Idiot question about setting timing

[Ian Vincent]

I have a strobe light for setting timing.  I have had it for at least 30 years.

I recently read the instructions and they state that you should remove the vacuum pipe from the carbs before checking the timing.  Why would that be?  It has always been my understanding that the vacuum take off for the distributor is in such a position that there is no vacuum until the butterfly in the carb is opened.  This is certainly the case with my car, if I connect a vacuum gauge to the take off when the engine is idling.

Rgds Ian

[Waldi]

Hi Ian,

that’s not a very stupid question

If you check timing with the vacuum hose connected and then disconnected (do not forget to blank off hose to engine), and the ign. timing  does not change, it apparently does not matter on your car, but on many other makes it does matter, it what I saw over the years. I simply follow the instructions in the WSM.

Besides checking timing at idle, it is more important to check if the advance increases evenly with increasing revs and also the maximum advance. The numbers for the different spec TR6-es are in the WSM (Brown Bible).

Cheers,
Waldi

[Z320]

Simple: the need for disconnecting the vacuum pipe for adjusting the ignition  is fake.

Edited July 3, 2022 by Z320

[Andy Moltu]

The vacuum advance is there to advance the ignition on light throttle/cruising which in theory helps fuel economy. 

If you are running a hot cam and or raised compression a rough rule of thumb is to have the ignition to about 32 degrees btdc at say 4krpm when max advance has been achieved with the vacuum disconnected.  Whether vacuum advance will be tolerated without pinking will decide if you can use on a modified car. 

On a standard car the static timing is all you need with minor tweets to adjust if it pinks.

[ntc]

35 minutes ago, Z320 said:

Simple: the need for disconnecting the vacuum pipe for adjusting the ignition  is fake.

Not so.

[Z320]

Tell us, then I tell you

Edited July 4, 2022 by Z320

[Z320]

No answer? So I start?

Years ago, as Ian, I noticed no difference at all when I adjusted the ignition on my TR4A with the vacuum pipe connected or not connected.

First I checked the vacuum box, stamped 2-6-3, and found it proper working due to the WSM and stamped data.

Next I checked the vacuum connection on my HS6 and playing with the revs at my garage there was no vacuum noticeable on my vacuum gauge!

Next I installed the vacuum gauge on the dashboard, connected with a hose and drove the car on the street.

And there it was, a lovely vacuum down from atmospheric 1 bar to 0.6 bar, never less.

This was the moment I noticed the advance unit only works with the car driving under load on the street,

with a reasonable consummation of air and petrol. It doesn’t works when not driving the car.

This is why in my opinion the demand for disconnecting the vacuum pipe for adjusting the ignition is fake.

Ciao, Marco

Edited July 4, 2022 by Z320

[Waldi]

I think it depends on the type/make of car. I just follow the WSM, there may be other reasons for the factory to require this (like when idle is too high but not noticed during adjusting?)

Cheers,
Waldi

[Ian Vincent]

Thanks for all the replies.  I concur with Marco, with SUs there is no vacuum from the venturi unless the car is underway.  This is not the same as a vacuum take off fitted into the inlet manifold where the vacuum is at a maximum when the throttle is closed but reduces as the throttle is opened.  Anyone who drove a 1950s Ford with vacuum powered windscreen wipers (100E etc.) will remember this.

Rgds Ian

[DRD]

On 7/4/2022 at 8:43 AM, Z320 said:

No answer? So I start?

Years ago, as Ian, I noticed no difference at all when I adjusted the ignition on my TR4A with the vacuum pipe connected or not connected.

First I checked the vacuum box, stamped 2-6-3, and found it proper working due to the WSM and stamped data.

Next I checked the vacuum connection on my HS6 and playing with the revs at my garage there was no vacuum noticeable on my vacuum gauge!

Next I installed the vacuum gauge on the dashboard, connected with a hose and drove the car on the street.

And there it was, a lovely vacuum down from atmospheric 1 bar to 0.6 bar, never less.

This was the moment I noticed the advance unit only works with the car driving under load on the street,

with a reasonable consummation of air and petrol. It doesn’t works when not driving the car.

This is why in my opinion the demand for disconnecting the vacuum pipe for adjusting the ignition is fake.

Ciao, Marco

That may be true for vacuum advance but isn't the case for vacuum retard.

[Ralph Whitaker]

On 7/5/2022 at 12:57 PM, DRD said:

That may be true for vacuum advance but isn't the case for vacuum retard.

Just done the timing on my TR3. On my car the manifold had a take off for the vacuum pipe and I was unaware of the one under the front carb. On idle the vacuum unit is pulled fully round to maximum advance. When accelerating hard the vacuum decreases, which allows the distributor to retard the ign to avoid pinking. I thought this was how they were all intended to work, I had never come across a system that was plumbed into the carb such that when the throttle is closed it cuts the vacuum to the pipe.

Ralph

[Lebro]

My experience is the opposite. up until last year I had always seen vacuum advance piped connecting to the carb.

Last year I was helping an MGB owner to get his car running nicely, his vacuum pipe did(& was supposed to) connect direct to the manifold.

Apparently it was to satisfy USA emissions regulations. most MGB owners change it to connect to carburetter.

TR's AFAIK always connect to the carb.

Bob

[RobH]

27 minutes ago, Ralph Whitaker said:

On my car the manifold had a take off for the vacuum pipe ........ On idle the vacuum unit is pulled fully round to maximum advance.

That must mean the basic mechanical advance setting due to dizzy position will be around 14 degrees less than it should be. Although the vac advance puts that right at idle,  when the engine isn't working hard and the vacuum drops the timing will be retarded a lot further than is correct.  The vac advance is supposed to work in addition to the mechanical setting - not as a substitute for it. 

[stuart]

When setting timing on a dizzy with vacuum advance you are supposed to disconnect it and plug it, set the advance and then reconnect the vacuum line.

Stuart.

[Ralph Whitaker]

3 hours ago, Lebro said:

My experience is the opposite. up until last year I had always seen vacuum advance piped connecting to the carb.

Last year I was helping an MGB owner to get his car running nicely, his vacuum pipe did(& was supposed to) connect direct to the manifold.

Apparently it was to satisfy USA emissions regulations. most MGB owners change it to connect to carburetter.

TR's AFAIK always connect to the carb.

Bob

Now you mention it I think the A35 has it connected to the carb too, I have a spare carb I will take a look.

Ralph

[Ralph Whitaker]

Just been out and had a look, and indeed the A35 SU has the vacuum take off on the carb, but the hole is on the manifold side of the butterfly, whereas the TR carb has the hole on the filter side of the butterfly when closed, although they are never properly closed otherwise the car would not run at all so it may not make any difference.

I guess I had better get the relevant connectors and couple my advance (or retard) pipe to the carb where it is supposed to be.

Ralph

[Ralph Whitaker]

19 hours ago, RobH said:

That must mean the basic mechanical advance setting due to dizzy position will be around 14 degrees less than it should be. Although the vac advance puts that right at idle,  when the engine isn't working hard and the vacuum drops the timing will be retarded a lot further than is correct.  The vac advance is supposed to work in addition to the mechanical setting - not as a substitute for it. 

Not so, as mine is set up with the take off on the manifold, if the engine is not working hard, say cruising, the vacuum will increase in the manifold, as Ian points out with his reference to vacuum wipers on old Fords, which will advance the ignition timing.

If I set my basic timing to 4 degrees as per the book with my vacuum advance disconnected, as soon as I reconnect the vacuum pipe my timing at idle jumps to 9.5 degrees BTDC. This of course drops back to 4 degrees as I accelerate away.

The mechanical advance comes in as the engine revs rise to a maximum of 11 degrees in addition, so fast cruising on light throttle openings I could have as much as 20.5 degrees BTDC.

Opening the throttle sharply, say to overtake, drops the manifold depression and retards the timing to prevent pinking.

The above calculations are based on the figures in the Haynes WSM for a standard TR4 distributor with 11 degrees mechanical and 5.5 degrees vacuum advance. All this assumes that the Powerspark electronic distributor I have has similar advance curves to the standard item.

A standard TR2/3 dizzy has 15 degrees mechanical and 8 vacuum, so the figures above will be even higher. As I understand it a standard system with the vacuum pipe connected to the H6 carb has no vacuum at idle, so timing at idle will be just 4 degrees BTDC, climbing gradually to maximum at maximum revs as both the mechanical and vacuum advances work together as air velocity in the carbs increases.

Sorry, I am not trying to re design the wheel here, just been trying to get my head around the 2 different systems as my set up seems to work fine, although I have ordered the parts to revert to standard.

Ralph

Edited July 30, 2022 by Ralph Whitaker

[RobH]

Just now, Ralph Whitaker said:

Sorry, I am not trying to re design the wheel here, just been trying to get my head around the 2 different systems as my set up seems to work fine, although I have ordered the parts to revert to standard.

I was describing what would happen if you set the advance to the correct figure at idle with the vacuum pipe connected ( as some have advocated above).  

As you say, the system will work provided you set the basic static advance correctly or do it dynamically with the vac pipe disconnected.  However, as your figures show, the engine will be over-advanced at idle when the pipe is connected but will work as designed everywhere else.  

[Ian Vincent]

3 hours ago, Ralph Whitaker said:

Not so, as mine is set up with the take off on the manifold, if the engine is not working hard, say cruising, the vacuum will increase in the manifold, as Ian points out with his reference to vacuum wipers on old Fords, which will advance the ignition timing.

If I set my basic timing to 4 degrees as per the book with my vacuum advance disconnected, as soon as I reconnect the vacuum pipe my timing at idle jumps to 9.5 degrees BTDC. This of course drops back to 4 degrees as I accelerate away.

The mechanical advance comes in as the engine revs rise to a maximum of 11 degrees in addition, so fast cruising on light throttle openings I could have as much as 20.5 degrees BTDC.

Opening the throttle sharply, say to overtake, drops the manifold depression and retards the timing to prevent pinking.

The above calculations are based on the figures in the Haynes WSM for a standard TR4 distributor with 11 degrees mechanical and 5.5 degrees vacuum advance. All this assumes that the Powerspark electronic distributor I have has similar advance curves to the standard item.

A standard TR2/3 dizzy has 15 degrees mechanical and 8 vacuum, so the figures above will be even higher. As I understand it a standard system with the vacuum pipe connected to the H6 carb has no vacuum at idle, so timing at idle will be just 4 degrees BTDC, climbing gradually to maximum at maximum revs as both the mechanical and vacuum advances work together as air velocity in the carbs increases.

Sorry, I am not trying to re design the wheel here, just been trying to get my head around the 2 different systems as my set up seems to work fine, although I have ordered the parts to revert to standard.

Ralph

Just clarify something, my reference to old Fords was that when you were cruising or at idle the vacuum in the manifold would be quite high and you had good wipers. The moment you came to a hill, or needed to accelerate and pressed the throttle, the vacuum dropped dramatically and your wipers nearly ground to a halt. 

Rgds Ian

[stuart]

2 hours ago, Ian Vincent said:

Just clarify something, my reference to old Fords was that when you were cruising or at idle the vacuum in the manifold would be quite high and you had good wipers. The moment you came to a hill, or needed to accelerate and pressed the throttle, the vacuum dropped dramatically and your wipers nearly ground to a halt. 

Rgds Ian

Correct, the minute you touch the throttle you destroy the manifold vacuum which is at its highest with the throttles closed.

Stuart.

[Phil Read]

On 7/29/2022 at 3:15 PM, stuart said:

When setting timing on a dizzy with vacuum advance you are supposed to disconnect it and plug it, set the advance and then reconnect the vacuum line.

Stuart.

+1

[Z320]

Hi,

This is from the Bosch "technical instructions", issue "battery ignition"

The vacuum for the advance unit is taken from the trottle, it is caused by Venturi by the amont of air flow.

"under low load the ignition must be earlier because the mixture burnes slower because is getting lean".

The vacuum for the retard unit is taken from the inlet maifold, maximum is on idle and while rolling the car braked by the engine.

"to improve the exhaust gases". As I know also to avoid jerking because too much advanced ignition (self tested).

Ciao, Marco

[d2alfa]

Hi Marco,

What setting have you used for your TR4a and what fuel do you use please?

Regards d2