No compression...why?

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Mar 19, 2005
As Alice said...curiouser and curiouser.

(1972, 750cc)

Finally the salt has been washed off the roads the sun was shining and the temp was a balmy 45 or so today, so I pushed the bike out and went to get it started. Not a fan of draining the sump, so, as I have done for 30 years, just gave it a few kicks through and with choke kicked it over. Naturally with the temp and the 20W50 it was hard to kick but the strange thing came after a couple of kicks. Suddenly, no compression. Never had this happen before. I mean you could have pushed it through with the pinkie. No TDC resistance, just an easy kick-through with no point you could say was the right point to kick from. Just compression. Kicked it though maybe 50 times with no compression, naturally a no-go as far as starting, and eventually the compression came back and I was able once again to find the TDC and got it started. Sump oil pumped up through the PCV Valve and into the tank for a good 45 seconds...lots of oil in the sump, but it blew no smoke out the pipes. Clean exhaust. Ran fine through a 50 mile run...nice but cool...lots of bikes out and about. No strangeness, ran perfectly normal and strong.

Now...why? Valves stuck open due to thick oil? You could hear the air from the rising piston coming out the exhaust on both sides, kind of a hiss. But why the open valves? Any ideas?

Had that happen a few times over the years usually when the motor had been flooded. As a teenager I was told a great theory of how fluid on the valve seats caused it, took it with a pinch of salt but hell you never know.
Never did any harm what ever it was so don't worry.

Did tickle the carbs, as always. Do I read this diagnosis as from "flooded" carbs or the fact of the sump being full up?

Worry , not, as it ran fine...but I have never had this happen before, so I am indeed curious as to the reason, even if it does usually kill the cat... :wink:
It seems it could only be one of two things. Stuck rings or two or more valves stuck open. I can't think of another thing that could release compression yet seem to repair itself. Usually stuck rings will smoke a while until they fully expand. And to have two valves stick in the guides at the same time seems odd.

I hope this is not a warning of impending failure!
No idea about the impending failure thing...but it is strange. Both sides too. Must have had something to do with the amount of petrol, as suggested, or the amount of oil in the sump, and it getting on the bores and such. Would though think this would cause the pistons to seal better, rather than not at all. It was the valves though, sound of air in the exhaust or the carbs, hard to tell with my Tinitus. Went away though and compression came back. Will have eyes open though for a repeat of this. Maybe just need to start it more often... :wink:
Never fails to amaze me. The nutty things that happen to a Norton. I probably would have been pulling the head off after ten kicks or so. What's really frustrating you can never conclusively prove what happened. And of course, you are haunted by the spectre of a return of the problem at a most inopportune time.
Could it have been a clutch issue?

Was the engine definitely turning?

The kickstart works through the clutch so if the clutch slipped it could seem like no compression.
Nope on the clutch. Motor was definately turning. It was, in retrospect, almost like the valves got stuck open, and unless I'm wrong, the exhaust valves. Strange as it seems, after a couple of kicks with good compression. Compression went then simply away for a while...mechanically only to explain by something stuck between seat and valve or....and it's a big or...the springs not being able to overcome resistance on the valve stem from a "lot" of oil? Or!!! This is my bet, now...So much cold thick oil in the sump that the lifters got stuck up in their bores for a bit? Could that be? Betcha I'm right...:wink:
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