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1949 M21 Carb Tuning
#826111 10/10/20 8:58 am
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Howley Offline OP
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Hi all,

The bike is now registered and I can ride it on the road! I've got a few questions about what I'm seeing and feeling.

Settings were:
276 carb
No 4 throttle cutaway
160 main
New standard needle jet
New needle on position 3
B6HS plug
Unknown pilot air

The bike starts first time with the carb tickled until fuel comes out the top of the float.

The running was a bit heavy and it ended up carbon fouling the plug.
I realised the needle should be on 2,so I set that and attempted to set the pilot air. I ended up 1 3/4 turns out, but:

1. Generally the pilot air setting only seems to make a tiiiny, almost imperceptible difference to idling. Is this normal?

After this initial tune it seems to run a lot sprightlier. Full throttle at lower speeds is OK but:

2. At around 70km/h (40mph) after a few seconds at WOT it starts to miss like its choking, maybe fuel starved? What are causes of this?

After getting it home from probably 10 minutes of riding and 5 minutes of idling the plug was sooty black again. The exhaust is sooty too.

3. Is this caused by rich running or am I not running long and fast enough to warm up the plug?

I tried revving on the stand with the needle at its lowest and the plug was still sooty.

4. Is there something else causing the richness here? I'd doesn't seem to be the needle position.

I suspected that the float height was too high causing richness, but the missing at high speed seems to indicate its too low. From memory if I go any higher the float contacts the tickler and the fuel never stops.,and if I go lower the tickler won't reach the float.

5. Any comments on the influence of float height? What is the standard setting?


The bike came with no cross-drilling on the bottom fitting for the fuel line banjo, so I drilled it myself.

6. Is it possible I need larger cross drilling to improve fuel flow? As it is I'd say there's more cross sectional area in the cross drilling than the fuel line. Is it possible to go too big?

7. From my readings it seems I may need a hotter or longer plug, a bp6hs or b5hs. With modern fuel is this required?

Any advice would be much appreciated. This is my first real dive into tuning these old carbs.

Thanks,

Luke

Last edited by Howley; 10/10/20 9:06 am.
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Re: 1949 M21 Carb Tuning
Howley #826136 10/10/20 4:56 pm
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maybe daft...... but a friend of mine had the same problem when he drives his empire star.....when i do drive his bike i do not have that problem....

Why? I pull the choke lever.... he did not 😉

Just a suggestion, because i do not see anything mentioned regarding choke..../ enrichment

And.... do you use an airfilter?

Last edited by Motolab; 10/10/20 4:59 pm.

Harold / Motolab.nl
BSA: WM20 '40 M21,B31 '55 B33 '54, ZB34GS '49, DBD34GS '58
Triumph 5T '49 + T100 + speedkit
Sunbeam S7 '48
Moto Guzzi LeMans '79
BMW R51/3 '51, R100GS '89
Re: 1949 M21 Carb Tuning
Howley #826160 10/10/20 10:32 pm
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Howley Offline OP
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I never use the choke, ie no enrichment.

I'm not using an air filter, just a small bellmouth.

Re: 1949 M21 Carb Tuning
Howley #826164 10/10/20 11:06 pm
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Never use it, or is it not fitted?

If it is fitted, and you do not use it, it is in use.... you have to pull the lever to disengage the choke..

For a 276 on a M21

Data is:

Size 1 1/16
Slide 6/4 (not 4 that is)
Main 160
Needle pos 2
Needle jet 106

Grade Plug has nothing to do with your problem.....

Is your float leaky? Weight? Correct float needle? Float needle undamaged? Float needle seat?


Harold / Motolab.nl
BSA: WM20 '40 M21,B31 '55 B33 '54, ZB34GS '49, DBD34GS '58
Triumph 5T '49 + T100 + speedkit
Sunbeam S7 '48
Moto Guzzi LeMans '79
BMW R51/3 '51, R100GS '89
Re: 1949 M21 Carb Tuning
Howley #826169 10/10/20 11:45 pm
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Howley Offline OP
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The choke is fitted and always pulled ie disengaged.

Your data on the throttle slide is different from what I've read elsewhere. My understanding was that 4 is standard for the m21.
http://amalcarb.co.uk/carburettor-for-a-bsa-m21.html
https://www.draganfly.co.uk/index.php/bsa/bsa/category/998-carburetter-and-air-filters

The float is metal and not leaking. The needle and seat are old but seal well. The carb won't overfill even if the fuel is on overnight.
The top of the float needle is a bit right but still seems to retain its setting.

Re: 1949 M21 Carb Tuning
Howley #826177 10/11/20 3:02 am
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How's the mag?

1 member likes this: Allan G
Re: 1949 M21 Carb Tuning
Howley #826178 10/11/20 3:20 am
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FWIW
I am in Sydney
run BP5 HS plugs for general running around , that would be a ride of 100 Km or less.
On longer rides like the 450 km Walla Walla to Newsted for the All British I run the colder BP6HS

If I run anything else then I have to starve the engine off because if not the plugs will foul then not firing and it will be a bugger to start

As for the pilot air screw ,the maximum you should be able to move it without causing the engine to stall is 1 turn, or 1/2 turn each way if you preffer .
SO your pilot passage ways are not clean or you have the slide stop up too high & you are idleing on the cut away and not the pilot passagways .
BEcayse the cylinder is so squat on he M series bikes they are prone to boiling he fuel in the float bowl.
That is why on a genuine M series v=carb, the float bowl sicks ou to the right side of the engine to get some cold air ( not easy in WA at this time of year ).
Also it should have a long inlet tract about 1.5"
Mine runs a 626 and needs 2" of space between the barrel & carb to prevent the carb overheating.


Bike Beesa
Trevor
Re: 1949 M21 Carb Tuning
Howley #826185 10/11/20 11:02 am
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Nick the mag is fine as far as I know. In my gloomy shed it makes a very fat bluish white spark.

I pulled the carb and checked all the pilot passages, and cleared them all completely by passing a fine wire through them. Everything, jet block included, is clear.

I'm idling with about half advance and dropping the throttle stop until it starts faltering. It seems to respond a little more to the pilot air than before but it's still very minor.

It seems to vary naturally in its idling rpm by a wider range than I can create with the pilot air.

Does the 626 differ in its pilot screw behaviour?

Is it always going to have sooty plugs if it's only idling from cold for a long period?
How long would a plug take to come to temperature at idle?

Re: 1949 M21 Carb Tuning
Howley #826187 10/11/20 11:16 am
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The problem is the mixture of volatile waste solvents sold down here as "fuel" .
Normally I could not stop the engine with the decompresor and expect the bike to restart reliably if there was less than 1 hours running .
ANd that is with a BP5
The direct equivalent of the std 1945 plug is a BP8 but they are way too cool to stay clean with the stuff we have to run on these days.
The Concentric idle works the same way as the 276 idle .
Try setting your idle with the timimg fully advanced
If you are always getting sooty plugs then you may be too far retarded to start with.
For the purpose of diagnosing the engine have a fiddle with the points gap
Increasing the gap advances the timing and decreasing the gap retards it.
If the bike runs substantially better with the points set wrong the you know the timing is off
This is a good test because you can do a dozen runs along the same road wih different gaps in a relatively short time .


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Trevor
Re: 1949 M21 Carb Tuning
Howley #826195 10/11/20 2:06 pm
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Originally Posted by Howley
The choke is fitted and always pulled ie disengaged.

Your data on the throttle slide is different from what I've read elsewhere. My understanding was that 4 is standard for the m21.

IMHO,

If you have a problem, go to the BSA specs as published by the factory.
They use the data i wrote down above...

As i do not know if you use the correct old one, or a new one or a assembled one, i did give you what BSA use to deliver..

6/4 is the trottle designation they used back in the day for M21 276 carb..

Later on monoblock the designation for slides was just 3 or 4 or 5 depending on the bike...

Burlen, now also uses the designation 4 i see in my AMAL stock... but still, you want something resolved.... then you have to know exactly what is needed (and not a 376 slide no 4 converted to 275 use) so double check make sense to me

Last edited by Motolab; 10/11/20 2:07 pm.

Harold / Motolab.nl
BSA: WM20 '40 M21,B31 '55 B33 '54, ZB34GS '49, DBD34GS '58
Triumph 5T '49 + T100 + speedkit
Sunbeam S7 '48
Moto Guzzi LeMans '79
BMW R51/3 '51, R100GS '89
Re: 1949 M21 Carb Tuning
Howley #826279 10/12/20 6:39 am
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G'day Luke,
I'm wondering if you have a mismatch of parts. There are two types of 276 carbs the early one with the four holes around the body above the large mixing chamber nut and the later one without these holes, the jet blocks for each type are not interchangeable.
On the early carb there should be a hole on the opposite side of the pilot air screw where you can see the point of the screw when it is fitted, some times people plug this orifice up and it upsets the pilot circuit. The later carb without the four holes has this orifice filled in.
Post a photo of your carb and lets see what you have.

Last edited by MattL; 10/12/20 6:43 am.
Re: 1949 M21 Carb Tuning
MattL #826287 10/12/20 9:21 am
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Matt,

https://flic.kr/p/2jSNE8S
https://flic.kr/p/2jSSi4c
https://flic.kr/p/2jSNEVJ
https://flic.kr/p/2jSNDvV
https://flic.kr/p/2jST8VS

Looks like the hole just inside the intake has no mate in the jet block. Is it supposed to?
https://flic.kr/p/2jSP31j

Edit, it appears its supposed to. Can this be drilled? Are there other holes I'm missing?

I had been wondering how the heck air gets in to the pilot circuit.

Last edited by Howley; 10/12/20 12:24 pm.
Re: 1949 M21 Carb Tuning
Howley #826347 10/12/20 8:18 pm
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Hi Howley,
You have the prewar jet block , it will not work with the carb body you have
It looks as though the jet block has been attacked with a hammer????
The je tblock needs to be airtight as well as a tight fit in the body or air / mixture will be affected
There are dents and marks on the jet block where it should fit tight in the alloy, if the alloy body is scored or damaged you nee to find another
It might be just easier to get another carb altogether??

John

Re: 1949 M21 Carb Tuning
Howley #826359 10/12/20 9:13 pm
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Well that explains a lot. Yeah the jet block sure has had a hard life, God knows how it ended up looking like it does, probably was sitting on a bench in London during the bombings.

The four radial holes are obviously redundant and the intake hole is missing, is there a chance that it could work if that intake hole was drilled?
It's not that bad where it fits with the body, it's probably airtight there.

I'm having a look at jet blocks. Is the bore measurement the diameter of the through hole (ie the intake diameter)?

Last edited by Howley; 10/12/20 9:29 pm.
Re: 1949 M21 Carb Tuning
Howley #826403 10/13/20 7:40 am
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Luke,

The jet block dia and the intake dia need to match. 1" dia for M20 and 1 1/16" for M21.

Re: 1949 M21 Carb Tuning
Howley #826410 10/13/20 10:37 am
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Thanks Matt. I've got a bead on one in Australia.

I drilled the old jet block and gained a small amount of control of the pilot circuit, but not enough.

I swapped the plug for a bp5hs and it's coming out sooty, but clean at the electrodes. I'm happy with that.

There's still a choke at higher speed at (I'd guess) mid to high throttle opening, could this be related to the jet block or is it something else?
I'm not experienced enough to know if it's a rich choke or a lean choke.

It feels like it feels when you whack open the throttle at too high a gear, but given how it rides in lower gears I don't feel it should do what it's doing.

Any ideas?

Edit. Aah bugger, I've got a 1" intake diameter. Looks like I'm in for a replacement body too.

Last edited by Howley; 10/13/20 12:10 pm.
Re: 1949 M21 Carb Tuning
Howley #826492 10/13/20 7:03 pm
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Hi Howley,
If I had your M21 I would look for an intake extension and a suitable Monobloc carb as fitted to the later M21's

John

Re: 1949 M21 Carb Tuning
Howley #826508 10/13/20 8:27 pm
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John,

I appreciate that that would be easier to live with, but I love the remote float carb and I want to keep the bike true to the period as much as I can.
Maybe I'll regret that later, but for now there's no way I'm upgrading to a new carb.

Thanks for the info regardless.

Re: 1949 M21 Carb Tuning
Howley #826610 10/14/20 10:06 pm
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I feel the need to be a little more straightforward in my language, but meant as advice...sorry for that to start with..

You are looking for help, but i have to say... nobody can help you with a crap carb... unmatched parts
Drilled and “opened up” by somebody who says about himself quote” I'm not experienced enough to know if it's a rich choke or a lean choke.“
And then.... Looking for a solution in the grade of plugs..

Sorry, again... it is not going to work as it can and should work...

Buy somewere a new carb or a really and proper refurbished one... jetted as BSA meant them to be jetted and you will have a good runner (if... the rest of your bike is better than its carb) Plug grades are details for later on, tested on loooong runs, not short trips...

all of my classics do their job on their normal and standard plugs.....


Harold / Motolab.nl
BSA: WM20 '40 M21,B31 '55 B33 '54, ZB34GS '49, DBD34GS '58
Triumph 5T '49 + T100 + speedkit
Sunbeam S7 '48
Moto Guzzi LeMans '79
BMW R51/3 '51, R100GS '89
Re: 1949 M21 Carb Tuning
Howley #826616 10/14/20 11:18 pm
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AMAL sells all the bits for a 276 rebuild.

http://amalcarb.co.uk/generic-type-276.html

maybe you can rejuvenate this one?

or go all the way

https://www.eBay.com/itm/276-1-1-16-AMAL-Premonobloc-Carb-BSA-M21-1946-54/323513641066?hash=item4b52ea686a%3Ag%3A43YAAOSwrGlbzyiD&LH_ItemCondition=3

Last edited by kevin; 10/14/20 11:25 pm.

I would rather be ashes than dust
Re: 1949 M21 Carb Tuning
Howley #826642 10/15/20 6:59 am
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Luke.
It might pay to place a WTB listing for a good used carb body in the sales and wanted section of the WD Motorcycle forum.
John Parker here in Melbourne sells new components for AMAL carbs, he's also a member of that forum,

Re: 1949 M21 Carb Tuning
Motolab #826651 10/15/20 8:01 am
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Hi motolab.

Thanks for your honesty. You're right, and I was ready for the fact I'd have to buy new parts.
I am two things:
1. Practical
2. Not well off.

I knew my jet block was junk, and drilling it out was free and gave me more insight into the working of the carb. I couldn't lose in trying it out.

I'm taking information from all of you here. I've accepted the Australian guys' info on the plugs because I'm using the same fuel as them.
I also completely agree with your assertion that there's no point asking for tuning tips if I have the wrong carb. Until Matt's comment I thought I had the correct carb!

I've since found and ordered the correct NOS body and NOS jet block. All the rest of my parts are new or in good condition so I'm hopeful this will sort the fuelling issues.

Again thanks for all the information and honesty. I'll be back with an update once the new bits are in.

Re: 1949 M21 Carb Tuning
Motolab #826656 10/15/20 9:21 am
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So how many days do you get above 30 C inthe Netherlands ?
Do you buy Australian fuel in the Netherlands ?

When we went from leaded to unleaded fuel the standard serice information was to go up one grade hotter plug and reatard the timing 5 %
This is because unleaded fuel burns colder and slower than leaded does .
On high performance engines you also needd to go up to jets around 10% larger to make up for the lower energy content of unleaded fuels.
When we went from unleaded to ethanol similar tuning information was made available from most fuel companies.

What is written in BSA's service sheets is only correct if you are buring 1954 petrol in 1954 air at sea level.
Otherwise it is nothing more than starting point for proper tuning.


Bike Beesa
Trevor
Re: 1949 M21 Carb Tuning
Howley #826692 10/15/20 4:53 pm
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Yeah you are right... we have still oldfashioned fuel in the Netherlands and the whole of europe.
We use still leaded fuel and there is no alcohol in our fuel. Also it is very cold here.... all year around even in the summer and most important we do not drive our british bikes, we have them only to look at.....

So i think am proposed to only give input, regarding the outside shiny bits of bikes...



On the otherhand... my M20 from ‘40 was designed for pool petrol, very low octane and low energy fuel, there was no lead in it.. we did only did get lead in our fuel in the end op the sixties... in europe that is.... but what do i know...

Last edited by Motolab; 10/15/20 4:58 pm.

Harold / Motolab.nl
BSA: WM20 '40 M21,B31 '55 B33 '54, ZB34GS '49, DBD34GS '58
Triumph 5T '49 + T100 + speedkit
Sunbeam S7 '48
Moto Guzzi LeMans '79
BMW R51/3 '51, R100GS '89
Re: 1949 M21 Carb Tuning
Howley #826696 10/15/20 5:39 pm
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DOPE
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Originally Posted by Howley
Again thanks for all the information and honesty. I'll be back with an update once the new bits are in.

do that.

it's a good looking machine, and its come a long, long way


I would rather be ashes than dust
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