Questions? Looking for parts? Parts for sale? or just for a chat,

The WD Motorcycle forum

WD Motorcycle forum
Start a New Topic 
M20 general performance/power expectations

Hello folks, in my pre- proper renovation work I know have a working and useable bike and have done 30 miles on it now, hurrah !

This is after getting new 3/8" chain on, full strip of carburetor and other bits and pieces. It brings me to the question of what I can expect performance wise. I have only had large Japanese bikes before and obviously 13 hp is a different ball game.

It seems to go along on the flat at 45 to 50 mph eventually; I read somewhere that it should do 70 mph but that seems unlikely. I hit one of our local large hills here in Brighton and I had to go into 2nd gear and flogged up it barely 20 mph. It makes me wonder if I'm getting all that power. By the way I'm not too heavy at 12.5 stone so hopefully not labouring it too much !

The plug is close to correct colour, with central electrode and I insulator being brown; outer plug part threaded into head is black but overall it makes me think carburation not too bad. Early posts concluded I have a non standard 276 carb for m20 and main jet also not standard, but the combo seems to work.

I bought the correct jet; a 170 as apposed to my 154. I was also thinking of experimenting with needle jet.

However before I tinker I just wanted to get an idea of what to expect or ways to confirm power output ok. I'm not doing this to hoon around at top speed, I just want ensure its running at optimum level.

Cheers as ever


email (option):

Re: M20 general performance/power expectations

A WM20 solo has a 19T engine sprocket as standard which is often changed to a bigger sprocket to up the gearing (20-21T) This is fine in Holland or Norfolk, but can lead to lots of down changing in hilly areas. I've upgraded my engine to M21 status and I still found the gearing too tall with a 21T sprocket and changed it to 20T. Ron

email (option):

Re: M20 general performance/power expectations

13 BHP combined with the M20s weight won't produce exhilarating acceleration or top speed...Designed top speed of the M20 was 60mph give or take...Some will do more than that but I treat claims of unusually high speeds from standard engines as dubious and I've had plenty of experience with them...

As Ron says there's little point raising the gearing much with such limited power...Gearing changes move the power around but it's still the same amount of power!!...What you (might) gain in one area you'll lose in another...

The M20s virtues are really strength, durability and reliability at which it excels in my opinion...
For more performance, enlarge the engine (a lot), get into the black art of tuning side valves, buy a different bike or forget it and enjoy the relaxing ride..:laughing: :laughing: ...I've run mine at 720cc for years and it makes for a reasonable tourer with better cruising speed and quite a bit more go up the hills. However, at the end of the day it's a glorified stationary engine with a wheel on each end and has the performance to match...Ian

email (option):

Re: M20 general performance/power expectations

Thanks for the replies Ian and Ron,

I'm not looking at upping the power or anything like that, I just wanted to get an idea how 'normal' it is with its present level of performance. I getting the idea that maybe it is normal. I'm quite happy to potter along at 50 mph if there is nothing amiss with the bike, that's fine. I'm not sure of the gearbox sprocket as dentistry yet but rear is standard at 46 tooth.

I still have a few things to check like tappets, there's a small rattle when hot. I've not checked timing yet either although don't have have tools to change it yet. It can't be too far out as idles very evenly when warm. At least when float bowl is not overflowing!!

That's probably subject of another post, intermittent sticking float. Sometimes good sometimes bad, depending on how much tickling has been done.

Cheers Neil

email (option):

Re: M20 general performance/power expectations

Twelve and half stone is fairly substantial by 1930s standards but it sounds as if your're a little down on power.

I'm not an M20 owner, but I've ridden with them a impression is that they have quite a gap between third and fourth so if they come off the boil in top, there is quite a reduction in speed, but unless they're forced to slow down by traffic, they should be able to pull third up most inclines.

They do though date from an era when most commercials travelled at 25 mph and a small saloon ran out of wind at 45...

Re: M20 general performance/power expectations

Haven ridden the bike as fast as it will go on a number of occasions, my M21 will do 60 mph on level ground. I have raced against probably 10 different M20's and in stock form they will do 55 mph.

On relatively long steep hills my bike will do 40 mph at full throttle in 4th gear. I have considered a bigger sprocket but there are lots of hills and mountains near where I live. I don't think the bike would be happy with a 21 tooth sprocket.

When I hear claims of a M20 being able to do 70 mph, I don't believe it!

I have one of Ian's 720 cc pistons but only recently have I found a 1953 barrel with no liner to put it in. We shall see how fast it goes then?

My M21 is happiest going 45 to 50 mph. It does not vibrate and runs very nice at these speeds. It will go 60 mph all day long if needed but does not like it. Fuel mileage on a low compression side valve at full speed is not good.

And finally the M21 will be put to the test tomorrow. The BSA club is having a ride in the morning that is 200 miles long and will go up to 9000 feet. I will probably only only be going 20 mph at that altitude.

Re: M20 general performance/power expectations

I had mine geared up way too far in order to try & keep up with the A65's
Fitting a 932 concentric carb made a big difference to the power ( or lack there of )
It would do close to 80 mph on the flat & downhills but anything that looked like a grade would have me down to 2nd desperate to avoid dropping into first .
It also guzzeled fuel which is no surprise as it was wearing a 932.
I don't do club rides any more so have reurned the bike to almost std gearing .
I find the trick is to down shift early
The massive flywheel has a lot of inertia but once it slows down then you are history so it is back to first & slowly build up momentum again

Reverting to a 626 has restored the fuel consumption to around 60 mpg ( was just under 30 ) and riding at a "gentlemans pace" has restored my sanity .

You just have to keep the engine spinning at around 3500 to 4000 ( which is around lawnmower speeds ) to ensure you are on the right side of the torque curve
Once you have gone much over 4000 all you are doing is blowing unburned fuel out the muffler.
From memory peak torque is around 3900 and you get the best performance just over that so as the engine slows a little the torque increases helping you maintian what ever speed you are doing.

Eventually you will work out that retarding the spark slightly at lower revs will give you a bit more power, but it is only a small amout that makes the big difference
Too far & all are going to do is coke up the engine

Check you timing on the road
On the level retarding the spark a little should reduce your road speed
If it makes little to no difference then you have set your spark too far advanced

Re: M20 general performance/power expectations

So you put a 32 mm carb on a 27 mm inlet port? That would not work at all unless you modified the port inlet up to 32 mm. You are gonna have a huge step where the carb mounts to the port. Which should have made the bike run very bad. Also the bigger carb I would imagine will do little if anything to maximum power as the breathing restriction is not the carb but the crazy path the air has to go through to get to the piston.

To go 80 mph you must have had a very big engine sprocket which would make going up hills very difficult. Then there is the problem that to go 80 mph you would need at least 20 HP. My 350cc Velocette which has 17 HP can only do 70 mph. A 1960's Triumph 500 twin which has 32 HP can only do 90 MPH.

Re: M20 general performance/power expectations

I agree, going 80 MPH on level ground with only 13 HP is impossible regardless of the gearing. The 13 HP is all you have, a bigger sprocket will not make more power. You will pick up some speed but not enough to do 80 MPH!

The bigger carburetor will not help as it is not restricting the engine but everything else on the inlet tract is.

Just for a reference, a Triumph 650 Trophy has only a 28 mm carburetor. If putting a 30 or 32 mm carburetor on the bike would have made it go faster, Triumph would have done it.

Re: M20 general performance/power expectations

Yes it was very overgeard on both the primary & secondary sprocket
As for the carb spacer it was a lot bigger and longer to get the carb behind the magneto
originaly done because I was supposed to lead a mystery ride and the carb would just not stop flooding so I grabbed what was on hand & shoved it together, carb from the B50 spacers from A 65's & B40's held togthe with a couple of bolts till I made up some longer studs
And it would not idle reliably either which was no surprise
As for the speed got dozens of witnesses who have ridden with me while that set up was there and we did way too much expressway riding ( another reason why I don't do club rides any more )
On the flat it would wind out on expressways that are 110kph and keep up with the traffic till it came to a rise when it was valve bounce in 3rd to try to avoid going down to 2nd
On all other roads it was run as a 3 speed because it would not pull in top on anything but flat ground
So yes it did do those speeds
It also guzzled fuel & blew a substantial amount of oil out the breather which was also no surprise as it spent a lot of time being reved way to hard .
You can say what you want but I rode that bike in that configuration for around 8 years over around 50,000 miles on every club ride + rallies up & down the coast including 6 BSA Nationals & 2 internationals
It was also very hard on clutches as every start required clutch slip to avoid stalling .
And because it was always being over reved also overheated so if turned off it had to stay off till it cooled down otherwise it would not start as the concentric sits behind the cylinder not poking out in the breeze like the type 27 bowl does .

However even when it was back to std gearing & wearing the 626 it did seem to go substantially better than other M20 so I suspect that the crank is not std and could even be M21 , don't know, never bothered to measure it and the last person who did major engine work was Barry Greaham a well know "restorer" of the any part that fills the space is the right one school.
Ihe invoice included machining a piston & fabricaating a gudgeon pin which at the time I thought was padding but since then found out he had a large stock of piston blanks
The engine is sitting in the container while the replacement barrel is out getting bored to take one of Ian's big pistons so I will probably get around to doing the make over during our winter, if it stops flooding here so all will be revealed then

There is just about nothing on the bike that is original but it is supposedly as the original owner who bought it as surplus rode it for 36 years till he sold it to the second owner who got posted OS for 3 years & put it in storeage then got another OS posting when he came back & sold it to me in 1994 so I had something to ride as the B40 had done its big end in ( never got around to fixing it )

Re: M20 general performance/power expectations

Tuning, or extracting more BHP is primarily about cylinder filling...The more fuel air mixture that reaches the combustion chamber the bigger the resultant 'bang'...That means improving gas flow/speed into the combustion chamber and that is achieved by a combination of features that do not work in isolation but in conjunction with each other...There's no point increasing the lift or duration of the cams for example if the carb/porting is too restricted to allow an increase in gas volume to pass...Rather the point Bruce was making...

Interestingly the BSA Gold Star did have a small difference in diameter between the carb and port size (smaller carb) in some years and it was noted in BSA literature this 'step' should not be removed..It was found that the step improved turbulence and gave a better fuel air mix. However, whatever the set up the limiting factor will be the single feature that most limits gas flow...That could be anything from cam profiles or port size through to crankshaft weight...
Raising the compression ratio is the exception, that will produce more power in isolation up to a point, though in a side valve reducing cylinder head volume to achieve this can actually restrict gas flow and the process becomes self defeating...Enlarging the engine produces that effect without changing the piston shape or head volume and is the main difference between an M20 and an M21, apart from a small increase in port size (1/16")...

Gearing allows you to move whatever power there is around, up or down the rev range, but you aren't making anymore power so what you gain in one place you'll lose in another..Also, the engine has to make enough power to pull very tall gearing unless there is a tow start available!!:laughing: ....That is why raising the gearing on a standard M20 will mean the bike may go a little better on level ground but will slow down when you see a hill in the distance...:grimacing: ...Beyond about 60mph wind resistance is the determining factor and that increases exponentially with each increase in speed...Without streamlining to improve airflow increases in speed beyond 60/65mph thus require an equivalent exponential increase in engine power to achieve them and that is something the dear old M20 just doesn't have...'You can't deny the laws of physics Captain' as they say:laughing: ...Ian

email (option):

Re: M20 general performance/power expectations

I completely agree with all Ian has to say.

If you say your M20 could do 80 MPH, I believe you. But there is no way the engine is in factory condition. Unless the bike is streamlined, you can't go this fast with 13 HP. It is impossible regardless of how much you rev it or the gearing. To go this fast you will need a minimum of at least 20 HP which no stock condition M20 has ever had.

Modern OHC 250 cc motorcycles which have 30 HP are barely able to do 80 MPH.

To go twice as fast, requires four times the horse power.

Nieuwe pagina 1