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Misc: Ignition ATD Hangup

Martyn Goodwin

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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

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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

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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

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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

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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?
 

Bobv07662

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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.
 

Martyn Goodwin

Well Known and Active Website 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.
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
 

greg brillus

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VOC Member
I had a Commando here before Christmas that had done about 26,000 miles on a set of new Premier concentric's, in my opinion they were near worn out. The wear in the slide guides was amazing, i had to dremel away parts of the bore to fit new slides.........
 

SteveO

Well Known and Active Website User
VOC Member
This may be nothing to do with it, but when I rebuilt my engine and set the timing up I failed to notice that one of the screws holding the ignition inspection plate onto the timing cover was slightly longer than the others (presumably fitted by PO unable to source correct screw). If it was fitted in the "one o'clock" position it was fine, but if fitted elsewhere it just caught the ATD, which freed it on the shaft and lost the timing so the engine stopped. It took me several rides to figure it out! It doesn't sound like your problem but might be worth a look.
 

Martyn Goodwin

Well Known and Active Website User
VOC Member
Ride feedback.

Out today with the air cleaner removed. When I came to a stop (motor should then idle) the motor started to gain revs BUT when I reached down and stuck my finger into the mouth of the carb and LIGHTLY pressed on the slide the revs fell back to a normal idle. Let go and the revs start to rise. I was unable to feel any movement in the slide when I pressed on it.

I now suspect wear in the carb body and/or slide allowing a small air leak around the slide.

Next job is to remove the carb and conduct a careful inspection.
 

Peter Holmes

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VOC Member
I dont post this to gloat, but many years ago, probably near 30, I was fortunate enough to purchase a brand new original pair of Type 29s, I kept them in a box for a short while before getting round to fitting them, also purchased a couple of adaptor rings from Ron Kemp to enable me fit K&N filters, those carbs have been perfect in every respect, with very little detectable wear to date. Type 29s might still be being made, but it seems like the quality is simply not there, appendages and buckets spring to mind.
 

vibrac

Well Known and Active Website User
VOC Member
Ride feedback.

Out today with the air cleaner removed. When I came to a stop (motor should then idle) the motor started to gain revs BUT when I reached down and stuck my finger into the mouth of the carb and LIGHTLY pressed on the slide the revs fell back to a normal idle. Let go and the revs start to rise. I was unable to feel any movement in the slide when I pressed on it.

I now suspect wear in the carb body and/or slide allowing a small air leak around the slide.

Next job is to remove the carb and conduct a careful inspection.
Try it with something thinner than a finger ... any knitting needles around?
 

greg brillus

Well Known and Active Website User
VOC Member
It wont matter, at 40,000 miles the carb will be due for replacement.........Oh well, at least you know what all the settings and jets should be. I've never been a fan of Mk 1 concentric's, I know there are lots of folk who swear by them, but to me they have always been made of rubbish material that wears quickly.
 

SteveO

Well Known and Active Website User
VOC Member
Greg, it seemed to be a sort of given that as Amal advanced in carb design, the quality fell off; so a Pre-Monobloc is better than a Monobloc, which in turn is better than a Concentric. Of course, none of the street carbs had/have the quality of the race carbs like the TT and GP series. Personally I quite like Concentrics, they're a very simple carb (ideal for me!) which is easy to tune. But their biggest advantage is comparative cheapness, so when they start to misbehave, it's best to just replace them. This was done at purchase of my Bonneville and Commando as a matter of course, so there's no reason to think it's not applicable on other bikes. I have a box full of worn Concentrics in the garage, I'm never going to use them so I often ask myself why they're still there (just too idle to get rid of them?)! What makes me laugh is seeing boxes of old sh**ged Concentics at autojumbles, who would buy them and why?
 

oexing

Well Known and Active Website User
VOC Member
Problem on many cheap carbs is they have zinc bodies AND zinc slides - which is not a good pairing of components. You get sort of seizures and a lot of wear, on slides and bodies. Amal had chromed slides, alu, brass and what not, better than all zinc. So a worn body will require fancy boring the slide fit and sleeve the slide with stainless, brass - or find a coating to take up oversize bore. Or maybe get oversize slides when available.

Vic
 

Bobv07662

Well Known and Active Website User
VOC Member
Ride feedback.

Out today with the air cleaner removed. When I came to a stop (motor should then idle) the motor started to gain revs BUT when I reached down and stuck my finger into the mouth of the carb and LIGHTLY pressed on the slide the revs fell back to a normal idle. Let go and the revs start to rise. I was unable to feel any movement in the slide when I pressed on it.

I now suspect wear in the carb body and/or slide allowing a small air leak around the slide.

Next job is to remove the carb and conduct a careful inspection.
Glad you found the issue. Not sure if anyone still sleeves the carb bores and slides. If the carb has enough time on it to have worn the slide that far, other parts are probably worn and in need of replacement. Since yours is a Comet you automatically qualify for a 50% discount! LOL.
 

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