Weird loss of power and sound

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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56G80S
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Re: Weird loss of power and sound

Post by 56G80S »

I certainly had a problem with a loose exhaust seat in the head which had rotated but fortunately stuck again! It stayed like that for a long time; now replaced by T&L. Apparently, though, the valve seats have an annular ring fitting into the head and unlikely to fall out.

I'm sure I read a post some time ago about someone simply peening round the edge of the seat to cure rotation of the valve seat.

If you do go for guide replacement be sure that the correct approach is used to extraction and fitting I had a disaster once entrusting this work to an car engineer when I should've done it myself.

Johnny B
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Expat
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Re: Weird loss of power and sound

Post by Expat »

Have some more details if you chaps could be so kind to peruse and comment on. It looks like there’s a fair bit of wear going in there but I’m still hoping I can get her running with a budget job as indicated previously.

Had another go at measuring, with exhaust guide coming in at 0.3735, valve stem 0.004 smaller, the used spare valve stem i have is 0.003 smaller, though the collet groove profiles aren’t great. Inlet guide 0.374 with valve stem at 0.372 so doesn’t seem too bad wouldn’t you agree? Both guides have what seems to be some bell mouthing at each end.

Cylinder bore front to back diameter half way down is 2.721as far as I can tell, with piston seeming to show the most wear I think at 2.713,
in the same plane as close as I could get.

Thanks for the tip John, I read up on the preferred method.

Thanks again, Steve
Keep shiny side up.

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Groily
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Re: Weird loss of power and sound

Post by Groily »

Just me Steve, but I think the guides would serve (the inlet particularly, as 2 thou is a good number I reckon), but I'd still replace that exhaust valve and sort the seat out.
Normally, bits that have run together for ages are going to be better than a mix of used bits that haven't been (like your spare valve) and one wishes to keep them - but the original valve is too far gone for me. The recession on the inlet seat isn't brilliant, but certainly not a killer and I personally wouldn't worry about the slight bell-mouthing or chamfer on the guides. (This if we aren't trying to go for 'perfect' - which is the enemy of the good anyway, some would say.)

Others more knowledgeable about piston to bore clearances will comment obviously. Whether there's wear enough to make a rebore necessary yet, or whether a hone and rings would be enough for now, I refrain from speculating beyond referring to what Spriddler posted (a link to the military G3L specs) in the thread on Burman CP gearboxes yesterday. I don't know if dimensions stayed the same from those days to the date of your beast, but if so there's some useful reading which might help as you compare dimensions with your own measurements. This is also a relevant thread: viewtopic.php?t=25751
The hassle with all this is that it needs really good measuring sticks and practised hands to get a good read - and most of us don't have them.
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Expat
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Re: Weird loss of power and sound

Post by Expat »

Thanks Bill, makes sense. I’ve no problem using precision measuring equipment, part of my trade when working but now all I have are a couple of mics, internal and external calipers and a digital vernier, so doing what I can.
I visited a couple of local shops today, one of them was operated by a older bloke who had probably been working on all types of engines for years and had no problems offering reboring and honing services, odd that I’ve had no luck sourcing valve seat cutters for reasonable prices, or someone to do that job. Maybe I’ll get away with just lapping in.
The link you provided makes interesting reading, so I’ll continue searching and will have to decide how far and how much to spend on this refurb.

I’m actually thinking now about a rebore, hone, new piston and rings and a new valve. I was hoping to retain the wire wound piston as I just didn’t expect to see it in the bike, so something of a curiosity as it seemed to be an unusual thing to have on a cooking machine, maybe keep it on a shelf as a topic of conversation. 😁
Just need to source and price it all.
Keep shiny side up.

These are my principles, if you don’t like them, I have others. (Groucho Marx)
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clive
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Re: Weird loss of power and sound

Post by clive »

Expat wrote: Tue Feb 07, 2023 2:10 am
I’m actually thinking now about a rebore, hone, new piston and rings and a new valve. I was hoping to retain the wire wound piston as I just didn’t expect to see it in the bike, so something of a curiosity as it seemed to be an unusual thing to have on a cooking machine, maybe keep it on a shelf as a topic of conversation. 😁
The factory fitted wirewound pistons from new even to the "cooking machines", but it is unusual to see one still in place.
clive
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Expat
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Re: Weird loss of power and sound

Post by Expat »

Thanks Clive, I was feeling sentimental on seeing the piston in good shape outwardly. 🙂 Always impressed with machines from the past that still run well, that’s mainly why I wanted to own the bike.
Now though, as the piston does seem to be pretty worn, I’m thinking of honing the cylinder bore, which doesn’t seem badly worn, seeing what size it ends up, then deciding to either buy a new standard piston and rings from the club or, rebore 0.020 over and fit corresponding piston and rings. Not a huge difference in the cost of parts between either, have to add cost of rebore though, don’t know what that would be at the mo.

Was thinking of buying a hone and doing it myself. Does anyone have experience of having done this? I feel confident enough with my background to have a go. What could go wrong?

Appreciate any comment, haven’t faced these options since teenage years. 🙄

Steve
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These are my principles, if you don’t like them, I have others. (Groucho Marx)
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Expat
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Re: Weird loss of power and sound

Post by Expat »

Hello all,
Finally making some time to get back to this and have purchased +0.020 piston kit and ex valve from club spares. Decided on this route after studying advice given here and from the guy who honed the standard worn bore. I will get him to rebore the cylinder and I’ve also been trying to decide whether or not to pursue the suggestion that the ex valve seat be lightly recut, then the valve lapped in.
It does seem that seat cutter kits are available from various sources at widely varying prices but also that the diameter of the mandrel in the kit would likely need resizing to suit the guide diameter in the head, which I’m unable to do myself.
I’m also reluctant to buy a cheaper kit as it seems these frequently have quality issues and the better quality kits are very pricey just to only use possibly one time.

I suppose my questions are: does anyone know of a reasonably priced suitable kit out there or who may be prepared to loan me a cutter, which I would obviously look after. I guess I could try lapping the valve first of all and then assess if that’s adequate.
Have others tried this approach?

I should probably add that I’m in the SE London/Kent area for a couple of weeks, and any tool loaned to me would be returned by secure method.
Worth an ask…….


Thanks for any and all replies. Steve
Keep shiny side up.

These are my principles, if you don’t like them, I have others. (Groucho Marx)
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Ralph
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Re: Weird loss of power and sound

Post by Ralph »

I have peened many valve seats in industrial engines and never had one move in years of service but dont think I fancy doing a road engine.
Ralph

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Expat
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Re: Weird loss of power and sound

Post by Expat »

Just fitted the rebored cylinder. Had a few problems, not done this job since doing it on a Fanny Barnet (two stroke!) in the ‘70’s. Interested to hear opinions on how others control con rod and piston movement when lowering cylinder onto piston, short of growing another pair of hands!
Got away with using a car type ring compressor, g clamp and parallel ally strips, but had a few pucker moments!
Keep shiny side up.

These are my principles, if you don’t like them, I have others. (Groucho Marx)
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1608
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Re: Weird loss of power and sound

Post by 1608 »

sometimes its easier to insert piston with rings into the barrel ( no risk to breakage of rings )and then lower barrel onto the conrod and then insert wrist pin etc. If its a heavy twin barrel ( example Ariel/bsa twin )then I have suspended barrel with pistons etc from the frame top tube with strap and then turned engine to raise the con-rod up into position. Alternatively one could prop the barrel (with piston in place) up from the c/case with wood blocks, but thats a bit unsteady.
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