Atlas

Previously gary_currier wrote:

I have a rebuilt atlas, I've timed it as close as I can, it started after many kicks, then ran at about 3000 rpm until I shut it down, checked the carbs, all seemed ok, but wouldn't start again. The mag is supposedly rebuilt, the carbs are good 930 Amals, can't seem to pinpoint the problem. This is the first Norton I've owned, and I'm a bit stumped. Is there anyone in the Worcester area can help ? I would be prepared to financially reimburse them for their time. I didn't rebuild the engine myself so am trusting it was done right by the previous owner, more fool me I hear you say.

well How is it that I can see the problem from hear the word is Amal 930, when it should be Amal Mono bloc 389/87 on a single carburettor you do not need twin carbs any more in this age as you never see any of the benefits of twin carburettors, but you see the benefit of a single carburettor in MPG instead of the MPH, a Amal 389/87 as a 420 main jet and a 20 pilot jet a No3 cutaway side 106 needle the Amal Mk1s tend to run lean So you have starting problems so you need to run he carburettor just a bit on the rich side, and fit some Bosch spark plugs like W8DTC or W7DTC

yours anna j

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The last of the Atlas bikes had 30mm Concentric carbs. These actually work quite well on this Norton once the knack of starting has been mastered. They generally are a little smoother and more efficient.

Anna is quite right about a single carb making life much easier with this bike for many reasons. It is much easier to tune a single carb and you do not have the hassle of two cables to adjust. Plus you will get around 60mpg instead of just 50 with the twin set-up.

The Atlas is a great bike once you get to know how to set it up. I would strongly advise that you make sure that both the carb and ignitionsettings are spot on. If the engine has been rebuilt then there is a high probability that any new pistons fitted are higher compression Commando types in which case the ignition must be reduced to just 28* BTDC.

High rpm with the throttle closed can be due to a carb slide hanging up, or the ignition timing incorrect. Perhaps the carb slides are open too much on the throttle stops.

Lots to think about. Get help from someone who has already owned one of these machines if you can.

It sounds like the throttle slides are not closing properly. Either the idle screws are too far in or the throttle cables are too tight, not letting the throttles close properly. The first thing to do is make sure the throttles shut properly. The Norton workshop manual (and many other manuals) give step by step instructions on setting up carbs. Just follow the steps. There is no reason an Atlas shouldn't run on a pair of 930 carbs, though I prefer a single 928 or 389 monobloc. My monobloc runs on 350 main jet and no 3 slide. Seems to work OK.

Do you have a magneto or coil set-up? Or other?

Commando flat top pistons are generally about 8.9:1 CR. With these inside the barrel you can get away with 31* Timing if you have an electronic ignition system like a Pazon. Otherwise, with the standard auto advance/retard unit calling the shots you must set a 28* advance.

One of the drawbacks of the early single trigger systems was the difference that often occured between the two cylinder firing points. It was quite common to find one cylinder was spot on but the other up to 4* off. This kind of difference can make a sensible idling speed very hard to establish. Even refurbished magnetos can sometimes show around 2* of difference. The Commando went to twin points and this was one of the reasons it was a better engine.

Previously gary_currier wrote:

It's mag alt set up as standard 1964

Well if you atlas is a 1964 it should of had originally Amal 389/87 monobloc on a Single Carburettor , as this is the best set up for todays roads you have loads of grunt low down and it will be right up to 100 mph before it start to peater out but for easy starting I found that these Triple electrodes spark plug work best Like Bosch W7DTC same heat value as champion N5 or W8DTC or Champion N4 are a bit colder ,or you can get NGK B7NTC are about the same as Bosch with Triple electrodes , from green-spark-plugs I been using them In my 650 Manxman now for the past 3 years and I have not taken one out yet, it all way start easy with in 3 kicks and runs nice a smooth even at top rpm so you can only give them a try out, Yours anna j

I may have this wrong, but I thought earlier Atlas' had a pair of Monoblocks, the right hand one of which had it's float bowl cut down & no float or float valve, but with a link pipe linking the jet holder tubes so that both carbs were supplied & controlled from the left hand carb. I've often heard them refered to as 1 1/2 monoblocks. Fine untill you laid the bike down in a long sweeping left hand bend & the right hand carb lost it's fuel.

I would agree that one carb is a much better option though.

Regards, Tim

My 1967 Atlas had a pair of handed monoblocs - a right and a left. Being prone to idleness and not inclined to fiddle about synchronising twin carbs, I fitted a single carb manifold and just used one of the original carbs. Didn't change the jetting in any way. The other is a handy spare. Timing with Commando pistons 28 degreees btdc. If you go to 31 Degrees, pinking is highly likely.

'Well if you atlas is a 1964 it should of had originally Amal 389/87 monobloc on a Single Carburettor'

Above statement is not quite accurate. By 1964 the Mark 1 Atlas (also known as the standard model), that had the single Monobloc carb, had been phased out and replaced by the more familiar Mark 2 (known as the sports model) This revised Atlas had a twin carb set-up. Check out the attachment. This pic comes from the 1963 brochure for export models to the USA.

The twin carb models had 389 monoblocs with the right side chopped. A few years down the line handed carbs arrived (389 & 689) and were fitted until Concentrics took over in 1967.

Attachments
1963-atlas-mk2-export-jpg

Previously phil_hannam wrote:

'Well if you atlas is a 1964 it should of had originally Amal 389/87 monobloc on a Single Carburettor'

Above statement is not quite accurate. By 1964 the Mark 1 Atlas (also known as the standard model), that had the single Monobloc carb, had been phased out and replaced by the more familiar Mark 2 (known as the sports model) This revised Atlas had a twin carb set-up. Check out the attachment. This pic comes from the 1963 brochure for export models to the USA.

The twin carb models had 389 mono-blocs with the right side chopped. A few years down the line handed carbs arrived (389 & 689) and were fitted until Concentric took over in 1967.

yes phil but in our days your much better off with a single carb , for many reasons , but they did do a single carburettor model

for sidecar work there bikes are very good with a sidecar on, lot of low down power

My 67 Atlas has twin Mk 1 Concentrics. It took me a long time and much heartache but they are now running well and tick over smoothly. It is possible to balance them. I bought a two column Morgan Carbtune which attaches to the two tapped outlets on the manifold spacers.

I had similar issues to yours.

My problems started with air leaks. These were both engine side and air side of the throttle. I threw away paper gaskets, which can add to the distortion issue and used a compound sealer. The air box back plate was not flat air side.

If you need loads of choke that may be a sign

Once I had it sealed it became apparent that despite my best efforts the pilot circuit had blocked over a period of standing and I hadn't managed to clear it. Sonic cleaning the carb bodies finally did that.

Overtightened mounting bolts had distorted not only the flanges but the bodies as well. This was causing the throttle slides to stick open which also worsened as soon as the engine started to warm up.

Just a point on timing settings. I believe that the setting could be confused by the fact that some references especially with magneto and mechanical auto advance units point to a setting with full advance as you fix the a/r unit fully open to set the timing?

Previously gary_currier wrote:

I have a rebuilt atlas, I've timed it as close as I can, it started after many kicks, then ran at about 3000 rpm until I shut it down, checked the carbs, all seemed ok, but wouldn't start again. The mag is supposedly rebuilt, the carbs are good 930 Amals, can't seem to pinpoint the problem. This is the first Norton I've owned, and I'm a bit stumped. Is there anyone in the Worcester area can help ? I would be prepared to financially reimburse them for their time. I didn't rebuild the engine myself so am trusting it was done right by the previous owner, more fool me I hear you say.

Hi Gary it sounds like a air leak on the Carbs have you overlooked some thing like the balance tube or have you a stuck slide it will be something simple like that get a can of easy start spray and try a small bit if it starts up then you know it's a fuel problem !Laurie .