OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Kirby H4-4A engine rebuild

Timing, I'm told, is everything. For the art of repartee it's 'better never than late'. For engines it's 'early is good'. So how early?

Anyone who has set the static timing on an old-style automobile engine will know that it is set a few degrees before top dead centre (BTDC), that is, it is advanced, and there is usually a mark on the flywheel to help. Line up the timing mark and rotate the distributor until the points just start to open - easy-peasy. Unfortunately, all single pot mower engines don't have distributors because they don't need one and also you usually have to remove the flywheel to access the points, so putting a mark on it won't help. Therefore, the timing has to be set using another method.

Some engines have a preset advance built into the mount for the points (eg: Briggs & Stratton) but on the Kirby engines the advance must be set and to do so, the head must also be removed from the block.

Start by fitting new points and setting the gap - at this stage it doesn't have to exact, just close - and don't connect them to the magneto. Wire up a battery (small torch battery will do perfectly) and an appropriate voltage light. Connect to the points so you can monitor when the points are open (no light) or closed (light illuminated). I just use my multimeter set to the ohms range and monitor the displayed resistance.

For the rest of this, you'll need a dial indicator and an appropriate mount.

Rotate the engine clockwise (looking at the magneto end of the crankshaft) until you find TDC. Here the piston will be at the top of its travel and both valves will be closed. Set up the dial indicator so that it is touching the piston crown and check that at TDC, the gauge on the indicator reverses direction as the piston starts to descend in the bore again. Next rotate the crankshaft counterclockwise until the piston has descended about 3mm BTDC. Now slowly rotate the crankshaft clockwise again until the dial indicator shows the piston at 1.65mm BTDC. For the H30 engine, this is the piston position on the compression stroke where the points should open and the spark plug fire. Rotating the crank backwards and then forwards like this takes out any slack in the big end bearing.

Loosen the magneto baseplate and rotate it until the points just open (light goes out or resistance goes to infinity). Fix the baseplate in this position. As these engines are governed to a maximum of about 1700rpm, a fixed advance is all that is required.

Finally reset the points gap to 0.50mm, replace the cover and flywheel. Because the magneto is on the inside of the flywheel on these engines, the air gap between the flywheel and the magneto is preset.

That's it for the timing. Fiddly, but once you get the hang of it, quite simple really. Hook up the wiring, replace the condensor if you have too and it's all done.



Timing cam and magneto mounting posts.



Points set and open at 0.50mm. Advance has been set.



Cover back on. This photo was taken prior to cleaning everything.

I didn't get any photos of the dial indicator in use, sorry. I guess I was carried away with the whole procedure as I've never had to remove the head to set the timing before

Next installment is the carburettor.

Chris