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Old 06-15-2014, 04:10 PM   #1
CookieMonster OP
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Airhead cherry

New to the airhead scene. Love it so far. Just bought a 74 R90/6 with 45k miles. needed a little TLC. Total rebuild on the Bings. (ouch!). Master cylinder rebuild working so far. I installed new pushrod seals as well ( leaking ). I thought it was running ok when I bought it but it would stammer in top gear under load. Come to find out when I dismantled the carbs one needle was set a two and the other at three. Runs better now. Still a little hesitant on the whole carb sync thing. I need to get a sync mate. Does anyone know what it means when I see double s in the timing window as Im checking the timing. Mabye the ignition advance mechanism is bad?
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Old 06-15-2014, 04:42 PM   #2
Jon-Lars
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What kind of ignition do you have? Sounds like a Dyna with a pickup that is off.
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Old 06-15-2014, 05:00 PM   #3
disston
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I double image or a ghost image in the timing window is not conducive to a well tuned bike. One plug may be in time and the other advanced or retarded. Makes for a loppy running Airhead that gets a lot of it's character from the fact it is even firing, or was.

If you have the stock ignition or an aftermarket it can be fixed. First we need that info, stock or what?
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Old 06-15-2014, 05:52 PM   #4
CookieMonster OP
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Stock ignition

It's a stock ignition and it does look a little old although the points and condenser and newer
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Old 06-15-2014, 05:53 PM   #5
CookieMonster OP
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Stock ignition

The manual says I might need a new ignition advance mechanism?
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Old 06-15-2014, 06:18 PM   #6
Bill Harris
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Quote:
The manual says I might need a new ignition advance mechanism?
Piffle. Priced one lately?

12 11 1 356 286 advance unit 1970 - 09.74 $ 282.40

Likely cause of the double timing marks is a worn timing chain. At 45K, that is possible. How does the "F" mark look? A double "S" mark is inconsequential. Another cause may be that the "quill"-- the extension of the camshaft that the advance mechanism rides on-- has gotten bumped and has some radial runout. It can be checked and straightened.

Make sure the pints are good and just don't "look" or "seem" good. Ditto condenser. And that the points cam has points cam lubricant on it and that the point gap is .016".

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Old 06-15-2014, 06:20 PM   #7
disston
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A new or another advance mechanism may work but I tried several when I had this problem and there was never a change with any other unit.

There are several theories about what has happened to make the timing off but it does not mater what causes the problem. The fix is the issue. Here is the article from Duane Ausherman that will give you the background and describe the fix. Duane is a /2 guy mostly and this article is about /2's I think but the principle is the same on your bike.

http://w6rec.com/duane/bmw/timing/index.htm

Some riders have had success measuring the runout of the camshaft quill and thereby been able to make more judicious adjustments. I did it by the hit or miss method described by Duane and mine has been fixed for 4 or 5 years.
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Old 06-15-2014, 06:27 PM   #8
disston
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You are going to try a new set of points, grease the rubbing block. Then a new timing chain and maybe a new cam gear. The crank gear is a given with Airhead timing chains, always a new crank gear.

Then you'll be back for the FIX.
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Old 06-15-2014, 06:33 PM   #9
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OK, I don't know from /2's I guess. I will try to give a brief description of the FIX.

Using a brass drift and a small hammer. Bang on the side of the advance unit enough to bend it slightly. Not much is needed. Check timing again. If marks are getting farther apart try another place to bang on. If marks are getting closer together bang some more.

If you have the tools to measure the runout it can be easier but the hit or miss method works also.
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Old 06-15-2014, 07:05 PM   #10
Bill Harris
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Random thwacking on the quill will end up being a major miss.

Standard procedure for ANY situation like this is to use a dial gauge, rotate and carefully tap the high side til the runout is zero'd.

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Old 06-15-2014, 07:06 PM   #11
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Diston,
When run out causes this, one lobe will open the points more than the other lobe. Lightly tap the "high side" towards the "low side." One or two light taps and Bob's your uncle. No need to guess.

edit: the lobe that opens the points more will also open them first. Tap the side that opens the points first towards the other lobe.
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Old 06-15-2014, 07:49 PM   #12
disston
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When I did this I tried to determine which way the tip, now know as the quill, was bent. If I could see which cylinder fired early and which fired later I would know which way the quill was bent. It didn't seem to matter or I was still not on the mark but I tried a few different parts to bang on the advance unit. Eventually it moved the two images closer together. One more good tap and I had a steady timing image.

I put a mark with a magic marker on the advance unit so I could tell from one smack to the next which way it was orientated.

I took several days of playing with this to get it right. Maybe 5 or 6 trials altogether. I didn't want to rush.

Some have done this with dial gauges and they have reported good results. I didn't have any luck setting up the old dial gauge I have. I don't have a lot of experience with this tool.
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Old 06-15-2014, 08:00 PM   #13
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Originally Posted by CookieMonster View Post
The manual says I might need a new ignition advance mechanism?
if you take off ignition advance to clean .. use a 1/4 drive in lb torque wrench and be really careful not to over-tighten nut!

be really careful not to over-tighten!

be really careful not to over-tighten!

airheads have lots of aluminum threads .. a 1/4in drive in lb torque wrench is a really good idea, if you don't already have one.
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Old 06-17-2014, 06:06 AM   #14
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Where in upstate NY are you located?
Before you start banging away at your fragile advance mechanism, remove it to make sure it's not rusted on to the quill.
Clean the quill and reinstall using Bosch Distributor Cam Grease.
http://www.pelicanparts.com/catalog/SuperCat/911M/#
Part # 5-700-002-025 (Lifetime supply!)
The thin shmeer of grease helps to "center" the mechanism on the quill.
What you see is called "timing differential", generally caused by some fault involving the advance mechanism.
As recommended, you should replace points and grease the felt and rubbing block.
At 45K I doubt your AM would require replacement. It's highly unlikely that your cam is bent.
Do you get the same "ghost" on both sides?
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Old 06-17-2014, 06:33 AM   #15
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Yes, the quill can get bent but it's not as common as folks think it is. Often it's poor mechanical judgment that causes problems with ghosting. Ghosting isn't all that bad unless it's a really long way out, in which case you NEED to find out why.

I find the ghosting is often caused by abuse to the cam quill and advance unit. Something as simple as not using a wavy washer under the nut holding the advance unit in place can be the start of problems. The wavy washer is there not only to help lock the nut in place but to also protect both the quill and advance unit from damage. Anyone that has had trouble getting the advance unit off after removing the nut can attest to that. Just running the nut on without the wavy washer will burr both the cam quill as well as the top of the advance mechanism, causing a kind of "rivet" situation.

Another way to damage the advance mechanism AND the quill is trying to turn the engine by means of either the nut on the end of the quill or by using a wrench on the advance mechanism itself (I've seen folks do both).

If the quill is bent, the best way to deal with it is (like what's already been said) is a dial gauge. To just beat on the advance mechanism or the end of the quill because you can't get the ghosting to stop is just plain silly. This is where you will find out that the quill and the cam are two separate pieces and yes, I'm here to tell you they can come apart! Once a quill has come out, it's not that easy to get it back in the correct position, it can be done but it's a lot of screwing around.
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