R25/2 - Tank Vacuum and Popping on Decel

Begonnen von Kurt in S.A., 09 März 2019, 21:10:29

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Kurt in S.A.

While out today, I began to feel the engine missing...made me cut the ride short.  But I thought about building a vacuum in the tank.  I have a clear fuel filter installed, so I took a quick look.  Hard to do and keep the bike straight.  When I cracked the gas cap open, I thought I saw the filter "fill up" and seems like the bike ran better.  I played with closing and opening the cap all the way home.  Too much traffic to play too many games.  Is there a way of checking the breathing of the cap?  I haven't checked, but maybe my R69S cap will fit???  Update...checked and the caps will swap, so that's a test point for the future.

I also noticed some popping out the exhaust (I'm pretty sure of that, but hard to tell) when I roll off the throttle to shift gears.  It was worse just after engine start riding out of the neighborhood...I didn't notice it that much after the engine warmed up.  Is that a timing issue or fuel issue?  I can never figure out which one...maybe it could be both.  I haven't checked the timing in a while but it wasn't that many miles ago and it was OK.

Thanks...Kurt
Kurt in S.A.

Kurt in S.A.

Kurt in S.A.

Herculestom

Hi Kurt,

what you describe here is a clear indication that for some reason your breather doesn't work as it is supposed to work. The effect will probably be less with a full tank. Warning: don't run the bike in this condition. At one point it might run too lean and secondary damage to be expected.

I am not sure, if the cap can be cleaned to restore the function. I have never done it, but I could imagine that it is possible to an extent by using solvents like carb cleaner. The problem is that you won't be able to clean it mechanically, unless you take it apart. Not sure, if this can be done in a normal workshop. I never tried.

Recommendation: Buy yourself a new one. Won't cost you an arm and a leg.

Good luck and regards
Tom
umuntu ngumuntu ngabantu

strichzwojan

Hi,
beside the cap, I would have a look at this:

there is a filter inside the tank. Clean it, and kick off the filter outside. I always had problems with them, sometimes they reduce the flow of gasoline because of "slimy carpets" they build inside !

Jan

OldsCool!

Agree to Jan. Depending on the fuel tap there is a filter already built in either on the entry side reaching into the tank or on the exit of the tap when you remove the nut of the output jack.
Should be checked for dirth anyway.

best regards,

Steffen
Ich bin auf dem Dorf aufgewachsen. Ich wurde nicht erzogen, ich habe ÜBERLEBT!

Kurt in S.A.

I've never been able to get this petcock off the tank.  I tried when I first got the bike, but it was on very tight and I was afraid of doing damage, so I left it.  I showed a picture of the petcock on the other thread:

https://bmw-einzylinder.de/forum/index.php?action=dlattach;topic=18077.0;attach=65630

Is this the type that has an internal (inside the tank) filter?  Hmmm...I just notice that the lettering on the petcock says "2 filter", so I guess I answered my own question.  Does anyone have a breakdown of this petcock and what it looks like internally?

Kurt
Kurt in S.A.

Taz

Hello Kurt.

Re. your tank's cap...the late odeon8 went through the process of taking one apart that had confirmed breathing issues ...the culprit had been identified as a piece of cardboard deep inside that had expanded due to moisture over time until it blocked all the channels allowing air in and out...he replaced that and puit it all together again, case closed.

Dismantling it shouldn't be too hard provided you do have a lathe at hand like he did, but as new caps are still available at a sensible price I would recommend the same course of action as Tom did...get yourself a new one and keep the old for scientific examination during the long and cold (or at least long in your case) winter nights  ;D

all the best, Taz
Ich hab noch nie einen Sarkasmus vorgetäuscht


auf Tour:

Zipfelreise (2019)
Cymru "Radnor Revivals" (2016)
Alba "Isle of Skye" (2013)
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Kurt in S.A.

Thanks...I'll try the R69S cap as a first test.  I looked at the original cap and don't see an easy way to take it apart.  If the R69S cap works well, then I'll certainly look to get another one.

Any thoughts on the decel/throttle closed popping noises?

Kurt
Kurt in S.A.

Taz

Hello Kurt.

Sorry...no idea re. that noises...

Taking the cap apart took Thomas some major thinking and as already mentioned a lathe...I am not sure exactly how he did it and it seems that the picture files he had used to explain the process are vanishing now one after the other (externally hosted)...sorry.

Good luck, Taz 
Ich hab noch nie einen Sarkasmus vorgetäuscht


auf Tour:

Zipfelreise (2019)
Cymru "Radnor Revivals" (2016)
Alba "Isle of Skye" (2013)
Austria Großglocknerhochalpenstrasse (2012)
Alba "Spittal of Glenshee" (2011)

Rütz

Zitat von: Kurt in S.A. am 10 März 2019, 16:39:51
Any thoughts on the decel/throttle closed popping noises?
Check nut and gasket on cylinderhead/exhaustpipe first.
I never dared to be radical when young.
For fear it would make me conservative when old. (Robert Frost)

Kurt in S.A.

RE: the cap, I noticed that the cork gasket is loose and I could almost pull it off, but I could also break it, so I left it alone.  There's no moving parts in the cap.  There are a few holes in the center which lead inside and it appears that the cap breathes through small holes all around the outside edge.  Seems like I could remove the gasket (maybe get another one) and then just soak the heck out of the cap.  It sure would be nice to have a test rig to see how plugged up the cap is.

Kurt
Kurt in S.A.

Taz

Hello Kurt.

Your thoughts re "holes in -holes out" is correct, that's the way the air is supposed to take, problem is the rather complicated way the air is supposed to take inside of the cap...Thomas had shown this very well and easily understandable, but his pictures seem to vanish...and also yes, there are no moving parts inside.
Removing the cork gasket will only result in a removed (and most probably destroyed) gasket and you being nowhere closer to the solution. The problem with the cardboard layer is that is doesn't have to be wet all the time...once soaked thoroughly or kept wet for longer periods of time will make it gain volume it won't loose when drying up afterwards...if you don't want to dismantle the cap you might want to start seeing it as a rather nice paper weight and get yourself a (new) working one...sorry.

All the best, Taz

Your test rig would be the bike, if it works then the cap is ok, and vice versa.
Ich hab noch nie einen Sarkasmus vorgetäuscht


auf Tour:

Zipfelreise (2019)
Cymru "Radnor Revivals" (2016)
Alba "Isle of Skye" (2013)
Austria Großglocknerhochalpenstrasse (2012)
Alba "Spittal of Glenshee" (2011)

strichzwojan

Ever thought about a smart test? Measure the flow of the gasoline. The tank has to be full, cap tightened.

Measure  1.: Fuel hose with the extra plastic filter.
              2.: Fuel hose without filter
              3.: Fuel hose without filter and without tank cap
if you want:
              4.: Fuel hose with the extra filter and without cap

Measure the time it takes to fill a 5 Liter can (or whatever you have)

As the R25 takes about 5 Liters at topspeed (105 km/h) it should be OK if the can is full within less than one hour.
When I made this measurements, it was full in 12 minutes.

Jan

Kurt in S.A.

Jan -

Yes, I've thought of a fuel flow test.  I've done that on my /7 and got something like 500+ cc/min, well more than the minimum needed which is something like 350 cc/min.  I'd rather not have to wait for 12 minutes!!  I'm sure I can see if there's any degradation in flow with and without the gas cap in short order.

Kurt
Kurt in S.A.

Herculestom

Hi Kurt,

another thing you could/should do after having fixed the real problem and you have made sure that there is a sufficient flow of petrol is to remove the fuel filter. Just dump it. My personal opinion. Even in fu&x$ing Africa the fuel is pretty clean nowadays. So far I never had a problem. Especially older fuel filters after long use might become a problem one day. If you measure it carefully, you'll find that the free flow through the filter is less than without.

Good luck
Tom
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OldsCool!

Hi Tom,

nowadays the fuel filter is not due to bad fuel-, but tank-quality  ;)

BR

Steffen
Ich bin auf dem Dorf aufgewachsen. Ich wurde nicht erzogen, ich habe ÜBERLEBT!

Herculestom

oho. Never thought about that. Might apply to some bikes.  ;)
umuntu ngumuntu ngabantu

Kurt in S.A.

I did some fuel flow testing.  Just to check, Jan said 5L in 12 minutes.  That's about 415cc/min.  Is that the bare minimum or just a number for high consumption.

I did the various scenarios with about 1/2 tank of fuel and turned the petcock to the "main" tank each time.  I found that the external filter does impede flow a bit but not enough to really worry about I think.  The original gas cap was allowing about 300-325 cc/min while no cap and the R69S gas cap were more like 350-390 cc/min.  So, it does seem that the original cap is restrictive.  On my next run, I'll use the R69S cap and see how things go.

Is 350 cc/min acceptable or should I really expect a faster rate than that?

Kurt
Kurt in S.A.

Herculestom

Kurt,

I'd say more than enough. Why? Let's assume your engine's fuel burn is 5 l/hour (5.000 cc/h). Then the theoretical fuel requirement will be about 85 cc/min. Therefore, more than enough and no reason to worry at 350 to 400 cc/min.

Regards
Tom
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Taz

Hello Tom.

Exactly.... thinking from the other end 300cc per minute make 18l per hour ... and to be completely honest I can't think of any operating state where one of these little beemers would be able to consume that much fuel....exept maybe setting it on fire...

@Kurt: so maybe your fuel cap might be no longer in top shape but I am confident that it will serve it's purpose for years to come...

Best regards, Taz
Ich hab noch nie einen Sarkasmus vorgetäuscht


auf Tour:

Zipfelreise (2019)
Cymru "Radnor Revivals" (2016)
Alba "Isle of Skye" (2013)
Austria Großglocknerhochalpenstrasse (2012)
Alba "Spittal of Glenshee" (2011)

strichzwojan

Hi Kurt,
assumed, that a fully accelerated R 25/2 will take about 10 litres per hour , 0,166 litres of gasoline per minute would be sufficent.
Problems with carburettors are mostly problems with ignition , and problems with ignition and electrics are problems on the fuel side...  ;)
What kind of ignition coil do you have? You need a good spark.
BTW: Have you had a look at the internal filter?

Good luck,
Jan

Kurt in S.A.

I went out for a ride with the R69S gas cap and didn't really notice any issues.  But as I was riding, I was thinking along the same lines as some of you.  I'm getting 50 miles per gallon and if I were to cruise around at 50 miles per hour, that requires 1 gallon per hour.  Doing the math, that works out to be a little less than 65 cc/min.  So, clearly fuel flow is not going to be an issue.

Jan, as for the internal filter, no I haven't been able to look at it.  This goes back to when I first got the bike, I tried to remove the petcock from the tank.  As hard as I tried, I couldn't undo the connection...I was afraid of doing some damage.  It might be corroded in place or possibly the previous owner used some kind of sealant.  To compound all this, my tank is developing a layer of rust inside the tank...my bad for not getting it sealed when I first got it...big mistake on my part.  So, a sealing event is on the horizon (I hope I can protect the paint) and I'll need to remove the petcock.  At that time, I'll fill the tank with water and use a heat gun on the petcock to try and loosen the threads.

Kurt
Kurt in S.A.

strichzwojan

The picture attached to answer #4 shows a petcock, that is connected with a nut, that has a thread turning clockwise at one and anti-clockwise on the ohter side. I have a different one there, because my tank is not original. But your version is the original one.

Can anybody tell Kurt which way he has to turn the nut to remove the petcock? 

You have to hold the petcock with the hand, only the nut has to be turned!

If there is rust in the tank, you have to get rid of it! Constant flow when testing it in the garage is hard to compare to the flow of gasoline when driving...

Jan

Kurt in S.A.

#22
It appears, although I'm not 100% certain, that the threads on the petcock side are "reverse" or left-handed while the threads on the tank stub are "regular" or right-handed.  I can easily be corrected.

Kurt
Kurt in S.A.

Herculestom

ZitatI can easily be corrected.

??? What do you want to correct? This is part of the mounting principle of many petcocks.
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Borgward

Kurt,
as Jan said it is important to hold the petcock with a wrench (better than by hand) and turn the nut open (clockwise as for regular nuts). You might even turn the petcock counterclockwise instead of only holding it when turning the nut.

go for it!
Hubi

Kurt in S.A.

I knew about the reverse threads on the petcock...had to deal with that on my other bikes.  As I recall, no matter what I tried, it resisted my efforts.

Kurt
Kurt in S.A.

Borgward

 :(
so you have to wait - as you said - for your tank inside restauration project. At least the petcock doesn't leak anymore  :thumbsup:

Hubi

Kurt in S.A.

Zitat von: Borgward am 06 Mai 2019, 13:15:32
:(
so you have to wait - as you said - for your tank inside restauration project. At least the petcock doesn't leak anymore  :thumbsup:

Hubi

Dry as a bone!  Thank you, sir!   :prost:

Kurt
Kurt in S.A.

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