Air leak, but where?

[jleyton]

I've read on here several times that the revs should drop when you remove the oil filler cap if the crankcase breathing system is all ok - still can't quite work out why though !

Anyway, on my car there is a very slight drop when I take the cap off but I think it's so slight that it points to a problem.

I've checked out or replaced the obvious (new breather pipes, new oil cap washer, new dip stick washer, new carb pedestal fibre washer) but I'll be b******d if I can think of what else to check.

Any thoughts anybody ? All advice will be very gratefully received.

Cheers

Julian

[V Mad]

Julian, the reason is that when you remove the cap more air can enter the inlet, and this weakens the mixture, hence the drop in revs. Perhaps your mixture is a bit too rich?
If you suspect other air leaks have you tried the large O-ring between the carbs adapter and the manifold? (I have a stock of Viton O-rings if you need a new one).

[jleyton]

Thanks Chris, now I understand.

It could be rich as I don't need any choke after the first few seconds, although I got the garage to adjust mixture for the MOT as I can't find my Strom tool and I don't have a gas analyser. They got CO below 3.5% which seemed ok.

And yes, I replaced the adapter to manifold O ring. Surprising story with that - I'd only changed it a few months ago when I did my inlet manifold gaskets. The other day I took the carbs off to check and took the O ring out but, although it still looked in good condition, it wouldn't fit back as, when I went to refit it, it was something like 4 or 5 mm too large in diameter - yet it had been fitting ok a few minutes earlier with no pinching or anything Luckily I had a spare new one.

Anyway, thanks for your advice - think I'll invest in a new Strom tool.

Any other suggestions ?

Cheers

Julian

[Arbman Al]

Hi Julian

Re your o-ring I noticed that once, and thought b****r, I haven't got another.
Carried on with something else, then had another look and it had shrunk back to the correct size

Re-fitted it, and all was fine.

Al

[jleyton]

Hi Julian

Re your o-ring I noticed that once, and thought b****r, I haven't got another.
Carried on with something else, then had another look and it had shrunk back to the correct size

Re-fitted it, and all was fine.

Al

Yeah, I think it's a mixture of heat and petrol that does it.

Luckily, I didn't throw it away (especially as it was only a few months old) so I've got a good spare one if I ever need it !

Cheers

Julian

[RogerP]

Hi Julian

Re your o-ring I noticed that once, and thought b****r, I haven't got another.
Carried on with something else, then had another look and it had shrunk back to the correct size

Re-fitted it, and all was fine.

Al

If you get an O ring like that and it's swollen, put it in the fridge for a few minutes and it will shrink back.

Rogerp

[jleyton]

If you get ..... like that and it's swollen, put it in the fridge for a few minutes and it will shrink back.

Rogerp

That's what my missus says

[Sheepdip]

Julian, I located an air leak by chance, while engine running, disconnected breather pipe and put finger over rocker cover hole - engine slightly pressurised and noticed oil spurting out around rocker cover screws, even tho tight the seals were knackered.

[jleyton]

Good thinking - thanks Sheepdip,

Cheers

Julian

[Sheepdip]

Good luck Julian, also thought - I put a clip on the rocker cover air conenction, it was rather slack and noticed pushing it further on affected engine speed.

As an aside, The air breathers connect to carbs and there is sort of valve inside each carb that is activated (or deactivated?) when the accelerator is pressed, whats the purpose of this?

[jleyton]

Great minds think alike I was wondering myself what that valve was last night and so I read up on it:

"Float Chamber Ventilation Valve. A feature of the 175 CD 2SEV type carburettor fitted to later engines is a mechanically operated valve which controls the route through which the fuel vapour in the float chamber is vented. The purpose of the valve is to allow fuel vapours to be drawn into the engine through the normal vent aperture in the front face of the carburettor under all running conditions except when the throttle is closed. In the latter case the internal vent is closed and an alternative one opened to route the vapours through an external pipe."

So now you know

Cheers

Julian

[V Mad]

Good luck Julian, also thought - The air breathers connect to carbs and there is sort of valve inside each carb that is activated (or deactivated?) when the accelerator is pressed, whats the purpose of this?

Just for clarity, this is not the same 'breather' that is connected to the cam cover. I presume you are talking about the float chamber venting that Julian mentioned.

[Sheepdip]

Thanks Julian, that explains their purpose better and I stand corrected - I mistakenly thought they were connected to engine breathing system.

Paul.