BSA B31 timing

[briancairns]

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

[idie]

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.

[briancairns]

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.

[briancairns]

Sorry but I have no clue how to check cams. Any advise?

Cheers

Brian

[idie]

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.

[briancairns]

Great thanks. Do I require any special tools etc? Pullers?

Cheers

Brian

[Derek996]

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.

[briancairns]

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

[idie]

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.

[briancairns]

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

[Derek996]

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.

[briancairns]

Thanks so much Derek,

Hopefully I'll not mess it up..... :



Thanks again

Brian

[briancairns]

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.

[Derek996]

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.

[briancairns]

Thanks again Derek.

Is there a specific torque when re-tightening?


Cheers

Brian