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Old 02-26-2010, 10:47 AM   #1
motoboyvfr
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Airhead Experts?

I just purchased a 1976 R75/6 from a friend in MA. He's convinced that it needs a timing chain because its "running a little lean" and "won't hold a tune-up". I'll probably just replace the timing chain since I have to get in there to look at it anyway.
My question is this: What other issues might cause the bike to run a little lean and not hold a tune-up?

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Old 02-26-2010, 10:52 AM   #2
Mista Vern
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To really gt at the core of the problem you/he must make sure everything is sorted out through a good tune up: torque heads, adjust valves, timing, look for leaks in intakes, carb synch, etc.). If that isn't done you will be just shoting in the dark. That said, sounds like the carbs or intake are wonky though.
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Old 02-26-2010, 11:16 AM   #3
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Yeah, as MistaVern says, more likely to be carbs and intakes than the timing chain..... but always a good idea to go through every thing... check compression, re-torque heads, adjust valves etc, and then get into the carb issues. Change all fluids as a prophylactic measure as well: engine oil and filter, tranny lube, driveshaft lube, final drive lube. Good idea to inspect and grease swingarm bearings and steering head bearings as well.

I would devote my energies to the above, rather than getting into the timing chain area first off......

Going through it all gives you a "base line".... you know exactly what is what, exactly when the fluids were changed, etc etc. These bikes are getting old, and there are a lot of things that maybe shoulda been done that weren't.....
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Old 02-26-2010, 12:22 PM   #4
opposedcyljunkie
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On my R100S, I could turn the alt rotor some 10deg even before the flywheel started to move. Then I knew the chain had too much slack and needed replacement.
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Old 02-26-2010, 12:46 PM   #5
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Quote:
Originally Posted by opposedcyljunkie
On my R100S, I could turn the alt rotor some 10deg even before the flywheel started to move. Then I knew the chain had too much slack and needed replacement.
Correct me if I'm wrong, but you really have a problem if you can turn the alternator rotor without seeing the flywheel move. Not just your cam chain is worn.
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Old 02-26-2010, 12:52 PM   #6
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In my case, it was just the chain. After replacement, there was absolutely no slack. The mark on the timing window was solid when viewed with a xenon light.
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Old 02-26-2010, 02:56 PM   #7
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Quote:
Originally Posted by opposedcyljunkie
On my R100S, I could turn the alt rotor some 10deg even before the flywheel started to move. Then I knew the chain had too much slack and needed replacement.
While you may well have needed a timing chain, unless your alternator rotor was loose or your crankshaft broken, the alternator and flywheel bloody well better move together, since they're both bolted directly to the crankshaft!
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Old 02-26-2010, 04:02 PM   #8
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Hey Steve! You? a BMW?

Hey if its got less than 70K or a million miles or so miles, it probably wont be the timing chain. Its fairly easy to diagnose those though- even though they are all less slack than OCJ's
It will show up as a timing mark with ants in the pants. It wont sit still no matter what. And it can sound like a bunch o rocks in the front cover.

It sounds like its got a intake vacuum leak someplace. Will be a little devilish to chase down but no big deal.

Have you even seen the thing close up?
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Old 02-26-2010, 11:01 PM   #9
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Quote:
Originally Posted by Grider Pirate
While you may well have needed a timing chain, unless your alternator rotor was loose or your crankshaft broken, the alternator and flywheel bloody well better move together, since they're both bolted directly to the crankshaft!
OK my mistake! I think I was turning the engine through the camshaft when the engine was out of the frame and the timing cover removed. That should explain it
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Old 02-26-2010, 11:09 AM   #10
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Quote:
Originally Posted by motoboyvfr
He's convinced that it needs a timing chain because its "running a little lean"


I would take his advice with a GIANT grain of salt. You only need a timing chain if there is rattly clacking coming from under the front cover and/or your timing marks get really jumpy under a timing light. There's no way your timing chain will affect your mixture. That's just a weird thing to say. I'd agree that it sounds more like a carb/intake/air leak issue.
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