Loss of compression

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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Re: Loss of compression

Postby REW » Wed Sep 27, 2017 10:24 pm

I would just check the tappet clearance again before major surgery. I know two 350's (only one of them mine) that suffered something similar and each had a slightly tight pushrod. I set them with just perceptible movement rather than just able to turn, when warm. Works fine.
Ron

1951 Matchless G3L, thumping round the Durham Dales

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Re: Loss of compression

Postby Pharisee » Wed Sep 27, 2017 11:01 pm

REW wrote:I would just check the tappet clearance again before major surgery. I know two 350's (only one of them mine) that suffered something similar and each had a slightly tight pushrod. I set them with just perceptible movement rather than just able to turn, when warm. Works fine.


My G80 went very 'rattly' recently and I thought it was excess tappet clearance. It was but not in the way I'd expected. When I took the tappet cover off I found that the nut securing the inlet rocker arm had worked loose and had fallen off. The arm was flopping about but fortunately, it had stayed on the splines so the engine still ran.

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Re: Loss of compression

Postby g80csp11 » Sat Sep 30, 2017 6:00 pm

Do I win a £1 , any more news

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Re: Loss of compression

Postby Bruno » Sun Oct 01, 2017 3:55 pm

I get to the job tomorrow ... with a bit of luck all shall become clear!

There's a whole £1 riding on this one and I have a feeling it might be the safe bet.

Shall let you know.

Cheers, Bruno

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Re: Loss of compression

Postby Bruno » Wed Oct 04, 2017 9:15 pm

Hello,

Well, I removed the head and it looks like the exhaust valve is giving the problem. I think it is pretty clear that it is sitting proud in the head and not sealing, hence loss of compression. With the valve springs removed, it doesn't move freely in the guide either, so I'm guessing some things are worn and will need replacing.

I haven't actually disassembeld everything yet to confirm g80csp11's diagnosis, but I think I will owe you £1 next time I see you at a club event!

Have some photos, but not managed to figure out how to post them yet - will keep trying. Thanks for everyone's guidance.

Cheers,
Bruno

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Re: Loss of compression

Postby Janet » Thu Oct 05, 2017 6:04 am

Bruno wrote:Have some photos, but not managed to figure out how to post them yet - will keep trying.
When you go to reply, look below the text box and you'll see two tabs. Click the one saying 'up load attachment' and it will show a button saying 'browse'. Click this and it will take you to your files on your computer. (I don't know whether it works on mobile phones and such devices) Find the photo you want, click on it and then 'open' and 'add the file' . There is a maximum file size of, I think 3MB.
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Re: Loss of compression

Postby Bruno » Thu Oct 05, 2017 6:53 am

Thanks for the guidance Janet. I did try that, so I'll have another go.

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Re: Loss of compression

Postby Bruno » Thu Oct 05, 2017 8:48 pm

Hello all,

A photo of the offending exhaust valve. It's not great, but I think you can see to the right of the spark plug threads, the valve is sitting proud of the seat in the head.

By the way - both valves look to be the same size, contradicting the guidance in the workshop manual which states that the exhaust valve is smaller than the inlet. My 350 G3 is a 1961 vintage ... can anyone shed some light on this?
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Re: Loss of compression

Postby JimFitz » Thu Oct 05, 2017 9:09 pm

May be a shadow on your pic but the valve does not look like it is sitting centrally in the seat - could it be a bent stem. If you take it out and roll it along a flat surface you may be able to check. Possibly spin it slowly in a drill or lathe chuck and check the valve head is spinning true.
Are there marks on the valve stem below the head where it may be chafing on the valve guide indicating that the valve is bent and are there any marks on the valve or piston which show any contact between the two?
Was the valve clearance ok before you stripped it?
As I said in the first answer to your question - if the valve guide has dropped it could prevent the valve seating and may also have resulted in the piston kissing the valve and possibly bending it.

Jim
Too old to Rock and Roll but too young to die.

1952 G80 rigid, 1960 G12 DL / Watsonian Monza, 1954 G80S.

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Re: Loss of compression

Postby 1608 » Fri Oct 06, 2017 11:05 am

Bruno, it may just be a build up of burnt oil on the stem. I have had this problem on several different bikes. I'm not sure, from memory, but I think that both valves have different diameter heads. I also believe that the exhaust valve is non ferrous and not affected by a magnet.
And John, in the photo of your G80 rocker arm bolt do I see a 376 carb fitted. Should it not be a 389. But if the bike performs well then thats fine. Correct me if I'm wrong but arn't the inlet tracts different sizes on those carbs.
Probably talking out of my hat, but well meant observations.

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