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Vacuum Advance

mojobaby

mojobaby

Enthusiast
Messages
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After reading a few posts regarding vacuum advance units, I decided to check up on mine on my GTL.

I took off the distributor cap and sucked and sucked on the pipe from the carb side, but nothing moved. When I disconnected the pipe at the vacuum unit, I discovered a screw blocking the pipe.

I have previously disconnected the pipe but have never noticed the screw, as it was pushed quite deep into the rubber pipe.

I presume that the vacuum unit is faulty/broken. Nothing moves at all in the distributor when I suck on the pipe, even with the screw removed.

My car runs perfectly at low speeds and high speeds, I have no problem at all. So my question is, should I just let sleeping dogs lie? It seems impossible to open the unit to check on any problems inside that I might be able to repair.
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The car will be much more 'nippy', smooth and economical on anything less than full throttle with a functioning vacuum advance.

You can get pinking on full throttle if the ignition is too far advanced, but that isn't so much of a problem on part throttle so the vacuum advance advances the ignition by up to 8 degrees or so on part throttle. That gives you smoothness, a lighter sound with less booming, makes the car go faster and improves fuel economy while cruising.

I would try to fix it. Looks Ducellier and replacements were available but not so much any more. Something that fits would probably do the job as there is adjustment for the vacuum advance inside the distributor.
 
Thanks Malcolm, sounds good so I'll make some enquiries. There are quite often 2nd hand distributors advertised on Le Bon Coin, so thats another option.
 
mojobaby....if you manage to find new vacuum advance for Ducellier 525187B R244D61 please let me know as i am interested to buy at least one....
 
Beautiful

Finally can say if my old vacuum advance working as it should or not.
Apparently not. :doh:

Thank you for everything.:waving:
 
Thanks, an excellent video. Very helpful.

Just one thing that perhaps you could clarify please. My understanding is that there is the most vacuum when the throttle is least open. At around 8:35 you say that when the vacuum advance unit sucks at higher revs it will advance the timing. But I didn't think that vacuum advance is dependent on RPM, only manifold pressure. So isn't it the case that the vacuum advance advances the timing when the throttle is closed or only partially open and has nothing to to with RPM?

Thanks
 
You are mostly right. At low engine speed the engine is sucking though small holes in the carburettor so the engine is sucking hard . To explain the high speed situation you need to think about when you want maximum power. You put your foot down on the throttle. Wide open throttle. (wot) Maximum fuel in, maximum air in to burn the fuel. Maximum vacuum then creates maximum distributor advance until you reach your cruising speed. You are sucking in as much air as you can but through bigger holes.
Then you back off the throttle, less fuel and air is sucked in which means less vacuum and less distributor advance. Everything is back to a "new" normal.
 
Thanks David. You've explained it well thank you. I was incorrectly thinking that the vacuum was related only to throttle position; but it is related to total air flow (volume), making it more complicated!

Thanks!
 
I know my 845 stumbles at low speeds when I forget to attach the small hose after work on the engine. I always feel pretty dumb, when forgetting it time and time again...
 
Haha, yes thats one interpretation. It actually talks to me with several voices at the time... The front driveshafts (I think), the brakes and some rattling noise from the engine bay.
And then the roof-rack that resonates at low rpm's.
 
Hi all,
The vaccum advance of my 4LC '74 had a hole in the ruber (Perforated membrane)...

You can tested this easly with a silicon pipe directly connected to Your mouth and sucking all air inside of this small vaccum system, it must stop. If u can suck more and more air without end, it's because the membrane is perforated. (disconneting the pipe in carburator side)

Also you can do the same with air compressor regulated with a very low pressure, and blowing residual air inside this small system (witouth leaks), it must fullfill the system and stop without ear any leak. In this case of blowing air, you must move the actuator with other hand making the rubber to move in oposite side, if air escape in any position during the movement, it's because it has a leak inside so it's faulty!

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Hello Bruno, yes I think sucking on a silicon tube, or any plastic tube is the best test. A compressor will be too strong.
 
MB, There are some pretty crappy home videos on Utube so it was a pleasure to watch yours. Nicely lit, no camera shake, an unhurried commentary, it all made sense. Why can't others copy your good example?
 
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