BMW R100R - Carbs with pod filters Slipping off

Discussion in 'Airheads' started by krikodium, Jun 14, 2019.

  1. krikodium

    krikodium Adventurer

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    Hi everyone, this is my first post in the forum and hope I could get the help I couldn't get from any arguably bmw expert garage here in London....
    First of all, the things I modified to the bike:
    1. ALTERNATOR DIODE BOARD ( Heavy Duty ) ( EnDuraLast )
    2. Upgrade Ignition trigger unit with electronic advance
    3. Mikuni VM34 Carbs
    4. Pod Air Filters
    5. motogadget m.unit

    Last time it did it was today after I left the garage, timing done, new sparks, valves clearance checked, etc, the bike was running as never before and suddenly before getting home (15' later I started it, 60ish C degrees in the engine, not warm enough), carb slipped again...

    So, the thing is when the engine didn't quite reach the operational temperature (ie. 100 degrees or so), sometimes, it does some kind of combustion explosion (don't know how to call it) and pushes off a carb (is always one, not both together), out of the cylinder head's sleeve. At first I thought it was because of the pod filters and the weight on the rubber grommets that connect the carbs to the cylinders, that they were not strong enough to hold the carbs, but it's so erratic that I honestly don't know what could be the issue.

    Any help, comment or suggestion more than welcome!

    Thanks!
    #1
  2. bpeckm

    bpeckm Grin! Supporter

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    You seem to realize that the weight of the carbs/filters is putting a strain on the mounts, the original design with the air tubes was integral to supporting the carbs. BUT, what you are describing sounds like a backfire/preignition. This could be caused by mis-timing, a hot-spot like a burned valve, or ???. I guess I would check timing first, check your valve clearances, and do a compression test (burnt valve?).

    Do both sides do it? They wouldn't do it at the same time, as these engines are something akin to two motors tied to the same crank. They have "wasted spark" ignition, i.e. the spark plugs fire on the compression stroke on one side and simultaneously on the other side during the exhaust stroke. Blowing off the carbs would be during the intake stroke, I should think. That's what makes me think mis-timing.

    Or, since you are running Mikunis which may not yet be properly jetted, it could be a mixture problem....or a combination of any/all! :p3rry

    Good luck
    #2
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  3. Disston

    Disston ShadeTreeExpert

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    There are many reports of carbs with pod filters falling off from vibration. What you report seems to be back firing and it's causes are timing or a lean mixture, usually.

    Check the timing and/or richen up the carbs. No idea how you do that on a Mikuni.

    Same thing bpeckm says I guess.
    #3
  4. dsmith181

    dsmith181 Adventurer

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    Why use he pod filters? Is there an advantage other the stock air filters?
    #4
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  5. infinityedge

    infinityedge Been here awhile

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    Pod filters were used on race bikes because it saves a tiny bit of weight.

    Then the cafe crowd started doing it to ape the look of racers.

    Ignoring the very slight weight savings, every aspect of pod filters is worse than the factory filter.
    #5
  6. Boxerbreath

    Boxerbreath 2017.5 GS Black Storm

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    It’s a ‘look’. There is little advantage to running pods if any at all.
    #6
  7. krikodium

    krikodium Adventurer

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    Thanks, and thanks to the rest of the guys who commented here.
    So, yes, I found out the painful way that original design was because of a reason, Germans don't do anything randomly, we all know that ;)
    The problem is assuming, as there are soo many customs using them, you'll not have an issue, and strangely enough when you start googling for solutions nothing comes up, believe me, I searched.

    Responding your question, before this last service it was doing it on the right carb only, with the Mikunis and also previously with the Bings that I was running with pod filters as well. After this last service where the guys told me they did timing and checked valves clearance and fix new spark plugs, it did it on the left one.

    But again, as I said, it only does it when the bike is still not entirely warm.
    Maybe I should do a compression test... have to check how to do that though :( is this a compression test btw?

    Thanks again guys!
    #7
  8. Big Bamboo

    Big Bamboo Aircooled & Sunbaked Supporter

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    Nope, that's a leak down test (also a valuable way to check the condition of the engine). A compression test uses a single gauge screwed into the spark plug hole, carbs removed or held open, and the engine turned over with the starter until max pressure is reached.
    #8
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  9. bpeckm

    bpeckm Grin! Supporter

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    You said: before this last service it was doing it on the right carb only, with the Mikunis and also previously with the Bings that I was running with pod filters as well.
    and: 2. Upgrade Ignition trigger unit with electronic advance

    Those are two good clues. I would pursue the ignition timing. It seems to me that "fire" is getting into the intake manifold somehow, I'm thinking that somehow the ignition fires way too early. A backfire (ignition of gasses on their way out of the engine) can be a sign of carburetion, but this blowing-off of the carbs suggests something in the intake. Intake valves don't usually burn or have hotspots, so I would have to go to ignition timing.

    I'll be all ears, as they say.... !!!

    :D
    #9
  10. Prutser

    Prutser Long timer

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    The carbs need support just like the original carbs do. They even come off on stock bikes sometimes.
    But there are shops who sold a rod to go over the gearbox which supported the carbs on the rear.
    #10
  11. leeroix

    leeroix Adventurer

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    Check rocker end play, and valve lash. Check for lean running idle condition. compression check.
    #11
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  12. misterpepper

    misterpepper Been here awhile

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    Consider the fact that the carb falling off might be causing the backfire. As the carb falls off an air gap opens up between the carb and the cylinder, letting in too much air and causing a lean condition. If this happens while riding you can easily get a lean mixture backfire that will quickly finish the job of removing the carb from the engine. I've had this happen. Replacing the rubber coupling hoses fixed the carbs falling off -- and backfiring -- for me, no further tuning required.
    #12
  13. AlpineRAM

    AlpineRAM PartsChaser

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    On difficult cases I use cheap hairspray to mount the carbs into the (new) rubber to the heads.
    It acts as a lubricant when still wet and as a glue when it dries out. Learned this from one of my turbo diesels where slightly increased boost made the hoses walk out under the clamps.
    #13
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  14. bpeckm

    bpeckm Grin! Supporter

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    The hairspray trick above also works quite well for grip installation....
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  15. bmwrench

    bmwrench Long timer

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    The backfire you experience during warm up is caused by a weak mixture. No mount in the world will prevent a carb blowing off when this happens. As far as the carbs slipping off when warm, use the 13 72 1 254 654 (intended for R75-R90), not the 13 72 1 338 360 BMW specs for R100. The 360 are too soft to support the carb alone and allow the car to resonate badly at speed.
    #15
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  16. krikodium

    krikodium Adventurer

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    Do you have any reference or link to share? Wasn’t able to find any, was about to 3D draw and CNC one myself! This could be very helpful! Thanks!
    #16
  17. krikodium

    krikodium Adventurer

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    Thanks a lot!
    Question here, as I couldn’t find any specs online. What’s the diameter of these rubber grommets? Are they good for the 40mm bings?
    #17
  18. krikodium

    krikodium Adventurer

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    So I found it on eBay
    https://rover.ebay.co.uk/rover/0/0/99?loc=https://www.ebay.co.uk/p/1151184675
    The thing is these are 52mm and the carb outlet is 42 or in my case 44 due I added a step up sleeve to fit other rubber grommets I was recommended to use. Any idea how can I make them fit to the Mikunis outlet diameter?
    I know, though one....
    #18
  19. tlub

    tlub Long timer Supporter

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    Why not just go back to the original air filter? It breathes just fine, doesn't cause trouble, the carbs stay on...
    Why break something that's not broken in the first place?
    #19
  20. krikodium

    krikodium Adventurer

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    You have a point there, and believe me I'm thinking about it, but again, if you always follow that principle then there wouldn't be any customs out there, right? now, honestly, I believe once I sort the backfire issue, the carbs will stop slipping off as I don't have any issues once the bike is up to operational temperature, and I've been pushing it and bike responded amazingly, the Mikunis rock it full time. Best!
    #20