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Old 02-26-2010, 09:47 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, 09:52 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, 10:09 AM   #3
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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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Old 02-26-2010, 10:16 AM   #4
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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, 11:22 AM   #5
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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, 11:46 AM   #6
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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, 11:52 AM   #7
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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, 12:34 PM   #8
motoboyvfr
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The bike runs very well. No issues other than lean and not holding a tune up. The PO sent me all of the maint. docs from the last 14 years and it looks like he did all the right stuff. Prior to that, a dear friend of mine owned the bike and I did all of the work back then.
I always check the basics when I get a new airhead, I guess I'm looking for something not so obvious. I'll be getting the bike in April sometime and I'll post pix and let you know what I find inside.

Thanks for all the sage wisdom.
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Old 02-26-2010, 12:49 PM   #9
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Quote:
Originally Posted by motoboyvfr
The bike runs very well. No issues other than lean and not holding a tune up. The PO sent me all of the maint. docs from the last 14 years and it looks like he did all the right stuff. Prior to that, a dear friend of mine owned the bike and I did all of the work back then.
I always check the basics when I get a new airhead, I guess I'm looking for something not so obvious. I'll be getting the bike in April sometime and I'll post pix and let you know what I find inside.

Thanks for all the sage wisdom.
Let's say the bike has been gone through and all the tune up stuff is done. How much has it been ridden, and if it has, has been on the open road for a while? Many times if all else if fine airheads will act pissy if they haven't been ridden and really warmed up once in a while - kind of like they are pssied off at having to sit around. Wouldn't hurt to run a couple tankfulls of good gas (Chevron or Shell) through it, and when you fill the tank throw in a tablespoon or so of Marvel Mystery Oil too.

Again, check for manifold leaks and synch the carbs only after the bike has done about 10 or more highway miles.
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Old 02-26-2010, 01:14 PM   #10
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Can't hold a tune up??

Does this mean you get it running well and soon after it does not run so well? How many miles in? Was the engine cold when you set the head torque and sufficiently warm when you set the carbs?
What is out of spec?
Most likely you have carb/ intake issues, like the others said. Maybe bad throttle shaft o-rings that effect the idle and transition charachteristics. Maybe diaphragms?
Have you checked to be sure the points are in good shape, gapped right and timing spot on?
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Old 02-26-2010, 01:56 PM   #11
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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, 03:02 PM   #12
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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, 04:53 PM   #13
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If it won't hold a 'tune' and is running lean, you have an air leak someplace.

Check the carbs, air tubes and connectors by spraying those areas while the bikes running. If theres a change, you have a leak and the rubber carb connectors do need replacement every decade or so.

+1 on the throttle shaft o-rings. If they need to be replaced, several of us here can walk you through that repair. It sounds like a bigger deal than it is and it's something that most owners neglect to do, even if they rebuild the carbs.

Check and come back with your results.
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Old 02-26-2010, 10:01 PM   #14
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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, 10:02 PM   #15
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I agree - there's insufficient data here to make any sort of evaluation.

Please let us know what is meant by 'won't hold a tune'. Will it idle smooth after an adjustment, then later it doesn't? Give us the exact problem.

It sounds like there's no specific data, just what was given by a friend. When the bike arrives, let us know what you experience and then we'll get somewhere! But till then, just study up. Perhaps hit an airhead tech day or two and watch the guys tune them up so that you know the drill when time comes to do yours.
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