Beating a dead horse

Jack70 · Jun 7, 2011

First Post! I've read EVERY post on PCV valves, HOWEVER... I have not been able to determine if this is an option on a 70' commando. All diagrams and posts seem to be for the 72' PLEASE put this to rest for me. Thank you

DogT · Jun 7, 2011

The 68-70 models had a timed breather off the left side of the crank just below the barrels. No PCV. It should have an elbow on it, goes to a rubber hose which will go to the top of the oil tank somewhere. Looks like you have a mix of Fastback and other parts there? What's the engine number?

Dave
69S

grandpaul · Jun 7, 2011

Welcome to the forum.

Hobot · Jun 7, 2011

The timed breather ain't up to venting the Norton twin crank case pressure spikes so it can be removed and a PCV installed for same benefits you've read about.

ML · Jun 7, 2011

hobot said:
The timed breather ain't up to venting the Norton twin crank case pressure spikes so it can be removed and a PCV installed for same benefits you've read about.

Better clarify that - removing the hose and fitting off the left crankcase and installing a PCV will not work whatsoever. The breather device is a slotted disc attached to the end of the camshaft inside the motor. The venting is through a drilled flange on the camshaft into the hollow centre and out by the disc.

There are 2 options on a '70 motor and that is - pull the motor down and drill breather holes into the timing side case as per 850 and add a PCV to the blanking plate on the old magneto apreture. or add a breather to the inlet rocker cover. Personally I would just add a breather to the inlet cover as it is simple and effective.

Mick

AussieCombat · Jun 7, 2011

G'day ML, Can you explain the Rocker cover breather a bit more or show a pic if you have one.
A friend of mine has a problem with oil up in the head of his newly biult 750 Domi, we pressume it's a
breathing problem. He would rather not strip the motor at this early stage.
Is it just a case of placing a fitting there somewhere and plumbing it away, with a valve.
AC.

pete.v · Jun 7, 2011

A simple matter of drilling and tapping a hole in the inlet cover and screwing in a barb. Run a hose out and down the back.
I wouldn't be so concerned with a PC valve. Just get it venting. Worry about proper updating next winter.

AussieCombat · Jun 7, 2011

Thanks PV, Yeah it's a racer so that'll do for now.
AC.

JimC · Jun 7, 2011

Any positive pressure in the crankcase will rob power and try to force oil out somewhere. The easiest way known to prevent this is to install a reed type PCV valve. That's with any four stroke engine. Race car engine builders go to extremes to accomplish this. As for the timed breather, better than nothing, but not nearly as effective as the reed type PCV valve. The closer (PCV valve) to the crankcase the better.

T.H · Jun 8, 2011

AussieCombat said:
G'day ML, Can you explain the Rocker cover breather a bit more or show a pic if you have one.
A friend of mine has a problem with oil up in the head of his newly biult 750 Domi, we pressume it's a
breathing problem. He would rather not strip the motor at this early stage.
Is it just a case of placing a fitting there somewhere and plumbing it away, with a valve.
AC.

G'Day AC,
can weld 3/8 fitting to inlet rocker cover and run pipe to catch bottle then to atmosphere.Helps a heap!

Tim

AussieCombat · Jun 8, 2011

Thanks to all invloved.
I'm on it.
AC.

Beating a dead horse

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