Misc: Ignition ATD Hangup

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
As a number of you will know recently I have been looking at and working on the Lucas ATD fitted as standard to our Vincent bikes.

After a full refurb of my ATD that included new and stronger springs I find that more often than not it performs perfectly for around the first 200 miles immediately after the refurb but after that, when the motor is cool it still works well but once the motor reaches operating temperature the motor is very reluctant to return to idle. With the bike in gear I can slip the clutch to get the motor to idle, but then it returns to much higher revs when the clutch is pulled in or the bike put into neutral.

I suspect that for whatever reason the ATD is not returning to its fully retarded position. I have rechecked the ATD springs and they are pre-loaded in the retarded position.

Anyone else had/having this issue? Any suggestions?
 
Last edited:

greg brillus

Well Known and Active Forum User
VOC Member
Have you enough end float in the shaft where the rear collar presses on........Secondly, often an engine that hangs up in the rev's can be a carburetor problem, even the newer Amal premier concentric's wear quite a bit in the slide/bore.
 

Robert Watson

Well Known and Active Forum User
VOC Member
Two things to look at. I had one that did exactly that. the new ATD gear was right on the Pitch dia but was about .030 oversize on the OD of the teeth. the ends of the teeth were all shiny where they had bottomed in the teeth of the breather gear (twin). In the lathe and the OD reduced. problem solved!
Next make sure that the whole ATD is not going too far on the mag shaft and the press on retainer is all the way on, and nothing is touching the housing causing it to drag.

As for the carbs, I had a new slide that did not have the recess for the throttle cable nipple to seat in. I tokk that thing apart about 5 time before I looked up inside the slide and concluded that he nipple was hitting the top of the jet block and no matter how far I slacked of the throttle stop screw it would not idle down on the front cylinder.
 

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
Hi Greg n Robert,

It idles just fine until the motor gets well up to operating temperature - around 10 miles of travel.

There is a good 0.010" end float where the rear collar presses onto the ATD.

The 'tits' on the underside of the ATD bob weights are well worn, almost non existent - I'm wondering if that is contributing to the problem. If it is, I have a 'fix' in mind.

I'm thinking of lifting the tank to make sure it not a cable issue.
 

erik

Well Known and Active Forum User
VOC Member
The magneto with the Coils is the resistance of the ATD unit.The resistance and the springs are in something like a balance.The resistance of the magneto wants to Retard the Timing. I hope you understand what I mean.Regards Erik
 

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
The magneto with the Coils is the resistance of the ATD unit.The resistance and the springs are in something like a balance.The resistance of the magneto wants to Retard the Timing. I hope you understand what I mean.Regards Erik
HI Eric,, yes I do understand. The issue is that until the motor get really warm the springs can overcome the magnetic resistance. BUT once really warm they seem unable to do so.

My remark about the 'tits' being worn down is more around the friction between the base plate of the ATD and the bob weights when the springs try to retract them.
 

Sakura

Well Known and Active Forum User
Non-VOC Member
A theory that may well be incorrect! A hot engine has less drag than a cold engine. Are you setting the tickover speed too slow with the engine cold? Have you tried setting the tickover lower when hot, which may mean it won't tickover on its own when cold?
 

Bobv07662

Well Known and Active Forum User
VOC Member
Martyn, I have a similar situation with my Rapide but as some here have discovered, it's not the ATD, but a carb issue. When cold or not fully warmed up all is well. When fully warmed up I get a slightly higher idle speed. I determined it to be slide and bore wear in my original rear carb. When it "hangs up" ( about 100 rpm) I can return the idle to normal by just touching the slide with a finger while the bike is at the faster idle. It immediately drops the idle speed back to normal. Not a huge issue for the limited use I give the bike. Someday I'll look at a new set of Amals.
Hope this is of some help.
 

Michael Vane-Hunt

Well Known and Active Forum User
VOC Member
I have had a similar problem with a fast idle on my Comet. It turned out that the end of the throttle cable was not seated properly in the top of the carb. Also saw the twist grip end of the cable not seated on a friends bike.
 

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
Martyn, I have a similar situation with my Rapide but as some here have discovered, it's not the ATD, but a carb issue. When cold or not fully warmed up all is well. When fully warmed up I get a slightly higher idle speed. I determined it to be slide and bore wear in my original rear carb. When it "hangs up" ( about 100 rpm) I can return the idle to normal by just touching the slide with a finger while the bike is at the faster idle. It immediately drops the idle speed back to normal. Not a huge issue for the limited use I give the bike. Someday I'll look at a new set of Amals.
Hope this is of some help.
Hi Bob,

I will remove the air filter then give that a try. My Mk1 Premier has been in use for over 40,000 miles.

btw I have checked the cable and there is some slack in it and, with the motor not running I can hear a definite 'clack' as the slide hits bottom with throttle closure

thanks, Martyn
 
Top