Misc: Ignition ATD Hangup

greg brillus

Well Known and Active Forum User
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

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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

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Non-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 Forum 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 Forum 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 Forum 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 Forum 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 Forum 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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