J.A.P 600cc engine adjustment


Please take care when attempting to start following adjustment, in particular to prevent kick backs ensure the advance/retard lever is working and that it is in the START position. 

Ensure that the starting handle is NOT grasped between your thumb and fore finger.  

Information supplied here is in good faith and is accurate to the best of my knowledge however I cannot be held liable for any loss, damage or personal injury as result of following these instructions.

Camshaft moves 360 degrees for two engine rotations.   

Make up a timing disk from a piece of thin aluminum or a piece of thin card. Using a protractor, mark top dead centre and a point 20 degrees clockwise from TDC. The disk should have a central hole slightly larger than the nut holding the sprocket onto the camshaft and two 1/4 inch holes that line up with the two timing cover screws either side. The engine rotates clockwise looking at the timing side, the camshaft therefore rotates anti-clockwise at half the speed.  Make a thin wire pointer twisted to form a loop bent to the shape of the nut. You may have to put a kink in the wire to miss the fixing screws.
Checking for TDC with the head in place is a bit fiddly, I use a bit of wire poked through the plug hole which unfortunately is not above the piston.
1/ Set the advance retard lever in the running position (lever towards the driver).
2/ Slacken the nut on the mag shaft so that the sprocket just turns freely on the taper.
3/ Fit the timing dial to the crank case using 2 cover screws.
4/ Rotate the engine to TDC firing stoke, ie. both valves closed.
5/ Fit the pointer onto the nut and bend it to line up with the TDC mark. 
6/ Rotate the engine back to BDC and then forward until the pointer lines up with the firing mark. 
7/ Rotate the mag by turning clockwise from the points side until the points are just opening.
8/ Hold the points and engine in this position and rotate the mag sprocket clockwise to take up the slack in the timing chain and push the spocket onto the taper.
9/ Tighten the sprocket nut finger tight.
10/ Rotate the engine back past the firing mark and then rotate until the points are just opening and check that the pointer is at the firing mark.
11/ Tighten the sprocket nut.
12/ Move the advance retard lever to the start position and check the the points have actually moved prior to starting.
On my dial I have also marked the rotary crankcase valve closing position and valve opening and closing . These should not need checking unless the engine has been dismantled, although I always like to check the rotary valve on new acquisitions because the  setting is crucial to ensure that crankcase vacuum is created to draw oil into the engine and expel surplus at the correct times.
Settings (600cc) are:-
Inlet opens on the exhaust stroke 16 degrees before TDC.
Inlet closes on the compression stoke 65 degrees after BDC.
Exhaust opens on the firing stroke 65 degrees before BDC .
Exhaust closes on the inlet stroke 25 degrees after TDC.
Rotary valve should be just closed 25 degrees past BDC.
To set the rotary valve I use a piece of soft copper wire inserted from below with the oil box cover and the access plug removed. Rotate the engine slowly until you feel the wire become trapped. Remove the wire and and check that the pointer should just be approaching the 25 degrees past BDC mark. Unfortunately to adjust the rotary valve you have to remove the camshaft cover and rotate the rotary valve without loosing the valve settings. Be careful if you have do this not to bend the decompression mechanism and that the valve timing is not put out!