Timing questions -- '78 Z

[FastWoman]

Hi,

Gee, it's been a while since I've actually had to time an engine -- long enough for me to forget a thing or two. I just replaced my distributor, and therefore I need to do this.

Here's what I did: Warmed up engine. Set idle to 800 RPM. Connected timing light. Set to 10 deg BTDC. Readjusted idle. Rechecked timing. Tightened set screw at base of distributor. Finished.

As I was doing this, I noticed that there was quite an RPM drop between, say, 20 and 10 deg BTDC. The engine seemed to run smoothly, but that seemed quite an excessive drop -- perhaps a few hundred RPM. Moreover, my engine vacuum is a few pounds below where it should probably be.

Stupid question :stupid:: Am I supposed to disconnect the vacuum advance before setting the timing? The Datsun service manual didn't tell me to do this, so I didn't.

Somewhat smarter question: At about how many deg BTDC should I expect idle speed to peak?

Another question: Does an excessive falloff in RPM between the "happiest" setting and the mfgr's specified setting suggest any particular engine problem?

Finally: Are there ever any reasons to set timing more advanced than the 10 deg Datsun specifies?

Thanks for any advice!

Peace,

Sarah

[TomoHawk]

Yes, disconnect the vacuum tube from the advance pot. The 800 RPM is with the transmission in gear, btw. I just had my timing done. Now it has a much better idle, especially when cold, better fuel mileage (the exhaust no longer smells like fuel) and more power when driving.

thxZ

Edited July 22, 2009 by TomoHawk

[FastWoman]

Thanks! I'll give it another go this evening with the vacuum advance disconnected! Regarding the transmission in gear... It's a 5 speed manual. I think I'll keep it in neutral!

[TomoHawk]

With a manual transmission, you use a different RPM to set the timing.

[FastWoman]

The service manual says 800 for manual and 700 (in gear) for automatic. I used the right RPM -- just had the vacuum advance connected. ;-)

[FastWoman]

Mmmmm... It turns out that it doesn't make any difference whether the vacuum advance is disconnected. It times the same at idle.

I'm wondering whether I have a good vacuum at the advance. I didn't have time to test today, as there was a beautiful sunset we wanted to take in on the water. The vacuum line to the distributor has a T connection also going to the carbon canister. That might be compromising the vacuum.

Question: Is the carbon canister really necessary? Does it do anything for engine performance? Would it still even work after 30 years?

I'll look into these and other matters tomorrow.