Loss of compression

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

Postby Bruno » Tue Sep 26, 2017 7:05 pm

I'm in need of some advice. Went to start my G3 yesterday .... usual procedure using decompressor just past compression. Good prod on the kick starter resulted in a very loud backfire. Went to look for compression turning over the kickstarter and couldn't find the usual resistance ... still some, but not as solid a compression as normal. I also had the sense that there was more airflow into the carb?

I should say that the same phenomenon has happened before a few months ago, but the loss of compression was very short lived and came back after kicking the engine over a few times. No problems since until now.

I decided to remove the rocker box - I had the idea that maybe a valve spring was bust or dislodged somehow. I could also check the valves for movement/sticking ... I thought a valve might not be seating properly? In short everything seems OK there and I could push the valves in and let them spring back OK etc.

So, I'm confused why I have lost compression ... could a piston be damaged by the backfire?

I thought some of you might be able to shed light on what is going on before I take any unnecessary steps, such as taking the head off. One thought - I didn't think to check the tappet adjustment before removing the rocker box ... is it possible that one of the valves wasn't closing fully due to poor adjustment and allowing compression loss?

Any suggestion welcome ... I'm hoping I've overlooked something very obvious.

Cheers, Bruno

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Re: Loss of compression

Postby JimFitz » Tue Sep 26, 2017 8:35 pm

I would check the tappets first. A valve guide may have dropped and is preventing the valve from closing.

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 g80csp11 » Tue Sep 26, 2017 10:15 pm

if its got an alloy head then its a valve seat , sometimes it reseats hence works again but needs replacing
your symptoms are typical of this failure

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Re: Loss of compression

Postby Janet » Wed Sep 27, 2017 6:33 am

g80csp11 wrote:if its got an alloy head then its a valve seat , sometimes it reseats hence works again but needs replacing
your symptoms are typical of this failure
Can't it be reground instead of replacing?
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Re: Loss of compression

Postby ajscomboman » Wed Sep 27, 2017 7:25 am

Janet wrote:
g80csp11 wrote:if its got an alloy head then its a valve seat , sometimes it reseats hence works again but needs replacing
your symptoms are typical of this failure
Can't it be reground instead of replacing?


I think Peter means that the seat has rotated in the head in which case a replacement or pegging is the only fix. I have to agree with him (my that pains me :rofl: ) but I believe his diagnosis is correct.

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Re: Loss of compression

Postby Triumph-Legend » Wed Sep 27, 2017 9:11 am

Ah, I had a motor where the comp progressively got less over time. Ah, sod it, best have a look. It transpired that the rings had stuck up in the grooves. The funny thing was that when the rings were freed off, the ring lands fell apart. The motor was still going OK, just down on power. :rofl:

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Re: Loss of compression

Postby Rob Harknett » Wed Sep 27, 2017 10:42 am

Had a members combo bike that lost power going to Jampot rally at Patley bridge nr Harrowgate years ago. That proved to be a rotated valve seat. It was decided we could fix it. Got the head out the members tent and put it on the bar B Q. That freed the valve seat, turned it, tested it held water in the head. All OK . then peened all round the seat to hold it tight. The deaf member came back when his head was on the bar B Q. You should had seen his face, " that my head " he said we just nodded. Got the bike all back together, then off he roared to catch up with the Sunday run. I followed in my, but never caught up with him. He made the 250 + mile run home OK and miles there after.

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Re: Loss of compression

Postby Bruno » Wed Sep 27, 2017 6:42 pm

Hello everyone,

Thanks for your contributions to solving the problem ... I think it sounds like the wayward valve guide, so I'll take the head off and investigate that further.

By the way, it is indeed an aluminium head. The motorcycle has been running great all year and in regular use almost daily through the summer.

I'm tempted to try your fix Rob depending on what I find - I shall report back once I have examined the head, valves and guides.

All the best, Bruno

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Re: Loss of compression

Postby g80csp11 » Wed Sep 27, 2017 6:50 pm

Rob knows what he is talking about in agreeing that its a valve seat insert . I'm prepared to bet up to £1 that is what you find
how confident is that !
I#ve seen so many aluminium heads with exactly those symptoms

Of course you need to find me to collect the £1 if i'm wrong !! , and then collecting it is a different matter . ask Rob

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Re: Loss of compression

Postby ajscomboman » Wed Sep 27, 2017 7:24 pm

g80csp11 wrote:Rob knows what he is talking about in agreeing that its a valve seat insert . I'm prepared to bet up to £1 that is what you find
how confident is that !
I#ve seen so many aluminium heads with exactly those symptoms

Of course you need to find me to collect the £1 if i'm wrong !! , and then collecting it is a different matter . ask Rob


Tell me about it!!!!!!!! :rofl: :rofl:

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