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<?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>
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previously:<o></o>
started immediately, idled at 1000/1200rpm, excellent fueling, steady/strong power to redline, no backfire/popping on decel, ran excellent. <o></o>
<o> </o>
now:<o></o>
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 )<o></o>
<o> </o>
checks:<o></o>
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 <o></o>
compression 150psi on both cylinders w/engine warm, throttle wide open – ok <o></o>
checked main and idle jets – were clear, ok
checked carb O-rings -- ok<o></o>
checked diaphrams and alignment tabs -- ok (were new this spring)
pulled and checked carb-to-engine rubbers – look fine<o></o>
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 <o></o>
drove for 20mles with lights off – no improvement <o></o>
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 3<SUP>rd</SUP> position down from top, tried 4<SUP>th</SUP> position down – no improvement, moved back to 3<SUP>rd </SUP>position down
<SUP><o></o></SUP>
<o></o>
tomorrow:
adjust valves <o></o>
remove float bowls and check fuel delivery
replace engine to carb rubbers<o></o>

any suggestions greatly appreciated!

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.

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.

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.

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

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.

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?

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.

HT coils breaking down under heat/load?

Possible bad plug wire and OR one or both plug CAPS.
Possible POOR connection(s) at coil(s).
Good Luck,
spo

Pardon my ignorance... does that thing have a condenser?

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.

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.

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!

she's runnin' as fine as ever, and i am stoked! here's the details:

my method to solving this running issue was based on the assumption that i had no significant internal anomalies, because of the low miles on my bike. i therefore started with checking for suitable spark (at the right time), an adequate fuel supply that was properly atomized, proper valve clearances, an unrestricted supply of filtered air. all these checks were done with nothing out of spec. during this period i tried varying the timing a smidge, tried differing brands and octane of fuel, enriched the fuel mix a bit and cleaning already clean components.

after all this, there was some improvement and she wasn't really running that bad. a slight rattling at 3500-4000 when on the throttle pretty hard, and some hesitation above 60mph. but she could do better.

thinking i was at the point of checking for signs internal damage, last weekend, i warmed her up, drained the oil and dropped the pan. super clean. hhhmmmmmmmmm, back to square one.

i double-checked valve clearances, finding the right cylinder off just a touch. was probably in a hurry previously, i found that timing was set a degree or two from tdc.

thinking the slight rattle could be caused by unbalanced carbs, i stripped and cleaned the carbs again, this time focusing on the mysterious idle circuits. i used carb clean followed by blasts from a can of compressed air i had just picked up.

i spent a good amount of time ensuring that the throttle cables had identical amounts of free-play, then balanced the carbs.

i checked all coil connections, finding one spade connection that needed a bit of a squeeze. just a bit.

a test run revealed that one or more of the above items has fix't the running issue. runs strong until redline, with light or heavy throttle, and no hesitation at the upper end. i burned up the tank of prermium until nearly dry then filled with regular and had a nice 160 miles run today thru southern vt. runs strong as hell, feels great, many thx!
bobp

I'm voting for the only slightly loose coil connection. Just because it is never that.

Congratulations.

You know to set the valves at TDC (OT) but the timing at idle should be set at the S mark and, most importantly, the timing at full advance should be at the Z.
That will make it happiest!

right now the timing is set such that the painted white line is in the middle of the window at idle, and starts to move upwards as engine speed increases. the "F" is just entering the window at +3K rpms, it's not centered in the window.

i can adjust the F upwards at 3000, but at idle the white line isn't centered, seems like either one or the other, not both.

according to my BMW service manual: "if full advance cannot be reached check the ease of the adjusting cam on the bearing journal".

not sure what this means but

The advance unit needs lubing. It is also the cam for the ignition points. It is held onto the tip of the cam shaft by a small (10mm wrench) nut. Be very carefull when replacing small nut to not over torque. It is the most broken part on an Airhead. The whole unit can be cleaned and lubed with a light oil. If taking the unit apart be careful with tiny pieces. Usually not necessary to disassemble. Use small bit of light oil on spindle when putting advance unit back on the cam shaft. Notice how it works. The advance unit rotates in operation, this opens the points earlier.

The advance unit is available new for something like $280 (it comes up in discussion, not sure how many sell at that price) But only as a complete unit. That is other than the tiny springs. You can buy the tiny springs new and they do need replacing sometimes.

There are also #'s stamped on the advance unit. Thru the years the #'s get higher. Ending in 005 are the /5's and the last edition were the 012s.

Other than the light oil for the advance unit the points rubbing block needs a light grease and usually this is called "points cam grease". There are universal ones and a special Bosch one. We''ll have to dig deeper to get you the info on this. you do need this grease or the points prematurely wear.

But lube the advance unit right now and we'll get back to you with info on the cam grease.