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

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, 200915 yr by TomoHawk

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!

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

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

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.