Adjusting dizzies to limit advance - possible?

[Zcat7]

I have been struggling with my engine running hot (read about it in this thread http://forum.earlybay.com/viewtopic.php?f=4&t=61130 ) and I have been trying to get to the bottom of it.

I have now addressed the lean fuelling by upping my idles from 55s to 60s (dual Kadrons) and it has made a great difference to round town driving and pulling up to about 45-50. The engine is definitely running cooler at these temps, indicating around 170-180 on a hot sunny day, and I have been able to reset my timing back to stock without the stuttering on in-gear acceleration I was getting before. In doing this however I think I may have noticed the problem - Trikky2 mentions in the topic above that with the vacuum advance disconnected and timing set to around 28 degrees max advance (which it is) I should get a max vacuum-connected advance of around 34 degrees. Well I noticed this morning that I'm getting about 42 degrees (!) which could well explain the hot running.

The dizzy I have has the serial number 0 231 168 001, which is for a 1600 transporter form 1973-75. I have stripped and reassembled it prior to replacing my 009 dizzy but don't recall seeing a max advance "restrictor" mechanism. Anyone know if it is possible to limit the max advance?

 

[Trikky2]

42 seems rather high though the vac advance will only work on light throttle/low engine load so its unlikely it would cause overheating.

The 34 total figure I quoted was a typical average but, for instance, there are dissys that will give 12 degrees of vac advance as per the ones fitted to 72 bus and pick up - number 34 on this list http://www.oldvolkshome.com/ignition.htm#B1971" onclick="window.open(this.href);return false; so you may well have one of these on your engine.

The amount of vac advance is controlled by the vac can fitted and cannot be reliably adjusted any other way.

As you can see from the list, some dissys were designed to work with certain models of carburettor whilst others can be swapped.

This is the summary listing http://www.oldvolkshome.com/ovhignbo.htm" onclick="window.open(this.href);return false;

To look at the details of a dissy click on the vehicle in the "application" list.

 

[Zcat7]

I have to say I don't entirely understand why but it is what I'm seeing:

Vac off at 900 rpm = 7.5 deg adv
Vac off at 3500 rpm = 30 deg adv
Vac connected at 3500 = 42 deg adv

Are you saying that under load the vac connected advance figure would go down? I always thought that that the whole point of the vac advance was to give you more advance under load. Hence I reasoned that my engine runs well within cooling tolerance (220F or below) while I'm at lower throttle positions but once I get up above 3000 revs on a long run, I have too much advance and it zips right up to 240+F (indicated of course).

I've just put my old 009 dizzy back on and took it for a hard run. Unfortunately it started raining so perfect for good cooling! However at a sustained 3800 rpm motorway run it never got above 220F. It wan't as long a run as I did yesterday so I know its far from the perfect test but I was expecting to see higher figures I have to say. Need to get out again on a hot day...

Incidentally my distributor doesn't appear on those lists; I think they might be a bit US centric. On another list I found it says it is from a 1600 Transporter from 1973-1975 but didn't give advance figures.

 

[Tofufi]

Are you saying that under load the vac connected advance figure would go down? I always thought that that the whole point of the vac advance was to give you more advance under load. Hence I reasoned that my engine runs well within cooling tolerance (220F or below) while I'm at lower throttle positions but once I get up above 3000 revs on a long run, I have too much advance and it zips right up to 240+F (indicated of course).

The vacuum feed should come from just above the throttle plate, meaning you get no vacuum at idle, a lot of vacuum when the throttle is only open a small amount, and less vacuum at wide open throttle.

Where are you taking your vacuum connection from? I'm guessing the Kadrons have been modified to allow a vac take-off?

 

[Zcat7]

I modified them as per Kaddieshack and Lowbugget's instructions - just to the side of the left carb like this:

I then blocked off the airway on the downside of the throttle plate to avoid full vac at idle.

 

[Tofufi]

Is that the same airway as mentioned here?

http://www.volkszone.com/VZi/showthread.php?t=426794" onclick="window.open(this.href);return false;

 

[Zcat7]

Yep, that's the one! Same process as he describes.

 

[jonboylaw]

The vac advance on the VW is a ported take off, the vac signal iss something like zero at tickover, jumping to high vac just on throttle opening, then falling back towards zero at WOT.
Your advance at 3500 with vac should be measured at WOT which should be around 30 degrees. The burn rate of the charge at WOT is a lot faster as it is running with lots if fuel in it, it needs less advance.
On over run the engine is pulling in air but throttle and main circuits are closed so the charge is lean and burns slower and needs more advance, vac signal will be high as the throttle is nearly closed, and the engine is sucking hard.

If you are getting flat spots then check the spring plate is moving freely and that the springs are not corroded.


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[Trikky2]

I have to say I don't entirely understand why but it is what I'm seeing:

Vac off at 900 rpm = 7.5 deg adv
Vac off at 3500 rpm = 30 deg adv
Vac connected at 3500 = 42 deg adv

Are you saying that under load the vac connected advance figure would go down? I always thought that that the whole point of the vac advance was to give you more advance under load. Hence I reasoned that my engine runs well within cooling tolerance (220F or below) while I'm at lower throttle positions but once I get up above 3000 revs on a long run, I have too much advance and it zips right up to 240+F (indicated of course).

I've just put my old 009 dizzy back on and took it for a hard run. Unfortunately it started raining so perfect for good cooling! However at a sustained 3800 rpm motorway run it never got above 220F. It wan't as long a run as I did yesterday so I know its far from the perfect test but I was expecting to see higher figures I have to say. Need to get out again on a hot day...

Incidentally my distributor doesn't appear on those lists; I think they might be a bit US centric. On another list I found it says it is from a 1600 Transporter from 1973-1975 but didn't give advance figures.

The other guys correctly describe the way a vac advance works. ie. none at tickover and none at heavy throttle opening.

The initial kick as you open the throttle improves driveability when pulling away. It's basically an economy device. It gives more advance at light throttle giving you more power at that throttle opening. If you were to apply that much advance at full throttle you would suffer pre ignition or detonation, knock/pinking as many describe it.

There's vast amounts of info on distributors on the net - for instance - http://www.vw-resource.com/distributor_questions.html" onclick="window.open(this.href);return false; - so there's little point in my repeating it all on here :lol:

 

[Zcat7]

Interesting stuff...

Jonboylaw - I'm not getting flat spots now that I've upped the idle jet, not even with the 009 that I fitted today, least not to any meaningful extent (just the regular 009 experience). Its all about the overheating.

My theory at this time is based around getting too much advance with the SVDA above 3000 rpm (as per the readings I'm getting) which is causing detonation/pinking as Trikky describes. Things start burning hot and up goes the temperature. Right now I can't figure out why I get that much advance at 3500 but I suspect that this is the underlying fault.

From what everyone describes it could be a carb fault. Lets face it, I have done this vac takeoff mod myself, based upon well advertised instructions, but I'm not a carb specialist and maybe I screwed it up somehow!?

I'll see how the next long run on a hot day goes with the 009 fitted, I reckon that will confirm things. In the meantime I have some more reading to do!

Thanks all

 

[Trikky2]

Well your idea of using the 009 will certainly tell you if there is an advance problem.

Did you check the accuracy of your temp gauge?

 

[Zcat7]

Didn't check the gauge yet, need to find a gap in the weather to drainil the oil and set up a kitchen outside the garage!
I've also figured that either way I am seeing such a drastic difference in temp between running at 3000 rpm and 3500+ that it it warrants inspection, but I will do it, maybe even today if I can.

 

[Zcat7]

Still didn't get a chance to test the gauge yet but did the same long test run as before at 3800rpm in pretty warm temps and it topped out at an indicated 235F (112.7C). That's around 7-8 degrees cooler than before and it took ages to get there whereas when it was reaching just over 240 (about a needle width past) with the SVDA fitted, it happened quickly and I spent the rest of the journey managing the temps back down (driving slowly!).

So despite it not being an exact science (slightly different ambient temps on the day) it does appear that the amount of advance is a factor in the heat buildup.

So is 235F too high or to be expected under the conditions; am I getting close to solving this mystery?

 

[Zcat7]

Been camping over the last few days. 4 people on board and loaded to the gunnels. The engine didn't miss a beat. Cruised along in the rain at 3500-3800 rpm and peak temp was about 225F. Heading home today in the hottest part of the day (warm but not scorching) and the needle only reached 235F again going up a long 5-6 mile incline. As soon as it leveled off the temps dropped straight back down to about 230F.

I have to assume that the extra advance of the SVDA was causing me overheating issues as this 009 dizzy is not causing me any at the moment... I so want to like the SVDA as I agree with its theoretical benefits but what can I do? Maybe I need to find an SVDA that delivers an overall lower advance but gives the extra benefit lowdown in the rev range. I think I will look up Trikky's list and see what I can hope to find on there.

 

[Trikky2]

Well the vac advance is only supposed to work on light throttle, Not when the engine is working hard. Hence it would not contribute to hot running at speed or up hills. Your symptoms would suggest your getting vac advance all the time.

Its possible the vac can is faulty (if say its internal return spring is weak or broken) but its unlikely since then the dissy base plate would not return correctly. More likely its the vac coming from the carb that's at fault.

It's very strange though since if you were getting 40 degrees of advance under load it would normally be audible as pinking/pre-detonation.

 

[Zcat7]

It's very strange though since if you were getting 40 degrees of advance under load it would normally be audible as pinking/pre-detonation

It definitely seems that way as that's how I'm reading the timing - I hold the revs at around 3500 rpm and I ready a steady 42 degrees advance. I must admit I thought this was "correct" but if it should fall off again at what is effectively part throttle it must be wrong.

I don't know why I didn't hear any pinking, maybe its because the engine is at the back and quiet pack exhaust I have isn't particularly quiet!

I checked the vacuum takeoff mod and so far as I can see it is as it should be. Ho hum, I'll keep looking but at least I have an interim solution for now.

 

[Trikky2]

It's very strange though since if you were getting 40 degrees of advance under load it would normally be audible as pinking/pre-detonation

It definitely seems that way as that's how I'm reading the timing - I hold the revs at around 3500 rpm and I ready a steady 42 degrees advance. I must admit I thought this was "correct" but if it should fall off again at what is effectively part throttle it must be wrong.

I don't know why I didn't hear any pinking, maybe its because the engine is at the back and quiet pack exhaust I have isn't particularly quiet!

I checked the vacuum takeoff mod and so far as I can see it is as it should be. Ho hum, I'll keep looking but at least I have an interim solution for now.

Revving the engine will always give you vac advance because there's no load on the engine.

Unfortunately you can't check the timing signal when the engine is under load at home. Needs a rolling road to do that.

 

[jonboylaw]

Try the advance springs from the 009 in the SVDA if they fit. You may be getting too much mechanical advance due to weak springs.


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