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#21092 - 02/08/07 9:28 pm My BSA B31 1955  
Joined: Feb 2007
Posts: 28
Hayden Offline
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Hayden  Offline
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Joined: Feb 2007
Posts: 28
Ebbw Vale
I have a 1955 BSA B31 in cafť race trim Can anyone please please please help?,
I have fitted: Mukini carburettor, converted magneto with Boyer ignition, Triumph four spring clutch, balanced fly wheel and a reconditioned engine. My gear box has STD for what I believe stands for standard.
The problem is with all this work I can only achieve 55 Mph which means I can not keep up with anyone when we go out.
I have played with primary engine sprockets and gear box sprockets to no joy; Iíve gone from one end of my timing adjustment to the other. Iíve tried jets in my carb from 120 to 220, Iíve tried straight through exhaust pipe. The only thing I havenít tried is cam timing, manuals say donít touch, tuning for speed says it can make a huge difference, has anyone tried changing valve timing.
Has anyone got any other ideas I will try anything.
Only one limitation I am a solider based in Germany so there is a lack of British motor cycle engineers please please please help some body.
Desperately hoping
many thanks Hayden.

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#21093 - 02/08/07 10:02 pm Re: My BSA B31 1955  
Joined: Jul 2001
Posts: 12,081
Lannis Online content
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Lannis  Online Content

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Posts: 12,081
Central Virginia
Hayden -

How does the bike act as it approaches 55 MPH?

Is it revving and pulling hard from 50 to 55 and then suddenly starts missing and popping and banging?

How does it act in 3rd gear? Will it rev higher in 3rd gear than in 4th or does it start breaking up at about the same RPM? If it will pull cleanly to 50 MPH in 3rd, it should pull to 70 or so in 4th.

Have you done a compression test or a leak-down test on the engine? It could be that, even though reconditioned, the piston was installed with too much clearance or that a ring is stuck, losing compression. Compression testers are cheap and can get you in the ballpark; a leakdown tester costs about $100 (or can be made for about $25) and tells you exactly what's going on in your combustion chamber - I wouldn't be without mine.

It's possible that your cam timing is off - it will take more of a B31 expert than me to tell you how to check that ....

Keep feeding the list information and asking questions and you will gradually zero in on the problem.

Lannis


OK, I admit it, I'm addicted to brake fluid.

But I can stop any time I want.
#21094 - 02/08/07 10:06 pm Re: My BSA B31 1955  
Joined: Sep 2001
Posts: 2,766
Gordo in Comox Offline
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Comox BC Canada
A published road test in 1956 showed a top speed of 72 mph mean speed for a B31 in standard trim. 55 mph does sound a little puny for a good B31, especially with all those upgrades. I would look at the carb as it seems that the engine is just running out of steam. A Boyer should be able to handle the rpm of a B31. What size Mukuni are you using?

Gordo

PS: How are you determinng the top speed?


Without frequent roadside repairs there is no fun in riding!
#21095 - 02/09/07 12:06 am Re: My BSA B31 1955  

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As Lannis suggests - you may have slowed the engine down too much, and unable to sustain speed in high gear.
Need: all 4 sprocket sizes + rear wheel OD.

#21096 - 02/09/07 12:09 am Re: My BSA B31 1955  
Joined: Feb 2007
Posts: 6
guzzi126 Offline
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guzzi126  Offline
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Posts: 6
Jondaryan Qld Australia
That doesn't sound good at all, I have a '51 B31 that is standard apart from having an AMAL Monobloc fitted, and it's geared down for the sidecar, and I can get 100km/h (60 mph) out of it ok - and more when I switch on gravity assist :-)

#21097 - 02/09/07 8:21 am Re: My BSA B31 1955  
Joined: Feb 2007
Posts: 28
Hayden Offline
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Hayden  Offline
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Posts: 28
Ebbw Vale
Hi, I am able to achieve the recommended 1st gear speed of 32mph the second gear is just short of its max recommended speed third gear I get 50mph and then 4th gear I get an extra 5mph. the bike through all the gears feels like it has a lack of fuel (a missing kind of feeling) Iíve ridden with the fuel cap off and have fitted a high flow petrol tap.
The compression seems good although I havenít carried out a compression test, I am a big lad 16 stone I cant get past TDC with out the aid of the valve lifter. I have had my friend of 11 stone ride the bike with the same top speed of 55mph but just got there quicker.
My Mukini card is 376/82 or Mukini equivalent, if I run the bike on a lean fuel mixture it accelerates better as it should but makes no difference to top end just runs hotter, if I run the bike rich the missing effect seem to get worse
I wish I joined a forum like this years ago.
Many thanks Hayden

#21098 - 02/09/07 8:58 am Re: My BSA B31 1955  
Joined: Aug 2001
Posts: 3,408
gavin eisler Online content
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gavin eisler  Online Content
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Posts: 3,408
argyll. scotland, uk
You have introduced a couple of tricky variables.
ignition timing and carb settings.
it would make life a lot simpler if you could find an AMAL Monobloc and jet to std specs. AMAL to Mic jet numbers do not cross relate well.
It could be your jet needle set is rich and your main jet is too small.
if you have access to a dyno test facility with air /fuel ratio sensors then that will help get your carb dialled in.
How are you setting the ignition timing,? have you set up a strobe mark so you know the Boyer is a) timed correctly and b] advancing correctly.
What exhaust, comp ratio, cams are you using, like the gents say even a stock B31 should exceed 60mph and maybe hit 70 on a good day.
Cheers
Pod


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
#21099 - 02/09/07 10:41 am Re: My BSA B31 1955  
Joined: Feb 2007
Posts: 28
Hayden Offline
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Hayden  Offline
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Posts: 28
Ebbw Vale
I have tried my pre-mono block carb and the problem still occurs, reference the Boyer timing issue itís a tricky old job getting the setting correct, what I did was to start with the timing in what could be called the middle position rode the bike adjusted to the left rode again the adjusted to the right of centre and carried on until I got the best performance.
The dark subject of cams this is the second set of cams for this bike, about 5 years ago I thought the speed problem could lie with the cams so I brought a standard set from C&D. My father also a British bike enthusiast has advised me to try and move my cam one tooth left or right but Iím to scared

#21100 - 02/09/07 2:09 pm Re: My BSA B31 1955  
Joined: Aug 2001
Posts: 3,408
gavin eisler Online content
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gavin eisler  Online Content
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Posts: 3,408
argyll. scotland, uk
Hi Hayden
If you get a degree wheel then you can check you cams are timed correctly, one tooth front or back will do no real harm, but its another random variable, better if you start with a known sotck setting.
likewise with the Boyer, you can set up a strobe timeing mark with the chaincase removed , mark a reference point in white marker fluid on the crank end, and fix a pointer to the cases , a bit of coat hanger wire with a point ground and attached to the chaincase innner. Rev the bike with strobe attached and check advance is correct.
you can also use the degree wheel to establish correct ignition timing.
once you have the ignition and cam timing sorted and you know the advance is correct then proceed to mess with the carb. if you dont get these base settings correct all bets are off for jetting.

First get a degree wheel if you need help setting it up and finding TDC just ask,
You will need a piston movement indicator, to establish true TDC bring piston to 1/2 " before tdc and note the reading at the degree wheel mounted on drive side crank end, next take the piston to 1/2" past TDC on the same indicator and note reading at degree wheel, then count the degrees between the two noted readings, divide the degrees by two and right this sum down, now move the motor this number of degrees between the two noted marks, leave the motor in this position and slacken the degree wheel mounting and set to zero degrees , this should be true TDC , lock the degree wheel now and then start to time your cams.

I have a file on Word that will help,
What the hell , here it is any way
excuse the flippant tone it was written for a child to understand.Please not the timing figures quoted are for an A65 substitute B31 cam timings for inlet open/ close and exhaust open and close.

Im going to try and walk you through this.
Read the first bit. Ive got to take the piss a little here but this method will work.
Print out from "ready to time " and have it with you when you are ready.

You are right in assuming the corrections are made by moving the cam wheel, to do this , with draw the idler wheel, move the cam wheel then re insert the idler.

Looking from primary side.
Starting conditions
Rocker lid off .
Spark plugs out, dont want to fight compression.
V/V clearances set at 15 thou.(this has to be done before setting to TDC , the way you would normally set v/v lash just a bit more clearance to get over the cam ramps).

Now get a good nights sleep, eat some fish , not farmed or tinned, and some fresh fruit so that you can be sharp in the morning.

GO to sleep repeating the mantra.
"I am going to set my BSA valve timing tomorrow morning" repeat 40 times . Use a piece of 6 mm cord with 40 knots in it to be sure you dont skip one. THis is strong juju and will guarantee success.

Sleep overnight, awake, breakfast on wholewheat toast and peanut butter, nothing too greasy with some good fresh coffee.
Before you start on your valve timing find an old Lucas component such as a rectifier or advance retard mechanism.
Take your favourite hammer and POUND the Lucas item to small pieces, this will remove any latent aggresion , help world peace and make your motorcycle more reliable, this is the strongest juju of them all.

Chanting "Collet of a shiny valve, in the crancase boil and bake, con of rod and pin of gudgeon, prepare to time this foul curmudgeon"

Now calm, deep calm...
Cigarette paper (rizla blue) to hand.

Soothing music on in the background, I always time my motor to Augustus Pablo , Big ****y Dub, failing this any quality reggae will do , if you like C and W stop now , you will not be able to time a BSA without reversing some of the earlier incantations, you may not be able to time it at all..
Burn some Castrol R scented candles and sprinkle WD 40 over your tools.

Now wearing your finest technicians brown shop coat , the one with the 6 inch rule and the inspection mirror clipped to the top LHS breast pocket prepare to time the motor.
Wash your hands, and keep some shop towels handy.
,
Motor at TDC exact,
degree wheel zeroed to pointer, pointer fixed to motor (sharpened coat hanger wire) from base nut on flange or somesuch).

Ready to time.

Looking from primary side, motor runs in anti clock direction.

So starting with an inlet v/v figure, rotate the motor clockwise ( reverse from running position)
51 degrees from TDC. Get this absolutely bang on, I repeat
rotate the motor clockwise counting 51 degrees from TDC.
Stop there , mark this spot on the degree wheel neatly with a fine marker pen.
Now go another 20 degrees or so, more.
Look at your inlet valves ( the springy things on the carb side of the motor), one of them should have v/v lash.
Put your cigarette paper between the rocker arm tappet and the v/v stem.

Hold it there and rotate the motor
anti clockwise ( normal running direction) untill the rocker pinches the rizla paper.

Note where this occurs on your degree wheel.
It should occur at the mark you made at 51 degrees before TDC.

If it does then yee ha! all is well.
If it doesnt , rotate the motor to exactly 51 degrees.
remove the timing idler gear.

Now move the cam , it spins the same way as the motor,( with your finger on the wheel )until the inlet v/v begins to open (pinched rizla)
, put the idler wheel back in (dont worry about timing marks we are way past caring about them at this point).
You may have to compromise by a degree or so at this point to get teeth meshed, but try to get as close as possible.
You should by now have the general idea.
If you dont then get a smart friend to help.
If you do get it.
continue to check all the Inlet close values and Exhaust inlet open values.

If you get the first one correct (procedure above) then the rest should all be within a degree or two.
If not then your method is wrong or you have the wrong camshaft.

If you mark up the card the way I described in the previous post then it will really help.

If this is away over your head, find a friend with a degree wheel , buy him a beer or two once you have checked it over.

I reiterate , if you dont know the valve timing is bang on and the ignition timing is bang on leave the carb alone.

Sorry about the long post , most was on file from previous enquiries, and sorry about the dietary advice , you may have a nut allergy for all I know.
Stay Calm
Cheers
Pod


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
#21101 - 02/09/07 5:24 pm Re: My BSA B31 1955  
Joined: Feb 2007
Posts: 28
Hayden Offline
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Hayden  Offline
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Joined: Feb 2007
Posts: 28
Ebbw Vale
Pod, i will try to set my valve timing in the morning, i feel confident many many thanks pod i will tell you the results

#21102 - 02/10/07 11:09 pm Re: My BSA B31 1955  
Joined: May 2004
Posts: 3,548
BSA_WM20 Online content
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BSA_WM20  Online Content
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Posts: 3,548
Sydney Australia
Hayden
Before we start tearing your bike appart, what is the compression ?
Your answer to Lannis of "feels good" is not good enough.
If you expect us to be able to diagnose your bike from thousands of miles away we need specific answers to specific questions and compression NUMBERS are very important.
I had a SR 500 compression readings of 120 psi "felt good" but indicated that the bore / piston was shot.
If the rings are wrong , shot , stuck worn out or the bore is glazed then you are not going to get any where near optimal speeds out of your B31.
There are hundreds on this list how can look at the numbers and tell you that they are good, bad or ugly, but feels good is not good enough.
Next, is the crankcase breather working properly ?
The flap valves do not last forever and if it is stuck then excessive crankcase pressure is going to rob you of top end.
Believe it or not I have actually seen a mismatched set of cases with no breather at all and that owner had a similar problem plus some nasty oil leaks.
You also have not answered Lannis' other question of "what actually happens as you approach top speed" this is very important for us to know.
Also important is what size is your Mukuni carb ?
If it is too small then you might just be at the limit of the breathing of that carb.
If it is too big then you don't get enough draught to volatalize the fuel properly & the carb can not work as intended.

You said you fitted the "original pre monoblock carb" and got the same result, which carb? what are the numbers on the side of it & what jets are fitted to it?

Rereading this post, it seem a little agressive, it is not meant to be but it is really hard to fix a bike that we can not see.


Bike Beesa
Trevor
#21103 - 02/11/07 12:25 am Re: My BSA B31 1955  

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376/82 Monobloc is 1" bore, OEM for 1955 B31 export. Original jetting:
pilot 30
slide 3-1/2
needle jet 106
needle C
main jet 200 (too small, IMH)
Equivalent size in a Mikuni VM is (roughly) 26mm.
What size is yours?
What parts are in it now?

Re: "move my cam one tooth"
If you move the pinion gear 1 tooth without disturbing the cams both cams rotate the same amount, which will move the power band up or down. Advancing it will increase cylinder pressure (torque) and reduce intake valve clearance to the piston, retarding it will reduce cylinder pressure but add peak power and reduce exhaust valve clearance to the piston. It will not affect overlap or valve-to-valve clearance during overlap
However, if you move only 1 cam (same comments apply but to the intake cam especially) you do change the valve-to-valve clearance, and overlap. Due to the small number of teeth on the gears (18/36) even 1 tooth will have a big effect (20 degrees). Minor changes are done using alternate keyways or offset keys.

#21104 - 02/11/07 9:42 am Re: My BSA B31 1955  
Joined: Aug 2001
Posts: 29
Josef2 Offline
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Josef2  Offline
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Posts: 29
Germany
hi,

maybe the limitation is not on the inlet side, but on the exhaust.
I had on my B33 a replica exhaust pipe with a poor design. There was a plate welded into it with a hole of some 9 mm to reduce noise, but it also limited the exhaust gas flow at higher revs.
I replaced it by a gold star exhaust pipe with no plate inside ( you can look straight trough) an it helped a lot.

Josef

#21105 - 02/11/07 2:47 pm Re: My BSA B31 1955  

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Anonymous
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IDT this is the problem because he's tried a straight pipe.
Even the wrong length or diameter won't have the symptoms you describe.
Very high starting effort is typically one of 3 things:
1. light flywheel (unlikely in stock B31)
2. high compression (unlikely in stock B31)
3. intake or both cams advanced
#3 will also cause high RPM power to fade out, and prevent full advance from being used due to knocking.

#21106 - 02/11/07 4:22 pm Re: My BSA B31 1955  
Joined: Aug 2001
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Mr Mike Offline
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Mr Mike  Offline
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Posts: 3,871
Cape Carteret, NC
Hayden,
Assuming all the things that others have suggested have checked out I had a similar experience with a Mikuni on my B44. It would idle and run well but would not obtain any top end speed...65mph tops. After about five jet changes I got it sorted out and it runs well abouve 75mph now and gets there quickly.

Mr Mike

#21107 - 02/11/07 4:35 pm Re: My BSA B31 1955  

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The easist thing ICTO on a VM is to put a small rubber stopper in the air correction hole (in the air-horn) with a piece of wire attached. Ride at 50 mph full throttle, and pull the stopper out. If it slows down you were too lean. If the bike won't run with the stopper, try a stopper with a 1mm hole. Free, simply, don't take anything apart, can't damage the engine.

#21108 - 02/11/07 7:11 pm Re: My BSA B31 1955  
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Posts: 544
joe a. Offline
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joe a.  Offline
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Posts: 544
minnesota
hi, just to ck a few items that sometimes get overlooked,does the slide open completely when viewed from the engine side? are you running a airfilter or v stack? do your wheels rotate freely? are your primary/secondary chains too tight? if all this is good then procede to the compression test(warm-throttle wide open)if low do a leak down to identify cause.cam timing on b's is fairly simple if the marks are present just want to make sure the key on the crank pinion has not sheared and pinion shifted...joe

#21109 - 02/12/07 6:51 am Re: My BSA B31 1955  
Joined: Nov 2006
Posts: 23
TTNZ Offline
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Posts: 23
New Zealand
If your motor runs fine and revs out ok then check the gearing on the bike you may find the rear sprocket is the wrong size also check the primary drive Good Luck


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