Crankcase oil leak as crankcase breather not working

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Durantc
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Joined: Fri Feb 06, 2015 7:31 pm
Location: Bristol England

Crankcase oil leak as crankcase breather not working

Post by Durantc »

I have searched the forum and the internet but cannot find an answer to my problem so I hope somebody can help if I explain the problem in detail.

I purchased my 1964 AJS 500 single in 1975 and rode it nearly everyday until I took it off the road in 1979 and stripped it down to thousands of bits. As with these things, I did not get around to rebuilding it until 2014 and completed the rebuild in April 2015.

I have ridden it over 4,000 miles without any issues and it has been brilliant. Then on May 2nd while riding with friends from Bristol to North Wales, after about 80 miles I noticed that there was oil on both of the frame downtubes below the front engine bracket. Could not work out where it was coming from. After cleaning it off after about 50 miles it came back. Eventually it transpired that it was coming out of the crankcase halves at the front of the engine. Assuming it must be a build-up of crankcase pressure, I took the top of the crankcase breather off the oil tank and checked if anything was coming out. No air what-so-ever. With spit, made a film across the pipe and even revving the engine did not break it. Thought the pipe might be blocked so took pipe off the copper pipe at the crankcase end and blew through it no problem.

So to me, it appears that the crankcase breather is not working at all. Has anybody come across this before? Is it because the bike was off the road for 36 years and it was about the only part that was not checked or replaced? I have bought a new crankcase breather diaphragm and will have a go at replacing it but is this the problem. Thanks, Chris
Mick D
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Re: Crankcase oil leak as crankcase breather not working

Post by Mick D »

Hi

The crank case normally breaths via a short copper pipe into the area of the secondary chain behind the primary chain case. The oil tank breather comes from the oil tank inside face via a rubber tube and short copper tube anchored to the frame and vents in much the same area as the crank case breather.

Is you bike different to this - are you confusing the breathers?

It's possible you have made an error when you re-assembled the bike after all those years - it's also possible I've gotten hold of completely the wrong end of the stick :)

Sounds like the breather is stuck closed - you could probably have cleared it by soaking in fuel / brake cleaner and then cleaning it, but if you've a new one on the way might as well fit that.

Regards Mick
Mick D
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Re: Crankcase oil leak as crankcase breather not working

Post by Mick D »

Hi

After re reading your post it seems like you have the crank case breather attached to the oil tank breather - in this condition neither will breath. This is probably why your breather has ceased to function too. The crank case and oil tank breathers should both vent directly to atmosphere.

Regards Mick
Durantc
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Location: Bristol England

Re: Crankcase oil leak as crankcase breather not working

Post by Durantc »

Thanks for your replies Mick. As a bit more of an explanation.

In the rebuilding of the engine the one thing I didn't touch was the crankcase breather and copper pipe, I just left them as they were. On the first start of the bike in April 2015 I checked the pipe that was connected to the brass pipe and it was blowing i.e. releasing crankcase pressure. On the 1964 model oil tank there is are two metal welded pipe coming out of the top. One is for the crankcase breather to attach to, the other is the oil tank breather. I connect the crankcase breather to the correct one, checked with a few people. For two years and over 4,000 miles they breathing has been perfect and there has been no (noticeable) oil leaks. Then this oil leak in May. So I took the breather pipe off the top of the oil tank and it should blow like it did in May 2015 but nothing and as I said the rubber pipe itself from the brass pipe is not blocked either. Just seems amazing that the breather should suddenly stop working.

I hope that explains it a bit more and open to ideas. To replace the diaphragm is not the easiest job as the chaincase, rotor, engine sprocket etc. needs to be removed to get to the crankcase breather and don't want to do all that if there is something simpler.
bjorn
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Re: Crankcase oil leak as crankcase breather not working

Post by bjorn »

Drive side crankcase.JPG
Hi Chris,
If the crankcase breather is not working, it could be because the hole inside the crankcase leading to the breather is clogged. The hole is only about 1/4" and could become clogged with remnants of gaskets and gasket goo. It is possible you could clear this plug without dismantling the engine if you can draw a sufficient vacuum on the breather pipe to suck the matter out.

Good luck
Bjørn
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Mick D
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Re: Crankcase oil leak as crankcase breather not working

Post by Mick D »

Hi Chris

I wasn't aware that the later models routed the crank case gasses through the oil tank, sorry.

As a 'clutching at straws' fix and if you have access to compressed air you could try blowing air back through the breather - short blasts without and with the engine running - fingers crossed.

Regards Mick
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Pharisee
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Re: Crankcase oil leak as crankcase breather not working

Post by Pharisee »

Mick D wrote:Hi Chris

I wasn't aware that the later models routed the crank case gasses through the oil tank, sorry.

As a 'clutching at straws' fix and if you have access to compressed air you could try blowing air back through the breather - short blasts without and with the engine running - fingers crossed.

Regards Mick
The diaphragm in the breather valve shouldn't allow you to do that. It's effectively a 'non-return valve'.
I'm from the Fens.... Gimme six.
Mick D
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Re: Crankcase oil leak as crankcase breather not working

Post by Mick D »

Hi

If the diaphragm is stuck - blowing compressed air in the reverse direction may clear it, (by unseating it), - like I said, just a chance.

Regards Mick
Durantc
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Joined: Fri Feb 06, 2015 7:31 pm
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Re: Crankcase oil leak as crankcase breather not working

Post by Durantc »

Thanks for all your replies and I'll be starting to look into it tomorrow and see which ideas I can try. I'll keep you posted.
Durantc
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Joined: Fri Feb 06, 2015 7:31 pm
Location: Bristol England

Re: Crankcase oil leak as crankcase breather not working

Post by Durantc »

Bjorn's idea made me think and I got my pump out which I use for pumping up airbeds for camping. the nozzle can also go on the other side for deflating airbeds.

Although it does not have too much suction and will let air in if there is no air to extract to save it burning out the motor I thought I would try it. Got a good fit and left it running for around 45 secs but nothing was sucked out and I did not know if anything happened. Next morning started the bike and after about 2 minutes I could feel it pumping air out of the breather so it appears to work. Took it out for a fairly long ride yesterday with the end of the pipe not connected to the oil tank but up near the petrol tank. It kept breathing and eventually a mist of oil started appearing. So as it stands it is working again and thanks for all your ideas.

Now I just have to keep my fingers crossed that it will continue working and I will still be wondering why it suddenly stopped working. If I ever have the rotor or clutch apart I'll get to the breather and fit the new diaphragm.
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