Lotus Elan

vacuum for distributor...where?

PostPost by: rdssdi » Tue Aug 07, 2012 9:36 pm

Where is there a suitable vacuum port on a TC engine for the vacuum advance on my new distributor?

I attached a hose to the small tube that comes off of the Zenith Stromberg 175 but I was told that is not a vacuum source and the vacuum ports at the front and rear of the manifold are for the headlamp pod and the brake boosters. I was also told those ports are not suitable for a distributor vacuum source as the vacuum wanes off idle.

Now I am really confused.

Bob
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PostPost by: cal44 » Tue Aug 07, 2012 11:27 pm

Bob,

(My car)the vacuum on the Weber conversion Stromberg head comes off a nipple screwed into one of the intake manifold tubes. The hose goes towards the front and the distributor hose is "teed" off, as the hose continues towards the vacuum for the headlights.

Can't say that it is correct. I look forward to other opinion.

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PostPost by: Henry VIIII » Wed Aug 08, 2012 2:02 am

When I first had my Elan in 1969, it had a distie with a vacuum gizmo on it, but the vacuum inlet was not connected to anything.
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PostPost by: elanern » Wed Aug 08, 2012 2:28 am

Bob, my '70 Federal S4 with ZS carbs has a fitting on the rear of the intake runner for #4 cylinder with a three-way distribution fitting - two for the bypass valves on the carbs and one for the distributor. The headlamp vacuum fitting is on the front of the #1 cylinder intake runner.

I've attached a page from the Elan workshop manual that shows the parts associated with the ZS emissions setup. The fitting I am talking about is to the far left of the picture. If you look closely you can see where the fitting threads into the #4 intake runner. The idle retard valve thingy is part of the vacuum line that goes to the distributor.

Hope this helps.

Cheers,
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PostPost by: rdssdi » Wed Aug 08, 2012 2:30 pm

I saw the same photo in my workshop manual. My car no longer has the emissions hardware. The rear #4 vacuum fitting on my car is a single nipple and feeds vacuum to the brake boosters. I attached the vacuum hose to the small nipple shown on the drawing coming from the carb. I now see that is not vacuum at all. I would prefer teeing off the vacuum port for the headlamps rather than tap into the booster vacuum.

The next question is where do I get a tee that is large on two legs and quite small (less than 5/32) on the third leg.

Bob
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PostPost by: rdssdi » Wed Aug 08, 2012 2:47 pm

I have attached a photo of my engine bay when it was first assembled. I no longer run those K&N filters as we found the forward filter rubbed the bonnet. I now have the original fiberglass intake plenum with trunking to a K&N cone filter in the nose.

You can plainly see the small black caps that cover the tubes protruding from the ZS 175 carbs. I attached the vacuum hose to the front tube. Apparently incorrect. The front #1 vacuum port has a 90" elbow and black nylon tubing that runs to the pod vacuum reservoir.

The manifold vacuum port at #4 is plainly seen with a rubber hose running to a metal tube that runs along the firewall and then to another rubber hose and onto the brake boosters. The metal tube was original to the car so I assume that was how the booster vacuum was plumbed. I remember the hex fitting at the hose / tube junction also functioned as a one way valve.The question is where did the " 4 way" fitting shown in the workshop manual attach?

A further question arrises as to why the vacuum for the distributor comes from the idle retard mechanism. I will source a tee that will allow me to feed the vacuum pod reservoir and the distributor capsule.

Bob
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Engine compartment as initially assembled
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PostPost by: rdssdi » Wed Aug 08, 2012 3:23 pm

Just completed teeing into the vacuum at the pod reservoir (#1) port. The vacuum advance now holds the timing at 30 deg. BTDC at idle and off the scale when revved.

I believe that is a bad idea to hook up there as the distributor will always "see" vacuum due to it being in line with the reservoir for the headlamp pods. It will never be at static timing.

I guess my only option would be to use the booster side BUT as the manifold is also part of the vacuum system will it also be under vacuum because of the reservoir. Is a one way check valve the answer?

Bob
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PostPost by: CBUEB1771 » Wed Aug 08, 2012 3:33 pm

rdssdi wrote:The next question is where do I get a tee that is large on two legs and quite small (less than 5/32) on the third leg.


I would start with a tee with three 1/8 female taper pipe threads and add the correct hose barbs two it. You can also get tees with one male thread port and two female ports, I think these are call "run" tees. That way you don't need a 1/8 nipple for the end that screws into the cylinder head. The normal location for the distributor vacuum take-off this the inlet runner to cylinder #1. You do want to sense the depression between the throttle an the inlet valve as this is the proxy for engine load.
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PostPost by: rdssdi » Wed Aug 08, 2012 3:55 pm

I have determined that the #4 port would be the only correct vacuum port to use as using the #1 manifold port would tee the distributor vacuum supply into the the reservoir thus providing vacuum advance at start up. As the one way valve which is integral (now I find out) with the # 1 vacuum nipple. If that valve is functioning then the manifold is not under vacuum hence the #4 port would be the way to go.

As the advance with the vacuum attached to the distributor was over advanced, I have decided to disconnect the vacuum pod on the distributor. With 10 degrees idle static timing (set with vacuum off distributor) climbs to 30 degrees with the vacuum added and at 3-4,0000 rpm's the advance is off scale and well over 45 degrees.

My only question, which I will pose to Pertronix, is, does the mechanical distributor and the vacuum distributor share the same centrifugal advance curve?

My readings for the timing with vacuum off was 10 degrees at idle peaking at around 32 degrees at 3500 rpm. Which as I understand is correct. Why the vacuum adds so much total advance is beyond me.

Bob
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PostPost by: types26/36 » Wed Aug 08, 2012 4:15 pm

rdssdi wrote:Where is there a suitable vacuum port on a TC engine for the vacuum advance on my new distributor?
Bob


I am a little confused here, having read the posts I gather you are trying to connect up a vacuum advance to the distributor. My question is why? T.C.?s were never fitted with a vacuum advance and the workshop manual expressly says do not connect a vacuum advance.
The Federal cars were fitted with a vacuum RETARD device for air pollution reasons but they usually caused flat spots, poor fuel consumption and lack of performance.
Is this some sort of modification and what do you hope to gain by adding a vacuum advance or are you adding a retard device which in my opinion would be step backwards.
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PostPost by: CBUEB1771 » Wed Aug 08, 2012 5:40 pm

There is a bit of misunderstanding about vacuum advance here. This helps elucidate:

http://www.corvetteactioncenter.com/for ... vance.html
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PostPost by: Henry VIIII » Wed Aug 08, 2012 6:12 pm

rdssdi wrote:Just completed teeing into the vacuum at the pod reservoir (#1) port. The vacuum advance now holds the timing at 30 deg. BTDC at idle and off the scale when revved.

I believe that is a bad idea to hook up there as the distributor will always "see" vacuum due to it being in line with the reservoir for the headlamp pods. It will never be at static timing.

I guess my only option would be to use the booster side BUT as the manifold is also part of the vacuum system will it also be under vacuum because of the reservoir. Is a one way check valve the answer?

Bob
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I think there is supposed to be a check valve between the engine & the headlights vacuum reservoir in any case.
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PostPost by: CBUEB1771 » Wed Aug 08, 2012 7:33 pm

Henry VIIII wrote:I think there is supposed to be a check valve between the engine & the headlights vacuum reservoir in any case.


Check valves are required in both the vacuum lines to the head lamp actuation system and the brake servos. Both are shown in the service parts list. Without the check valves you have the chance of fuel vapor getting into both of these systems not to mention having the headlamps go down at large throttle openings. The vacuum take off for the distributor must located between the inlet manifold and the check valve.
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PostPost by: rdssdi » Wed Aug 08, 2012 9:27 pm

To clarify. I wanted to replace my original distributor in an attempt to improve engine run quality. The original distributor had been rebuilt with the addition of a Pertronix ignitor.

I just spoke with Pertronix tech and they said the vacuum advance distributor is a universal unit made for many applications not only the TC engine. This is certainly counter to their listing the distributor being for the Lotus TC engine. Anyway, I was told the vacuum "street" unit is the same as the mechanical "track" unit if the vacuum advance is not connected. So it goes. He had no answer as to why the advance was excessive with the vacuum attached.

I will run this new distributor without a vacuum advance and see how it goes.

I also understand that the removal of the emissions system and blocking off the ports, and removing the secondary butterflies is not equivalent to the UK twin ZS 175 non-emmision system Beside the lack of the secondary butterflies and cross-over pre heat tubes, the manifold does not have the balance tube of the UK units. I do not know if the carbs are any different with the exception of those small tubes that the emission system used but the manifold sure is.

Has anyone replaced the emissions ZS manifold with the balance tube UK version? Has run quality improved?

That may very well be my next option.

Running a new plug and play Pertronix with vacuum not attached. Some improvement. 10 degree BTDC static (idle) timing.

Bob
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PostPost by: rgh0 » Fri Aug 10, 2012 12:22 pm

I am stetching my memory a lot of years but I recall from earlier carbs with twin Strombergs and vacuum advance ( first generation Saab 99 and Hillman Imp GT- as it was called in Australia) that the vacuum advance required a specific port on the carbs that only saw manifold vacuum once the throttles had opened a specific small amount as you did not want the vacuum advance at idle but only at cruising throttle openings to improve fuel economy which is what it is all about.

Vacuum retard as used by Lotus was all about emissions reduction and generally ruined engine performance

Do the ports on your Strombergs see manifold vacuum after the throttles are opened a specific amount?

cheers
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