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Old Book
by Morganmike - 21/09/26 07:54 PM
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Jun 14th, 2026
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Martyn, Where can I find the vacuum can?
I do have a condition where I am travelling at about 55 mph, maybe 60, and as going up hill climbing in fourth gear the engine or carbs clinks. It seems that I am lugging the motor up the hill. Shifting to third gear at a higher rev would fix this, but the revs seem a bit high. Is this normal for the TR4 engine?
'67 +4 tourer - broken white - RHD
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Gone to Porsche Part of the Furniture
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I think I need to explain distributor function traditional type ignition.
In the body of the distributor, attached to the shaft, are some weights and springs. The function of the weights and springs is to advance the ignition with revs and the springs to return the timing to the static figure when the engine slows down and stops.
On road car distributors, there is also a vacuum can which is aimed at putting more advance in when the inlet vacuum is high. An example of this would be accelerating to cruising speed. While accelerating, the can does nothing, but when cruising speed is reached, the driver lifts off the throttle, a high vaucuum is formed in the inlet manifold and the can further advances the ignition on what the weights have done. You don't need or get extra power, but you gain in fuel economy. Put your foot down to overtake, the vacuum disappears, the ignition goes back to the weight only setting so you lose the extra advance. This is fine as the weights and springs give the advance for load, and the vacuum can only the further advance for economy.
No where though does the distributor retard timing, it advances the timing with revs, and it advances the timing with vacuum.
If the balance weights stick, the most common symptom is the timing remaining advanced as you slow down, and if you then press the throttle, the car 'pinks' but doesn't do it from rest. Cure is usually as simple as a spray of light oil on the weight mechanism.
Calling the vacuum can an advance and retard device as many do distracts from its real purpose, an economy device. Although a good explanation, i think a little complex for some one who doesn't even know where the "CAN" is, also you have not mentioned about actually setting the Ignition timing from the body of the distributor with strobe over Crank wheel, this would be necessary to evaluate whether the ignition timing is advanced or retarded, now this even confuses the situation more as you Quote, "no way can you retard the timing you can only advance" when we used to service a vehicle, the Distributor Advance "CAN" as you say, would be checked and freed off, ie by squirt of WD and a little light oil, once it is proved the mechanism is working, then the Timing would have to be set, a little more complex than a Mechanic showing you in 5mins, I THINK
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JohnV6
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An excellent and accurate description as always! 
Graham (G4FUJ)
Sold L44FOR 4/4 Giallo Fly
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'90 LR 90 SW
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Timing can be set reasonably quickly staticly and in days of old - that was the only way most cars were set. One simply assumed the various advancing methods were doing their job correctly. In terms of the explanation - I was trying to fit the level to the audience. It's often a thankless task as one person will say too simple and another will say "what?". I stick with my argument though, from the static position, all the various mechanisms do is advance a bit, or not. If you have 8 degrees static, that is the base point - it's all advance from then on. Modern cars, though may be more complicated, but the questioner hasn't got one. I'm not giving instructions, just a bit of background. If anyone doesn't know what the bits are, look here (vacuum can is the disk shaped object on the right in the first picture): Lucas Distributor rebuild Manual ignition timing control on the 1930 car is much more fun...
1930 Super Sports Aero 'The Elk'
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Martyn, Where can I find the vacuum can? I get very worried when people ask advise on how to mess around inside engine bays or cars in General, making it sound easy can be dangerous as there are lots of high speed spinning things in their that could hurt your fingers if you are not competent enough 
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I appreciate all the notes, comments and advice. I know enough not to mess with parts I know nothing about unless I am willing to pay for fixes of my fixes. I know I have a lot of learning to do and experience is a good teacher.
I like this forum and all the help people are willing to give.
'67 +4 tourer - broken white - RHD
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Indeed, it is bit of a problem and why I don't generally write "how to guides" as we all start from different bases. Whilst I often criticise Haynes books, I'd not want to write one.
Vitruvius - I hope my earlier follow post and photo's in the link answer your question.
1930 Super Sports Aero 'The Elk'
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I found you write up added to my (lack of) knowledge although I used to tinker with timing many years ago on Morris Minors, Heralds and Spitfires.
I never did find out why the weights reduced to shavings of the Morris Minor distributor. My fix was a 30 UK pounds "recon" engine.
Ian
2011 Plus 4
Wild Mogs Reims 2019, Goodwood 2018, Isle of Man 2017, Carried on to Caramulo" 2016
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once it is proved the mechanism is working, then the Timing would have to be set, a little more complex than a Mechanic showing you in 5mins, I THINK All I wanted to say is that the Mechanic would show which bolt to loosen and which way the distributor has to be turned a small fraction to advance or retard the ignition - and give the appropriate health and safety warnings (as good Mechanics do). This method is good enough to find the point just before the beginning of pinking on an engine which runs o.k. otherwise. To be very thourough we maybe had to start with adjusting the valve timing BTW not every distributor has a vacuum mechanism. And on many highly tuned engines (Loti) it is disconnected.
1967 4/4 1500 Comp.
1935 Super Sports Barrelback
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