Testing the motor

Oil Change

Having refitted the motor and before attempting to start it I set about changing the oil. Draining the oil is a lot less messy if you insert a hose joiner as a spigot into the drain hole. The oil then drains clear if the motor and frame.

Using a hose joiner as drain spigot

I could then try starting the motor,  it did run but only at full throttle and wouldn't run on. In fact it soon stopped running completely! 

There was also a tremendous vibration when it did run. I had wanted to replace the points and reset the timing as these were original anyway so I removed the flywheel again. To my surprise the Woodruff key had actually sheared and this had allowed the flywheel to shake around, generating the vibration and ruining any attempt at accurate timing. I think this came about because I hadn't fully torqued up the flywheel nut. This needs to be tightened to 21 ftlb (28Nm). I hadn't thought this would matter for a short test run and although my nut was tight, it wasn't at full torque- Big mistake! This shaking had actually sheared the key and also chewed up the taper on the crank and its matching partner  inside the flywheel. New flywheels are no longer made and the taper is part of the crankshaft which is available only at vast cost.

Damage to end of crankshaft, taper is now scored.

I was therefore forced to try and rectify this damage as best I could and  I approached this first by removing obvious burrs on the crank taper with a fine file, and then  using coarse and fine valve grinding compound in sequence; applying it to the magneto internal taper and then rotating it on the shaft. It helps  to remove the back plate while you do this. Luckily this returned the tapers to a close fit and although it didn't remove the scoring damage, it did give a considerable improvement.

I then set about removing and renewing the contact breakers (points) and started by taking several pictures of the setup before it was disturbed.

General view of old cb in position.

Close up

The moving point is retained on a pivot pin which is part of the base plate fixed point. The moving points are held onto this pin by a split clip and washer. Remove the split clip with small snipe nosed pliers and lift off the tiny washer. In my case this was very worn where the clip had been positioned.

Remove the tiny washer- note the wear marks where the clip had rested against it.

I then removed the base plate retaining screw so that both points could be removed as its then easier to get at the connections to the moving point.

removing the contact breakers by removing the base plate retaining screw so that the points can be lifted off complete.

Unscrew the nut on the rear of the moving points and remove the black wire connection. You can then lift the moving points off the spindle on which they move.

Black wire contact to the moving points removed, note the loosened nut replaced on the bolt. Note  also the two nylon insulation sleeve washers that sandwich the mounting bracket and ensure that there is no electrical connection between the mounting bracket (base plate) and moving points via the bolt or spring.

Note that this connection is to the moving point only, there is no electrical contact between the retaining bolt and the  the base plate through which it is screwed, or via the spring and the bracket. This is achieved by a number of insulating washers and plastic sleeves which isolate the bolt from the bracket through which it passes. These must be refitted correctly.

Moving point fixing bolt, washer press on the spring of the moving point, the nylon sleeve washers remain attached to the mounting bracket.

Replacing the points is pretty much the reverse and its necessary then to set the gap when the points are on the top of the cam or tdc. This can only be done when the flywheel is refitted and its damned awkward. In fact the centre of the cam coincided with one of the flywheel webs in my case and this correspondence cannot be changed. Fortunately the points are open over a wide area and I was able to set them  at tdc. My manual suggests that they should be set to 12 thou with the timing set to 2.5mm btdc. These settings  served me well 40 years ago so I have stuck to them: I'm told that with modern fuels and synthetic oils (which can be used at 30:1 instead of 20:1) its advisable to set the gap more narrowly to 8 thou and the timing to 2.1mm btdc although some needle changes may also be necessary in the carb.  I'm using mineral oil and I'm going to try and at least get it going with standard settings before experimenting with new oil ratios or ignition settings.

I refitted the flywheel with a new key. This time I also used Loctite retaining compound on the taper. I didn't want to use this because of the difficulties it will cause in stripping once more but I couldn't risk shearing another key or causing yet more damage to the tapers. I refitted the flywheel and torqued up the nut before leaving it for 24hrs for the Loctitite to cure.

Flywheel refitted and torqued.

I use a spark plug hole dial gauge to determine the timing position. This screws into the plug hole and allows me to determined tdc and 2.5 mm btdc quite accurately.

You can then slacken the two magneto back-plate retaining screws and rotate the plate slightly such that the points are just about to open when the motor is a  2.5 mm btdc. I use a timing box from "the magneto guy" which connects across the points with crocodile clips and indicates exactly when the points begin to open. This connection is awkward, one clip can just go to earth anywhere but the clip that attaches to the moving point is awkward and tends to pop off as you adjust. When I rebuild this motor I will insert an insulated fly lead that emerges from the magneto but connects to the moving point and allows a timing box connection when necessary. In the absence of this lead, adjustment is a fiddly and awkward process,  its hard to move the magneto back-plate without also moving the flywheel and thus the motor a little. It would be good to have a locking system to fix the motor in the desired position whilst you move the back plate.  However I persevered and by checking, adjusting and rechecking I found that I could obtain an opening point of 2.5 mm btdc- but this used all of the adjustment in the back plate slots. I don't think it will be possible to set this at 2.1 mm even if I were to try and do so.

Its not possible to time the motor dynamically but I can at least check it while it runs. I set the motor to tdc and then marked across the flywheel to a convenient datum on the crankcase. Marking both with white paint. I was able then to rotate the motor to 2.5 mm btdc (remembering the the motor rotates anticlockwise) and made a second mark there. When setting these positions it's important to turn the motor in its normal direction. Ie as this motor turns anticlockwise, you need to rotate it clockwise (backwards) past the point you want to mark and then back to the desired position in an anticlockwise (forwards)  direction to take up any backlash in the shafts and gears. I could check the timing as I peddled the bike using a neon timing light connected in series with the spark plug.

Timing marks indicated on the flywheel. Note the white spot datum on the crankcase. The two marks are righthand TDC and lefthandside 2.5mm BTDC. As shown above the motor is set to top dead centre (remember it rotates anticlockwise so has already passed the left hand mark which is before tdc.)

It would be nice to report now that having renewed the points and reset the gap and the timing, and  having fixed iall in place with Loctite, that the motor now runs well. Sadly that is not the case;  it doesn't want to run at all! It will start and run for a short while at full throttle and then stop. After a few of these cycles the motor stopped running completely even in this unsatisfactory manner. 

Removing the spark plug showed that it was very wet and it seemed likely that the motor was being fed too much petrol. I have re examined the carburettor  and found that the float chamber was overflowing, presumably because the float needle was not cutting off the flow of fuel completely. I cured this by regrinding the needle into the valve in the chamber lid with fine valve grinding paste. Although this stopped the overflow, it failed to affect any improvement in running. The fact that the motor did run for a short burst at full throttle indicates to me that the ignition timing is at least in a functional condition. The timing can now be checked dynamically and this shows its still firing about the correct position. I am wondering if I actually have two problems; firstly a leaking float valve and generally worn carburettor giving a vastly over rich mixture, and secondly a blocked or defective fuel tap/filter that reduces fuel flow such that the small quantity in the float chamber isn't being refilled fast enough to keep up with use at full throttle. I have therefore decided to renew both components and bought new items from Krippl in Germany . They are marginally cheaper there but I expect prices will soon rise significantly courtesy of Brexit!

I replaced the carburettor, fuel tap and filter but obtained no difference in running suggesting that my old components were probably serviceable. I'm now having to contemplate a more serious problem with this motor- in fact after the last run  I noticed exhaust fumes emerging from behind the flywheel. Stripping off the flywheel showed there was now a lot of oil around the crankshaft and over the coils on the back plate. Turning the motor over allowed me to test around the crank with a hanging tissue. This showed distinct puffs of compression emerging from the crank and implied that the seal here has broken. This would allow the gases to exit the crankcase rather than enter the cylinder and confirmed the suspected breathing problem. 

This is a new problem, I hadn't seen it before. Its possible its of my own making, resulting either from running the motor at the wrong timing or using Bradex easy start. Alternatively it may simply have been a weak seal that gave as soon as the motor ran at anything like full throttle. There is now no alternative but to strip the motor and replace all seals and gaskets since if one is weak the chances are that all are in poor condition. I will renew the bearings and some of the bushes whilst I'm inside the cases as it seems daft not to renew them whilst it's stripped.