Vacuum / timing questions...

[jakub421]

Hi, I have some questions about the vacuum system on the earlier E21s (1978).

- Why is the vacuum advance only activated in 4th gear instead of all gears, is there a functional reason?

- If i put my car in 4th gear with the clutch in while stopped, I don't notice any change in idle speed (I'm assuming you have to be at a higher RPM for it to pull enough vacuum to advance). What would be a good way to test if the switch is working?

- Why do we adjust the timing with the retard vacuum line unplugged?

- I don't have a belt on the air pump, so it is currently not functioning. I noticed that the white vacuum line going to the blow-off valve from the manifold did not hold vacuum, so I just disconnected it from the valve and plugged the nipple on the manifold. Any negative effects to doing this?

- What is the vacuum in the intake manifold supposed to be if checking with a vacuum gauge?

I'm sure I'll think of some more later haha, but that's it for now.

Thanks!

[cgifool]

Good questions! I'll look forward to the answers..

The only question I'm going to attempt answering is #3- (hopefully someone will correct me if I'm wrong on this):

Timing should be adjusted with BOTH hoses disconnected (and plugged so they don't create vacuum leaks) because the timing mark is a baseline setting and any issue with advance or retard systems would give a false reading.

Only after timing is set properly at a known RPM without advance or retard, would you be able to begin to check for proper advance or retard action.

My own car has been messed with in the past, various unknown bits removed (no idea how it was setup originally) etc, and I've tried various things, and so far the best combination of cold starting behavior, power, and hot temperature seems to be NO vacuum retard, manifold vacuum connected directly to the advance, and the timing retarded just slightly from baseline to eliminate some mild pinging at part throttle (im thinking this was necessary to compensate for the lack of vacuum retard). Keep in mind this on an M20 engine. Not sure how relevant this is to an M10. Seems to me like all the factory settings should be taken with a grain of salt as soon as any of the equipment has been removed or altered.

The reason I disconnected the retard was that although it starts easier with it connected, the engine ran hotter when hot (and I live in Texas). Some day I want to try a temperature controlled switch so the retard only works when cold.

-David

[epmedia]

I'm most certain that only 4th gear uses the vacuum advance is because vacuum advance 'all the time' would cause smog test to fail.

Yes - the throttle needs to be opened a little for vacuum advance port to start sucking. This vacuum advance connects to the canister on the ignition distributor, and to the EGR valve, via a thermal valve and t-connector. For testing the 4th gear vac advance while the engine is warm - you can try raising the rpms with your foot while the shifter is in 4th (also clutch pedal to the floor), then see if the rpms drop a couple hundred rpms when the shifter is moved to neutral. If no rpm changes, must find out if vacuum leak on one of the components, also electrical connection to the 4th gear switch and electric/vacuum switch on the firewall.

Vacuum retard only works at idle and a teeny-bit more open position on the throttle body. This helps with smog testing and lower rpm engine torque. While the engine is cold, this vacuum retard is not functional for at least several seconds (depending how cold it is).

Do a vacuum test on the air-pump blow-off valve and ignition advance/retard canister. Both of mine failed vacuum test over a decade ago... My EGR valve has been fine.

[Somjuan]

If you need spare early smog parts I’m in Sacramento with the whole lot. All worked when the engine was pulled, it passed smog not horribly long before the head gasket blew

[jakub421]

I did just recently pass smog with flying color, had to order a new air pump though cause mine was locked up and I decided to remove the belt after the testing, thanks anyways!

Vacuum advance and retard both hold pressure, but the blow-off valve leaked slowly. I blocked off the vacuum line on the manifold and want to make sure that I won't damage anything doing that.

Since the 4th gear advance switch is just an emissions thing, would it be a ok to bypass it? Might help a bit with power?

[epmedia]

Since the 4th gear advance switch is just an emissions thing, would it be a ok to bypass it? Might help a bit with power?

I'd surely give it a test run for science purposes. The electric/vacuum switch on the firewall has the two hoses connected to it. Just couple those two hoses together for test run and for science purposes. Not for long term purposed of course