Clutch issue

[Simon Dinsdale]

What you are describing is a positive feedback loop... Advancing the ignition timing causes the engine to speed up which causes the ignition to advance which causes the engine to speed up which causes the ignition to advance... etc etc

One other point is that if the combustion chamber mixture is ignited such the combustion is complete before the piston reaches TDC, then isn't the power from the combustion trying to push the piston back the way and thus slow it down?

What you describe above in a feedback loop creating a never ending increase in rpm though is not what is happening. On a machine with an atd as in the case of kurtfly's the rpm initially increases due to the throttle been opened. The atd reacts to the increase in rpm by the weights advancing the ignition against the springs.
When kurtfly closes the throttle the engine is slow to settle back to a steady tick over rpm. IF the atd is sticking and thus the timing stays advanced the engine rpm will drop a bit due to the throttle been shut (but not fully) but will still idle fast and not settle to the correct idle due to the ignition still been to advanced. If the springs are weak this will also have the same effect, but slowly the atd may return to fully retarded and the tickover will settle.

The effect of ignition advance on tickover rpm can be easily demonstrated as previously described on a bike with manual ignition control. With a bike with manual ignition ticking over correctly, you then advance the ignition using the lever on the handlebars, the tickover rpm will increase slightly. This is the same situation where kurtfly is in with the throttle closed and a possible sticking atd staying advanced. The only difference is you don't need to initially open the throttle to advance the atd weights as the ignition can be adjusted manually.

Conclusion: advancing the ignition at tickover via manually or a sticking atd will increase the tickover. A sticking atd or weak springs can give a high or irratic tickover. I had exactly the same problem with my Rapide and since fitting a new atd it now returns to a steady tickover as soon as the throttle is shut, but I did check for air leaks and carb problems first.

The above is only one possible cause of unstable tickover. A carb running to weak due to incorrectly set or air leaks at the induction manifold joints can also cause the problem.

As for combustion theory and piston directions etc, I don't know, I'll let somebody else explain combustion theory, but I believe burn time against ignition point has something to do with it. All I report above is what I have actually seen and experienced.

 

[macvette]

What you describe above in a feedback loop creating a never ending increase in rpm though is not what is happening. On a machine with an atd as in the case of kurtfly's the rpm initially increases due to the throttle been opened. The atd reacts to the increase in rpm by the weights advancing the ignition against the springs.
When kurtfly closes the throttle the engine is slow to settle back to a steady tick over rpm. IF the atd is sticking and thus the timing stays advanced the engine rpm will drop a bit due to the throttle been shut (but not fully) but will still idle fast and not settle to the correct idle due to the ignition still been to advanced. If the springs are weak this will also have the same effect, but slowly the atd may return to fully retarded and the tickover will settle.

The effect of ignition advance on tickover rpm can be easily demonstrated as previously described on a bike with manual ignition control. With a bike with manual ignition ticking over correctly, you then advance the ignition using the lever on the handlebars, the tickover rpm will increase slightly. This is the same situation where kurtfly is in with the throttle closed and a possible sticking atd staying advanced. The only difference is you don't need to initially open the throttle to advance the atd weights as the ignition can be adjusted manually.

Conclusion: advancing the ignition at tickover via manually or a sticking atd will increase the tickover. A sticking atd or weak springs can give a high or irratic tickover. I had exactly the same problem with my Rapide and since fitting a new atd it now returns to a steady tickover as soon as the throttle is shut, but I did check for air leaks and carb problems first.

The above is only one possible cause of unstable tickover. A carb running to weak due to incorrectly set or air leaks at the
induction manifold joints can also cause the problem.


As for combustion theory and piston directions etc, I don't know, I'll let somebody else explain combustion theory, but I
believe burn time against ignition point has something to do with it. All I report above is what I have actually seen and
experienced.

Hi,
I have two 1930's bikes with mag dynos and manual ignition advance. With a normal tick over speed, advancing the ignition causes the tick over speed to rise. I also have a series D which is fitted with a distributor with atd. If the timing is advanced by rotating the distributor at normal tick over, the tickover speed increases. Of course, the fuel side must be in good condition.
This is well known and in fact, I ran the Corvette in my avatar for 20 years. These are fitted with an 8 lobe distributor with atd and vacuum retard. They also have a crankshaft mounted damper on which there are timing marks. Over time these marks become unreliable because the rubber in the damper deforms. The method of setting the timing was to plug the vacuum retards , run the engine at 1500 rpm and advance the ignition by turning the distributor until the revs just begin to drop and turning it back until the revs returned to maximum. The conclusion is that at idle, if the engine slows on advancing the ignition, it is too advanced, all other things being equal. Equally, if the ignition is retarded enough at idle, the engine will stop. Consequently timing an engine with "sticky" atd can cause it not to start or to run too fast at idle depending on where the atd is sticking

 

[bmetcalf]

Tuck in a piece of cloth below in the opening to prevent dropping in the spring !

This is good advice everywhere you are working on a component through a cover and it would be a lot of work to retrieve a fallen item.

I started using a short piece of fuel line to wedge open my ATD after reading about it on this Forum.

The late Roy Price made new heavy duty ATDs and the Club Spares Co has taken them over, but at last report hadn't sorted out producing more. Watch their website and order one when they are available. Mine made my bike's slow return to idle go away. The cleaning, lubing and new springs will help, though.

 

[kurtflys]

Did you replace your springs yourself? Do they make spring tools that small or did you just use a small screwdriver ?

 

[bmetcalf]

I probably hooked some wire or dental floss around the spring loop and pulled it over the peg, maybe with needle nose pliers.

 

[Martyn Goodwin]

Did you replace your springs yourself? Do they make spring tools that small or did you just use a small screwdriver ?

I replaced them myself using a small screwdriver BUT I did it with the ATD off the bike.

 

[mercurycrest]

Did you replace your springs yourself? Do they make spring tools that small or did you just use a small screwdriver ?

It's easy enough; 1st, back the nut off a bit... don't go any further, or the fit on the taper will come loose and you'll have to reset the timing. Next, remove the horseshoe shaped keeper and the big washer. Now the springs are exposed and easy to replace. Reverse what you just did and it's done.
Have fun......... John;-)

 

[kurtflys]

Thanks John
On page five I have a picture of my ATD. I only see one nut and nothing that looks like a horse shoe. Can I back the nut of two turns? three? are the horse shoe items under the aluminum disc? Boy I wish I had a schematic to look . I'm sure once I have done this it will be a no brainer. But sometimes these things take on a life of there own. Today I took off the fuel bowl on my Amal 930's and one of the screws fell behind the chain under the battery in no mans land. 20 minutes and I still could not find it. The thought that it had fallen in behind the chain and into the case scared the daylights out of me. I finally laid there and turned the back wheel backwards and after a few seconds out came the nut kind of jammed in the chain. WOW!$#@$#@ was I glad to get that screw. I think the springs will be here friday so I can give it a try.
Thanks

 

[Martyn Goodwin]

Thanks John
On page five I have a picture of my ATD. I only see one nut and nothing that looks like a horse shoe. Can I back the nut of two turns? three? are the horse shoe items under the aluminum disc? Boy I wish I had a schematic to look . I'm sure once I have done this it will be a no brainer. But sometimes these things take on a life of there own. Today I took off the fuel bowl on my Amal 930's and one of the screws fell behind the chain under the battery in no mans land. 20 minutes and I still could not find it. The thought that it had fallen in behind the chain and into the case scared the daylights out of me. I finally laid there and turned the back wheel backwards and after a few seconds out came the nut kind of jammed in the chain. WOW!$#@$#@ was I glad to get that screw. I think the springs will be here friday so I can give it a try.
Thanks

Go take a look at this link and also the photo's at the bottom of the page that you see when you open the link. Should make all clear.

http://brightsparkmagnetos.com/library/LucasATDspecifications.htm

Martyn

 

[bmetcalf]

What Martyn said, plus this sort of larger pic: