Author Topic: Unstable RPM during carb tuning  (Read 445 times)

Offline Bill Parker

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Unstable RPM during carb tuning
« on: December 05, 2020, 01:45:37 PM »
I encountered an interesting problem with my 1984 R65 while trying to use the shorting method to sync the carbs. I find it almost impossible to set the throttle such that the engine holds 2500 rpm. It either revs to 3500 or drops to 1500, but won't stay at 2500. Setting it at 3500 and shorting one side yields a progressive drop to 1500 which then fails to regain 3500 when the short is removed. Is this normal or do I have problems with carb settings or the advance mechanism? Thanks.
1984 R65, 2005 R1200ST

Offline dogshome

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Re: Unstable RPM during carb tuning
« Reply #1 on: December 05, 2020, 05:19:27 PM »
This one has electronic ignition and it is advised not to use the shorting method.

Mine ran ok with slightly leaking choke gaskets, heavy floats and a leaky carb top! With fully rebuilt carbs she is very smooth though. They will run around idle with a multitude of problems. My ignition was also several degrees retarded and the engine didn't want to stop.

What caused you to try and balance the carbs? If it was hanging onto revs, then worn advance is said to do that. I'd like to know when the carbs were last overhauled first though!
« Last Edit: December 05, 2020, 05:21:53 PM by dogshome »
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Offline mrclubike

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Re: Unstable RPM during carb tuning
« Reply #2 on: December 05, 2020, 07:14:11 PM »
I am pretty sure its ok to short the secondary out
What you definitely DO Not want  to do is kill the cyl by removing the plug wire 
I think the manometer method is safer
That spark can knock the crap out of you  :beehive:
Kind of sounds like you have other issues going on
Sticky advance or carburetor issues 
1982 R65 running tubeless Snowflakes
2004 R1150R

Offline Barry

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Re: Unstable RPM during carb tuning
« Reply #3 on: December 06, 2020, 05:18:08 AM »
I think you may be aiming a little high at 2500 rpm.  The objective is to balance the throttle cables pulling at the same time so any speed above idle will work and the adjustment will be more sensitive at say 1500 rpm. Worth a try.

As an insight into the problem, whatever revs you use to do the balance, the carbs will still be on the idle circuit because the engine is under no load.  So the idle circuit including the transition ports is the place to look for issues.
 
« Last Edit: December 06, 2020, 05:26:53 AM by Barry »
Barry Cheshire, England 79 R45

Offline Bill Parker

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Re: Unstable RPM during carb tuning
« Reply #4 on: December 06, 2020, 04:39:01 PM »
The carbs were completely overhauled a year ago. I resync the carbs after every valve adjustment and usually use a vacuum device. However I watched a video in which Matt Parkhouse advocated using the shorting method to balance cylinder power instead of vacuum. Since my R65 is a bit more "vibration" prone compared to my other BMW, I'm always looking for a way to make it smoother. I was just asking to see if any other R65 owners had noticed the same RPM instability between idle and ~3000 or if this was an indication of a problem.
1984 R65, 2005 R1200ST

Online Tony Smith

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Re: Unstable RPM during carb tuning
« Reply #5 on: December 06, 2020, 05:18:08 PM »
The plug shorting approach to  carb balancing is a hangover from days gone by. You can build your own manometer for under $10 or buy a set of gauges or sticks for under $100. Following a post by another member I am getting ready to build an Arduino powered days total carb balancer. It will not work any better than a couple of drink bottles and a few metres of hose and it certainly won't cause world peace to break out, but it is an interesting and cheap project.

The problem is that modern motorcycle engines (yes even R65s) are tuned for a far higher specific output than Grandpa's Indian with 6.5 to 1 compression and teeny little carbs.

The R65 has such small throttle opening at idle you need to have all the planets line up by trying  to balance the carbs by running the thing alternatively on single cylinders.

And in any event, that will (if you succeed) only balance at idle. Don't know about you but I am more interested in the balance at moderate opening than I am in the idle balance.

All that said, your problem sounds like a vacuum leak to me. How old are the rubber trumpets connecting the carbs to the cylinder heads? And if they have been replaced, are they tight?


I set ignition timing at 3,500rpm and care not what the idle ignition timing is.

I do set carb balance at idle initially, but then I look at what is happening at progressively larger throttle openings and adjust accordingly.

Oh, and I do valve lash before doing either.
1978 R100RS| 1981 R100RS (JPS) | 1984 R65 | 1992 KLE500 | 2002 R1150GSA |

Offline mrclubike

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Re: Unstable RPM during carb tuning
« Reply #6 on: December 06, 2020, 07:38:30 PM »
I encountered an interesting problem with my 1984 R65 while trying to use the shorting method to sync the carbs. I find it almost impossible to set the throttle such that the engine holds 2500 rpm. It either revs to 3500 or drops to 1500, but won't stay at 2500. Setting it at 3500 and shorting one side yields a progressive drop to 1500 which then fails to regain 3500 when the short is removed. Is this normal or do I have problems with carb settings or the advance mechanism? Thanks.



I think I understand what your  asking  now

Yes
It does seem like  it is difficult to lock  the throttle to  a specific rpm in the range you speak of
But I don't know about the RPM not recovering after spark is restored
Its possible it all has something to do with the CV type carbs


The throttle is very sensitive with out a load on the engine
 Does it run fine other wise

As far as the vibration
Make sure your motor mounting bolts are torqued up to 68nm or 50lb/ft
1982 R65 running tubeless Snowflakes
2004 R1150R

Offline Justin B.

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Re: Unstable RPM during carb tuning
« Reply #7 on: December 06, 2020, 09:10:32 PM »
...my R65 is a bit more "vibration" prone compared to my other BMW...

The R65 is known to have a bit more vibration at certain RPMs than the other Beemers and got a bit of bad press about it at the time.
Justin B.

2004 BMW R1150RT
1981 R100RT - Summer bike, NEKKID!!!

Offline Barry

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Re: Unstable RPM during carb tuning
« Reply #8 on: December 07, 2020, 04:34:03 AM »
On the vibration I had some success by using non standard torque settings.  I had noticed that the mono model had lower torque settings which I tried and it made the engine feel smoother in every day use by pushing the vibration period up the rev range.   The period of vibration is a resonance between engine and frame and the theory goes that the coupling between engine and frame can influence the resonance. For example it's well known that the very same engine in a different frame can result in very different amounts of vibration.  The other option is rubber front engine mounts although they are said to affect the handling. Even the front rubber mounts don't isolate the engine from the frame, they just change the coupling which for me confirms the theory that different torque settings can have an affect.  It's a free experiment to try.
Barry Cheshire, England 79 R45

Offline Bob_Roller

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Re: Unstable RPM during carb tuning
« Reply #9 on: December 07, 2020, 08:18:44 PM »
I had the vibration issue when i first got my '81 R65 in 01/81, around the 4200-4500 rpm area .
I installed a Luftmeister rubber forward engine mount isolator kit, no difference in vibration level  .
Seems like at 95,000 miles I seriously can say I cannot detect any increase in vibratio0n level at those rpm levels .
'81 R65
'82 R65 LS ?
'84 R65 LS
'87 Moto Guzzi V65 Lario
'02 R1150R
Riding all year long since 1993 .
I'll give up my R65, when they pry my cold dead hands from the handlebars !!!!!

Offline mrclubike

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Re: Unstable RPM during carb tuning
« Reply #10 on: December 07, 2020, 08:24:24 PM »
On the vibration I had some success by using non standard torque settings.  I had noticed that the mono model had lower torque settings which I tried and it made the engine feel smoother in every day use by pushing the vibration period up the rev range.   The period of vibration is a resonance between engine and frame and the theory goes that the coupling between engine and frame can influence the resonance. For example it's well known that the very same engine in a different frame can result in very different amounts of vibration.  The other option is rubber front engine mounts although they are said to affect the handling. Even the front rubber mounts don't isolate the engine from the frame, they just change the coupling which for me confirms the theory that different torque settings can have an affect.  It's a free experiment to try.

That is what I have noticed also
When I ran the mounts at a lower torque the vibration was softer below 4000 to 4300 ish RPM but harsher above that
I now have them tight and find the vibs are less above and a little more below. I find this more comfortable
I also have a 37/11 final drive and I am sure that has an effect on it also 
1982 R65 running tubeless Snowflakes
2004 R1150R

Offline Bill Parker

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Re: Unstable RPM during carb tuning
« Reply #11 on: December 08, 2020, 07:21:36 AM »
Thanks for all the advice. I found that one carb was leaking fuel past the float valve at idle, enough to cause that side to run very rich. A removal and thorough cleaning/adjustment seems to have solved that problem. But now my Harmonizer (vacuum sync) has broken and I'm waiting on a Twinmax to re-sync.
1984 R65, 2005 R1200ST

Offline dogshome

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Re: Unstable RPM during carb tuning
« Reply #12 on: December 08, 2020, 05:11:17 PM »
Hey I got one right for a change?  8)

Regarding shorting secondary coils, I think this will result in a higher primary current going in. Instead of building current to saturate the coil and then levelling out on primary resistance, it also now has to supply the secondary which takes current whilst charging (when is shouldn't). I don't know the exact characteristics of these things, but short on a secondary means increased current on the primary. I'm also not sure what the current does when when the transistor goes off with a shorted secondary. It probably reduces and if so, not a problem. Surprisingly I can't find this kind of measurement on the web.

Since the primary probably current goes up and the transistor has fixed instantaneous and long term limits; it may fail immediately or after several cycles depending on components. Contacts are harder to break - they would normally weld in after abuse in say a relay, except this big cam comes and forces them open rather than a spring.

I'd be interested if someone has a graph of primary vs secondary currents when normally loaded AND when the secondary is shorted out. I think it goes up in a nasty way whilst saturating.

EDIT:- I found something, but on modern transistor ignitions which will have protection (current limit). But yes, a shorted secondary makes the average current in the primary go up. Enough to blow up one of our simple transistor ignitions? I don't know.
« Last Edit: December 08, 2020, 05:33:40 PM by dogshome »
肉(r?u)包(bāo)子(zi)打(dǎ)狗(gǒu) (meat+bun(2nd and 3rd)+hit+dog)
* Literally: To hit a dog with a meat-bun.:-O

Offline mrclubike

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Re: Unstable RPM during carb tuning
« Reply #13 on: December 08, 2020, 11:39:07 PM »
 

Regarding shorting secondary coils, I think this will result in a higher primary current going in. Instead of building current to saturate the coil and then levelling out on primary resistance, it also now has to supply the secondary which takes current whilst charging (when is shouldn't).                       

I'd be interested if someone has a graph of primary vs secondary currents when normally loaded AND when the secondary is shorted out. I think it goes up in a nasty way whilst saturating.

Remember with a dual tower coil the secondary is not completely shorted when only one side is grounded

But I think you may be correct
You got  me curious
Not sure if it is  enough to overload   the ICU
I have a trace of the  normal current in the secondary
I will have to get the scope out this weekend and  see how high it does go when shorted

I think it would be safe to say  "Do not completely short or open the secondary"  :beerchug:
« Last Edit: December 08, 2020, 11:47:24 PM by mrclubike »
1982 R65 running tubeless Snowflakes
2004 R1150R

Offline Barry

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Re: Unstable RPM during carb tuning
« Reply #14 on: December 09, 2020, 04:25:44 AM »
While the average primary current may change I can't see there is any change to the maximum value of primary current as that is limited by the primary resistance. What shorting the secondary might do is change the inductance so that the rate of rise of the primary current and therefore the measured average is different.  I doubt it's a big deal in terms of current but there may be other transient effects.
Barry Cheshire, England 79 R45