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No vacuum advance

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Jim Lee

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Hi all,

Has anyone either had their vacuum advance stop working or plugged it up purposely. Mine is not working I think because of a bad diagphram or stuck something and I am wondering how much of a performance difference it would make on a TR3A.
I have read that some folks plug them up purposely only experience a lack of fuel economy. I have also read that they play an essential part in the performance of the car.
I am having performance problems with some really aggravatin g backfiring under load and am trying to figure out if some of it might be because of the vacuum advance not working. The mechanical advance is working.

Thanks very much,
Jim Lee
 
Take the distributor cap off and also the fitting for the vacuum advance that stick out the body of the dizzy aiming towards the front of the car. Get a similar fitting and piece of tubing or slide a rubber hose onto the tube for the vacuum advance (if that's what you have), put the tube into your mouth and suck. Look into the top of the dizzy while doing this and see if the plate gets sucked forward. If it does, it's working. If it doesn't move, it needs attention.
 
If you have backfires when you are under load with sustained wide open throttle (not at the advent of WOT), I doubt that your back fires are being caused by the vac advance system. If you get a back fire at the advent of WOT, I think at least part of your problem would be the non working advance system.
 
Different car, model and engine, but after I got my distributor back from Advanced for a rebuild and recurve, I blocked off my vacuum retard unit and it works well.

I don't think that I would want to block off the vacuum advance though. Is your distributor actually advancing the timing mechanically? Are the weights free to move and advance? It may be all rusted up in there. If you look at the pics on my site describing the overhaul, you'll see what I mean. Second picture down shows the weights and springs.

https://www.74tr6.com/distributor.htm
 
I was just looking at your dizzy pictures. Very interesting. I have a wire that looks alot like your ground wire before picture, I cannot remember if it the ground wire. It seems pretty plain that I need to remove the points plate to have a fighting chance of really checking out either the mechanical or vacuum advance mechanisms up close and personal. I am thinking that the mechanical advance is working based on using a timing light and seeing the mark move quite a few degrees to the left (counter clockwise at the front of the car looking towards the rear).
when I goose the accelerator momentarily. Is that a sign that the mechanical advance at is working? It certainly may
not be working as well as it should. I need to get in there and make sure everything is as free as it should be.

Anyone with a TR3 know if there are only the two tiny phillips head screws I need to remove to get the points plate off? Also, are all 25D distributors the same or do I have to get one from another TR3 or TR4?

Thanks very much,
Jim Lee
 
Jim, that's the problem as these cars get older and things bind up or just start to break done from old age. Kinda like me!

I'm not sure, but I think that they all had just two screws, but I could be wrong. If you notice in picture #2 on my page, the spring at the bottom is stretched more than the top. Not good!! But again, old age.

If you have any questions about the distributor, feel free to contact Jeff at Advanced. He frequents this forum a lot and offers a lot of free advice. He'll be glad to help through this, I'm sure. www.advanceddistributors.com
 
Hello Jim,

firstly, the vacuum advance unit is inoperative at heavy load so discount it. (It is purely an economy device and the engine is set up so that a slightly weaker mixture can be run smoothly, i.e the set up is weak mixture at cruise\light throttle and the extra advance that is required to burn a weak mixture is provided by the vacuum device)
You are right that the timing point alters as you rev the engine, and your manual should give total advance, i.e static plus mechanical advance probably when the engine is running over 3,000 rpm so do a timing check at that speed or higher (Vacuum advance disconnected or you won't get a true reading)

The points base plate is the same from any 25D distributor, the bits that are specific to a particular model are cam, weight springs (Note the two springs are different to each other) and the vacuum unit.)

The other thing to bear in mind that even with a fully functioning distributor you may need to adjust the timing from standard to suit your engine and the fuel you are using.

Alec
 
First, in answer to your question, yes for the TR3/4 engine you anly have to remove the two screws to get the plate off. Second, the vacum on these cars (the TR3/4) is not the same as the vacum on more modern cars. From what I have obseved measuring the vacum as a function of RPM it seems that the vacum port on this car is placed such that it only increased in vacum at the moment of throttle opening and it returns to lower vacum once the open throttle has been that way for a seonc or two. That being said (or vmore correctly observed) I imagine the vacum unit on the dizzy only changes the timing at the instant of throttle opening from then on it is the mechanincal advance that is doing everything. This agrees with what Vettedog was saying (as best I can tell, not wanting to puit words in his mouth).

On both my TR3 and TR4 I don't see a whole lot of advance going on with a timing light as you describe, but I have concerns about my timing light itself.

I still think you should make sure your mechaincal advance is working and you have the correct springs in there. One of the springs has thicker wire then the other and one of them is actualy loose when the car is not running. The reason for these two things is the advance curve for the car is not linear and it (the curve) has two break points. The slope of the curve changes at the two break points and that slope is determined by the spring constant of the springs (for the begining part of one spring and for the later part of the two springs combined).
 
The holes in the mechanical counterweights (moon-shaped) can become worn and oval. Also the pins that they are supported by can become worn (smaller in diameter). These problems can throw it all off. Check this out. If worn, replace them. Or try to bush the holes in the weights and redrill the holes to the correct size. Check this and see if there is some way you can remove, replace or repair them. Finally, if the top bushing is worn, the drive shaft for the dizzy coming up from the camshaft can wobble off center adding further complications.
 
Distributor - How specific to year?

How specific is the distributor to the year of the car?
I am thinking of getting a backup.
Would I have to make sure that it is from a 1959 TR3A with the exact same part numbers as mine?

Thanks very much,
Jim Lee
 
Re: Distributor - How specific to year?

Jim, The distributors are universal in fit, but quite specific in application. The ignition curve will be different for each car and 5-digit model number. I may actually have an original distributor for your car if you're interested. Contact me and I can help you out one way or another!
Jeff@advanceddistributors.com
 
Re: Distributor - How specific to year?

This link has a lot of info on Lucas distributors and their applications...

https://www.telusplanet.net/~chichm/tech/lucas.pdf

I carry one spare dizzy that I have used in both my TR3A & TR4. Haven't noticed any performance change when the back-up was on there (I think it is a TR4 distributor) but the original dizzys would not have been exactly the same.
 
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