The Airheads Beemer Club is a non-profit club reclaiming the 'Legendary Motorcycles of Germany'

Topic-icon Advance Unit - When do I know it's shot and needs replacement?

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1 month 2 weeks ago #6425 by jnicks01
'78 R80/7.

First of all, I've only timed a moped prior to my BMW. So if you am doing it wrong and completely lost, please tell me so.

Using Boxer2valve's Youtube as I was doing it.

I've been trying to set my timing and just can't get it right. Points gap set with northwood tool, new Noris points, new AHU springs (correct ones). I took apart the unit, gave it a good cleaning, and lubed it up properly. Took off the plate and cleaned that up too. You can eat off the points area.

Static timing doesn't seem to want to be as easy as Boxer2valve makes it out to be. I set it at S, but I get a TON of rotation before the points close and my light goes off.
Decided to try and get it as good as possible and started to focus more on full advance instead. The F rides up and out of the window at around 2400 to 2500 RPM. Plate is full clockwise as it is really to only way to get it to start and idle. If I turn it counter slightly, the bike begins to sputter, backfire, and die.

Before and after replacing the springs, one of the lobes on the cam still likes to hang lower than the other. Not as tight as I'd assume it needs to be. Kind of "floppy". Not sure if the advance unit may be the culprit and I need to drop the $200 to get a new one. I'm okay with that, but don't like to buy parts just to buy parts.

Or should I reset my brain, start over, confirm points gap, and see how that works again first?

Ugh. I need a tech day. Dang Covid19.

Electric ignition is another topic for another day...

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1 month 2 weeks ago - 1 month 1 week ago #6427 by Wobbly
Held in your hand, Yes the auto advance unit will feel sloppy and floppy.... It's how it feels mounted in the motor in the running position that counts.

The bob weights will feel sloppy because that's an easy way to keep them from seizing. However, the ignition cam should show no signs of wear, each cam lobe should measure the same height, and when mounted to the engine, the cam should only have play in rotation and axial directions. Any play in the radial direction would make the point gap vary with RPM and thus adversely affect the ignition timing. Good, solid ignition timing is totally dependent upon the ignition cam interacting with the points the exact same way (exact same position) on every revolution.

In the old days all you could do is set the timing statically, and then wish and hope. These days you can visually check the spark with a stroboscopic timing lamp. That slight jumping around of the timing mark that you see is variation in ignition timing. Even when the bike was new, there was some variation, or visual jumping around. It had to be this way because of the dependence upon a loose mechanical system to trigger the electronics.

► There is also another common wear point which you may be seeing. The bob weights slam into the Full Advance position 'stops' every time the throttle is opened. When an advance gets this old, the stops have simply been hammered to death. (It's a very small hammer, but when you do it a billion times...) So because the 'stop' gets elongated, the timing may advance too far. This is one reason we keep telling people to set their ignition timing at Full Advance.

What I would suggest is force a small chunk of rubber or wood into the advance bob weights and 'chock' them open so that the advance mechanism assumes the Full Advance position. The advance's spring pressure will be fighting you on this. This move will allow you to statically time the ignition to the 'F' mark. Then you can remove the chocks and crank the bike. The strobe should show the 'F' mark much closer in the window. The points plate will still need a tweak, but you'll be much closer than you were.

• Point gap (measured at the highest part of the ignition cam) is important.

► I highly advise you NOT to invest any big money in renewing this stock system. $200 is approaching the price of EI, which has zero moving parts (and thus no chance for wear and no required maintenance) and zero variation in ignition timing. Also, the advance curve built into the software of the EI is much more realistic when running modern fuels. So conversion to EI on an Airhead offers only good.

Hope this helps.

Don't hide 'em, Ride 'em !!
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Last edit: 1 month 1 week ago by Wobbly.

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1 month 2 weeks ago #6428 by jnicks01
Fantastic Info Wobbly, as always. I will look into all this and review my conditions. I love the trick of blocking the weights. That is something I am yet to read on the numerous sources of information. If I can't get it or something is wrong, EI is worth the $$ for me. I'll report back on my findings in a few days. No chance to work on it at moment. Essential business and all.

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1 month 2 weeks ago #6431 by Wobbly

jnicks01 wrote: No chance to work on it at moment. Essential business and all.

You just tell them at work that the directors of this Club have designated Airheads as an "Essential Hobby". :P

Don't hide 'em, Ride 'em !!
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1 month 2 weeks ago #6432 by jnicks01
Unfortunately my 4 bosses all ride Harley Davidson. And not 1 tattoo. Go figure

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1 month 1 week ago #6434 by jnicks01
Update....
I got a cool 45 minutes at lunch to fiddle with it.

Well it seems like a little additional research and a reset has given me the solution. And the solution is..... I didn't do it right.

Reset the points gap. A little tighter at .15 in lieu of .16. This now allowed me to adjust the plate without running out of adjustment. Got it static timed to where S is just a hair above the hole, but still visible in the window. I needed something to give my brain a benchmark.
A quick test with the timing at light gets the F in the window. Same hair above, but close. Jumps around a little, but not concerning.

I'll readjust it in the next day or 2 too get it more "perfect". It runs amazing compared to what it did.

So, looks like $200-$400 bucks saved for now.

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