Seagull Engine Construction Diary

Making a 10cc twin 4-stroke to the E T Westbury design

Compression Test

2020-02-22 & 26 – Making a compression tester

This consists of a fitting to screw into the sparking plug hole, and a sleeve to carry the a pressure gauge. The fitting has a 116″ hole, 516″ long, then opening out to 18″ diameter. A 332″ ball is dropped down the 18″ hole to make a non-return valve. This arrangement keeps the additional cylinder head volume incurred by the tester to a minimum. Externally, the fitting has a 516″ O-ring near the outer end, and a 38″ x 32 thread. The pressure gauge sleeve screws on over the O-ring, allowing the gauge position to be adjusted to face up. Flicking the flywheel by hand fleetingly registered pressures up to 20 psi. (5 hours);

2020-02-29 – Motoring the engine

For motoring the engine over, I have a suitable variable speed drive with an adaptable mounting arrangement; in the form of the lathe. I re-fitted the flexible coupling to the flywheel, and made some mounting plates and spacers to allow the engine stand to be fixed to the cross-slide boring table. Running on and off, I seem to be getting plenty of compression on No2 cylinder, but little on No 1. The ball valve in the tester is only working sporadically. (3¾ hours)

2020-03-15 – Improving the breather

I have been motoring the engine occasionally for brief periods. The crankcase is breathing partly through the exit hole for the timing wire. I need to make a plug. Removing the timing cover it is good to see that there is oil around the top of the timing case. The magnets have collected a small amount of dirt. I made and fitted the plug, removed the silencer, and checked the tappet clearances.

I don't have a spring small enough to load the ball in the compression tester, but have found that putting a loose pin behind the ball helps it to to seat. At a nominal 625rpm I am now getting up to 54 psi on No 1 cylinder, and, very inconsistently, up to 38 on No 2.

The engine is chucking a lot of oil out, especially at the flywheel. I suspect the breather valve is forming an oil seal and the six holes are not providing enough area to break the seal and lift the valve. Rummaging in the engine's jig box, I found the chucking ring for the breather and cut a shallow annular groove to join the holes. This has made a big difference, and the breather valve is now the noisiest thing on the engine. I briefly tried a roughly made leather disc instead. It is certainly much quieter, and might do the job if better made.

I also modified the pin and ball coupling to take a Meccano shaft (8 SWG or 4.1 mm), ready for starter gearing experiments. (5½ hours)

2020-03-22 – Getting compression

Still motoring the engine, running at speeds up to 2000rpm for brief periods. Above 1500rpm it gets quite warm. I have put the plastic crankcase breather reed back, at least for now. I have reset No 2 exhaust tappet clearance, but it is still a bit on the high side at 0.003″

At 1500 rpm, I am now getting compression readings up to 50 psi on No 2 cylinder, and up to 70 psi on No 1. This is enough compression to be able to go back to the starter experiments.

Test Running

2021-01-21 – First light

The engine ran for the first time today.

2021-01-22 – Early testing

A short early run. The engine is motored by the lathe via a sprag clutch (a form of freewheel) so that it can outrun the lathe, and a torque limiter to protect the engine in case it decides to seize up.

It took a while for the engine to lift fuel. The carb is still a long way off - too lean when open and too rich at idle. It also looks as though the choke will be needed, as I had to open the jet before the engine would start, and then screw down again as it warmed up and before it would run well. Once it was warm, I did not try to go to full throttle, I won't do that without a load. I have yet to learn how the engine wants to be asked to start.

The main difficulty is that circulation in the thermo-syphon cooling is hopelessly inadequate. After a couple of minutes running the cylinders are too hot to hold. I shut down this run immediately when I saw a bubble in the water outlet hose. I was expecting I would need a water pump, but not expecting the engine to heat up so quickly. I have a pump more-or-less designed.

At higher speeds the exhaust is blue, so the splash lubrication is working. There may be a bit too much oil in the sump, and hopefully the piston rings will bed in. As a precaution I might strip the engine down to check all is well before it gets much more running time. I think, and fervently hope, the horrible chirruping noise is coming from the cheap double-hooke-joint drive between lathe and engine.