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E: Engine Unsteady Idle

Martyn Goodwin

Well Known and Active Website User
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?
 

Robert Watson

Well Known and Active Website 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 Website User
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 Website 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 Website 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
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 Website User
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?
 

van drenth

Well Known and Active Website User
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?
not enough play at the moving parts when hot.
van Drenth JC
 

Martyn Goodwin

Well Known and Active Website User
VOC Member
not enough play at the moving parts when hot.
van Drenth JC
Than you - I checked and there is around 0.012 inch end float in the ATD so I think it is not an issue with its moving parts.

Have checked my existing Amal today and there are signs of much wear, so a new carb looks like it will be the fix.

As my Comet is a rider, not a show pony, I am looking at the Mikuni options.

Anyone fitted a Mikuni to a Comet?
 

Martyn Goodwin

Well Known and Active Website User
VOC Member
In a recent thread about carbs I posted regarding an issue I have had with an unsteady idle speed with my Comet. With luck our moderator will spot those posts and move them into this thread.
 

Martyn Goodwin

Well Known and Active Website User
VOC Member
My Comet runs a Amal Mk1 Premier carb, 30mm, that has seen around 40,000 miles of use. I also run the original (refurbished) Lucas magneto and the Lucas mechanical ATD. I have tuned my ATD to give 4 degrees BTDC for starting and 30 degrees BTDC at full advance.

For a long time the motor ran and idled well - nothing unusual being observed.

Around 5,000 miles ago I noticed that after covering around 10 miles that the bike would no longer idle reliably. When I came to a halt the motor would idle as expected but after just a few seconds the idle speed would increase and the only way I found to get it back 'under control' was to put the bike into gear, apply the brake then slip the clutch, thus slowing the motor. But alas, after 5 to 10 secondes I needed to repeat the same process. The situation has been slowly but progressively getting worse.

Suspecting an air leak I removed and partially disassembled the carburettor, cleaning all the jets etc and also replaced the sealing 'O' ring on the carburetor flange. Following reassembly and a test ride - nothing had changed! So again, cursing, I removed the carb and on removing the slide (again) and with the aid of a powerful LED torch and a magnifying lens I was able to see some very small burs in the carburettor body - low down in the channel in the side of the bore that the locating pin on the side of the slide travels in.

Taking care not to add to the 'damage' to the bore of the carb body, with a very small jeweler's file I carefully removed the burs, then reassembly and a test run. The idle issue was improved but not resolved. Now the motor would idle reliable for around 30 seconds before the revs started to rise but the reves did not rise to their earlier level. Just the same I needed the slip the clutch technique to get the revs to drop.

I next turned my attention to the Lucas ATD - a seemingly simple device that relies on centrifugal force to advance the ignition as the engine revs rise and then springs to retard the ignition as the revs drop.

Those of you who have ridden a bike with manual ignition advance will have noticed that when the motor is idling, if you do nothing more than apply a little advance the engine revs will rise - then if you retard the ignition the revs will fall. With this in mind I turned my attention to the ATD.

Looking at the exploded photo of a Lucas ATD I noticed 2 very small shims/washers that go between the rocking arms and the rocking arm ins in the middle of the bob weights. These are tiny; approx 8mm OD x 5 mm ID x 0.25mm thick. I was fortunate enough to locate a source of hardened steel shims/washers of this exact dimension from a local supplier, http://www.precisionshims.com.au/ .

When I disassembled my ATD I noticed that these washers were missing. I also noticed that there are small 'dimples' on the underside of the bob weights that are apparently there to minimise friction as the bob weighs slide back and forth over the back plate.

ATD Parts 1.jpg
With the aim of reducing as much friction as possible I polished the surface of the back plate that those dimples slide over and also gently polished the face of the dimples as well.

ATD Parts 2.jpg

On reassembly I fitted the new 0.25mm washers, and as I had obtained extras I fitted them on all of the pivot points in the atd, 6 in total. My aim was to minimise friction wherever possible. As the ATD springs appeared sound I did not replace them.

On reassembly of the ATD I made sure that there was a generous 0.010" end float after I pressed the retaining ring back into place. A check for free movement within the ATD showed no trace of any unwanted friction with the unit returning to its retarded position with a satisfying 'clunk'

All that remained was to refit the ATD, reset the ignition timing as detailed above, refit the Timing Case cover (needed to remove it in order to remove the ATD), check oil levels then go for a test run.

Well over a 20 local mile run just completed with multiple stops to verify the idle I can report that the problem of idle speed advancing all by itself appears to have been resolved. In a few days time I will be doing a 100 to 150 mile day run and that I suspect will be the ultimate test of my handy work.
 
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