Unable to correctly tune my BMW R27

Begonnen von Tiago, 20 Juli 2017, 14:33:47

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Tiago

Hello everybody,

I am new to this forum and relatively new to old BMWs, but I have messed around with motorcycles for a long time. I bought last year a BMW R27 which was restored previously but not ridden much afterwards. Although it started and ran very well when cold, it had the following "minor" issues which I attributed to a bad tuning:
1) When running, the RPM would stay high with no throttle. Almost killing the engine would bring the RPM down again (bad advance springs?)
2) After the engine was hot, it would be extremely difficult to start it again.

I did a complete maintenance to the motorcycle, including replacing the ignition advance springs (I then confirm they were operating with a stroboscope), cleaning the contact breaker, checking the advance timing (it was OK), torquing the head bolts and adjusting the valves, and finally cleaning and replacing gaskets on the carburetor. I also installed a helicoil on the cylinder head due to a broken thread on the intake flange connection (which I assumed was producing an intake leak), and to make sure this was not a problem I also applied sealant.

After putting everything together, all that was left to do was to adjust the carburetor, on the mixture screw. To my understanding and after following several websites, you should set the mixture screw to 2 turns out to begin, and basically after that try to find where the RPM drops either screwing in and screwing out, and more or less set it to half way between these 2 points. In my case, turning the screw in I could find the point where the RPM decreases and almost stalls the engine (the "rich" point), but turning out the mixture screw the RPMs only go high and up to a point where it stays high, never drops.
Applying throttle is not sounding great, as it always initially almost feels like it gets "out of air" and only afterwards it increases the RPM.

I was hoping to get your help in trying to see what can be the problem and how to fix it. If I am doing something wrong, or if you confirm that what I am doing is correct, I would highly appreciate everybody's help!

Cheers
Tiago

Tiago


rolf

That was the mysterial of Bings ;D....every body copied that ..." screwing the mixture screw until the RPM's are highest...that was the correct micture"... I always have the same problem as you....unscrew the mixture-screw....the rpm getting higher and higher until the screw lays on the floor ;D
if you want to set the mixture:
the mixture was right when you turn the "Gasgriff" a little (!!) bit for increasing the rpm's a little (!!)....the rpm should getting a little bit higher without any discreasing at the point  "idle rpm-> a little more than idle...try this by screwing a liitle bit the mixture screw (about 1/4 turnaround....in both directions....) then you can see which direction you must go further (in 1/4 turn steps)....then take 1/8 turn step until it is perfect.

Dappsull

Welcome Tiago,

from what I hear and read and definitely know from own experience: it seems to be almost impossible to get a stable idle on the R27 (maybe because the head  and caburettor are shaking like hell). I recently did a tour of a couple of hours and every time I had to stop I had a different idle - anything between 400 and 900. So it takes some patience and getting used to to find a reasonable setting.
Cheers
D.

Elbsegler

Hi Tiago,

I would not say that it's impossible to get a stable idling for BMW R27, may be its a little bit more difficult and time consuming to get it.

I tested two carburetors on my R27: 1) Blitz, 2) Bing. With the Blitz carburetor a stable idling was impossible, with the Bing it is fine.

Follow the steps Rolf described. I attach also a video which demonstrates 1) a stable idling and 2) excatly the topic Rolf is talking about: the point from idling and increasing RPM whith very, very little throttle. If you listen carefully you can hear, that the transition from no throttle and idling to a very little throttle is not ideal for my R27: The RPM is decreasing a liitle bit: Not dramatic, but you can hear it. Therefore I need to adjust the carburetor seetings next time again.

By the way: I'm not using an electronic ignition: There is often the opinion that stable idling is only possible with an electronic ignition. If your mechanic (centrifugal) controller of your ignition is in good shape it should work.

Best regards, Thomas



rolf.soler

Hello everybody,
it IS possible to get stable idling on R27 - Dappsull, you have to visit me, guess we'll have to check what is wrong...

if idling is very difficult / impossible to adjust, could be that the main jet holder (Mischkammereinsatz) has crept up a bit in the last decades...this problem and its solution is described in tips & tricks, under http://bmw-einzylinder.de/forum/index.php?topic=8369.msg109397#msg109397 with nice picture of main jet holder too high up...

if RPM don't come down when closing the throttle grip, this can be due to:

throttle cable or throttle slider in the carb "hanging", not moving free. Check visually, remove carb from flange and rubber, let it hang on the cable, turn it sideways so you can see inside, open/close throttle grip and see it slider moves free, no resistance felt on turning grip

centrifugal advance not working working properly i.e. weights not tucking in when RPM are reduced - springs are only one factor, weights can also get temporarily stuck if the articulation is bad...holes in weights too worn, not moving nicely on the bolts. Check visually with running engine, nice and smooth movements out and in when changing RPM ? and with strobe, smooth shift from S to F and back ?

hight main jet holder, see above

jets, mixture screw not well adjusted

I strongly suspect the first 2 reasons...

in engine does not start well whe hot, can be:

wrong starting procedure...do NOT flood with the buttton before starting, open throttle very (!) slightly and kick

bad condensator (can work fine when cool, but badly when hot

the same is true for the coil (but i have NEVER heard of a bad coil in R27...because it does not get hot because outside of generator... (it IS placed on the frame, right ?)

try these and good luck

Tiago

Hello guys,

Thank you all for this information, I have to say it is very precious  8)
I am facing a small but annoying fuel leak from the fuel tap  :schimpf: which I have to fix first before trying to run the engine again... I see there is always something to get you "entertained" on these bikes.

I will definitely try Rolf's method to set the mixture and see from there. I will also re-confirm again that the throttle slider is not getting stuck...

The main jet holder is fine and where it should be (I inspected it throughly while cleaning the carburetor). The centrifugal advance unit seemed to be working OK, I changed the springs but I did notice there was some play with the weights (holes a bit worn), however the movement when rotating seemed to be ok with no jams. After I get the carburetor setup Ok at idle I will do another check for the advance unit.

When I restart the bike when hot, I try not to flood the engine (sometimes wit no throttle or button at all), so it is strange that it is so difficult to restart. Do you guys know if there is any possibility to test if the condensator is not OK? And yes, the coil is placed on the frame  ;D

I need to fix these things soon to enjoy the sun :bike: I will give you feedback when done!

Thanks again!
Tiago


rolf.soler

Zitat von: Tiago am 27 Juli 2017, 13:33:55
Do you guys know if there is any possibility to test if the condensator is not OK?
1. yes, exchange it...
that's the fastest and cheapest way to find out  ;D unlike the coil a condensator is very cheap, maybe 1-2 £ I guess, yo can use any old fashioned petrol engine condensator from VW beetle, Landrover, British motorcycle etc.
You can also see what happens when you lift the contact breaker while ignition is on and engine off. There should be only a very small, barely visible spark between the contact points. if there is a fat spark  it's bad. Let the engine run and watch the contact point gap again - no "firing", should be visible.
Problem is that you would have to check when it's not working, i.e.when it is hot; with a R27 this means burnin your hands or forearms on the exhaust pipe when trying to manipulate the contact breaker in this its small housing, at least that's what happens to me... :schimpf:
some people are even able to measure tha capacity of a condensator, depends on your multimeter. But what figure is good I don't know, check google
2. dripping fuel is no big problem...just drive fast enough  ;D that's what I did when I drove the 700 km to the Edersee meeting
remember: any drop of fuel not burnt reduces CO2  ;D
3. "some play with the weights" is OK, even with brand new ones. As long as they move in and out smoothly and the range is not too much (original range is 35°, from 7° BTDC (S, idle) to 42° BTDC (F, >3000 RPM). However, it increases with use, up to about 40° range is OK, just adjust F for high RPM, not S for idle.

just some more ideas: you did try a different spark plug, right ? ALWAYs try that first in any ignition problem ( 1st rule of rolf the Great)

If hot starting remains difficult even with new plug and condensator your coil might happen to be the first bad R27 coil in the literature  ;D (no, it's possible but not common...)

Tiago

So here's the follow-up, with some news...

Previous feedback - Rolf.Soler, thanks for the added input. Yes, I ordered a new condenser which is cheap, and confirm that I have installed a new spark plug before doing all the tuning stuff  8).

2 days ago
Fixed the gas tank tap (Frankfurter model) leak (at least this type of leak is a must-do  :)). A couple of more turns and now it is fine and aligned... Replaced one more gasket on the carburetor and very carefully re-tightened the rest (I know, I have to be VERY careful with these aluminums...). Looked into the generator area for a visual recon of the condenser. Ordered the condenser and some new bits for the carb, just in case...

Yesterday
Time for a small test drive and re-tuning... So it started on the first kick  :juhuu:
1. Throttle response with initial dip in power but would pick up very soon after, and ran OK, although with more "bass"
2. When stopped, it would accelerate by itself again, 1400-1800rpm.
3. Mixture screw with gasoline come out from here - is this even possible?...
4. And finally...  :kopfhau: observed that the spark plug and cap was vibrating more than the head  :schimpf: Stopped the engine and confirmed that the spark plug thread insert was loose on the head, although it could keep the spark plug torqued, and even sometimes did not feel loose at all  :galgen: Well, that may now explain the rpms going high (air leak)

Check the photo, were you can see light damage from the vibrating spark plug.

So, now, need to either re-weld the head and drill and tap a new thread, or get a larger insert fitted... I could not find a good thread on this matter, can you guys advise me which is the best, longer lasting option?

Also, it would be my first time removing this head, I already removed the exhaust nut carefully (it was not properly tightened...), if you guys have some tricks or special advice (besides the manual, which I have of course  :poet:) it would be highly appreciated!

Wish me luck, hope all else is OK  :rip:

rolf

#8
The ONLY way is alloy-welding a new thread (after get the old steel part away)....make it in a long thread....it cost the same....and it will lives longer than a short thread.


Normally the thread is a steel part...casting into the head...when this going loose....see above

Tiago

#9
So the head went off yesterday. All OK, just that  :schimpf: spark plug thread problem...

I am now looking for someone experience on these heads. In the meantime, do some of you have photos or drawings of the "long thread" modification that you can post? The added thickness is applied on the outside, correct :)? I am guessing the final thread is directly on the head, with no other insert this time.

Which spark plug are you using afterwards?

And finally, can someone help me interpret the numbers on the piston "+0.040" and "3.85"? Is this an aftermarket part, oversize, etc?


rolf

all correct...and a long thread spark plug....like Bosch W4CC

Tiago

#11
Finally got the repaired cylinder head yesterday :) It was repaired by a professional classic car machinist but he opted to fabricate an insert with a longer thread so that it runs through all the short spark plug thread (I asked for Rolf's solution but he insisted on his experience with these inserts :p).



Yes I noticed that the inside diameter is big and is near the valves, but in reality it looks good and sturdy and it came with a  guarante to last a very long time :)

Anyway, went home and installed the head and everything else and started it today. It idled beautifully at 700 rpm!!! Yeah!!! Very, very stable, never been better! AND no rpms going up with no throttle :D
So there was the main problem - air leak through the spark plug insert.

I still need to fine tune the carb and will be going for a test ride in the next days. I will give you guys some feedback (hopefully all will go well :p).

SteveMKentucky

Zitat von: rolf am 20 Juli 2017, 15:04:46
That was the mysterial of Bings ;D....every body copied that ..." screwing the mixture screw until the RPM's are highest...that was the correct micture"... I always have the same problem as you....unscrew the mixture-screw....the rpm getting higher and higher until the screw lays on the floor ;D
if you want to set the mixture:
the mixture was right when you turn the "Gasgriff" a little (!!) bit for increasing the rpm's a little (!!)....the rpm should getting a little bit higher without any discreasing at the point  "idle rpm-> a little more than idle...try this by screwing a liitle bit the mixture screw (about 1/4 turnaround....in both directions....) then you can see which direction you must go further (in 1/4 turn steps)....then take 1/8 turn step until it is perfect.

I am also experiencing a problem with my R27's idle.  When the throttle is closed the RPM tends to come down slowly, often hanging at a higher RPM before finally settling down to a 'kicking' idle during which the engine often tends to die.  I can't seem to get a nice, even tickover.

I know that this is usually an idle mixture problem so have attempted to adjust the idle mixture, but the results to date have not been good.

So, I've read your adjustment description a couple of times but still don't completely understand.  The procedure you describe (if I am following correctly) is as follows (?):

- Adjust idle mixture until a slight opening of the throttle results in an increase in RPM without a lag or decrease in RPM
- Adjust the idle mixture either way to eliminate a 'lag' in RPM?
- Then (I presume) adjust the idle speed (bolt that bears on the slide to raise or lower) until it's high enough not to inadvertently kill the engine.

rolf

#13
Hallo Steve,
You have understand the description...but change the order:
- adjust the idle speed (bolt that bears on the slide to raise or lower) until it's high enough not to inadvertently kill the engine.
- Adjust idle mixture until a slight opening of the throttle results in an increase in RPM without a lag or decrease in RPM
- Adjust the idle mixture either way to eliminate a 'lag' in RPM?

because ,to do it very exactly, you have to readjust the mixture after changing the idle RPM's...every RPM needs a own mixture ....like all other RPM's by throwing the cable to reach higher RPM's....there you change the mixture too by the "Nadeldüse" and "Düsennadel".


BUT....it seems that you have 2 problems
first ....as described above
2. your 27 comes down too slowly...that should be really another case

first: your centrifugal advance  getting free?  Check it by spreading the weights by hand....they must go back with a "klong"  (don't be worry if there was another noise like "schnapp" or somethig else ;D )

try this first (both...idlesetting and "klong")....than post again....and we will search for eventually other reasons (par example this: http://bmw-einzylinder.de/forum/index.php?topic=8369.msg109397#msg109397 )

rolf

Hallo tiago,
the sparkplughole has a leak instead the new insert?

Tiago

Zitat von: rolf am 08 September 2017, 08:46:08
Hallo tiago,
the sparkplughole has a leak instead the new insert?
The air leak was before installing the new insert. Now, like I wrote, the idle RPMs are incredibly stable. Almost like it's fuel injected :D

I will read your tuning advice again before starting to play with my carburetor for the final fine tuning   thanks for all the details!

Tiago

Did my test drive some hours ago.
- Started very well (not at first kick but it was ok)
- Again, idle RPMs were very stable
- Went for a very nice ride :) no problems, even managed top speed of 100 km/h for a couple of seconds just to see if it could get there
- Stopped for some fine tuning. I could find with the mixture screw the miraculous "middle point" between rpms decreasing at rich mixture / lean mixture. However, at any mixture setting (apparently), starting to twist the accelerator the engine first reacts with that "engine almost dying" sound before picking up and increasing the RPMs. Is this normal???

I did not mention before but I also replaced the condenser due to the fact that the bike was almost impossible to start when hot since some months ago. Well, that problem is gone :))) while tuning sometimes the engine died but I always managed to start it again while hot - hurray!!!

Again, thanks everybody for all the help and support. Both of us are going to an event this weekend so it was crucial to get to this almost perfect point!

Tiago

#17
My first event :) Hope you like the photo with a Cessna 140!



Still need to fine tune. And it is now a bit stubborn when starting hot. Work with these machines is never finished!

Borgward


berndr253

"However, at any mixture setting (apparently), starting to twist the accelerator the engine first reacts with that "engine almost dying" sound before picking up and increasing the RPMs. Is this normal???"

If the engine is cold that is normal - in case of an hot engine (temperature on "working-level") not!
If it is not possible to change this effect by turning the mixture screw "inside" change the jet to an higher value (from 35 to 45, for example)

Hope that helps

Regards

Bernd
Leben und Leben lassen

Tiago

Zitat von: berndr253 am 12 September 2017, 09:40:22
"However, at any mixture setting (apparently), starting to twist the accelerator the engine first reacts with that "engine almost dying" sound before picking up and increasing the RPMs. Is this normal???"

If the engine is cold that is normal - in case of an hot engine (temperature on "working-level") not!
If it is not possible to change this effect by turning the mixture screw "inside" change the jet to an higher value (from 35 to 45, for example)

Hope that helps

Regards

Bernd
Thank you for the advice. I may try that and see what happens, it is getting impossible to solve that issue just by the mixture screw.

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