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Old 08-05-2011, 06:55 PM   #1
noman OP
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Question R75/6 Bile's Not Runnin' Right, tried my best but.......... need help!

my bike has just started to run poorly and i haven't as yet figured it out. discussed the issue with my local/well-known bmw shop and we are a little stumped. he's booked until october, so any offerings will be greatly appreciated.

my bike:
1976 r75/6, 40,1xx miles, stock bings with stock jetting, aftermarket oem-style mufflers, dyna electronic ignition


previously:
started immediately, idled at 1000/1200rpm, excellent fueling, steady/strong power to redline, no backfire/popping on decel, ran excellent.

now:
starts immediately, revs freely to redline when in neutral w/no noises, has engine knock at 3500/4000 when accelerating, low power especially uphill in 5th, surges at 55/60mph, both plugs show lean condition (not noted previously, but might not be new )

checks:
checked timing – ok, no change (as expected)
checked spark -- fat, blue, nice
“gen” light not on -- ok
in-line fuel fileters -- ok, still clear
charging system checks (12.0v at idle, 13.6 +v at 3000rpms and above) – ok
compression 150psi on both cylinders w/engine warm, throttle wide open – ok
checked main and idle jets – were clear, ok
checked carb O-rings -- ok
checked diaphrams and alignment tabs -- ok (were new this spring)
pulled and checked carb-to-engine rubbers – look fine
little screws that are removed to connect manometer still present -- ok
choke cable/levers still ok

attempts to fix:
changed plugs (were very lean) – no improvement
drove for 20mles with lights off – no improvement
cleaned air filter (was covered in fur, mice made nest) – no improvement
tried 3 different full tanks of different brands of reg fuel – no improvement
needles were at 3rd position down from top, tried 4th position down – no improvement, moved back to 3rd position down


tomorrow:
adjust valves
remove float bowls and check fuel delivery
replace engine to carb rubbers

any suggestions greatly appreciated!
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Old 08-05-2011, 07:24 PM   #2
DoktorT
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[QUOTE=
[SIZE=3][/SIZE]

tomorrow:
adjust valves
remove float bowls and check fuel delivery
replace engine to carb rubbers

any suggestions greatly appreciated!

++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++

Sounds like you know what you are doing. For sure veryify valve adjust and check the float valves as well as the float levels.

If nothing found there, reset mixture and set it a high spec, or if you don't have a CO meter, just set it a bit richer, then try it and report.

When you say you checked the main jets, did you remove them and pull the stack?? If not, do that as crud in there can cause your issue and is not at all uncommon.
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Old 08-05-2011, 07:33 PM   #3
disston
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Sounds like fuel delivery problem. Clogged fuel lines, screen, float needles (both at same time? strange but could happen)

When you were trying those different brands of gas, what did you do with suspect gas?

Let us know what it turns out to be. The suspense is killing me.
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Old 08-05-2011, 08:38 PM   #4
Wirespokes
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Somehow I get the idea it's ignition. Something to try would be disconnecting the Dyna and run with the stock points system.

Is the advance working ok?

Just because there's a good looking spark at atmosphere doesn't guarantee a good one under pressure. It's possible some ignition connections have higher resistance due to corrosion.

If the problem isn't localized to one side or the other, the problem would have to be something common to them both. And I didn't get that it's running rough, so that would pretty much rule out individual carbs or coils, wires or caps.
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Old 08-05-2011, 09:07 PM   #5
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I'd give the carbs a whore's bath with spray carb cleaner up the main jet stack and the pilot jet. If that doesn't fix it, remove jets and inspect the condition of the o rings as well.
I suspect crud lurking in one or both carb crevices.
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Old 08-06-2011, 12:46 PM   #6
noman OP
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no improvement as yet

many thx for the replies!

timed w/timing light, advance is working and unchanged since last winter's fettling

carb O-rings are new, and still look new


this morning:
checked valve clearances, and are unchanged since last adjustment: 0.006 and 0.008

took float bowls off and checked fuel flow, is unrestricted

suspicious that the air filter still might have restriction after cleaning/vacuuming yesterday, removed air tubes and ran but no change

suspicious that the carb-to-engine rubbers might look ok but have a tiny split that i couldn't see. taped them with scotch 33 electrical tape and ran w/no change

ran with gas cap loose, no change


next up is pull the carbs for cleaning. will post
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'97 900ss/cr (full sp spec)

noman screwed with this post 08-06-2011 at 04:16 PM
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Old 08-07-2011, 12:10 PM   #7
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not runnin' right

stripped the carbs fully down and everything looked fine. sprayed w/carb clean and compressed air anyway. jets clear, new o-rings still look new, no tears in the diaphrams and were installed right, all passageways were clear, floats were set right. idles nicely, revs freely when in nuetral but soft knocking/rattle when shifting at 3500/4000 and low on power.

am stuck. implausible that 3 different brands of gas were crap, but it seems my last option is to run this tank out and try premium (although it never needed it before).

if that fails, it's off to the pro.
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Old 08-07-2011, 02:08 PM   #8
disston
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I thought we were supposed to run premium. I always have in my R90/6. The bike can be set up to run almost anything. I think people in Mexico say they use a thicker or multiple base gaskets to lower the compression ration and use lower octane gas. Or the pistons can be changed for lower compression. But I do think a stock /6 needs premium. Am I throwing away my hard earned cash for octane I don't need?
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Old 08-07-2011, 05:39 PM   #9
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fuel

have never used premium before today. this afternoon i drained the tank and filled w/premium with slight improvement. now no rattle if i roll on the throttle slowly, and surging is reduced. pulls to 75mph slowly, with more in her but perf def down.

at idle (800/1000rpm) the white line is about centered in the window but the "F" mark is a tad low at full advance/over 3000rpm. i think i'll adjust the timing so the "F" is centered at full advance and check perf again.
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Old 08-08-2011, 05:15 AM   #10
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HT coils breaking down under heat/load?
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Old 08-08-2011, 05:52 AM   #11
spo123
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Possible bad plug wire and OR one or both plug CAPS.
Possible POOR connection(s) at coil(s).
Good Luck,
spo
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Old 08-08-2011, 04:24 PM   #12
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Pardon my ignorance... does that thing have a condenser?
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Old 08-08-2011, 05:41 PM   #13
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The /6's normally have points and condensers but this bike was converted to electronic ignition. I'd second that it could be something with the coils, plugs, caps or sparkplug wires.

I was running dual dyna coils on my /5, and the wire connecting the two coils together was bad, which was giving me the poor running at speed like you describe. I just soldered up a heavier gauge wire with new connectors and the problem was solved.
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Old 08-08-2011, 08:03 PM   #14
noman OP
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r75/6

many thanks for other areas to check, i am !

it's an 0400 morning for me tomorrow, so it'll be a coupla days before i try a few things. will adjust the timing a smidge so that it's perfect at 3000rpm's instead of at idle. during the last winter's layover, i sanded/cleaned all ignition wiring connectors, but a simple matter to check again. gonna do that.

it's not a heat-related coil failure, 'cause it behaves this way right off the bat.

am leaning away from a prob with caps/wires 'cause the spark is so bright, huge and snappy. and no change with new plugs. i do have some nice, new NGK 5 ohm caps laying around (were for my honda cb750k refurb, but that'll wait). maybe i'll stick 'em on just to elimanate another possible source.

it's amazing how many things i've tried w/out success.

still have a positive attitude, tho. i'll find it.
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'97 900ss/cr (full sp spec)
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Old 08-09-2011, 11:33 AM   #15
Grider Pirate
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Does it still have a knock between 3500 and 4500? Have you drained some oil to look for the dreaded metallic sheen? Sorry to bring this up..... and I certainly hope it's nothing like that!
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