Now, I expect everyone with a pre-war Douglas has thought of this already – but I was quite pleased with the idea!
Some of the smaller pre-war models, such as my A31, do not have a plugged drilling in the cylinder head to allow a marked rod to be used for measuring piston position, so setting the ignition timing with a degree disc is the only real alternative. Fitting a disc to the end of the crank, however, would involve removing the flywheel and primary drive, which felt like a lot of extra work. The alternative was to cut the centre out of an old degree disc and attach it to the flywheel with self-adhesive velcro pads. Clearly, it won’t fit anything else in future but it saved a lot of work!

Timing Disc on Douglas External Flywheel
Timing Disc on Douglas External Flywheel

The next challenge, of course, was to establish top dead centre (TDC). Again, in the absence of a drilling in the cylinder heads, I had to improvise, using a long blunt rod through one of the valve caps to establish when the piston ‘rocked’ at the top of its stroke. After a few attempts, I think I got pretty close.

Close enough? Ultimately it’s a low-compression side valve. Accuracy is limited by the keyed drive pinion on the magneto, so it will always be ‘the closest tooth’. And, if, my crankshaft is typical, TDC differs from one piston to the other! I think it’s close enough …

4 thoughts on “Timing a Pre-War Douglas”

  2. You don’t even need a timing disc – most prewar side valve machines were timed at 36 degrees before TDC, so just measure the circumference of the flywheel and divide that by 10 – that gives a timing mark for setting the magneto. If the correct setting comes between 2 teeth, err on the advanced side. Bear in mind that the figures given in the handbooks were for the fuel available prewar, so some adjustment may be needed when running on the modern unleaded rubbish. You also have manual advance/retard for any fine adjustment that may be needed.

    Like

    Reply

  3. I’m struggling to get my 3.5hp to run after some re Building of engine. The timing marks don’t align, after some advice from aussie douglas guys I made a handmade disc , however not certain its still timed correctly. I agree the tdc and bdc appear to occur slightly differently on disc. I also think magneto points opening to early, any tips please?

    Like

    Reply

  4. I just use a bit of bent welding wire trough a valve cap and marked it 3/16 from from TDC. The bikes starts easy and seams to run fine. As you said it`s a low compression side valve with manual advance/retard so how accurate does it need to be. Don`t forget to set the timing to fully advanced when setting.
    On my Royal Enfield the Indians only give a height before TDC so I calculated the relevant angle and set it up accurately and the damn thing would not run right. I cut a small notch in a pencil and set the timing through the plug hole and it started first kick and runs fine. So I think we worry to much about accuracy on some of the old bikes.
    Just enjoy riding it as long as it goes OK,
    I know it`s a different game if you`re racing or running modern stuff.

    Like

    Reply

Leave a comment