If the original springs were doing their job they should look like the new springs just older (if springs gain patina with age).

If the old springs performed as they should and now you have installed new springs that physically look different and the advance is bonkers, MY BET is that they have sent you the wrong springs.

I'm not much as a mechanic, but I can hold my own with wagers Cool


  

Joe Wilkerson
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Barry
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Re: That good old hanging rpm topic again
Reply #47 - 10/09/17 at 08:08:51
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The new springs are obviously stronger so the bob weights can't raise sufficient centrifugal force to extend them to the stops until 4000 RPM.

So how much are the springs too strong ? 

Well it's not 1000RPM/3000RPM or 33%.  The relationship between RPM and centrifugal force (or centripetal force if anyone wants to be pedantic) has a square function in it so the spring is more like twice as strong as it should be.

Wilcom could be right, they sent the wrong springs.
  

Barry Cheshire, England 79 R45
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georgesgiralt
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Re: That good old hanging rpm topic again
Reply #48 - 10/09/17 at 10:09:03
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Errrr, Barry,
My guide says that the maximal advance is at 3500 RPM So a little less than the 4k observed...
  
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Barry
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Re: That good old hanging rpm topic again
Reply #49 - 10/09/17 at 12:14:23
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My riders handbook and BMW workshop manual both say maximum advance is at 3000 RPM. 

Perhaps it changed to 3500 RPM for the electronic ignition models.  If it did then ideally there should be two spring rates available.  Motobins only supply one type of spring which would be a compromise even if accurate.   Which brings us to the point that there are no genuine BMW spares for the bean cans. Any springs available are after market parts which may not have been accurately reproduced.  To check I would measure the old and new springs. The spring wire diameter, number of coils and overall spring diameter should be the same. If they are then they both have the same spring rate and it can only be the overall length that is causing the problem.
  

Barry Cheshire, England 79 R45
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Mrclubike
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Re: That good old hanging rpm topic again
Reply #50 - 10/09/17 at 19:46:21
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Take it back apart and give the spring anchor post a little bend in towards the center

I replaced mine with the springs Moto bins sells and they are working fine after I gave them a little tweak
  

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Justin B.
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Re: That good old hanging rpm topic again
Reply #51 - 10/09/17 at 20:27:20
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I must have missed something but why were they changed if the timing advance was spot on?
  

Justin B.

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tunnelrider
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Re: That good old hanging rpm topic again
Reply #52 - 10/09/17 at 22:06:37
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Justin B. wrote on 10/09/17 at 20:27:20:
I must have missed something but why were they changed if the timing advance was spot on?
                   

Because the revs took a long time to return to idle once the engine is hot.  Cold yes the timing advance works fine.  Sorry I've not had the chance to reply to any of your good replies yet. My rider's handbook that is the bike's original book says full timing advance at 3000rpm, so yes, apart from the slow drop in revs once hot it was working as it should.  But it was not working as it should when at normal operating temp.
Probably not much use, but for the experience, plus the fact I haven't test ridden it with the new springs put on 'straight off the shelf' I'm just about to take it for a test ride. I expect there will be a hit in low end performance.

Mrclubike wrote on 10/09/17 at 19:46:21:
Take it back apart and give the spring anchor post a little bend in towards the center


Aww taking that thing apart and putting back together is about as enjoyable as going to the dentist!
  

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Justin B.
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Re: That good old hanging rpm topic again
Reply #53 - 10/09/17 at 22:24:14
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In my experience the slow return is crud/rust causing the weights to be sluggish/sticky.
  

Justin B.

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tunnelrider
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Re: That good old hanging rpm topic again
Reply #54 - 10/10/17 at 02:20:27
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So a good hilly test run proved the new bean can springs are too tight, the bike was pretty sluggish increasing revs from 2000rpm especially in 4th/5th under load.  There's not a massive difference but my bike needs all the performance it can get.  On the plus side, the revs do drop back to idle properly!

When I got back I checked the timing again, advance with the new springs starts at approx 2000rpm and finishes at just over 4000rpm.  So the bean can will have to come off and entered again.  Firstly though I might try mucking around with the timing and see if I can set full advance at 3000rpm.

Mrclubike wrote on 10/09/17 at 19:46:21:
Take it back apart and give the spring anchor post a little bend in towards the center

I replaced mine with the springs Moto bins sells and they are working fine after I gave them a little tweak

What was your experience with the Motobins springs when you installed them Mrclubike?  Were they too tight as well?  By bending the spring anchor post in towards the center a little I presume you are decreasing the length of the spring.  Do you mean bending the anchor post on the weight arms?  How much did you bend it, about 1mm?  And have you got your advance starting at 1500rpm and finishing at 3000rpm now?

I'm half tempted to put the old ones back on and cover the slower decrease in revs to idle by dropping my idle speed below 800rpm, which pretty much does the trick especially for open road riding.

I don't think they are the wrong springs they sent, I doubt if they sell other springs like these.


  

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Barry
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Re: That good old hanging rpm topic again
Reply #55 - 10/10/17 at 05:01:53
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After looking at them on the Motobins site I'd agree they have not sent the wrong springs as the other pre-bean can springs they stock are quite different.  It must just be they are after market springs and too short which will cause your problem even if they managed to get the  spring rate correct. Advance not beginning until 2000 RPM is a sure sign they are not right as BMW say advance should start at 1500 RPM.

I'd be inclined to refit the old springs provide there is no slop in the bob weights once fitted. You could check at what RPM advance then begins.  If there is slop because the old springs are too long then you have to aim in the middle either by bending the spring anchors or stretching the new springs.  Stretching springs is tricky to say the least and risks ruining them so bending the anchors may be safer.
  

Barry Cheshire, England 79 R45
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georgesgiralt
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Re: That good old hanging rpm topic again
Reply #56 - 10/10/17 at 05:06:17
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Apologies,
I've just looked at the factory course material for model AFTER 1985.
They say that the maximal advance is achieved "around 3000 RPM" and for a total advance of 32° before TDC.
So maybe your settings are not that bad regarding the 3000 RPM but too slow the slope considering your riding results... Are you sure of the static setting ?
  
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Mrclubike
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Re: That good old hanging rpm topic again
Reply #57 - 10/10/17 at 19:39:04
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I would try bending them about 1 mm

I cant remember exactly what I had when I started
I had them bent out for the old springs so I had to bend them back for the new springs

You will not have to disassemble the shaft just remove it from the housing

I am wondering if it can be gotten to thru the little window if you knock out the cover

Bosch had to tune them some how
  

window.jpg ( 130 KB | 6 Downloads )
window.jpg

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tunnelrider
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Re: That good old hanging rpm topic again
Reply #58 - 10/10/17 at 22:36:03
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Mrclubike wrote on 10/10/17 at 19:39:04:
I am wondering if it can be gotten to thru the little window if you knock out the cover

Bosch had to tune them some how


Now there's a thought Mrclubike. If possible and not too fiddly to do it would sure beat pulling it apart again. What's going on with your extra big window in your photo above by the way?
Barry the old springs did not have any slop.  Unfortunately I did not test the timing advance start and finish rpm when the engine was hot with the old springs, only cold, but I'd say it was still about where it should be when hot.
You could say, based on this, that there was some crud holding the weights up but it was pretty clean in there and the weight pivots were definitely loose enough to not cause any problems.  It is tempting to think that just by opening it up it may've dislodged something that was causing the problem which is possible so I should refit the old springs.
So what to do next.  Firstly I'm still going to try setting full advance to 3000rpm and see what that does.
Presuming that is no good I've got a couple of options - try and work through the window to push the spring anchors inwards a little, probably very fiddly.
Maybe take it apart and put the old springs on again.  Maybe take it apart and push the anchors in a little.  Whatever I do, I don't really want to be going in there again more than once, I've also got a four day bike trip coming up in a week.  I'm going to read Snowbum's thoughts on it all too.
 
  

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tunnelrider
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Re: That good old hanging rpm topic again
Reply #59 - 10/11/17 at 02:52:52
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Time for some more useless info.  Trying to set max advance at 3000rpm didn't work, as turning the bean can to the stops anti clockwise did put the advance line in the timing hole at 3000rpm but further revs caused it to disappear above the hole.
It does seem like it'd be possible to bend the spring anchors through the inspection window, one way would be to chock one weight arm spring anchor against the end of the hole with a screwdriver or something then turn the driving dog against it.  Not exact science by any means. I didn't attempt it yet.  But my bean can does have some existing marks that suggest this might've been done before but these may be from a previous entry too, before mine.
Since you all appreciate pictures, below are the original springs.  One has stretched approx 1mm more than the other, perhaps giving my original problem of slow return to idle when hot.  Or maybe a PO had stretched them, who knows really.
  

original_springs_1.jpg ( 77 KB | 4 Downloads )
original_springs_1.jpg
original_springs_2.JPG ( 139 KB | 6 Downloads )
original_springs_2.JPG

 1985 Black R65  -  2001 DRZ400 dirt only
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