More on Old Bikes

[Grip Fast]

A friend was telling a group of us at the gliding club about the technique for starting the engine of his motor glider, while in flight (obviously has too much money). He said there is a decompression lever in the cockpit, which when pulled, allows the propellor to spin in the breeze and turn the engine over. Once spinning at a sufficient rate, he releases the lever to reintroduce compression and the engine fires, with a bit of luck.

Sounds like a valve lifter on an old motorbike, says I. So we started to discuss how a valve lifter works and realised none of us has ever owned a bike with a valve lifter, so we didn't know what the technique was for starting.

Anyone here know?

Did you lift the valve to allow sufficient decompression to kick start it? But then would it start with the valve lifted, and was it both inlet and exhaust valves or just one that was lifted?

Or as was suggested, you get the engine to the compression stroke with the valve lifted, then close the valve(s), then kick it over TDC with full compression back on?

[dave the german]

A friend was telling a group of us at the gliding club about the technique for starting the engine of his motor glider, while in flight (obviously has too much money). He said there is a decompression lever in the cockpit, which when pulled, allows the propellor to spin in the breeze and turn the engine over. Once spinning at a sufficient rate, he releases the lever to reintroduce compression and the engine fires, with a bit of luck.

Sounds like a valve lifter on an old motorbike, says I. So we started to discuss how a valve lifter works and realised none of us has ever owned a bike with a valve lifter, so we didn't know what the technique was for starting.

Anyone here know?

Did you lift the valve to allow sufficient decompression to kick start it? But then would it start with the valve lifted, and was it both inlet and exhaust valves or just one that was lifted?

Or as was suggested, you get the engine to the compression stroke with the valve lifted, then close the valve(s), then kick it over TDC with full compression back on?

The last bit - thats how it works with my SR500 - can be abit of a bugger if you don't get it right!!

[nab 301]

I did as mentioned before have an Enfield bullet . They use a separate cable operated decompressor which is threaded into the head /exhaust port like a sparkplug.
Starting technique is to ease up to compression with the kickstarter and use the decompressor to get just over TDC. This allows you maximum rotation before compression (exhaust and inlet stroke) and hopefully the momentum to get over TDC on the compression stroke .
Older bikes would have ign advance /retard levers to play with also.
Only the exhaust valve would be lifted on engines with that system and modern singles have varying types of automatic valve lifters with electric starters......

[StrokeB]

I did as mentioned before have an Enfield bullet . They use a separate cable operated decompressor which is threaded into the head /exhaust port like a sparkplug.
Starting technique is to ease up to compression with the kickstarter and use the decompressor to get just over TDC. This allows you maximum rotation before compression (exhaust and inlet stroke) and hopefully the momentum to get over TDC on the compression stroke .
Older bikes would have ign advance /retard levers to play with also.
Only the exhaust valve would be lifted on engines with that system and modern singles have varying types of automatic valve lifters with electric starters......

Agree, I had a 1953 500 single BSA B33, when I was 16, which you had to use the decompression lever to get just past compression and you would also retard the ignition fully then kick start and pray you don't get a kick back which could break ankles or send you over the handlebars! You advanced the ignition manually as you rode and I remember forgetting to do this once one cold winters night which meant that when I stopped the exhaust pipe was glowing nearly white hot! It was a great bike full of character.

[Corvus]

Honda singles based on xbr500 and also cb250rs had an automatic, cable operated decompressor. Activated when kicking over. Not entirely sure how they work though.

How about the Ducati panigale. I think they built excess clearance into the desmo system at very low rpm, to enable a smaller (therefore lighter) starter motor and associated drive. The excess clearance is taken up by a centrifugal device once the revs build above a certain rpm. I think! Something like that anyway.

[oyster]

My 1955 AJS 350cc single. The de-compression lever, a small hooked lever located between the clutch lever and headstock on the handlebar, turned a cammed arm in the rocker box that depressed the exhaust rocker, opening the valve. To start engine: Crank the kickstart until compression; lift decompressor lever, gently move kickstart to take the stroke just over TDC, release decompressor; ensure mag advance is fully retarded, fuel on and if cold, tickle carb float; ensure good stance across the bike, hold both handlebar ends securely, swing down firmly and steadily on the kickstart, feather throttle as engine fires. I could do this quite casually having owned the bike for years, mostly without the need of the decompressor, there was a knack to it, most of my mates could not start it. The decompressor was essential to stop the engine as it fired on a magneto. Some bikes had a magneto earth switch, killed the spark through earthing it. I have seen someone hoisted clean off an old bike when trying to kickstart it without knowing about the compression cycle. Know as 'kickback'. Painful.
These days technology lets me merely press a button to start the engine. Does anyone remember using (shotgun) cartridges to fire up an engine?

[Bruno]

My Ariel VB 600 sidevalve uses the same technique as Oyster's AJ. The Ariel has a posh two-part lever that controls both ignition advance and the valve lifter.
I had an Ossa trials iron that used a separate decompression valve in the cylinderhead. It was a 250 two stroke. It helped combat engine braking on slippery downhills.

I've only seen the shotgun cartridge thing in the movies; Flight of the Phoenix.

[slparry]

A friend was telling a group of us at the gliding club about the technique for starting the engine of his motor glider, while in flight (obviously has too much money). He said there is a decompression lever in the cockpit, which when pulled, allows the propellor to spin in the breeze and turn the engine over. Once spinning at a sufficient rate, he releases the lever to reintroduce compression and the engine fires, with a bit of luck.

Sounds like a valve lifter on an old motorbike, says I. So we started to discuss how a valve lifter works and realised none of us has ever owned a bike with a valve lifter, so we didn't know what the technique was for starting.

Anyone here know?

Did you lift the valve to allow sufficient decompression to kick start it? But then would it start with the valve lifted, and was it both inlet and exhaust valves or just one that was lifted?

Or as was suggested, you get the engine to the compression stroke with the valve lifted, then close the valve(s), then kick it over TDC with full compression back on?

The last bit - thats how it works with my SR500 - can be abit of a bugger if you don't get it right!!

When I had my GN400 none of my mates could start it as they've all become reliant on starter motors.

I mentioned this to my daughter, Lianne, and being bloody minded she was determined to "learn" the technique.

Sure enough she mastered it and took joy in showing my mates up with her new found skill

[nab 301]

I've only seen the shotgun cartridge thing in the movies; Flight of the Phoenix.

I couldn't think of the name of the film when I posted..

[Grip Fast]

Thanks for the interesting and informative answers, folks.

I did a quick bit of reading about the 'shotgun cartridge' method of starting aircraft engines. It seems the Coffman (shotgun) mechanism was one of the most common, and used to start Spitfire engines amongst others.

Do not try at home - the lead shot can make a mess of your boxer.

I also saw that jet engines can be started with a cartridge which supplies a blast of hot gas to spin up the turbine. Starting a Canberra:

http://m.youtube.com/watch?v=g9fkmqPHTDE