Hello All

I have a carburettor question on my M20. I have been renovating and cleaning it after recent advice on starting difficulties. It was in a bit of a state. I’ve just got jet block back in after heating body and cooling jet block, that method worked a treat. Getting it out was another story. During all this noted evidence of some sealant or other on the base of jet block and threads at base of body where large nut thing holds jet block in. Do I need to have sealant here and on the threads when I re-assemble? See pic.

Also, I’m sure I read somewhere that it is important to keep the small cutaway at the bottom of the threads open. Is this the case? See pic.
Also on a more general question, does anyone know of a good source to explain all the various codes on the carb? There are codes on flanges, main jet, jet block, throttle slide etc.





Cheer Neil

email (option): nlhclarke@gmail.com

There shouldn't be sealant. The composition washer is all that is necessary. 99% of any leakage there is caused by too high a float level or poor seating of the needle due to wear or dirt.

The small opening on mixing chambers with an 'R' suffix should not be blocked. It is an overflow.

Jet block markings are a bit of a puzzle but the others are significant...That said, there are early carb numbers and then later separate numbers for mixing chambers, float chambers and complete assemblies...If you post what numbers you haven I'll try to match them to Amal literature...there are some sources on-line but they're not always that easy to find.

Hello Rick, thanks for reply. Here are the codes I could find:

Flanges 1: AW 5288
Flange 2: 276 ER/1DB
Throttle slide: G/2
Jet block: 30T
Main jet: 158
Float chamb: 64-195


I couldn't find an R next to an orofice so not sure what mixing chamber is.

Cheers Neil

email (option): nlhclarke@gmail.com

The carb for an M20 is 276C/1B but basically a 276 carb can be configured to most applications. Yours is set up wrong.
The Throttle slide should be a 6/4 (You might be mistaking the 6 for a G) The number after the 6 denotes the amount of cutaway at the mouth of the carb. That number represents the amount of 1/16" cutaway ie 4 = 1/4"

Your main jet should be a 170 and the float chamber should be vertical (no angle at the banjo).

Also with the jet block removed, it's advisable to poke some wire through the little hole in the throat at the flange end and also it's corresponding drillings in the jet block. These can easily get clocked with verdigris. See pic attached.

Also I just noticed in your picture that you have the long version big union nut at the bottom of your carb. An M20 has a short one. The best firm I've found for carb parts is https://www.hitchcocksmotorcycles.com/partsbook/179/amal-pre-monobloc-type-276 Ron

email (option): ronpier@talk21.com

At the link provided, a new item, air intake assembly, gauze and clip item 51, is available. Is that filter intended primarily to avoid back fire flame starting a fire? Or is it just a dust filter?

Thank you.

email (option): unpob@yahoo.com

Yes Henri they are not classed as a filter, more an air intake/flame trap. They usually consist of 2-3 layers of fine gauze, so will stop fly's getting sucked in. https://accessories.hitchcocksmotorcycles.com/Amal-Carburettors/Air-Intake-Assembly-with-Gauze

Ron

email (option): ronpier@talk21.com

Hello Ron,

many thanks for all the info, it looks like my carb has some non-standard parts, potential wrong jets, throttles, and float chamber angles/positions. I don't have much info on bike history, but it has been laid up for years so maybe unchanged for a few decades at least but ran. Obviously, I have no idea how it ran. Once i get it running, I’ll try some of the tips indicated in the AMAL doc in the technical section on this site.

I noted the needle position was at position 4 (4th from top)
So, recommend should be:

Throttle 6/4
jet 170
Needle position 1 (top)

Mine is:

throttle 6/2
jet 158
needle position 4 (4th groove down)

Do you think that some of these mismatched might compensate for each other? E.g. needle higher up (richer) offsets wrong jet or wrong throttle cut out?

Also, you mention float height and leakage. Is there a procedure to set float height? I placed it towards the top of the needle shaft as it was around that position on dismantling.

Cheers
Neil

email (option): nlhclarke@gmail.com

Hi Neil
With this assortment of parts you've really jumped into a fairly deep part of the pool. Some of these components would compensate for others in the sense that they'd affect the running but how much good that'd do is open to question. Assuming the motor is in standard condition it'd be better to just buy the appropriate parts & fit them. As others have noted the bits can be had. And there's lots of information here.

The recommended needle position is 2. The original float needle has only one groove which is set for the brass float. The modern needles come with two grooves. The top one is for the brass float, the bottom one is for a modern nylon float.

The big union nut on your carb will alter the float height which is why you need the short one. If you email Hitchcocks with the current details of your carb and that you want it set up for an M20, they are usually more than obliging. Ron

email (option): ronpier@talk21.com

Sorry Neil, I missed your reply at the weekend.

The 276ER/1DB is listed as being fitted to the 1950 - '53 A10 Golden Flash and it's a 1 1/16" body so someone has apparently played with the settings to achieve decent running on the M20 which ran a 1" body.

Flat level can be checked by running a clear pipe up from the float bowl drain screw. Correct level is in line with the pilot air screw and certainly not higher

Hello folks,

again, many thanks for all the useful comments and recommendations. There's a lot to unravel here, it looks like the deep end indeed. I need to retire to spend proper time on this LOL.

So, it would appear I have the completely incorrect carb off another bike the A10; isn’t that a 650 cc twin?? Maybe all that needle/main jet/cutaway modification is to allow running a 500-cc engine?? You say throttle slide bore for this car is 1 1/16" . I measured the carb throttle body bore and slide with digital callipers and comes out at 30.06 mm and 29.89 mm respectively (mean of 4 measurements each). This doesn’t quite match the conversion of 1 1/16” which comes out at 26.99 mm which seems a long way from 30 mm ish?? Something adrift here??

In any case is it possible to add parts to such a carb body I have to return it towards some sense of a standard form? It seems to me that you would need to start with the correct body (1” bore) and float which seems like a new/used carb of correct type? Sounds expensive. Do you think Hitchcock’s would be able to advise on using existing carb body and what might work with it?

On the float question, I have a brass float and needle does indeed have only 1 groove. I have a feeling I did not seat the clip in the groove; need to check so height will be out. On initial fitting it leaked fuel from around that banjo union (big nut; item 41 in Hitchcock’s list) and or the large cup item 26. (Union nut shallow). I was unsure how tight they should be and used old seals so schoolboy errors apparent here. Ill get a seal kit give them a bit more torque with correct float needle settings.

cheers
Neil

email (option): nlhclarke@gmail.com

No Neil the bore is the passage into the cylinder. Measure it with your vernier at the flange end. I don't think It will make much difference if you configure it correctly for an M20. Or if you feel like spending about 300 quid you can buy a complete new carb. I don't think you can buy a new body on it's own. Ron

email (option): ronpier@talk21.com

Hello folks, a bit of an update.

Well I got her going today after a bit of faffing about. Initially I could not get it going apart from with a squirt of stater spray in the intake upon which it would run for 5s. This indicated fuel starvation to me. I have no tank on at the minute and am topping up open float chamber with a jug. I thought I'd adjust float and make it higher by moving the clip off the needle's single groove. I guessed a bit as haven't figured an easy way to attach clear pipe to it as Rik suggests. Lo and behold the bugger started and ran ! Bloody amazing; 1st time in 38 years. It ran really smoothly with a little bit of advance, no choke. Oil recirculating, ticks over very evenly. There is no smoke at all and engine seems very quiet mechanically, almost like a Japanese in terms of no knocks, clunks or whines.

Next step is to temporarily put tank on with my new tap and filter to give it a run to see how it goes through different engine speeds. Check plug for colour, not sure if that's a good guide on such an engine or does it take many miles to give clear indication?

Anyway thanks to all for the help provided so far, I'm sure I'll be asking more.

Cheers Neil

email (option): nlhclarke@gmail.com

just a thought, if you are not worried about originality you could get a new concentric carb from Hitchcocks (or elsewhere) for around 140 quid and be pretty sure that your carb problems are over.:slightly_smiling_face:

Hear hear

John 👍

email (option): john.kater@chello.nl

I did however, myself, treat my B33 to a new 289 for 300+ quid a few years ago.:grinning: moment of madness after a tax rebate!