AJS & Matchless Owners Club

Long time since last post, life has been vying for attention involving retirement and moving to another state, though always checking in to see what’s going on. Some great conversations and tech stuff.
My G3 issues just happened and I will be interested to hear any comments forthcoming from you guys.

Went for short, twenty mile ride and bike was fine going out, cruising around fifty, sixty mph as per usual. On the return journey she became reluctant to increase speed/revs above forty/forty five mph and was noticeably down on power. Got home, bike idled and sounded okay, so I switched off and made a mental note to check carb/timing etc.

On trying to start a few days later…..no compression and no starty !!! Eventually tried the time honored test of squirting oil down the plug ‘ole to find some compression returned and she started!! So not a stuck valve issue?
But….strange and unusual sounds now coming from engine. Not knocking or screeching but tinny sounds from cylinder area. No smoke, no change when clutch lever pulled and first gear selected but I’m definitely not feeling that I want to try riding. Still idles okay.

So what has happened and how did compression disappear after switching off, only to come back to some degree when oil was introduced?
I’m feeling that the cylinder has to come off at least to investigate but will welcome any opinions as to where the problem might be.
Btw, engine is all original and has covered little under 30k.

Thanks in advance for any comment.

Steve

A case of timing loss causing pinking?

If not then I'd investigate possible broken piston ring.

I was thinking along the same lines. Ignition slipped and retarded would explain loss of power in the return journey. This could have resulted in overheating the engine and one of the rings became trapped in the ring groove as a result explaining the loss of compression after it all cooled down. Tinny noise piston slap? If you take the barrel off slide some cloth to cover the crankcase in case Andy is right and it is a broken piston ring. Good luck.

Could behead gasket leak.

if its a aluminium head , possibility of loose valve seat insert that can re-seat and appear Ok , certainly not unusual
head off is the only answer

Thanks all for your inputs, interesting and I certainly hadn’t thought of any of your suggestions, (obviously, that’s why I asked!), especially the possibility of a valve seat insert playing up.
Oh well, I suppose I was hoping against hope for some kind of magic solution not involving a strip down, (we know that never happens don’t we), coz that’s gonna have to wait until SWMBO is happy with adjustments to the new, to us, retirement home!
Will definitely come back and reveal what seems to be the issue though.

Now I’ll have to see if the newly issued insurance policy can be put on hold!

Cheers

Steve

Finally got the G3 up on the lift to investigate power loss and strange sound from engine.

Cylinder is off ( after rag stuffed in crankcase mouth just in case! ). An amazing sight, to me anyway, no damage anywhere in bore, wire wound piston shows no sign of seizure at any time, no broken or seized rings, no rock in small end, very slight sideways movement of con-rod, no discernible up/down play in big end!

So what is the problem? Did check that the timing hadn’t slipped, also inverted the head on the bench and filled combustion chamber with white spirit. Some has quickly leaked past the exhaust valve, so that bears checking I reckon in case valve is burnt or seat is loose as was suggested earlier.

Interested in any suggestions from those with more experience than moi in these matters, best way forward regarding piston and cylinder re use especially. Bearing in mind engine never used to smoke noticeably but oil disappeared quite rapidly, definitely leaking here and there when not running.

Hone bore and fit new rings? Measure bore and/or check ring end gaps first, is this done using new rings? I’m obviously keen to retain the piston if possible, unless of course a rebore proves necessary.

TIA, Steve

Hope this pic works to show piston.

And the other face.

In your very first post you wrote:

Expat wrote: ↑Sun Sep 18, 2022 3:53 am Went for short, twenty mile ride and bike was fine going out, cruising around fifty, sixty mph as per usual. On the return journey she became reluctant to increase speed/revs above forty/forty five mph and was noticeably down on power.................

On trying to start a few days later…..no compression and no starty !!

Expat wrote: ↑Mon Jan 30, 2023 12:56 am .........filled combustion chamber with white spirit. Some has quickly leaked past the exhaust valve,...

Since the piston looks O.K. with no broken rings I'd first investigate the zorst valve and that the seat and valve guide are secure before considering anything further.
Since it ran O.K. for 20 miles and the ign timing hasn't slipped it's highly unlikely that the bore and rings etc. would've worn so quickly as to be so noticeably down on power after 20 miles.
(If you re-use the copper head gasket be sure to anneal it first).