My B31 is starting ok but seems to struggle with power. Getting 50-55mph max on the flat but really struggles on hills.
I checked timing etc and it's set at about 58° BTDC. I changed the points and retimed on full advance to 38° BTDC. I haven't started bike yet as I want to check will still increase the power output and to check I haven't done anything stupid.
Points set at 12 thou at full open
38° BTDC set on compression stroke.
Spark plug gap set between 4 and 5.
I'm new to old British bikes.
Cheers
Brian
My B33 which is a similar although bigger engine has the same settings. I use the old way of doing timing 3/8" BTDC spark plug gap 18 thou I don't understand 4 or 5. Have you checked your cam timing, one tooth out it will start and run but will be flat.
According to manual : 7/16th BTDC ( 38° I think) . Spark gap is between 0.45 and 0.50mm, I set it at 0.48mm . Not sure what you mean about the cam? No markings on magneto gears.
Sorry but I have no clue how to check cams. Any advise?
Cheers
Brian
If you take of the cover plate shown in your photo you will see the exhaust and inlet cam they will have dots and dashes stamped on them. Make sure that they all line up correctly.I think it is a hard to see the dot on the pinion on the crank shaft that lines up with a dot on the inlet cam. This in turn has a dash which lines up with one on the exhaust cam. As the exhaust and inlet cams are the same on a B31 the cams will have both dots and dashes. If you find that one is out when you reset it you will have to re time the engine ignition.
Great thanks. Do I require any special tools etc? Pullers?
Cheers
Brian
No social tools required but if you have to reposition the inlet cam you will lose the ignition timing and have reset it. One way round this is to make the mark the magneto pinion and the idler pinion with Tippex or a similar marker and try to keep then lined up while you remove and reinstall the cam. If they have only moved by a few teeth it's not difficult to get them back in line.
Great Derek,
So once I remove supporting plate the cams can be removed and re-installed by hand into correct positioning?
As you can tell I am very new to DIY mechanics but wish to learn.
Many thanks
Brian
Make sure that the cam follower is on the back of the cam by turning engine to get the right place, otherwise the cam will not slide off.
Great!
Thanks all!
I will obviously have a check 1st without moving cams as they all might be set perfectly.
But could be the answer to lack of power.
Cheers
Brian
This what the timing marks should look like if they are set correctly. Note the dash on the crank pinion lines up with the dash on the inlet cam and the dot on the crank pinion is lined up with the dot on the exhaust cam. The cams have a dot and line on each because they are interchangeable and can be used for either inlet or exhaust, just lining up the appropriate marks.
Thanks so much Derek,
Hopefully I'll not mess it up..... :
Thanks again
Brian
Sorry to disturb you all again.
I am trying to check cam timing marks on cam wheels are aligned correctly.
Stupid question ..........but how to i remove the main shaft nut? how to I stop shaft turning?
I really apologise for all the questions.
Easiest way is to put the bike in top gear and get an assistant to hold the rear brake on. Fit an appropriate sized socket to the nut and give end of the breaker bar a whack.
Thanks again Derek.
Is there a specific torque when re-tightening?
Cheers
Brian
Not that I know of, I've always just tightened up until it feels right to me. But I would think that around 45Nm wouldn't be far off and about 24Nm for the screws retaining the support plate.
Cheers Derek
Thanks
Brian
Hi,
Set timing according to book ( at fully advanced, checked that it was definitely advanced) :
7/16th ( 38degree) BTDC.
Spark plug 0.48 mm
points ; 0.3mm at widest opening. ( set magneto timing just as points opened)
Now bike hard to start, no real power and if i retard even the slightest bike splutters and pops.
new to this , but what i am doing wrong?
Thanks
Brian
Were all the cam marks were lined up correctly? Did you change anything?
Checked the cams all perfect . Only thing I changed was new points and adjusted magneto as originally timing was set at 57degree BTDC, point gap was 0.6 mm . However bike started ok but couldn't get over 50mph and struggled with the slightest hill
It seems like you have the mag points gap and timing set correctly so I'm struggling to see where else you can check. Is you retard cable tight or slack for full advance? If the cable is on the cylinder side of the mag it should be slack, if it on the side away from the cylinder it will be tight for full advance. It is possible that you are setting it for the wrong advance direction.
Ok Derek .
I will recheck.
Cheers
Brian
Just to clarify, the points face cam in the magneto should be rotated fully counterclockwise for the magneto fully advanced setting, regardless of the advance cable configuration. At 7/16in before tdc, the points should be just starting to open with the face cam rotated fully counterclockwise.
Ok thanks Jim,
Not sure if I did that but or not.
Will re-time .
Cheers
Brian
I would strongly recommend that you visit the technical section of the WM20 website at http://www.wdbsa.nl/.
much of the information is also applicable to the OHV models.
with regard to timing the sections linked below are very informative and should help you with any such problems.
http://www.wdbsa.nl/ignition_timing.htm
http://www.wdbsa.nl/download/Ignition%20setting.pdf
good luck
Pete
Thanks so much Pete
Cheers
Brian
Hi Brian
so long the ignition and cam timing are OK the only other issue could be the carb. What carb have you fitted? The settings are pretty basic to get it going reasonably well.
You mentioned at the beginning that the bike is OK up to around 55mph but bogs down on hills. The B31 should be good for around 70mph max but the engine does need to be spinning to get the best performance. Unlike a B33, it isn't really a slogger.
cheers
Pete
Hi Pete,
It's an Amal carb.
When bike was able to reach 50-55mph, it would really dive at the slightest hill.
I'll check your links and go through process again.
Thanks again
Brian
Hi Brian
You could have one of a variety of Amal carbs :( , pre-monobloc, monobloc or concentric. Do you know which you have? They all work OK but your tuning strategigy might be a bit different for each.
this link is a good start for tuning info. https://amalcarb.co.uk/downloads/
Concentrics are cheap and easy while pre-monobloc or monobloc are more "authentic" Enter your bike details here in "vehicle search" to get info on which carb would be "correct" for your bike https://amalcarb.co.uk/
Great Pete!
I have a look in the next day or so .
Thanks so much
Cheers
I once had an engine with the same problem. It eventually turned out to be that the valves were only opening 1/8" due to cam wear. Also, a blocked exhaust can cause the same symptoms. Just a thought.
However, there seems to be something strange here. To start at the beginning, if your timing was set to 56 degrees fully advanced, this would be about 26 degrees fully retarded. I doubt if you could start the engine at that. I think it may kick back violently, if it fired at all. Well, you say it did start and ran at 50 to 55 mph. So you must have only been running with only a very small amount of advance on the lever. Fully advancing it would have caused extreme pinking and maybe stopped the engine. So it doesn't really make much sense.
You then set it to 38 degrees, and say it was hard to start. (Later, without making any further changes, you say it starts OK).
You say it splutters and pops when you retard the ignition. Is this at tickover, or on the road? I think the popping may be relevant. In my experience, this usually indicates an ignition problem.
Being new to all this, are you using a cigarette paper to check points opening? Also, (sorry to state the obvious) remember that when viewed from the right hand side of the engine, it turns clockwise, and when viewed from the left hand side it turns anticlockwise.
I wonder if you could paint marks on the cush drive and crankcase at TDC and a few degrees BTDC and see what's going on with a stroboscope?
Having said all this, if the motorcycle gets up to 55 mph without pinking, the timing can't be far out.
I wonder if you might check the fit of the points carrier on the brass cutout on the armature. These can wear badly. Is your armature coming apart due to loose endcaps?
One of the ways to check if it is advancing correctly, is to ride along with a small throttle opening at full retard. If all is well, the bike should surge forward when you advance the ignition.
It might be worth investing in a Spark Gap tester. A 5/16" gap will show a voltage of 15,000 volts. But don't go more than 5/16".
Finally, I once had an engine that ran fine, but faded going up hills unless I closed the choke. Turned out I had a bad air leak at the carburettor flange.
Hope all this may be of some help to you.
Thanks for the advice, currently rechecking everything and taking carb off to clean in ultrasonic bath. I'm probably doing something daft !
Will let you know.
Thanks
Brian
Quote from: Pete C on 26 Mar, 2023, 20:49
Hi Brian
You could have one of a variety of Amal carbs :( , pre-monobloc, monobloc or concentric. Do you know which you have? They all work OK but your tuning strategigy might be a bit different for each.
this link is a good start for tuning info. https://amalcarb.co.uk/downloads/
Concentrics are cheap and easy while pre-monobloc or monobloc are more "authentic" Enter your bike details here in "vehicle search" to get info on which carb would be "correct" for your bike https://amalcarb.co.uk/
Hi Pete, carb is a pre-monobloc 276DR/AT. I got the tuning guide but link I have an air leak at flange. Taking carb off to give a good clean and fix air-leak.
Thanks Brian
Quote from: Steve.S on 30 Mar, 2023, 17:52
I once had an engine with the same problem. It eventually turned out to be that the valves were only opening 1/8" due to cam wear. Also, a blocked exhaust can cause the same symptoms. Just a thought.
However, there seems to be something strange here. To start at the beginning, if your timing was set to 56 degrees fully advanced, this would be about 26 degrees fully retarded. I doubt if you could start the engine at that. I think it may kick back violently, if it fired at all. Well, you say it did start and ran at 50 to 55 mph. So you must have only been running with only a very small amount of advance on the lever. Fully advancing it would have caused extreme pinking and maybe stopped the engine. So it doesn't really make much sense.
You then set it to 38 degrees, and say it was hard to start. (Later, without making any further changes, you say it starts OK).
You say it splutters and pops when you retard the ignition. Is this at tickover, or on the road? I think the popping may be relevant. In my experience, this usually indicates an ignition problem.
Being new to all this, are you using a cigarette paper to check points opening? Also, (sorry to state the obvious) remember that when viewed from the right hand side of the engine, it turns clockwise, and when viewed from the left hand side it turns anticlockwise.
I wonder if you could paint marks on the cush drive and crankcase at TDC and a few degrees BTDC and see what's going on with a stroboscope?
Having said all this, if the motorcycle gets up to 55 mph without pinking, the timing can't be far out.
I wonder if you might check the fit of the points carrier on the brass cutout on the armature. These can wear badly. Is your armature coming apart due to loose endcaps?
One of the ways to check if it is advancing correctly, is to ride along with a small throttle opening at full retard. If all is well, the bike should surge forward when you advance the ignition.
It might be worth investing in a Spark Gap tester. A 5/16" gap will show a voltage of 15,000 volts. But don't go more than 5/16".
Finally, I once had an engine that ran fine, but faded going up hills unless I closed the choke. Turned out I had a bad air leak at the carburettor flange.
Hope all this may be of some help to you.
Hi Steve, I checked ignition timing and it was out ( don't ask me how) but have reset. I think I also have an air-leak at carb flange . Taking carb off for a good clean and fix air leak.
Many thanks
Brian
Brian, I wonder if your drive pinion might be slipping on it's taper? If you were to paint a mark on the points housing adjacent to the points when they are just opening, would this be a quick way of checking whether or not the pinion had moved?
I use wet or dry on plate glass to resurface the flange. You'll soon see if it's warped. If you still have a problem, it might be worth dribbling a little oil over the flange with the engine running. If it slows down, that indicated an air leak. You could also try the same on the inlet valve guide, or perhaps use spray grease?
Steve.
Thanks Steve, I was going to do what you said regarding flange. Hopefully will get it sorted soon.
Thanks again all for your help
Brian
Make sure the timing pinion hasn't got a hair line crack on it. It will slip if it has.
Thanks , will check it out.
Brian
Hi Brian
Have you determined the cause of your apparent power loss? If yes, please let us know. I am interested and others are probably also.
Jim
Sorry Jim,
No updates yet, I'm very busy at work and haven't much time at the minute. I rechecked timing again and it was spot on. Currently I have the carb off and giving everything a good clean in sonic bath. Will probably skim the flange as I think it's slightly warped as think previous owner seemed to have over tightened it. There seemed to be an air leak from flange.
Hopefully I will have it ready for a test ride by end of next week and will give an update then.
Thanks for your help
Brian
Hi All,
Delighted to report bike is working fine, took carb off and cleaned in sonic bath, although it looked absolutely fine.
I think issue was a mixture of things, timing, contact breaker gap, spark plug and dirt in carb.
Thanks so much all for your invaluable advice!!
Thanks again
Brian
Glad to hear that it's all sorted. Having various item not right can be a bit baffling but working on one at a time usually gets you there. Well done :)
Thanks.
No doubt I will be back for more advice!
Cheers
Brian