Vacuum/Timing questions

[ac78]

Hey all-

Got a 78 FI Hydr Westy. Something has been off the last couple weeks. Stumbling a bit, back fire on decel etc.

Recently replaced plugs, wires, cap, rotor, points.

Vacuum is sitting pretty steady at 13 at the plenum. I have followed the hose diagram and reset connections. I am getting a reading of 0(zero) at dizzy/throttle body no matter what happens to the rpms. What should this reading be? I have been searching but I must not be phrasing it right.

Another curious piece is that the timing light on #1 shows 0-TDC at idle, not the 7.5 btdc that its supposed to with hose off. If I cant figure out why there's no vacuum at the distributor, I don't want to start messing with advance.

Should I be looking at valve adjustment? I know they are hydr, but I don't know what they are set to. Would that throw my vacuum off that much?

Thanks in advance.
_________________
78 Westy 2.0l hydr. FI

VW Campmobile
For people who believe a family's
car should be it's castle !

[busdaddy]

If you don't know what the valves are set to you should do it just to have a base line for next year when your yearly reset to monitor valve and cam health, log how many turns each one takes to reach zero lash and watch for changing numbers.

But I doubt that has anything to do with your current issue, start by removing the distributor hose from the plenum and sucking on it, will it hold a vacuum?, how about if you disconnect the EEC valve on the air filter and block that leg of the Tee, hold vacuum now?, does the points plate in the distributor move when you suck hard?

Let us know what you discover.
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[ac78]

[busdaddy]

Hold off on the valve messing for now, lets concentrate on isolating the cause of the issue without masking things with too many fixes at the same time.

So you've already discovered the failed EEC, now see if the distributor will hold a vacuum with that golf tee replacing the EEC, if not try a short piece of new hose directly on the distributor to rule out porous lines leading to it.
_________________
Rust NEVER sleeps and stock never goes out of style.

Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!

Слава Україні!

[ac78]

[Randy in Maine]

This seal?

http://www.airheadparts.com/vintage-vw-parts/vanag...22133073-a

I think I put a dusting of hylomar on the seal for no real reason.

Make sure you tighten the 2 screws that hold it in there eash a little at a time.

[ac78]

[airschooled]

[ac78]

[Randy in Maine]

Why are you at 5º ATDC?

Remove the vacuum hose(s) and rev the engine until it will not longer advance centrifically no matter how much more gas you give it and make that point to be 28º BTDC.

Do it a couple of times so that you are confident all of the centrifical advance has been accounted for.

Let it loaf back to idle and plug in the vacuum hose. Adjust idle speed to the correct spec. Note the initial timing but don't change it. Go for a ride and report back your findings.

[ac78]

[ac78]

[Randy in Maine]

I assume you own a real timing light. You have to take the vacuum advance out of the equation by removing the hose. rev the engine up until the timing light no longer advance with additional gas you give the engine. Make that point to be the 28º BTDC.

Plug in the vacuum hose and set the idle speed to the correct 850 RPMs. Note the initial timing. It should be close to 7.5º BTDC, but don't change it. Just note it.

Humor me and confirm the timing mark is this one as described by Ratwell....

http://www.ratwell.com/technical/FindTimingMark.html

The only mark you would ever make on the fan is for something like this "ll" 180º from the TDC mark for Bottom Dead Center (useful in adjusting valves).

This one below is BDC at 10º ATDC.

[ac78]

[SGKent]

I am having trouble following the question but would like to throw 2 cents in here just in case it hasn't been covered.

There are usually two sources of vacuum for devices on these engines.

1) Plenum. This is the vacuum that is on the intake valve side of the throttle body. It is the vacuum that the valves see.

2) S-boot side of throttle body. This source varies wildly. When the throttle plate is closed on decel or at idle it would be almost no vacuum. As the throttle plate is opened the air begins to rush into the plenum. The more the plate is opened the closer the plenum and S-boot side get to being the same. At low RPM and high loads the vacuum would be close to almost nothing. At high RPM's and light load the S-boot side would be very high - maybe 16 - 20 inches.

The devices that need to follow the vacuum the engine sees are hooked to the plenum side. These are the deceleration valve and the Fuel Pressure regulator. The devices that need to follow engine load hook up to the S-boot side. These are the EEC as it only opens when the engine is under load - essentially when the fuel vapor can be burned without greatly affecting the mixture, and the distributor, which changes its advance to match the engine load. The brake booster hooks directly to the plenum for safety reasons.

Many things can affect the vacuum at each of these sources. On the S-boot side for example, a broken S-boot would greatly affect vacuum there at idle and mixtures at all RPM. If the PCV valve is leaky along with a loose valve cover or oil filler cap it can inject significant air into the S-boot that has the same effect as a broken S-boot.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[ac78]

[Randy in Maine]

Time it as I suggested and then take it for a ride. Report back.

[ac78]

[SGKent]

[Wildthings]

The ported vacuum connection for the vacuum advance will not have vacuum at base idle, but should have vacuum if you rev the engine a bit. I guess the port in the throttle body could be clogged with something, but haven't experienced it myself. Can you suck through the vacuum advance hose with it attached to the throttle body, if not remove the throttle body then try blowing through the port both ways with compressed air.