Engine Breathing

[Mknight702]

I thought I would post up my latest trials and tribulations.

My engine has been breathing heavily through the oil filler cap since I replaced the SUs with the Weber. The Weber manifolds don't have a vacuum port for the breather circuit so I thought that a catch tank was in order. I did flirt with the Evacusump system from Vizard's book but that didn't seem to work for me so I relented and had some stubs welded onto the inlet manifold that I plumbed to the catch tank to give me negative crankcase pressure. (I had also previously modified the route of the breather system so the timing cover vented to the rocker cover which in turn vented to the catch tank.)

Fired the car up in the garage and it seemed good, finally I had vacuum in the crankcase. Took the car out for a quick run and after 5 minutes I suddenly did a James Bond and vanished in a cloud of smoke. Pulled over to the side of the road and popped the bonnet, I now have so much vacuum that it was hoovering the oil out of the rocker cover and had filled the catch tank and was in turn getting to the inlet. OK so now I quickly rerouted the breather to remove the rocker circuit and just take from the timing cover, better but not perfect so turn round and drive home looking like a Red Arrows display.

Today I have fitted a PCV valve to the vacuum side of the system to partially restrict the peak vacuum at idle and cruise, plus rerouted the breather circuit more permanently to exclude the rocker. I may also have to fit a restrictor to the vacuum if it doesn't have the desired effect, unfortunately the weather wasn't the Mae West for further testing today so I haven't managed to test it extensively, maybe tomorrow.

One day the car will be finished!

[techbod]

"One day the car will be finished!

Then what on earth will you do with your time?

[sgrant]

Matt - what were the original symptoms? I do get a bit of oil weeping out under the rocker cover gasket, but it's nothing uncontrollable and not enough to make me have to top up after a run. Just not sure if this is something I need to worry about.

[Mknight702]

My original sysmptoms were loss of oil from the filler cap (being sprayed out the vent) and loss of oil from somewhere else (no idea where, I was just using loads still). The vent cap you could put your hand over and feel little pulses of gas out as the engine ran, don't get that now I have vacuum.

Still not tested with the restrictor yet, but the PCV did help reduce the vacuum, I am still filling the catch tank though. I have made a restrictor with a 3mm hole and I will run with that and go smaller if necessary.

As for what will I do when the car is finished, well driving it would be nice!

[sgrant]

oh I know the feeling oh so well!!

I took mine down to Airey Tuning, who did a fantastic job rejetting the carbs and I had oooh I don't know, lets say 30 minutes of wonderful trouble-free driving until I noticed that the revs were at 1500rpm at idle. Then I hit traffic and it overheated. Eventually I got home, and I might even have sorted the overheating with a lot of banging of hoses, releasing coolant at the heater tap blanking plate and blowing into the header tank, but I'm still struggling with the idle revs. It's the carb not shutting down fully when I release the throttle. It's not the throttle stop on the carb, that much I think i've worked out, so it's either a sticky cable or the linkage or a problem with the carb itself. I've detached everything (ie, cable and linkage) and I still have to gently push the carb closed by hand - just the last ~1/8th inch won't close on its own, but i'm hoping that, with a new cable in place and if I make the linkage a little longer, then the cable will exert enough positive pressure on the linkage onto the carb that it'll shut properly.

Else i'm going to be using even more fuel on the way down to Le Mans than last year

stephen

[erictharg]

I don't understand why, even with strong vacuum, you'd get so much oil pulled out of the rocker cover and into the catch tank. Even curiouser that things improve just using the breather from the timing case, becasue that must have a similar amount of oil in it - but I suppose the oil separator means that most of the oil drains back into the timing case. Is your rocker cover filling with too much oil for some reason? I can't think why it would through. You can run an A series without the rocker cover at idle without too much oil appearing. What's your hot oil pressure? Maybe the relief valve sitcking and giving too much oil to the rocker cover fills up more than it should?

[Mknight702]

The oil from the rocker seemed to be just oil mist that was hoovered out. The outlet to the catchtank was not shrouded other than by the indentation for the mounting bolt so I think that any droplets of oil flung about by the rockers was just sucked into the catch tank. On a similar note, the filler cap on the alloy rocker doesn't seem to be shrouded in any way which is why oil is sprayed out of there. The normal filercap for the pressed steel unit seems to be better shrouded (but wont fit the alloy rocker).

I have now fitted some more restrictors in the vacuum hose and I think solved most of my problems, new restrictor is 1.5mm so I guess that's answered the question as to whether the small vacuum takeoff I have for the ECU must be sufficient.

Now need to fnd out why the car seems to be running on 3 cylinders and we're home free!!

[brian]

Right - numpty question of the week.
At present I don't have a rocker cover breather but having spoken to MED (who made the engine) it seems as if it would be a good thing to do.
I have a catch tank (with the timing gear cover breather going into it plus a spare port that I can use for the rocker cover.

So how do I go about fixing a breather pipe take-off point in the ally rocker cover? I phoned the local motor engineers who said that they wouldn't be able to do it as they weren't able to obtain 'breather pipes'. I suspect I got hold of the one person there who knows less than I do...
Is it as simple as getting some suitable o/d tube on a flange and rivetting/sealing it over a suitable hole in the cover?

Thanks in anticipation of help!
Brian

[Mknight702]

I just got some alloy tube and drilled and threaded both the hole and pipe, bit of PTFE and Robert's your mothers brother.

Not sure how a professional would do it though, plus I have had problems with too much oil being taken to the catch tank as there is no baffle in the rocker. Now I have the restrictor working I may give the vented rocker another go, but I may wait until I can get the car on 4 cylinders first though.

[brian]

Thanks for that. More straightforward than I thought. What sort of pipe diameter would you recommend?