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Old 04-24-2015, 03:20 PM   #1
SidecarThumper OP
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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 04-24-2015, 05:09 PM   #2
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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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Old 04-24-2015, 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 04-25-2015, 04:58 AM   #4
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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 04-25-2015, 05:07 AM   #5
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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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Old 04-25-2015, 07:10 AM   #6
mcma111
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Check the float level. I had a similar issue with my R100gs. Found one float set to low and it would run out of gas after running at high speeds.
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Old 04-26-2015, 06:10 AM   #7
Beemeup
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I suspect electrical. I think carb issues would cause stumbling but as far as I know a misfire is a timing or and electrical problem. The only time mine did it is when I had a loose, but not obvious connection on a coil.
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Old 04-26-2015, 06:16 AM   #8
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If cruising ok at say, 70 mph, and then you accelerate but the thing just doesn't go at all...then slowly back off the throttle to the 70mph again. If it picks up to the previous nice lopping speed then I'd suspect a diaphragm.

I'm not sure if a bigger main jet to match the mufflers would help. What's the air filter?
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Old 04-26-2015, 06:30 AM   #9
Jim K in PA
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Sure sounds like fuel flow restriction to me. Start at the fuel bowls and work your way up to the fuel taps.
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Old 04-26-2015, 07:38 AM   #10
therealbatman
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Quote:
Originally Posted by mcma111 View Post
Check the float level. I had a similar issue with my R100gs. Found one float set to low and it would run out of gas after running at high speeds.
Bingo!
I had exact same issue at high speed. Adjusted float level so there is 24 mm fuel In each bowl...all is good now.

Sorry I cannot comment on the wet issue
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Old 04-26-2015, 10:55 AM   #11
wmtunate
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I had identical symptoms. Would buck at WOT on the highway, near 70-80mph. Turns out it was a flipped fuel enricher plate on one of the carbs.
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Old 04-27-2015, 04:50 AM   #12
Beemeup
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Bucking is one thing but he said it was backfiring.
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Old 04-27-2015, 04:54 AM   #13
SidecarThumper OP
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Thanks to all for the help and suggestions. I fitted new diaphrams and cleaned carb out about 18 months ago, but will check float chamber jets and anything on fuel flow line Tank to Carb. How do I check float height?Also, about a year ago I did a 1000 mile trip round Scotland, BMW ran fine and while I was doing motorway miles I steadily increased the speed so I was cruising at 75mph. Misfire started to creep in after about 10 miles so I kept reducing throttle but it got worse until I was down to a cruising speed of about 45-50 mph, which was about 40 miles on. I eventually came to a halt and pulled off the motorway almost running on one cylinder. I checked for spark etc and everything seemed ok, I continued on the bike on back roads albeit running on 1 cylinder and down to 30-35 mph and after about 10 minutes everything kicked back in and it ran great for rest of trip, but I didn't take it above 65mph and kept to back roads.Thanks for your help and I will continue to check LT electrical conections and fuel delivery.

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Old 04-27-2015, 05:05 AM   #14
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Seems you've done a bit on the fuel delivery side, but maybe it's worthwhile spending an hour.......

  • Empty the tank.
  • Remove fuel taps.
  • Dismantle the taps, replace any worn bits.
  • In line filters? clean, replace or ditch.

Fuel maybe be getting to the bowl, check the level.

Then check that there's no obstruction to fuel uptake, eg from flakey shit from the tank or deteriorating fuel line.
  • Float bowl off.
  • Remove the jet stack. Dismantle and check the central hole is clean and free of any debris.

Replace the stack but be careful the atomiser is correctly seated within the carb orifice.

This small piece of fuel line blocked my main jet 1500kms from home.



After dismantling the bike searching for all sorts of problems, I checked the main jet. BINGO!
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Old 04-27-2015, 04:13 PM   #15
Warin
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The wet running problems .. high probability of the HT leads/spark plug caps faulty ...

Test .. at night, while the bike is running, spray water onto;
  • the spark plug caps - check running ok
  • the HT leads - check running ok
  • the ignition coil - check running ok
Why at night .. you may see the HT leakage!

-----------------------------
Have you check the ignition timing? Particularly the advance.

------------------------
To check fuel flow ..


Remove one float bowel
Place old ice cream container under it



Turn on fuel and check fuel flow is constant for at least 2 litres.



Repeat for the other fuel bowel. This checks all of the fuel supply system for performance.
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