Metro again

Also if it is the case that there is no vacuum being fed through the smaller nozzle, could this be indicative of problems with the carb (more than just a blocked vacuum nozzle)?

if it does not advance that extra bit then it will be less economical, the power will be as it should though.

mrcheerful

You do realise this is ported vacuum - i.e. vacuum increases with throttle angle. (it will be zero at idle)

Tim..

No, 'cos Idon't know what that means. ;-)

I know there's going to be bog all vacuum at idle and increasing vacuum with increasing throttle, yes. But even at reasonably high revs the vacuum being supplied to the vacuum advance seems (based on previous experience, and based on what's being fed to the hot/cold air changeover flap) to be pathetically weak.

Another noddy question: the vacuum splits two ways at a T-piece at the vacuum advance - into the vacuum advance, and then on to terminate in the top of some big black plastic cylinder (sort of the size of an extra big can of beans) next to the battery. Bunch of other what look like fuel hoses going in and out, some of them into the carb. I have no idea what this cylinder is or might be doing, and the Haynes manual gives no indication of it being present on this engine. What is it? I've only seen anything like it before on injected engines (and not known what it was then either).

Sorry for the idiot questions, I'm trying to learn as I go here.

Is it possible it's a charcoal canister for evaporative emissions? The Haynes only talks about it being fitted to injected engines and controlled by the ECU, and the purge control valve only being opened about engine temp of 70 degrees and engine speed above 1500 rpm. But I'm thinking you could achieve the same thing with the vacuum that's fed to it (and to the vacuum advance) - there's a thermostatic switch in the line that only passes vacuum to the canister and vacuum advance once the engine gets above 70 degrees. Supposedly this is to prevent retardation when the engine's cold, hence increasing the temperature of exhaust emissions and aiding heating of the catalytic converter.

The message from "Tim.." contains these words:

Mostly it's the other way round.

No, vacuum is at a maximum at idle, vacuum reduces as you open the throttle, at high revs with a light throttle the vacuum will get higher and the vacuum advances the ignition to give better efficiency.

Odd valves and things are used to increase the control of the vacuum so that it varies with engine temperature, air temperature etc.

So you need to know what the valve is supposed to do before you can tell whether it is operating or not.

The important thing (for economy) is that the vacuum advance system works and actually advances the ignition at a vaguely appropriate amount and time.

The engine should be fully hot, with vacuum disconnected and plugged you check the initial advance at the specified speed, reconnect the vacuum with it running at a couple of thousand rpm and you should see the timing marks move as the system advances the ignition. As long as this occurs and the quantity of advance is ok then all is well on the timing department.

Have you actually checked the cam timing as that is more likely to be wrong than the ign. timing.

Likewise give the car a service, especially check tyre pressures and whether any brakes are binding.

Lastly or maybe firstly check your driving style.

Mrcheerful

The message from "mrcheerful ." contains these words:

That depends which side of the butterfly the port is. Downstream and the vacuum' highest at idle, upstream and it's highest flat out full throttle.

why would there be a port outside the carb? it would have atmospheric or very close available all the time unless there was a blockage in the air intake system, so I suppose you could use it as part of an 'air filter blocked' warning system. (not on a metro as std. equipment)

mrcheerful

The message from "mrcheerful ." contains these words:

Undoubtedly, that's why I said earlier "Mostly it's the other way round". There is a port upstream on Metros, IIRC, it's for the crankcase ventilation.

No. Guidance on how to do this would be much appreciated as the Haynes doesn't say.

Have checked plug condition and gaps and compression - all fine, plugs show slight carbon deposition, i.e. rich running - changed the air filter, tyre pressures are all good, so sign of brake binding but will check for any wheel rims getting overly hot next time I'm out. Carb dashpot has been topped up. Clutch is high but not slipping.

I'm a pretty economical driver, not heavy on the right foot, I don't overuse the choke and get it in as soon as is reasonable, I always tend to get about the published combined cycle MPG on any car I drive. I'd tell you whether I'm getting it on this one if only I could find it. :-(