Hundred mile stone reached (part 2)
…and it started up first kick! I kid you not, nobody was more surprised than me.
The correct thing to do after starting a rebuilt engine is to ride it straight away. Letting the engine idle does no good, it just generates heat which the new piston rings can’t get rid of because they are not yet worn in.
However I have an important check to do before can ride anywhere - I need to check the ignition timing. When I fitted the electronic ignition system, it included alignment markings to get the timing about right, but not spot on. Having had the engine failure before while running it ‘about right’ I was keen to check it before putting the rebuild engine through its paces.
An over-advanced ignition can damage the engine because the spark plugs set fire to the mixture while the piston is still moving up, causing a lot of pressure on the piston and rod and generating a lot of wasteful heat. It can even melt a hole right through the piston.
The timing is checked using a stroboscope - a simple strobe light with a pick up that detects the no 1 spark plug firing and flashes the strobe at same instant. By shining the strobe at the timing plate on the end of crankshaft, you can ‘freeze’ the timing marks and check if the plugs are firing at the correct point.
They are not! The ignition is well advanced from where it should be, about 10°. Not healthy. It’s adjusted by simply rotating the plate with the ignition pickups in the housing until the marks are correct. On this bike, only a fully advanced timing mark is provided so you have to check it while revving to 4000rpm in your garage, which is LOUD!
Timing sorted, it’s time for a test ride and the beginning of the running-in process. I have filled the engine with special running in oil, which is actually a very basic mineral oil that compared to the normal stuff is not all that slippery.
The idea is to let the piston rings rub against the cylinder walls and actually get worn down very slightly, making them a closer and closer fit. This prevents gasses escaping between the rings and cylinder walls, improving the compression and giving a strong, healthy engine. Before starting up, the compression was already 130psi, which is pretty good and shows the machining was well done and the rings are good quality.
All the new and re-machined engine parts need some time to bed in and get to know each-other, so running in is done in stages by gradually increasing the engine load and revs over time. Run it too hard too soon and the heat can glaze the cylinders, meaning the rings will never bed in properly.
Unfortunately, this all proves rather academic, as I only get a few hundred yards when I suddenly lose power and get a lot of tappet like noise from the top end. Things seem to have gone pear shaped again. I nurse the bike up and down the street but it fails to free up. Back to the drawing board.
Disappointed, well disgusted really, I park up and lock up for the night. This is not the correct frame of mind to be trying to fix things, I need a clear head.
In the morning I take another look. Could a valve have got stuck? Peeping in the adjuster holes they look fine, and the clearances are spot on. Compression is fine too, so what gives? The bike starts fine and runs briefly, then the problem returns. There’s nothing for it but to strip it down and take a look inside.
Seat off, tank off, carb off, exhausts off, rocker covers off, cylinder head off… I am getting quite quick at this now. And what’s this? The exhaust valve guides don’t look right, they are sticking out of the head by about 5mm, preventing the valves closing fully. Removing the valves, I tap the guides back into place, which is done far too easily. They are supposed to be a tight interference fit. As they are iron and the head is aluminium which expands faster, the fit gets looser with heat. Clearly the original overheat, or perhaps the machining work, has loosened them.
Unfortunately I can’t fix this myself. Sure I can buy new guides and bash them into place, but I can’t guarantee the correct interference fit, I can’t ensure they are dead straight and I can’t machine them down to the ideal 0.001” clearance needed for best valve function. The head will have to go back to the experts HT Howard, while I source some best quality solid Colsibro valve guides.
Andy at HT Howard carefully cuts down the old guides before driving them out to minimise damage to the head, then cleans up and measures the holes. I have the new guides made to a size 0.0025” BIGGER than the hole they are to be driven into, and send direct to Andy who presses them in, reams them out and recuts the valve seats. All in all an annoying and expensive delay.
Meanwhile I have done some reading up, particularly this excellent article by guru John Healy explaining all about valve spring strength and fitted spring length, and how this is changed when the cylinder head is skimmed. Checking my old springs, I find they are about as strong as a saggy mattress, opening at 40lbs instead of 65lbs. Plus when fitted the spring length is too long by several mm. All this means there is no way they were strong enough to close the valves quickly and securely.
A new set of quality LF Harris valve springs is just £40, and I also order some shims that sit between the head and the bottom spring cup to bring the fitted length back to spec.
Now it just remains to reassemble the engine once more. I anneal the head gasket again and use new pushrod tune seals. It all goes together easily now I have practice and I have the head torqued down, valves adjusted and the bike whole again in just a couple of hours.
Again she obligingly starts on the first kick, and this time runs and rides as she should. I can’t open up the throttle just yet, but already I can feel the engine is much more lively and responsive. I trouble-free 50 mile bimble later and the running in process is going great.
Once the engine is cool, everything needs to be re-tightened again - barrel nuts, cylinder head bolts and rocker covers. This is quite an involved process as the rocker covers have to come off to access the cylinder head bolts, and the re-torquing process puts the valve clearances out of spec each time. Still I am getting faster at it - good job as it needs doing two more times to ensure everything is settled down and tight.
After 300 miles of gradually more spirited riding it's time to change the oil and oil filter. This gets rid of any tiny metal particles from the run-in process, and allows me to use the proper oil - a vintage spec 20W50 endearingly called ‘Golden Film’. Satisfyingly, compression is up significantly to nearly 160psi. It doesn’t get much better than that.
While I am still building up to full power and can’t thrash it yet, I can now rev the bike cleanly up to 8000rpm and feel most of the power. Which since you ask is around 45bhp. Not a lot compared to modern bikes, but don’t forget it only weighs 170kg. A power-to-weight ratio that would still put most sports cars to shame.
Sorry for the lack of pictures - I did find this charmingly dated magazine review of my bike when it first appeared in 1971, from which I stole this:
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