Oil loss through Breather Box - 1976 LeMans 1

[Sam-mikeleitch]

Thanks Phil. I’ll try that if the new pistons don’t reduce the crankcase pressure and the problem still persists.

[Mike_H]

@banquo

I don’t know what is the cause of your problem, but if it was running fine before you sent the heads off, then what is the point of changing the pistons and barrels? Nothing has changed there, and they were working fine before the rebuild, so how can they possibly be the cause of the problem now?

Piston rings gaps measured as 32 thou., (0.032") should be about 17 thou., (0.017") so not ‘running fine’. Rather, implies, totally clapped out.

Guessing bores are plated, (Nigasil?) if so can’t be rebored oversize, so I’d be looking at new barrel & piston kits too. Will also include new piston rings & gudgeon pin clips.

[Mike_H]

@Phil

I am running a T4 with pod air filters. Had to renew the breather box because the old one rusted internally. Found new replacement breather boxes (I tried 2) had too much spring pressure on the NRV so crankcase pressure was forcing oil up the return hose from the crankcase into the breather box and this was then running out the vent to atmosphere hose and coating the rear wheel. Followed advice in Guzziology, ditched the breather box, looped the breathers high up behind the steering head and then routed them down along the swingarm to the rear wheel spindle. Works for me!

On the LM breather box you can easily disable the so-called ‘flap valve’ (basically, a washer on a rod with a coil spring behind it, it closes off the end of the crankcase breather tube and is pushed off by crankcase pressure), by slipping a suitable sized star spring washer (off the top of my head, sort of 4 - 5mm hole size), over the rod and pushing back the washer, the star washer then keeps it open. You then fit a ball valve in the metal pipe coming out of the bell housing. I have done this.

A tip I read recently, the end of the small diameter return tube (metal) into the sump should be below the surface of the oil, else blow back and overflowing the catch box can occur. If you have a sump extension fitted, you may need to extend this tube with a bit of hose over the end, that will reach down to dip below the oil surface. Hope that makes sense.

Another trick I did that worked quite well, is extend the hose that comes out into open air, I think normally it points out to the right side below the gearbox and is secured by a clamp. I extended mine by adding a longer piece of hose to run along the left edge of the rear mudguard and end just in front of the number plate. Never lost any oil that way. However I did start to lose oil when, after some years of the former set-up, I decided to rebuild it all with new catch box (+ disabled flap valve), new hoses and new ball valve. The vent hose hung below the gearbox, as per standard. Almost. Then I could lose oil level if the bike was ridden hard. It hardly happened using the extension along the mudguard, best as I can remember.

(Edited due to typos.)

[Don-Spada]

@Mike_H

A tip I read recently, the end of the small diameter return tube (metal) into the sump should be below the surface of the oil, else blow back and overflowing the catch box can occur. If you have a sump extension fitted, you may need to extend this tube with a bit of hose over the end, that will reach down to dip below the oil surface. Hope that makes sense.

That doesn’t sound right. The oil is being forced back up this return pipe by the positive pressure in the sump. If the pipe is clear of the oil in the sump, it can’t get pushed back up.

There should also be 2 small breather pipes off the rocker boxes that go to the breather box and 2 oil feeds to the cylinder head. I assume you haven’t managed to get these two mixed up somehow?

[Chris950s]

@Mike_H

On the LM breather box you can easily disable the so-called ‘flap valve’ (basically, a washer on a rod with a coil spring behind it, it closes off the end of the crankcase breather tube and is pushed off by crankcase pressure), by slipping a suitable sized star spring washer (off the top of my head, sort of 4 - 5mm hole size), over the rod and pushing back the washer, the star washer then keeps it open. You then fit a ball valve in the metal pipe coming out of the bell housing. I have done this.

I got my breather box chromed (I know show pony!) and that process disabled the sprung flap, works for me.

[Mike_H]

@Don-Spada

That doesn’t sound right. The oil is being forced back up this return pipe by the positive pressure in the sump. If the pipe is clear of the oil in the sump, it can’t get pushed back up.

Thinking about it I would agree with you, but that’s what the guy wrote.

There should also be 2 small breather pipes off the rocker boxes that go to the breather box and 2 oil feeds to the cylinder head. I assume you haven’t managed to get these two mixed up somehow?

If you’re asking me, no.

[Don-Spada]

Hi Sam
Did you ever find the cause of your over venting troubles?

[Superheat6k]

There was no top hat valve, but there was the disc valve in the box itself. I have ordered the top hat valve from Gutsibits along with the replacement advance control springs which are now back in stock.

I think a dremel will get the old valve out

[Mike_H]

On mine I pushed a spring washer onto the rod, can’t remember the correct name but it looks like a shakeproof washer or spring washer with ‘ears’ on the inside, …

Starlock washers! These things…
.

[Superheat6k]

I punched the pin over, the disc valve detached which I have left inside and retrieved the spring.

All wire brushed off and re-sprayed gloss black, and now re-mounted.

[Roy_V7]

Hi Mike, you mentioned fitting new cylinders and pistons , if the bores are good ,not scored or stepped , I would look at fitting new rings.
If you contact Cox and Turner engineering, they can supply rings that are slightly oversize it is then easy to file them down to the correct gap

[Sam-mikeleitch]

Hi
I had a spare set of new 88mm barrels and pistons and fitted those! Problem cured and now hopefully ready for a run to Mandello in June. Neil Wof has my old set and your advice is good and will be passed on!
Ciao