Weird loss of power and sound

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
Post Reply
User avatar
Les Howard
Posts: 1017
Joined: Sat Jan 07, 2012 9:57 am
Location: Somerset UK

Re: Weird loss of power and sound

Post by Les Howard »

As G80 has suggested above, seems more like head trouble, and that means valve sealing in some shape or form...Position the head stably upside down and level on a bench and pour in petrol to fill the combustion space. Monitor for a drop in level and using a torch to see if the inlet and exhaust ports show leakage of the petrol...report back from findings....
User avatar
Expat
Member
Posts: 1033
Joined: Thu Nov 12, 2015 2:22 pm
Location: Tennessee, USA

Re: Weird loss of power and sound

Post by Expat »

Thanks Spriddler, will focus on the head when I next get to it later this week.

Expat wrote: ?Sun Jan 29, 2023 8:56 pm
.........filled combustion chamber with white spirit. Some has quickly leaked past the exhaust valve,...

Les, thanks for the tip. You can see earlier in my posts I have already tried your suggestion and there is possibly a problem there. Will report back with more as it’s revealed.

Steve
Keep shiny side up.

These are my principles, if you don’t like them, I have others. (Groucho Marx)
User avatar
Expat
Member
Posts: 1033
Joined: Thu Nov 12, 2015 2:22 pm
Location: Tennessee, USA

Re: Weird loss of power and sound

Post by Expat »

Took the exhaust valve out first and looking at the sealing face and that in the head, it seems like it wouldn’t be sealing very well. See pic, what do you reckon? Also, a build up of carbon on the valve stem seems like it could prevent it closing fully. Inlet valve and seat look better, though looks like both valves would benefit from a decoke and lapping in.
Neither valve stem or guide look scored up, though there is some rocking clearance evident with each valve half inch or so off its seat. There’s bound to be some I guess? The inlet valve throat area has a hard, glossy, varnish like deposit coating, not sure if pics show it well.
Could replacement guides be called for?
Stems measure 0.2695/.2701 dia with mic, haven’t measured guides yet.


[/attachment]
618BD083-9771-4D13-8C72- 8924112FA79F.jpeg
A3C996A9-D97C-432B-B3E0-5F725AB0F89F.jpeg
865C-589E873CD413.jpeg
94BF36F8-2439-482E-A702-A7EA8FA405C4.jpeg


Cylinder bore isn’t scored at all, though there’s a ‘soft’ wear edge at the bottom of the stroke that can be felt with a finger. Will have a go at measuring with inside calipers and vernier, not my preferred method but needs must!

Thanks for any comment and advice chaps,

Steve
You do not have the required permissions to view the files attached to this post.
Keep shiny side up.

These are my principles, if you don’t like them, I have others. (Groucho Marx)
Groily
Member
Posts: 2151
Joined: Thu Jul 04, 2002 1:00 am
Location: NORMANDIE FRANCE

Re: Weird loss of power and sound

Post by Groily »

Think as a minimum the ex seat needs a light cut, and the inlet a good grind-in, and the ports a good clean out. That's if the seats are properly secure when hot, which is part of the question of course..
The exhaust side didn't get like that in just a few miles, so given the sudden loss of compression I wonder what the head joint / gasket is like, mating surfaces etc etc, and was it torqued down OK when you undid things?

Question: are the valve stem diameters really 0.2 . . .? I know very little of singles but G3 dia is stated at 0.3715 in the places I have looked? If it's actually a 3 not a 2, then yours are worn by a couple of thou, and no doubt the guides are a bit tired too.

I'd replace the ex valve as a matter of course and in any case I think I can just see the beginnings of a 'notch' or flat burning its way in at about 7 o'clock in your pic.

Stem to guide clearance should be no more than a very few thou max (data will probably be in one or other of the books in the archives but haven't managed to locate it in a brief look-see). Always hard to measure properly too, I usually make and mike up bits of silver steel bar to determine actual diameters of bushes and guides and stuff. If you want to 'do it right, and only once' you'll clearly need to measure them accurately and replace if out of spec. (Think I had new cast iron guides in one of my my alloy head twins at about 2 thou clearance with new valves, but memory is fuzzy.)

Personally, because I'm an anorak and hate revisiting things, I think I'd be inclined go the whole hog and 'do' the lot - seats, guides, valves, springs. Might be overkill though, I do accept. Along with getting the bore measured properly as regards possible rebore / hone / new rings given your comments about oil consumption - although that could in part be a worn inlet valve and guide.

That said, these things last for ages with a bit of bore and piston wear, and oil isn't that expensive in reasonable quantities, for moderate mileages. One man's 'worn out' is another man's 'nicely run in' after all!
User avatar
clive
Member
Posts: 5657
Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Re: Weird loss of power and sound

Post by clive »

Well the inlet(?) valve and seat had a definite ridge round them which will affect performance although I admit to simply lapping in similar wear in the past it could do with being recut. The other valve shows very poor sealing too but neither are likely to result in sudden loss of compression. This takes us to two likely culprits. Leaking head gasket or valve guide moving. You might see signs on the head gasket if it has blown by, a cause for me in the past has been not tightening down on the gasket because swarf in the bottom of the thread in the barrel was causing one cylinder head bolt to bottom out rather than fully compress the gasket. You could test the valve guides by heating the head in the oven and see if either guide them moves easily.
I think Groily is right if you want certainty you will need to replace quite a lot, if you only intend to do 100s of miles a year ratherthan 1000s the question of cost will raise its ugly head.
clive
if it ain't broke don't fix
User avatar
Duncan
Member
Posts: 2118
Joined: Thu Jul 10, 2003 1:00 am
Location: HAMPSHIRE UK

Re: Weird loss of power and sound

Post by Duncan »

Expat wrote: Tue Jan 31, 2023 12:54 am The inlet valve throat area has a hard, glossy, varnish like deposit coating, not sure if pics show it well.
Do you know if the petrol tank has/or had an older type lining installed, modern fuels can dissolve some of them and redeposit them in the carburettor and around the valves causing them to stick.
User avatar
Ralph
Member
Posts: 87
Joined: Tue Jan 07, 2020 7:00 pm
Location: UK

Re: Weird loss of power and sound

Post by Ralph »

Wouldn't be surprised if a lump of carbon had not come loos and stuck under
the exhaust valve and held it off it's seat, allowing a compression leak and increasing the valve clearance.
I would clean the valves and head up and a light recut if you can then a good regrind.
Ralph

Image

Image
User avatar
Expat
Member
Posts: 1033
Joined: Thu Nov 12, 2015 2:22 pm
Location: Tennessee, USA

Re: Weird loss of power and sound

Post by Expat »

Thanks so much for your replies and suggestions chaps. Will find someone with the tools to re cut valve seats, or maybe source them locally and buy. Haven’t got around to having a go at measuring guides yet. Bill, I was wrong, of course valve stems are as you say, 0.370 dia. not 0.270. 🙄

Head was torqued down, no signs of gasket blow by. Clive, will check if guides may have moved. I do have spare pair of valves but not sure if they’re better shape yet. I don’t have lathe but do have quite a selection of plug gauges and metric/imperial drills as I often used to check bore dias as Bill describes.
Duncan, a very interesting and coincidental theory, as another bike of mine has suffered from just that issue. It had a tank made from GRP that had been lined by the po but something was leaching into the petrol and causing the carb slide to stick! Made for a couple of interesting events.

Ralph, very possible I would imagine, thanks.

Will report back with further discoveries.
Keep shiny side up.

These are my principles, if you don’t like them, I have others. (Groucho Marx)
User avatar
1608
Member
Posts: 2493
Joined: Mon Oct 05, 2009 1:00 am
Location: ESSEX UK

Re: Weird loss of power and sound

Post by 1608 »

Looks to be burning oil and the top land of the piston has been scrubbing the cylinder. It suggests simple bore and ring wear to me. But what has it measured up like ?. This may be a side issue, but why the problem manifested on the home run is another matter. My guess would be sticking valve when got hot considering how much carbon build - up there is.
Typed this before seeing page 2, but there you go!
User avatar
Expat
Member
Posts: 1033
Joined: Thu Nov 12, 2015 2:22 pm
Location: Tennessee, USA

Re: Weird loss of power and sound

Post by Expat »

Want to measure everything again as best I can just to try and confirm or otherwise figures scribbled down yesterday. Fingers were getting numb out in the shed despite having heater on. 🥶
Also looking for size specs of valves, guides, piston etc to compare to, can’t seem to track them down in Christian’s archives as yet.
Should I also be looking elsewhere? Have seen some specs in these pages in the past but have no record of them.
Still want to check for loose guides/seats and my future plans for riding the bike are inclining me to say, replace guides, rings, maybe ex valve and hone the bore.

Contemplating any more work/expense would really mean going all out for a total rebuild to make it worthwhile and be more certain of some long term reliability, as has been suggested already.

Will return with more findings and no doubt, asking for even more helpful advice. Thanks
Last edited by Expat on Thu Feb 02, 2023 5:54 pm, edited 1 time in total.
Keep shiny side up.

These are my principles, if you don’t like them, I have others. (Groucho Marx)
Post Reply