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View Full Version : Switched from vac retard to vac advance (1500)



Guest
09-29-2010, 10:15 PM
Was getting b/w 23-25 MPG w/ vac retard disconnected. Put in a vac advance unit, better part throttle response, less pedal needed to cruise, does 70 MPH with ease, exhaust allot quiter at cruise (4K)- can barley hear it. Last tank got 23.26 MPG, Travelled 31.7 miles @4K with some stop and go and got 26.84. I haven't gotten that high in a while but it's cooled off and I have run the car a bit harder in the past too. Hard to tell this soon how big a difference it will make, but you can definatly feel a difference. I usually get poorer milage when I travel mostly highway and don't get above 25 MPG.

Refilled and leaned a flat. Will post more results once I run a full tank through it. The good thing is now I don't have to push the car as hard to accelerate at the same rate. Seems to handle heatr soak back vapor lock better. Hard to tell if more advance makes it easier to light the weaker hot mixture or it's better cause of temp drop.

Car is allot more civilized and highway friendly. A big plus, I recommend it.

dklawson
09-30-2010, 07:18 AM
With all those carbs you have... I've forgotten which are on the car at the moment. Are these the dual SUs or single SU?

bigjones
09-30-2010, 07:59 AM
Kellysguy,

Thanks for posting - very interesting!!

Is this on the stock distributor (1500)? Where did you get the advance unit from? Is it a straight swap?

I'm getting 25 mpg from the single HS4, which equates to 30 mpg in UK terms. Only yesterday I was looking at the vacuum retard unit (plugged, of course) and thinking why isn't that a vacuum advance.

Cheers!

Guest
09-30-2010, 08:45 AM
With all those carbs you have... I've forgotten which are on the car at the moment. Are these the dual SUs or single SU?

Duals.

Yeah, straight swap onto the stock dist. Any Lucas 45D unit will work. Mine is from a B ( I think). There was a N.O.S one on ebay for $40. Takes about 5 minutes to swap. Retime once you do.

bigjones
10-06-2010, 05:36 PM
Slight problem folks.

I pulled a vacuum advance unit from an inexpensive distributor from the UK, supposedly for the 1500, and fitted it to the stock distributor. Unfortunately, when I hooked it up to "ported" vacuum, the timing shot up from 12 to 24 degrees BTDC! This was at an idle of 800 rpm. Am I missing something or is the vacuum unit, plain and simple, unsuitable for this application?

("ported" I take to mean coming from a hole in the carb that is basically closed (by the throttle disc) at idle)

Cheers!

TOC
10-06-2010, 05:41 PM
If it shot your timing up at idle, it ain't ported.....

Guest
10-06-2010, 06:29 PM
If it shot your timing up at idle, it ain't ported.....

(Dad beat me to it.)

bigjones
10-06-2010, 07:13 PM
Thanks for the replies.

Well, just looking at it, you would think it is ported:
https://i12.photobucket.com/albums/a241/AdrianJones34/advancepickup.jpg

Plus, it looks like a pick-up for a vacuum line rather than, say, PCV, which I think is lower down, towards the bottom of the pic, and, for some reason, blanked off by the factory.

Let me take the carb apart and check that the throttle disc actually covers the hole at idle.

Cheers!

Trevor Jessie
10-06-2010, 08:31 PM
Did you leave the spring out of the vac advance assembly?

TOC
10-06-2010, 08:36 PM
Yes, I think it's ported.
You'll need to remove the carb to see where it goes......another thing, put a vacuum gauge on that port and tell me what you have.

Guest
10-06-2010, 09:28 PM
Yes, I think it's ported.
You'll need to remove the carb to see where it goes......another thing, put a vacuum gauge on that port and tell me what you have.

I'd say it's ported too. MIne looks just like that.

Trev, no spring in these. ( at least not one you can get to.)

Did you leave the dist loose and it got moved when you pulled on the line?

bigjones
10-06-2010, 09:31 PM
Trevor,
I think you may have something! I don't remember no spring! Where abouts does it go?

Thanks, TOC - I'll get a reading first thing tomorrow.

Cheers!

Oh, OK, I see what you are talking about Trevor. Actually, in both distributors it is a solid bar, not a spring. But, yes, it is connected. I double-checked by using my MityVac on it before installation, and I remember it does come back when the vacuum is released.

bigjones
10-06-2010, 09:45 PM
Did you leave the dist loose and it got moved when you pulled on the line?

No, I tightened the clamp up after setting the timing to 12 BTDC. I'll take another look tomorrow but...

Trevor Jessie
10-07-2010, 08:25 AM
Wait, what type distributor are we talking about? Is it a 25D with an adjustment wheel?

bigjones
10-07-2010, 09:12 AM
Trevor,
No, this a 1500 engine which came with 45D distributor fitted (in the US) with electronic ignition. No micrometer adjustment (unfortunately).

This is how it looks originally. The block between the dizzy and the vacuum unit is the ill-fated electronic unit. Now fitted with a Pertronix:

https://i12.photobucket.com/albums/a241/AdrianJones34/Opus.jpg

Cheers!

Guest
10-07-2010, 09:41 AM
O.K, wait a minute, you have the Opus (45D4E?) which is completely different than mine (45D4M). Not all 45D's are created equal. I haven't taken one of those down yet, so I can't tell you how much is involved with the swap. Mine is just two screws on the side. IIRC, you have a '79 which should have the 45D4M unless someone swapped it.

bigjones
10-07-2010, 10:18 AM
Kellysguy,

Yeah, I have the Opus 45DE - it is a 1975 car.
After removing the (defunct) Opus I had to drill and tap 2 holes to mount the new vacuum advance - that's about all that is involved.

OK, at 900 rpm, the "ported" vacuum is 12 inches Hg, 600 rpm is between 9 and 10 and with the idle screw totally backed out it gives 500 rpm and 8 to 9 inches (and the timing drops from about 12 to 10 deg BTDC).

I think the next step is to dismantle the carb (single HS4, brand new) and take a look at the exact location of this "ported" outlet, unless someone has any ideas.

Cheers!

Morris
10-07-2010, 10:59 AM
I would hook the vacuum gauge to the carb then slowly ease the throttle. If the vacuum gauge makes a big jump at some point, you have ported. If it increases gradually as you open the throttle, you do not have ported.

Also, if you have a vacuum pump, hook it to the advance and ensure that the things are moving in the right direction.

If find that you do not have ported vacuum and your car will idle okay at 24 advance. Then leave it and enjoy. Low idle advances are product of pollution control measures. If you are not running said pollution gear, there is no need to worry about ported vacuum.

The following is an excerpt from an article by John Hinckley, a corvette guru.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-
40 years of controlling vacuum advance with full manifold vacuum, along came
emissions requirements, years before catalytic converter technology had been
developed, and all manner of crude band-aid systems were developed to try and
reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these
band-aids was "ported spark", which moved the vacuum pickup orifice in the
carburetor venturi from below the throttle plate (where it was exposed to full
manifold vacuum at idle) to above the throttle plate, where it saw no manifold
vacuum at all at idle. This meant the vacuum advance was inoperative at idle
(retarding spark timing from its optimum value), and these applications also had
VERY low initial static timing (usually 4 degrees or less, and some actually were set
at 2 degrees AFTER TDC). This was done in order to increase exhaust gas
temperature (due to "lighting the fire late") to improve the effectiveness of the
"afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the
A.I.R. system; as a result, these engines ran like crap, and an enormous amount of
wasted heat energy was transferred through the exhaust port walls into the coolant,
causing them to run hot at idle - cylinder pressure fell off, engine temperatures went
up, combustion efficiency went down the drain, and fuel economy went down with
it.

bigjones
10-07-2010, 12:25 PM
Morris,

Thanks for the input.

The vacuum increases steadily (no jumps at all) from 12 to 18 max, then trails off.

With the vacuum line hooked up to carb and dizzie, the timing goes from about 26 BTDC to way over the scale when throttle slowly increased from idle.

The way it is, I don't see any advantage to this vacuum line - I mean I could set the base timing to 26 BTDC.

No pollution equipment left on the car (except for hose from valve cover to air fiter housing)

I'm mucho confused.

PS. The Hinckley article doesn't make sense - Kellysguy has just reported a big advantage to having an operating vacuum advance system, activated by ported vacuum.

Morris
10-07-2010, 04:58 PM
There IS a big advantage to vacuum advance. There is just not necessarily an advantage to "ported" vacuum.

If you set your base timing to 24 BTDC, you would end up with something like 42-46 BTDC when the weighted advance is all in.

All I am saying and all that the article is saying is that if your car will run okay with vacuum advance at idle, it's fine. You do not need to worry about ported vacuum. All ported vacuum does is turn the vacuum advance off at idle.

bigjones
10-07-2010, 06:39 PM
Morris,

Sorry for being such a numbskull but if I set the timing to 10 or 12 BTDC(vacuum disconnected and plugged at the carb) and then re-connect the vacuum, then I should be good to go? And have the benefits that Kellysguy obtained?

Cheers!

Guest
10-08-2010, 01:07 AM
You should, if your engine will idle with that much advance. That's the same port I have on my HS4 that mine is hooked on and it's ported vacuum. I also have dual carbs, so less vac/flow in one of mine than in yours. Cause yours is feeding all cylinders, and you might have to set idle open enough that you are already comming into the port area at idle.)

What he's saying is mainfold vacuum is o.k if your engine will idle with that much advance. Mine wont. It'll putt puut puofff put the exhaust with that much initial advance.

The advantage is, vacuum advance senses engine load and retard timing when you need it retarded. If you set the base at 26*, it would knock/ping itself to pieces once you put a heavy load/throttle on it.

Morris
10-08-2010, 10:10 AM
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Sorry for being such a numbskull[/QUOTE]

You are not a numbskull... if something is difficult to understand, it's always the explainer's fault :wink:

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]if I set the timing to 10 or 12 BTDC(vacuum disconnected and plugged at the carb) and then re-connect the vacuum, then I should be good to go? And have the benefits that Kellysguy obtained?[/QUOTE]

I would plug your vacuum advance, then set your timing to 28-30 BTDC at about 3500 rpm. The place that timing advance is most critical is when the car is under load. The 1500 makes max torque at about 3500rpm, so this where you want your timing advance to set accurately. If your timing advance is off here, it can cause damage to your engine or seriously affect your performance. You have a lot more leeway at cruise and idle (these are the areas where your vacuum advance come into play). Get it close for cruise and idle, and you will be fine.

bigjones
10-08-2010, 07:22 PM
Kellysguy,

I took the carb off the car and had a good look at the advance pickup hole. Even with the throttle disc fully closed there was still about 25% of the hole exposed to the manifold. (It is a small hole by the way). As you say, there are 4 cylinders sucking on it so I think that is why there is vacuum at idle. Anyways, it is immaterial now.

Morris,

You are a true gent. Thanks for the explanations. I need to get a new timing light anyway - I'll get the model that will allow me to dial in the timing and then look at the O deg mark. When it arrives I'll check the timing at 3500 as you suggest. I'll put the old timing light in the trunk - it is a good trouble-shooting tool!

Cheers!

Guest
10-08-2010, 09:10 PM
As long as you don't have solid core plug wires (like me) you'll love your new light. Mine can't get a clean signal and is picking up EMP's from all five wires. My light will flash from about 3 feet away from the car.

drooartz
10-08-2010, 09:44 PM
As long as you don't have solid core plug wires (like me) you'll love your new light.
I have trouble with this as well. Funny thing is that I have an old Heathkit timing light that my father-in-law made years ago. It seems to work fine. My nice new one is all over the place. Solid core plug wires on my car.