Xs Vacuum Advance Unit

[miamichris 9]

Just got a new vacuum advance unit for my Xs from Peugeot today as my old one was knackered.

 

I've fitted the new one but it doesn't seem to be getting a vacuum from the carb to operate the unit, anyone know why this would be? What creates the vacuum in the carb, is there possibly a diaphragm in there that has perished as well?

 

I have my dizzy rotated to almost full advance (or is it retard? ) to get the car to run right (why I bought the new vacuum unit in the first place!) and when I remove the pipe from the vacuum advance there is no change in engine speed (which there should be I think..). But when I disconnect the vacuum hose from the carb and suck on the pipe the engine speed rises, so this confirms to me that the new vacuum unit works but its just not getting a vacuum.

 

Any advice?

 

Also, what difference will the new vacuum unit make to the car once I have it working right?

 

Cheers, Chris

[miamichris 9]

Was hoping there'd be an answer to this by the time I got on here today

 

I've been researching how the vacuum is created, it's to do with the venturi effect inside the carb. So, I reckon I must have a vacuum leak in the carb or else the vacuum take off is blocked maybe?

 

Next move is to take the carb off to see if I can figure this out, could be that I need a gasket and diaphragm kit for it.

 

Also, researched a bit on here about the vacuum advance and what effect it has. There seems to be a lot of debate, some people say it makes a lot of difference and some say a little. I've gathered that it helps the mpg on part open throttle and should give a bit of extra performance, I have a slight hesitation on low revs when I put my foot down so I reckon it will cure this too.

 

I'm sure I'll figure this one out and I'll post up what was wrong once I fix it, so that anyone else having this head scratcher of a problem will know what to do!

[AndyJ 1]

Was hoping there'd be an answer to this by the time I got on here today

 

I've been researching how the vacuum is created, it's to do with the venturi effect inside the carb. So, I reckon I must have a vacuum leak in the carb or else the vacuum take off is blocked maybe?

 

Next move is to take the carb off to see if I can figure this out, could be that I need a gasket and diaphragm kit for it.

 

Also, researched a bit on here about the vacuum advance and what effect it has. There seems to be a lot of debate, some people say it makes a lot of difference and some say a little. I've gathered that it helps the mpg on part open throttle and should give a bit of extra performance, I have a slight hesitation on low revs when I put my foot down so I reckon it will cure this too.

 

I'm sure I'll figure this one out and I'll post up what was wrong once I fix it, so that anyone else having this head scratcher of a problem will know what to do!

 

Easy way to check you are getting a vacuum at the take-off is to pull the pipe off and stick your finger over the take-off. If the advance unit has been gone for some time someone might have blocked it up to correct the air leak this would have caused.

 

The vacuum at this point (in the intake manifold) is the caused by the cylinder/pistons sucking against the closed throttle plates, which is why you have a high vacuum at idle (which pulls on the advance unit to retard the timing) and a low vacuum at full throttle which advances the timing.

You are referring to the venturi effect which is at work in the carb throat itself which is what pulls the fuel through the jets, but not what actuates the advance capsule.

[miamichris 9]

Easy way to check you are getting a vacuum at the take-off is to pull the pipe off and stick your finger over the take-off. If the advance unit has been gone for some time someone might have blocked it up to correct the air leak this would have caused.

 

The vacuum at this point (in the intake manifold) is the caused by the cylinder/pistons sucking against the closed throttle plates, which is why you have a high vacuum at idle (which pulls on the advance unit to retard the timing) and a low vacuum at full throttle which advances the timing.

You are referring to the venturi effect which is at work in the carb throat itself which is what pulls the fuel through the jets, but not what actuates the advance capsule.

 

Nice one Andy, thanks. I checked for a vacuum at the take off by putting my finger over it but there was no suction. I'll have to take the carb off some night this week and see if theres anything blocking it.

 

Cheers for explaining the vacuum advance thing, I was a bit confused but that clears it up!