No compression

C

Comet Rider

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VOC Member
Off topic

Hi Pete,
Whilst I maybe the new S/O for the Herts & Beds I'm not the rally organiser. That honour remains with Ray for the moment (depending on how Ray's feeling):D

Back to the leakdown tester. If you are able to do the test in a quiet area, it can be possible to detect where the leak is, so identifying rings/liner valvles and or head joint. When racing we used to use a cylinder of CO2 so I could hear where the leak was.:rolleyes:

Neil
 
T

Tom Gaynor

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Comets with no compression

A local rider had a Comet with almost no compression. It would cough, but wouldn't really run. There were two things wrong with it.
One was that the valve timing, although correct to the marks, was wrong. I believe that Comets with timing marks that are wrong are far more common than twins similarly afflicted. A quick check is that both valves should have more or less equal lift at TDC.
(The other was that the muff was so slack on the liner that there was evidence of gas transit between the two. (The piston to bore seal was actually pretty good.) How slack? When it was sent to have an o/s liner fitted, the fitter put the barrel on his bench and lit the gas axe to heat the muff. As he did, the muff slid off the liner under its own weight.)
Prior to all of this the bike had been running: not well, but loudly.
 
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macvette

macvette

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Non-VOC Member
This is definitely off topic. In the early 70's there was a significant explosion on one of the nations then leading petrochemical company's facilities. It was thought to have been caused by a crane's diesel engine ingesting hydrocarbon gas, bouncing it's valves as a result of over revving and backfiring though the intake into the gas cloud. I was charged with coming up with a solution to this. Problem was the theory was wrong because it was impossible to over rev to valve bounce speeds without destoying the engine. The actual mechanism was that the engine govenor racked back to compensate for the gas in the atmosphere and the engine ran quite smoothly on the gas and diesel mixture. The gas retarded the normal ignition point with the result that flame fronts travelled quite quietly out of the inlet manifold and ignited the gas cloud during the valve overlap period.
I and another friend came up with a device that detected levels of gas in the atmosphere and shut down the diesel by injecting co2 before the situation developed dangerously. We are both named on the patents covering this( no cash because it was before the law was changed to compensate the inventors).
Back on topic, to stop the diesel we had to inert the engines and decided to use co2 fire extinguishers as a source of gas. Our first attempts resulted in fractured pistons as the freezing gas hit the hot pistons. The cure turned out to be simple. We turned the cylinders upside down ( they feed from the bottom) and this cured the problem. Of course , we were stopping large equipment engines so there was avery significant temperature difference between the co2 and the pistons. Nevertheless I wouldn't do a leak down with co2 on a hot engine and would make sure that on a cold engine, the co2 bottle is upside down.
Regards Mac
 
mercurycrest

mercurycrest

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Ady,
You just described a near "text book" case of piston seizure. If you had "holed the piston, or seized it really bad, there would have been oil coming out from your oil cap due to the crankcase being pressurised. Hopefully, a quick job with a hone and a new piston and you'll be back on the road. Don't always rely on the settings listed in books, especially. when using new carbys.
Cheers, John
 
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