Dunno about earlier carbs, but I had similar problems with a monobloc which was traced to a partially blocked idle jet. Mine was backfireing as well and it wasn't till after continual cleaning would the bike run properly.
Spitting back in the carb at low throttle settings is indicative of a weak mixture so check first for any possible air leaks at the carb flange, cylinder head joint and timing setting bolt. Assuming the exhaust valve seat and valve face are in good condition also check that the exhaust tappet clearance is correct and that the exhaust valve lifter is set correctly..If the valve is held off its seat air can be drawn through the exhaust on the induction stroke.
The needle should be set on the middle groove and the pilot air adjustment screw should be set 1-1 1/2 turns out as a base setting.
You should have a 170 main jet fitted and a No4 slide cut away.
If the bike will not run on these settings suspect an air leak as mentioned, a fault with fuel supply or dirt/ contamination inside the carb itself. As you can cure/improve the problem by richening the mixture using the pilot screw it is unlikely to be an ignition related fault..Ian
email (option): ian@wright52.plus.com
You were right and after checking my main jet it was a 160. I tried to replace it with an amal #170 new (gold), but it was too short and pushed the needle into the carb body. I am including a photo of the old (silver) alongside the new (gold) main jet. You can notice the difference in length. Why are they so different and how do i fix please?
https://picasaweb.google.com/lh/sredir?uname=115674442219018009194&target=ALBUM&id=5607440870031716545&authkey=Gv1sRgCND8woW4vanwGw&feat=email
The needle should be set on the middle groove and the pilot air adjustment screw should be set 1-1 1/2 turns out as a base setting.
You should have a 170 main jet fitted and a No4 slide cut away.
If the bike will not run on these settings suspect an air leak as mentioned, a fault with fuel supply or dirt/ contamination inside the carb itself. As you can cure/improve the problem by richening the mixture using the pilot screw it is unlikely to be an ignition related fault..Ian
email (option): jeff@infinitymedspa.com
Hi Jeff..I can't get your link to work..I might be able to later after my computer man has been to sort out some problems.
However, I think your problem may be you have the wrong jet. The fittings on the bottom of the body on 276 carburettors vary in length for different applications and there are two lengths of main jets to suit the different types.
Is the jet you had fitted originally the shorter of the two?...As a matter of interest a 160 main jet is for an M21. You should have a 106 needle jet. The main jet size won't be directly related to your problem if the spitting back is occuring at low throttle settings. Carb function is controlled at that end of the range by the pilot air screw and the throttle cut away.
It is best however to return the carb to the correct base settings as described in my previous post.
When this is done it is important that you establish that you do not have any air leaks before proceeding with attempting to tune the carb.
Potential problems are carb settings, contamination in the carb, restriction of fuel supply and air leaks and you have to start eliminating each possibility in a logical sequence...Ian
email (option): ian@wright52.plus.com
Your doing something wrong Jeff. I'm getting your baby and wedding pictures.......Errr shouldn't the wedding come first
Ron
email (option): ronpier@talk21.com
This is the picture by the way.
Right hand main jet is still screwed into the (late war) stainless steel nedle jet.
Yes, that's a concentric (or monobloc) jet on the left.
The one on the right is the correct type for a 276 with the needle jet screwed into its lower end. Take care when ordering a replacement for the jet on the right that the correct one is ordered...these are the ones that come in two lengths for different applications...Ian
email (option): ian@wright52.plus.com
I believe I ordered the correct one and received it today. (looks the same length) However, shouldn't the new #170 have a larger hole in it than the older #160? It is marked 170 (I checked), but it looks as if the hole is a smaller diameter?? Thanks for all the help
The one on the right is the correct type for a 276 with the needle jet screwed into its lower end. Take care when ordering a replacement for the jet on the right that the correct one is ordered...these are the ones that come in two lengths for different applications...Ian
email (option): jeff@infinitymedspa.com
Hi Jeff, I am assuming you have determined there are no air leaks or restriction to the fuel supply and that the carb is clean internally BEFORE attempting to tune the carb.
Tuning the carb is a pointless exercise if you have an underlying fault.
The needle position and main jet size govern performance from approx 3/4 to full throttle. Main jet size should be correct at 170 so fit that and set the needle position in the center (3) position.
Lowering the needle (or raising the clip to a higher groove) will weaken the mixture. Raising the needle will enrichen it.
Over weakening the mixture will tend to make the engine run hotter.
Your original posting indicated weakness at the lower end to mid range..ie pilot screw adjustment or throttle cutaway. Have you set the pilot screw to its base setting and checked the throttle cutaway is correct? Also have you checked you have a 106 needle jet fitted?...Ian
email (option): ian@wright52.plus.com
Hi ian. I've cleaned and rebuilt the carb. How do I double check the needle jet? I don't see any markings. Thanks
Tuning the carb is a pointless exercise if you have an underlying fault.
The needle position and main jet size govern performance from approx 3/4 to full throttle. Main jet size should be correct at 170 so fit that and set the needle position in the center (3) position.
Lowering the needle (or raising the clip to a higher groove) will weaken the mixture. Raising the needle will enrichen it.
Over weakening the mixture will tend to make the engine run hotter.
Your original posting indicated weakness at the lower end to mid range..ie pilot screw adjustment or throttle cutaway. Have you set the pilot screw to its base setting and checked the throttle cutaway is correct? Also have you checked you have a 106 needle jet fitted?...Ian
Hi Jeff..The needle jet is screwed into the bottom of the main jet. It should be marked on one of the faces of the hexagon (very small!) 106....Ian
email (option): ian@wright52.plus.com
Oops now I am really confused. If you look at my picture I thought the enitre silver part was the main jet? the only numbers on the original silver piece is the very bottom jet that is screwed in and it says #160 (Is that the needle jet and the larger piece is the main jet?) you're help is appreciated.
Hi Jeff, That's it, the upper piece is the main jet, the lower piece the needle jet. The needle jet will unscrew from the main jet. The pilot is just a drilling in the carb body metered by the tapered screw that goes horizontally into the side of the carb body...that controls air supply, not petrol....Ian
email (option): ian@wright52.plus.com
Hi Ian
I thought the main jet screwed into the bottom of the needle jet 
cheers
Pete
http://www.amalcarb.co.uk/AmalExplodedImage.aspx?ProductId=2586&carb=PreMono&Spares=true
email (option): petercomley@web.de
Your quite right Peter..I must stop making postings so early in the morning...
.Thanks for the prod..Jeff would have ended up even more confused..Ian
email (option): ian@wright52.plus.com
and I thought I must have mounted my carb upside down
Pete
email (option): petercomley@web.de
That's one way of stopping the leaks out of the lower fitting.. ...Ian
email (option): ian@wright52.plus.com
I have one small question. When I removed the #160 jet, the diameter of the hole was larger than the #170 jet I installed. Possibly someone installed the #160 and then drilled it out. Should I leave the #160 jet with the larger hole in or continue and replace the main jet? Once again thanks for everything.
...Ian email (option): jeff@infinitymedspa.com
Hi Jeff, Replace the jet with the 170. If the 160 jet has been drilled out there is no way of knowing how much fuel it is passing without a lengthy investigation comparing it to other jet sizes to determine where it has ended up....which will be wrong anyway.
The standard Amal settings for the M20 (or any other bike) will get the bike running. Then if necessary fine tuning can be carried out to suit the characteristics of the individual engine. The standard settings can be regarded as a 'base' setting.
Once this is done if there is still a major carburation fault then other causes, such as air leaks, wear, or contamination, for example, are likely to be the cause....not the basic settings.
In diagnosing any fault on the bike, if it is not obvious, a logical sequence of elimination of the possible causes must be followed. It is then inevitable the fault will be identified.
There are really only two faults that will be found with carb settings...richness or weakness of the mixture...and each symptom has a number of causes which need to be worked through once one of the two has been identified as the cause of the problem.
The Amal carb works in stages throughout the throttle range, with each staged controlled by a different part of the carb..ie Pilot air screw, Throttle slide cutaway, Throttle needle and Main jet.
Each stage 'overlaps' a little with the next.
I would advise obtaining some information on the operation of the carb (it is well covered in BSA factory worksheets which are available in reprint) and acquiring a firm grasp of how the device functions. With that knowledge the diagnosis and rectification of faults will become much easier....Ian
email (option): ian@wright52.plus.com