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

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5 days 18 hours ago - 5 days 15 hours ago #5557 by Blue-1
Replied by Blue-1 on topic More Timing Mark Questions
...4 engine rotations to adjust 4 valves.

I just watched Chris Harris's Airhead Valve Adjustment video on YouTube.
First, he sets the engine at OT, then adjusted the valves, intake and exhaust on one head.

He then turns the engine over 360º, to OT again, and adjusted both the intake and exhaust valves on the other head.
Just two engine rotations.

What am I missing here, why do I need to do 4 rotations?

Update: I followed Chris Harri's procedure and my engine now barely runs at all. Very hard to start.
Last edit: 5 days 15 hours ago by Blue-1.

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5 days 11 hours ago - 5 days 11 hours ago #5558 by 8053
Replied by 8053 on topic More Timing Mark Questions
I watched the Chris Harris video. While his procedure is perfectly correct, what he fails to explain is the "why" of the need to rotate the motor and the need to understand the opening and closing of the valves.

Wobbly's alternative method is bulletproof. If the motor were a parallel twin like a Triumph or BSA, the rockers would be side by side and the mechanical function would be obvious. When one cylinder is on the compression stroke, the other is on the exhaust stroke. The piston is travelling up in the cylinder. It's the same for the intake valves. When one is open, the other is closed because the cylinder is on the power stroke. The piston is travelling down in the cylinder. The ignition and spark plug will have just ignited the air fuel mixture in that cylinder. It is essential that the rockers be adjusted on the cylinder that is on compression and/or power stroke. That is when the valve train is slack and the valves are as closed as they can be. So, when one rocker arm (either exhaust or intake) is raised, and the valve pushed open, the corresponding rocker for the other cylinder will be closed.

The cam lobes are ground and shaped to achieve what is called valve overlap. At the end of the exhaust stroke, when the exhaust valve is closing, the intake valve is starting to open. If you are concentration on parking the OT mark in the aperture, you can not be observing the valve rockers moving at the same time. If a person sets rocker clearance during the valve overlap phase, the full lift of the cam lobes will be reduced, and the motor will not operate efficiently if it runs at all. The Chris Harris video does not cover any of this. If your motor was running well before the most recent valve adjustment, this may be what has happened.

If you employ Wobbly's method, the cylinder that is transitioning from exhaust to intake will present to you the opportunity to set the valves on the opposite cylinder in 1/2 a revolution of the motor. Then in 1 rotation the valves will present the opportunity to set the clearances for the other cylinder. The difference is you will be setting one rocker at a time instead of 2.

By The Way, I believe that the arrow in the illustration in your original post would be the static timing mark.

James Strickland, IL. Airmarshal
Last edit: 5 days 11 hours ago by 8053.

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5 days 10 hours ago - 5 days 9 hours ago #5559 by Wobbly
Replied by Wobbly on topic More Timing Mark Questions
Your issue seems to be that you have not fully digested the discussion over the FULL thread. In it, 2 completely separate methods of how to achieve the correct crankshaft position in order to allow correct valve adjustment are outlined.

Blue-1 wrote: He then turns the engine over 360º, to OT again, and adjusted both the intake and exhaust valves on the other head.
Just two engine rotations.

He's a very experienced mechanic and thus he makes setting the crankshaft position using the electric starter look easier than it really is. If you do not work on motorcycles for a living, then I highly suggest you use the more methodical method of removing the spark plugs, placing the gearbox in high gear, and then bumping the rear wheel in order to position the crankshaft for valve adjustment.

And 'Yes', this static method will only require you to set the crankshaft in 2 positions. Once for the first cylinder, and a second position for the other.

Blue-1 wrote: What am I missing here, why do I need to do 4 rotations?

There are 4 valves in an Airhead engine. When you use the individual valve method (the SECOND method outlined in this thread), you will need to rotate the crank 4 times, one for each of the individual valves.

Blue-1 wrote: Update: I followed Chris Harris' procedure and my engine now barely runs at all. Very hard to start.

Most probably because the stater moves the crank too fast, and usually too far.

When you use the OT mark to set one cylinder's valve adjustments, you absolutely MUST use the very next appearance of the OT mark to set the valves on the opposite cylinder. My friend, you have simply fallen prey to total dependence upon a very small mark appearing in a very small window. That method is filled to overflowing with pitfalls for the novice mechanic. Mainly... adjusting the valves on the wrong engine stroke. This is your engine's current problem.


My friend, you are placing WAY too much importance on a silly mark. The mark is simply an indication aide, but in your case it is not helping you at all. So I would tell you to TOTALLY IGNORE THE MARK and watch only the motion of the rocker arms and valves.

With the plugs removed, both valve covers removed, and the gearbox in high gear... use the rear wheel to turn the engine forwards or backwards until you see one of the rocker arms depress a valve to the full open position. Stop turning the rear wheel and go adjust the same valve clearance on the OPPOSITE cylinder. Say for instance the LH exhaust rocker arm has completely opened the LH exhaust valve, stop and go adjust the clearance on the RH exhaust valve. Then dab a tiny bit of white grease on that valve to indicate it's been adjusted. Finally, rotate the engine until all 4 valves have been adjusted in this way. Yes it will require 4 movements of the crankshaft, but it is a fool-proof method and your motorcycle will be back on the road.

Hope this helps

Don't hide 'em, Ride 'em !!
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Last edit: 5 days 9 hours ago by Wobbly.

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5 days 7 hours ago #5560 by Blue-1
Replied by Blue-1 on topic More Timing Mark Questions
Thank you Wobbly,

Chris Harris's video sure is misleading, for as you say a "novice" like me.

I was thinking, he was somehow catching one cylinder at the compression cycle when both valves on one head have to be closed at the same time. Then it seems then you could adjust both valves on the one head at the same time.

But you're right, I don't fully understand the engine mechanics.

I have been using the rear wheel bump method to rotate my flywheel. Not my electric starter. With the spark plugs out it works pretty good.

With your explanation, I agree, you need to rotate the engine four times to adjust four valves. I was just wondering how Chris could do it in the video with only two revolutions?

Then again, I'm not sure if my old eyes are good enough to catch when a valve fully is opened, or fully closed as I rotate the wheel. Especially when you're adjusting the gap to 1/1000 of an inch.

That's why I've been so dependant on the timing marks to help me and my 65-year-old eyes out.

I'm going to go back and see if I can decipher your adjustment procedure from your detailed explanations of how a 2 cylinder runs. It can be a bit overwhelming for a novice. But what you said about watching the valves go up and down makes sense. I'll give it a try and see if I can get my engine to at least start and run again. Right now, it's totally out of wack! Ha, that's an understatement!

Thanks again...

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5 days 33 minutes ago - 5 days 22 minutes ago #5561 by Wobbly
Replied by Wobbly on topic More Timing Mark Questions

Blue-1 wrote: I was thinking, he was somehow catching one cylinder at the compression cycle when both valves on one head have to be closed at the same time. Then it seems then you could adjust both valves on the one head at the same time.

That's exactly what Chris is doing. But it's far easier said than done !

The 4-cycle engine takes 720° (2 full rotations of the crankshaft). Since the 2 cylinders fire 360° out of phase with each other, one cylinder's worth of valves can be adjusted every 360°. But ONLY one very specific cylinder !! What's happening to you is that the obscure OT mark flies by in the tiny window, and that sets you up to adjust the wrong 2 valves !

But like I said, the OT mark is only an indicator, not the actual item. It's the same as seeing checks in the checkbook and assuming there's still money in the bank. The checks and the account are associated, but not directly linked. In the same way the OT mark is associated with valve movement, but not directly linked. You need to be using a true indicator, and that should be your rocker arms.


Blue-1 wrote: I have been using the rear wheel bump method to rotate my flywheel; not my electric starter. With the spark plugs out it works pretty good. With your explanation, I agree, you need to rotate the engine four times to adjust four valves. I was just wondering how Chris could do it in the video with only two revolutions?

I'm going to go back and see if I can decipher your adjustment procedure from your detailed explanations of how a 2 cylinder runs. It can be a bit overwhelming... But what you said about watching the valves go up and down makes sense. I'll give it a try...

STOP! Let's come up with a fool-proof way to adjust the valves without needing the OT mark, AND which only requires 2 crankshaft positions. Sound good ? Then here it is...

Your New Personal Valve Adjustment Method
1. Ignition OFF. Spark plugs out. Both valve covers removed. Gearbox in high gear. We are going to move the crankshaft by "bumping" the rear wheel in ONE direction, forward or backward. We are going to watch all 4 rocker arms while we bump the wheel.

2. Bump the rear wheel until you see BOTH rocker arms on either cylinder moving at the same time. That is, if you bump the rear wheel forward and then backward (say 10°) thus making the crankshaft linger within a small portion of rotation, and both rocker arms on one cylinder are moving at the same instant, then you have found Top Dead Center for both cylinders.

3. Now you can safely adjust both valves on the cylinder that DID NOT have the moving rocker arms.

4. To adjust the valves on the opposite cylinder, keep bumping the rear wheel in the original direction and fairly soon the 2 rocker arms you just adjusted will both start move at the same time. It is now safe to adjust both valves on the cylinder that DID NOT have the moving rocker arms, which will be the opposite cylinder.


It's that simple. You'd need to work harder than that to fall off a bar stool !

Don't hide 'em, Ride 'em !!
#15150
Last edit: 5 days 22 minutes ago by Wobbly.

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4 days 23 hours ago - 4 days 23 hours ago #5562 by Wobbly
Replied by Wobbly on topic More Timing Mark Questions
Why does this system work ? It's a funny quirk of the 4-stroke engine, but once every 720° of crankshaft rotation, both valves are open for a very brief instant. (One opening and one closing.) And this happens exactly at TDC (what the BMW flywheel mark distinguishes as "OT").



As you can see from the simplistic diagram, the purple cam followers (both intake and exhaust) on Cylinder A have ridden up on their respective cam lobes. Therefore, the rocker arms for Cyl A will be in motion holding both their valves partially open.

On the opposite end of the spectrum is Cylinder B, and both it's cam followers are riding the base circle of the camshaft. Therefore, both Cyl B rocker arms will be relaxed and both their valves are closed. It's only when the rocker arms are in this relaxed state that the valve clearances can be properly adjusted.

Then, with a simple 360° rotation of the crankshaft, the cam lobes point the opposite direction, and the valves can safely be adjusted on the opposite cylinder.

So by watching the rocker arms you can tell what the crankshaft position is with great precision, and thereby know when to adjust the valves.

Watch the rocker arms because they always tell the truth. Know that the flywheel marks are lying to you 50% of the time.

Don't hide 'em, Ride 'em !!
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Last edit: 4 days 23 hours ago by Wobbly.

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