Breathing Problems

Information relating to the Matchless G85 500cc Heavyweight, AJS 7R, Matchless G45 and Matchless G50
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Breathing Problems

Postby Peter Baker » Thu Jul 07, 2016 6:15 pm

I have just re-commissioned a 1961 G80CS scrambler imported from the states and refurbished there (but I don't know when). It's running well now but I have an oil leak from somewhere. The engine leaks when running but is quite dry when left to stand overnight which suggests its a breathing problem. As far as I can tell without dismantling, it seems to be from somewhere between the crankcase and the primary chaincase (alloy).

Before I go any further with the investigation, does this ring any bells with you experts? I'm not clear as to how the engine is meant to breathe and the workshop manual is not particularly helpful on this one. Any tips or thoughts before I start tearing things apart would be much appreciated. I'm running the beast on 30 grade Castrol. All I can see is a pipe from the rear of the chaincase directed towards the back chain, which is presumably to relieve the chaincase of any oil pressure, but I'm not clear how the pressure is meant to get there in the first place.

Also, I have changed the inlet cam to SH (as per exhaust) to calm things down a bit! (Edited by the Advertising Editor to remove an advert which is not allowed on this part of the forum. Guest members can place an advert in the for sale section for a charge of £5, it's free for full members - Clive)

Cheers

Peter

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Re: Breathing Problems

Postby clive » Thu Jul 07, 2016 6:20 pm

any reason why this is in the G15/model 33 part of the forum? If not it could be moved to either the G80 or competition sections

ps I would have thought 30 grade was too thin for the UK but will now hide my head as an oil debate ensues :headbang:

pps how much does it leak? I imagine even for a late model there is still a crankcase breather fitted to the crankcase behind the primary case. It has a small pipe and a flapper valve which allows the crankcase to breathe as the piston comes down the barrel. This generally liberally coats the underside of the engine with oil mist when the engine is hot. Its worse if the rings are not totally free in the ring grooves as a result of a minor overheating seizure. How do I know? numerous problems through thrasing my G80 with double adult sidecar.
clive
if it ain't broke don't fix

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Re: Breathing Problems

Postby Rob Harknett » Thu Jul 07, 2016 9:45 pm

30 in winter 50 in summer. I use straight 50 all year round, don't ride the bikes much in winter. Never use a multi grade, which invites sumping when standing. Straight 50 will not sump so much when standing. I used 30/50 in a newly built twin engine and it sumpped buckets of oil.

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Re: Breathing Problems

Postby Don Madden » Fri Jul 08, 2016 4:10 pm

All the singles breath through a non-return flap valve located just behind the primary case. A large, thin washer between the two drive side main bearing prevents excessive blowby into the primary case. A tube connects to the breather valve, bent downward. Late scramblers had a hose to connect the tube to the oil tank. Be sure the three screws that hole the primary inner to the crankcase are very tight a they tent to work loose. There should be external star lock, (shakeproof), washer under the screw heads & a gasket on crankcase. Cheers, Don.

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Re: Breathing Problems

Postby Peter Baker » Fri Jul 08, 2016 6:09 pm

Thanks so much for all the expert advice to one who is new to the G80CS. I'll have another careful look at it over the weekend and also change to the right grade of oil. Is there any merit in introducing a vent to the rocker box cover to relieve the pressure at the top end as well? As suggested, part of my problem may well be bedding in a new piston and rings and I am hoping to see some improvement in running oil leaks once we have covered a few miles.

Cheers

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Re: Breathing Problems

Postby ajscomboman » Fri Jul 08, 2016 7:28 pm

Many people put a vented rocker inspection cover on but specialist Ken De Groome who also raced singles reckon it's a waste of time.

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