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Autor Thema: R25/2 - Tank Vacuum and Popping on Decel  (Gelesen 2041 mal)

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Kurt in S.A.

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R25/2 - Tank Vacuum and Popping on Decel
« am: 09 März 2019, 21:10:29 »

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
« Letzte Änderung: 09 März 2019, 23:03:50 von Kurt in S.A. »
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R25/2 - Tank Vacuum and Popping on Decel
« am: 09 März 2019, 21:10:29 »

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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #1 am: 10 März 2019, 06:46:59 »

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

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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #2 am: 10 März 2019, 13:30:13 »

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
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #3 am: 10 März 2019, 13:47:22 »

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
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #4 am: 10 März 2019, 13:58:13 »

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
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #5 am: 10 März 2019, 14:30:05 »

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
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #6 am: 10 März 2019, 16:39:51 »

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
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #7 am: 10 März 2019, 20:38:12 »

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 
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #8 am: 10 März 2019, 20:55:09 »

Any thoughts on the decel/throttle closed popping noises?
Check nut and gasket on cylinderhead/exhaustpipe first.
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #9 am: 10 März 2019, 22:53:54 »

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
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #10 am: 11 März 2019, 10:40:21 »

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.
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #11 am: 11 März 2019, 20:32:05 »

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
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #12 am: 11 März 2019, 22:46:57 »

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
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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #13 am: 13 März 2019, 18:58:08 »

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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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #14 am: 13 März 2019, 20:36:55 »

Hi Tom,

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

BR

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Re: R25/2 - Tank Vacuum and Popping on Decel
« Antwort #15 am: 14 März 2019, 04:56:48 »

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

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