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Old Yesterday, 03:20 PM   #1
SidecarThumper OP
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Joined: Apr 2015
Location: Askam
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BMW r100/7 misfire - HELP

Had a BMW R100/7 for 2 years and I can't get rid of a top end misfire. Bike runs fine on 1/4 throttle, through gears and cruises at 50-60mph just fine. If I accelerate hard through gears get to 80 and big misfire, shut down and I have to shut off, bike slows then I open to 1/4 throttle and she runs ok at 55-60. Also won't start in wet. Changed points, condenser, ht leads plugs caps and coils - still the same, misfire at high revs, load & speed. Bike is on standard jetting but has modified silencer cut and shut into megas with minimal baffle/back pressure. Any suggestions?
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Old Yesterday, 05:09 PM   #2
Hennepinboy
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Check the fuel flow, you may have the red coat tank coating flanking clogging the in tank fuel strainers. If the strainers are missing the petcocks may be restricted. Start with the simple.
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Hennepinboy screwed with this post Today at 04:44 AM Reason: spelling
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Old Yesterday, 07:10 PM   #3
Tim McKittrick
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If it only fails under high load/rpm then I'd also suspect it's being starved for fuel. Poor flow could be due to clogged fuel screens in the tank, occluded fuel filters, blocked vent in the fuel cap, or otherwise blocked fuel lines. Does it run better with a full tank than a nearly empty one?

Hard starting in the wet sounds more electrical, so you may be chasing more than one problem. All of my airheads were sensitive to tight valves and cold starting, so if you haven't checked lash clearance that is another easy thing to rule out.
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Old Today, 04:58 AM   #4
Hennepinboy
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Thinking about your high speed miss.

1. Check the fuel flow
2. Check the diaphragms for small holes, if you have never replace the diaphragms it would be a good idea as a preventive maintenance item.
3. Remove and clean your main jet, air/fuel mixing chamber, water in the fuel cause a build up of nasty brown crud that restricts fuel flow through the main jet.

Purchase the carburetor book from Bing, everything you need to know about your carbs.

Wet starting, with the items you have replaced, check the wiring spade terminals, you may have corrosion, wet it allows voltage to leak to ground.
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Old Today, 05:07 AM   #5
ozmoses
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Sounds like a carb issue, though it may be a combo.


There will be more specific help in the Airheads sub.
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