Timing and driveability

[240ZMan]

Lately I've been running my 73 240 with roundtops as lean as I can to deal with some idle issues. One side effect is that applying the throttly gently, such as going around a corner slowly in 2nd gear, results in a jerky response from the engine. It's just not possible to accelerate gently and smoothly at the same time.

So I was playing around with things and removed and plugged my vacuum advance. I also advanced my timing from about 8 degrees to 15 at idle. Interestingly, the jerkiness at part throttle is almost completely gone. And I find it pulls to 6k rpm just a little stronger as well (due to the extra advance, maybe?).

So my question to the group is what am I giving up? At the risk of sounding a bit cynical, rarely in life is there a free lunch:finger:

[gramercyjam]

I don't think 8 is enough. 15 should be getting it close. Tuning by the seat of the pants, I wound up liking the performance best at around 18 BTDC at idle. That is close to what my Chilton's says it should be too.

[240ZMan]

I would like to go with more, too. I tried 20 degrees, but was getting pretty strong pinging around 4500 rpm, and maybe higher, just couldn't quite hear it clearly over the engine.

I'm just wondering what I give up by not getting the vacuum advance at part throttle. I'll guess it translates into a bit less fuel economy, but given I can set the carbs a bit leaner and still keep it driveable this way, perhaps it's all a wash.

Wish I had a real dyno...

[gramercyjam]

you should try to get the vacuum and centrifigual advance right so you get more total advance when you rev it up or performance will suffer. There is someone on the zcar list that seems to know a lot about this - Kim Blough of Idaho Z car. Maybe you can get his advice.

Here is a link to an interesting discussion on distributor curves

recurving dizzy

[240ZMan]

Thanks John, that's an interesting discussion. I respect Kim's opinion a lot, and I think I'll be doing some work on my dizzy in the not-too-distant future.

In the mean time I'll have to see if I measure a change in mileage without the vacuum advance.

[LanceM]

By setting your static timing higher you are getting pretty much the same thing as the vacumum advance is giving you. The trade off for setting a higher static timing is typically harder starting when the engine is hot. The best way to set timing with out using vacumum advance is to set the static timing for full advance at around 2000 rpm. 34-35 degrees is a typical full advance at this rpm.

To set the timing this way warm the engine up and set the idle up to around 1800-2000 rpm and adjust it for 34 degrees. To find out where your 34 degree timing mark is check out this site

http://www.thedirtforum.com/timingcalc.htm

This will tell you where to put the mark on your front pully.

If you set your timing up this way you will know that you have full advance out of your distributor at speed.

Anyway, if you increase your static timing you shouldn't miss your vacumum advance.

One of the things you loose if you go to after market carbs like webbers is vacumum advance so running without it is nothing new. I am wondering too if your vacumum advance in your distributor is leaking causing some of your low speed problems, typical of a vacumum leak??

[240ZMan]

Lance, thanks for the timing suggestion. I think I'm going to try that this weekend.

On the vacuum leak idea, what's strange is that it acts like it's way rich, which would be the opposite of a vacuum leak, unless I had the leak the rest of the time and it stopped when warm. I tried that theory for a while, but just couldn't find the source, so I gave up on it.

To be honest, I suspect the fuel in the float chambers is overheating. The symptoms fit. It's just that it happens when things are no where near as warm as I would think they needed to be.

BTW, if you're interested, here's the link to the latest from this afternoon.

http://www.classiczcars.com/forums/showthread.php?s=&threadid=12323