Distributor vacuum advance

[Viterbo]

I'm running a 5K engine with a 4K engine carburetor from a Starlet. This carburetor doesn't have the vacuum exit on the bottom for the vacuum advance so I'm running the advance from the intake manifold.

I usually use a timing gun to set the timing and usually run it at 8 degrees. 

A strange thing I noticed a long time ago was that if I unplug the vacuum hose from the dizzy the car shuts down.

I'm taking mechanic and electronic classes and this week my teacher showed me how to set my static timing with a probe light.

Basically I connect the probe light to the "-" side of the coil and to the ground of the car. Rotated the engine till the timing mark on the pulley sets at 8 degree. Then rotated the dizzy till the light turns on.

Few things I noticed: the car starts much much better and revs like a maniac, so freely.

But then comes the strange thing. At idle if I point the timing gun it gives me above the scale of the timing cover. It's around 25 degrees at idle!!

If I take out and block the vacuum hose it reads 8 degrees as I have set!

The hose is also pulling air very hard.

What's wrong here? I'm affraid to suffer from detonation from such and advanced timing, but the engine now runs very well

[ke70dave]

1 hour ago, Viterbo said:

 This carburetor doesn't have the vacuum exit on the bottom for the vacuum advance so

Why not? 4k carby should have as the 4k had vac advance. 

[Viterbo]

I don't know, but there is no vacuum port there.
I think it came from a flat front KP61

[altezzaclub]

What you is quite correct.

Cars used to run no vacuum advance at all, it was added as an anti-pollution device in the 70s.  The first ones came off the inlet manifold like yours, and the timing was set as you did. Take the vac line off and block it. (or the massive air leak into the inlet will lean out the mixture and the motor dies) Then set to 8deg advance, put the vac line back on and adjust the idle speed on the carb to suit.

So you have it idling at 25deg- this is needed as a lean mixture is difficult to burn so you start the fire earlier. THAT is why there is a vac advance- you boot the car to accelerate and the vacuum collapses from the open throttle and the ignition retards. Then when you have speed up you back off the throttle, lean the motor out and the vacuum advances to burn the mixture thoroughly.

If it is too advanced and you get pinking as you boot it, it won't be the vac advance, it has no affect when the throttle is open. The weights do the rest of the advance curve, and the usual way to find optimum advance was to floor the accelerator and listen for pinking.  If there was none you advanced it a few degrees and did it again. You timed the acceleration with a stopwatch and picked the fastest time, as over-advancing will slow it down overall.

Finally, 4Ks will take massive advance, I had one of mine idling at 18deg without vac and it drove fine.

 

 

[Banjo]

We all talk about static timing as being 8 or 10 or 12 deg BTC, as being important, but really it is only an initial "static" setting, that allows us to get the car started.  The most important ignition setting is the dynamic advance setting, which is the advance whilst the car is running, under various load conditions. However, few of us ever get to check the dynamic setting.   We just assume it gets there.  The maximum dynamic setting, is the sum/total of the static setting (rotating the distributor at rest until the points just open) plus the maximum centrifugal advance, which is the function of the little "bob weights" under the dizzy plate, that don't get looked at often.  If you do ever pull your dizzy to bits one day, you will notice that the little bob weights, or the adjacent plate, have a little number stamped on them like 14 or 18 or 21 or something similar.  This number is the maximum number of degrees of advance, those particular centrifrugal bob weights & springs can provide,  Some people play with these weights, end stops & springs, to increase the maximum advance possible.  You will also notice that the two little springs are different, one being "lighter" than the other.  This is so one bob weigh moves out first, The other heavier one then adds to it, as revs increase, to provide a 2 stage advance curve.

So lets say your static advance ignition setting is 12 deg., & lets say the bob weight/s have 14 stamped on it.  The total advance possible is therefore 26 deg.  The maximum advance in our little K series engines is all achieved by the time the revs have reached say 2500 - 3000 rpm.  So the most important thing to determine, to my mind, is to ensure that when your engine is running at 3000+ rpm, that the advance reaches this possible maximum.

So what about this vacuum advance you/we are discussing here. As vacuum decreases, with the engine under load, with W.O.T. (wide open throttle) the vacuum actuator on your dizzy, actually retards the advance.  This is because under load, the engine "sucks" harder, and with greater density, it ignites easier,earlier, (not as lean), so not as much advance is needed.  That's it in a nutshell, but there are plenty of places on the net where you can read about that in more detail. 

So this is why when you set your static advance setting, you always take off the vacuum line & block it, so it has no effect, on the setting.

So once you get your motor running, you should set the dynamic advance to the maximum recommended possible.  As we can't change the bob weights action, without modifying them, the maximum achievable is still adjusted by rotating the dizzy, with the clamp just nipped slightly. However this is done at 3000+ rpm, rather than at idling. The reason this method is not used widely, is that you need a timing light to check that you are achieving say 26 deg, as in the example above.  However, a timing light is a very good addition to your toolbox, if you are serious about getting the most out of your K series motor.  The problem with this technique is that to read 26 deg of advance on the timing chain cover is difficult, as the markings usually only go to 20 deg BTDC. Now you can guess where say 26 deg is, or pre-mark it on the case, or get a adhesive degree strip, to stick on the edge of your crankshaft pully, but it must be a strip designed for that precise dia. pulley.  The timing light manufacturers have overcome this issue, by adding a knob you can turn to retard the flash, until it lines up with TDC instead of say 26 deg.  The 26 deg, is then read out on a little LED screen.

From your description, it sounds like the current p/u point on the inlet manifold is causing your problems, or maybe the vacuum actuator on the dizzy is faulty.  Try the sucking on a plastic tube connected to the vacuum actuator, & see if the dizzy contacts plate moves.  You might also have an air leak on the inlet system somewhere that is causing the engine to idle fast. The first bob weight usually starts to move around 1200 - 1500 rpm.  The idle must be down around 800-1000 rpm, to ensure that the bob weights are not advancing the timing at idle.  The possibility is that a bob weight spring has fallen off and there is nothing holding back one or both of the bob weights. 

Let us know what you find.  Sounds like your teacher is doing well, and created a great interest in you to question why things work as they do.

Cheers Banjo

 

 

Edited June 2, 2017 by Banjo

[Banjo]

Quote

I'm running a 5K engine with a 4K engine carburetor from a Starlet. This carburetor doesn't have the vacuum exit on the bottom for the vacuum advance so I'm running the advance from the intake manifold.

The effect on performance of the vacuum advance on your distributor, is quite different, when the vacuum pickup point is on the manifold (manifold advance) as opposed to connected to the carburetor point (call ported vacuum). When connected to the carby, the vacuum point is basically blocked off by the butterfly at idle.

That is probably why your car  "revs so freely".  As Altezzaclub has mentioned above, the K series engines will accept a large amount of advance at standard compression ratios, without detonation & pinging. I've been running "manifold advance" vacuum into a transducer which produces a 0-5Vdc signal "mapped" to a programmable ignition, and I regularly obtain 45 deg of advance, without a sight nor sound of a ping.

Never under estimate the difference even a small change in ignition timing can make to the overall performance of your normally aspirated K engine, as this video clearly demonstrates.

All good fun.

Cheers Banjo

   

Edited June 2, 2017 by Banjo

[Viterbo]

There are a few different hints you gave and I'm kinda lost here.

Altezza you say the way it's working right now it's the way it's supposed to be so everything is alright?

 

Banjo, I cannot praize you enough for taking so much time to explain everything the way you did, thank you.

However I think you didn't fully understood my issue, maybe I expressed myself wrong.

The car idles perfectly at 900 or so RPMs, the issue is, I think (I just think because I don't know if it is wrong or not) I'm running too much advance because I think I'm getting too much vacuum. With vacuum on I'm reading 20+ degrees at idle, with vacuum off and hose blocked I'm perfectly running the static advance of 8 degrees.

 

 

I forgot to say I'm running a bit of compression, between 12 and 13 bar in all cylinders so I'm affraid of getting detonation with so much advance

Edited June 2, 2017 by Viterbo

[altezzaclub]

You shouldn't get detonation, the motor is doing no work at idle.  Then when you open the throttle to get it to do some work the vacuum collapses and the ignition retards. You will hear any detonation or pinking anyway, its not like running a turbo, and it will be at wide-open throttle with the cylinders as full as possible and the ignition running purely on the centrifugal weights. 

More modern carbs run with ported vac, a port that goes into the airflow right at the throttle plate closing spot, so it gets cut off or very reduced at idle.  It still retards as you open the throttle, but doesn't carry so much vac advance at idle.  It is more active at high airflow speeds.  You can run both ports at once on a duel diaphram distributor.

What fuel do you use??  I just United's 95 all the time, modified 4K, 4AGE with cams and the 3SGE in the Altezza all run on it.

You can run it w/o the vac at all, its just an anti-pollution thing.  All our Corollas probably take up 20 modern cars with thier clean exhausts, so an extra bit of nitrogen oxides won't matter!

[Banjo]

Altezzaclub is quite correct in saying that your result is normal, with a static advance setting of 8 deg, but when you plug the vacuum hose back on, it rises to 20 deg dynamic advance.  At 900 rpm idle the bob weights should be tucked up against their stops, held there by the springs, and having absolutely no effect at all.  The 20 deg of "dynamic" advance you are seeing with the timing light is the sum of the static advance plus the 12 deg advance contributed  by the vacuum actuator.  You are never going to detonate at 20 deg advance when there is no load at idle, when the mixture is very lean. Don't forget that a lean mixture requires a lot more advanced spark, as it is harder to ignite, than a rich, dense combustion mixture.  The vacuum advance existing in your inlet manifold has little or no relationship to the revs of the motor. It is purely a simple way of  practically feeding back to the ignition system, what the load is on the engine is.

You would not be surprised to know that when you have got your car into top gear, driving on a flat road at say 60-80 klms per hour, with no passengers, & no load of cement bags in the boot, you are probably only using/need about 30 -40% of the power your engine is capable of.  Your foot is not flat to the floor, and the mixture is reasonably lean, and you need all the advance you can get to make the engine run as efficiently under those conditions.

In years gone by it was popular to fit aftermarket vacuum gauges to your dash.  (I had one)  I think there were a couple of car brands that fitted a gauge in the dash, which they called an economiser.  It didin't have units on it.  It had a green & red zones at each end, and a zone in the middle called the economiser zone, where you tried to keep the needle whilst you drove.  It stopped you from stamping on the accelerator, which usually shot the gauge to the end.

The most important thing, about ignition advance is to ensure that the components in the engine that create advance & retard are working as intended.  Remember our Rollas are around 40 years "olde" & getting tired.  Things do wear out & break, and forgotten about, like those little bob weights that hide in the bowels of the distributor & rarely see the light of day, let alone get oiled. I've pulled some distributors out of Rollas, & after removing the contact plate been faced with weights rusted and pivots worn so badly they didn't have a hope in hell of ever performing as they were intended. When I've cleaned them up, oiled them & put it all back together, the owners have indicated it was like a whole new car.

Vacuum advance units do fail, but are very easy to test.  ( A piece of plastic tube & your mouth )

Hope that helps.

Edited June 3, 2017 by Banjo

[Viterbo]

Now everything is clear.

Thank you to both of you!

 

I'm running 95 octane gas. Tested the car on the road and it's flying!!

Do you think the fact of the it being so retarded could be overheating my oil, about the problem I reported in another topic? My water thermostat is faulty and let the engine cool while running so I'm not getting excessive water temp. But I have no way of knowing that with the oil, and I know retarded advance causes overheating.

[Banjo]

Here is a good read, regarding your concerns about excess retarded ignition timing causing overheating.

https://www.smokstak.com/forum/showthread.php?t=70385

There was someone on here recently that had the timing way out, and had combustion happening in the exhaust manifold, such that the whole manifold was glowing pink.

Frightening sight if you lifted the bonnet at night.

Cheers Banjo

 

 

 

[Viterbo]

Apparentely the oil issue is also solved... 

[rebuilder86]

as a side note, i can personally guarantee this, vacuum advance can never cause detonation in 4k.
Upon acceleration, on this engine, the vacuum drops so far that the advance goes to 0 almost immediately.For this reason, ported vacuum is almost useless on this engine.
Manifold vacuum is great because it allows the idle mixture to be right out at 1.5 turns where it belongs from factory. With mixture at 1.5 turns, and manifold vac running the advance, it should pur, so long as your slow jets are ok.

[Banjo]

Anyone who is interested in learning about ignition advance in our K series engines, and its importance & possible implications, cannot read a better short write up than this one by Julian Edgar on the AutoSpeed website.  It is so crammed full of general & specific information, obvious written out of experience, rather than theory.  I've read the article several times, & gained more insight with each read.

http://www.autospeed.com/cms/article.html?&title=Getting-the-Ignition-Timing-Right&A=109132

I just love the photo in the article showing an engine on a test bed/engine dyno, with the advance obviously very retarded, such that the combustion is taking place in the exhaust manifold/header, with it literally "glowing".

One of the many indicators of a well tuned engine at maximum power output, according to this article, is the lowest exhaust temperature possible.  Has inspired me to put a couple more temperature sensors on my air intake & exhaust system, & just see what is going on in my engine, under different environmental & driving conditions.

Cheers Banjo

 

 

[rebuilder86]

hmm that last point is interesting, thats different from what i learned flying choppers
I learned that in an aeroplane with an EGT gauge, the best performance is achieved while leaning the mixture but stopping halfway to peak EGT.
This is shown graphically on  a typical horizontally opposed air cooled engine here
https://aviation.stackexchange.com/questions/21466/what-appears-on-egt-gauge-if-the-mixture-is-rich-or-lean