Vac advance & ign timing problems

[Murph]

First things first, where should my vac advance be hooked up to? First pic shows where it was originally hooked up when I first got the car, 2nd pic shows how it has been up until recently (please excuse the shithouse pic....it's all I have that shows it). I assume the change happened when the new carbs went on (they were nicked, I didn't install the new ones).

Is there really any difference between using either position?

Now, it had been running fine and dandy up until late last week when I installed a crane XR700. When I first took it for a drive after setting the timing I had forgotten to hook up the vac advance. So with it running, removed my plug (from the nipple on the carb) but oddly it didn't run badly with the vac leak, then hook up the advance and revs increase as expected. Ever since then however, I don't seem to get any vacuum on that nipple at all. I shoved a piece of wire down there to try and clear any blockages....but that made no difference. Since then I hooked it up to the nipple on the manifold where it was originally and it seems to work fine.

However, the car does not feel the same to drive and does seem to be a little down on power. Mind you, with the XR700, it runs and idles soooooo much more smoothly than it ever has.

While I'm at it, where should the ignition timing be? It's a 74' 260Z, but the dizzy is from 75' 260Z 2+2. The hanes manual suggests either 7 or 10 deg it seems to run better at 7 than 10, but still feels down on power a bit.

Any help here would be great.

Justin

[240ZX]

I can't provide you with an answer to your original question but, upon observing the pics of your SU's I see that you have the float bowl vents tied together with a section of hose. From this observation I would ask you if the section of hose is vented to atmosphere...as in, are there any holes somewhere along the hose that vent the two float bowls? Just something to think about! The float bowl require venting.

[240ZMan]

The way you have it currently set up (bottom picture) is correct. You won't see any vacuum until you crack the throttle plates, which then gives you the advance you want at part throttle.

As for initial timing, I'd be surprised if you could feel any differnce between 7 and 10 degrees of advance. What really matters is how much TOTAL advance you land up with.

At this stage in the life of an engine, there are many factors that can determine the max advance you can run. Regular vs. Premium fuel is also one. Unless you are going for absolute maximum power, I'd set it at 10, listen for ping, and if you don't ever hear any, you're probably fine.

As for being down on power, why do you say that? Are you comparing it to some earlier time, or perhaps you just expected more? Hard to advise without knowing more.

[beandip]

Justin , the correct port for the vacume advance is the one right in back of the front carb. Manifold vacume to the vacume advance , will make it go to full advance at start up and retard when you depress the throttle and the manifold vacume drops. The secound picture is correct. 10 BTDC I find on my car is spot on. Gary

[Murph]

Thanks guys.

240ZX: The vents line just runs over to the other side of the engine bay (open to atmosphere), just to keep it away from the exhaust should there ever be a malfunction and the bowls overflow.

240ZMan: My comment on it being down on power is based on how it was running a few days ago, before I installed the XR700. It's not much of a difference, it's just sort of lost its edge you could say.

[240ZMan]

My comment on it being down on power is based on how it was running a few days ago, before I installed the XR700. It's not much of a difference, it's just sort of lost its edge you could say.

Did you by any chance note the timing before you started the swap? My own experience is that if you have a lot of advance, the engine feels like it responds to the hit of the throttle more sharply. You can always try adding more advance at idle, say 5-10 degrees, and take a test drive. I'm not saying you won't run into pinging problems with too much advance though.

One of the things I like about the Z is that you can experiment with these settings and as long as you don't go overboard you won't damage the engine. And you can learn a lot. That's the fun part! Good luck.

[Murph]

Timing was last set by my mechanic, to what he said was stock....I assume 10.

When I first installed the XR700 I didn't have the timing light at my place....so i set it to what sounded fairly retarded then drove it to my mates place. It felt down on power on this drive. Turned out it was at around 15º!

I change the vac advance back to where it should be, seems to run fine. Timing it still sitting at 7º. I'll change that back to 10º later in the week.

[Murph]

Arrrhhh! I hate that car at times!

I went to change the timing from 7 -> 10 degrees today, chuck the timing light on.....15 degrees!?

So...I figure I just knocked the dizzy when undoing the locking bolt. Put the timing back to 10....feels dead.....put timing back to 7, feels really dead. Like it didn't want to rev over 4k sorta dead. Put it back upto 15, feels sorta like it did before.

What the hell is going on?! What can cause this? Maybe the vac advance mechanism is binding and isn't able to fully return? I checked and it is still working btw.

[240ZMan]

There are some who have advised me to get rid of the vacuum advance all together. You don't have to have it, although you may see some reduction in mileage if you drive at steady speeds on the highway.

To try it, put a plug on the nipple on the front carb where the vacuum hose connects and just remove the hose from the dizzy. Now you can dial in more advance at idle without as much concern for pinging at part throttle accceleration.

Try it and see. Just listen carefully to your engine for pinging in which case you need to back the timing off.

[Murph]

That's what I do (no vac adv) on the A12 in my sunny.....runs a million times better without it. Mind you, from the factory it wont allow any vac advance unless the engine is up to temperature and you're in 4th gear.

It still doesn't explain why the zed is running like a dog at WOT, with the base timing at 10 degrees? I think I might have to pull the dizzy down and give it a really good clean up, check that everything moves freely etc.

On the topic of pinging, I've never heard the sunny or the zed do it, ever. If the timing is over advanced they just lose power. Maybe my ears just aren't well tuned to it on these engines; although I do hear it find on my mates various turbo imports.

[gramercyjam]

2 Things about the XR700 - Both these things have a tremendous impact on performance.

1. mine never ran right with the ballast resistor. Although the instruction say to use one with a coil without an internal resistor, taking it off was a big improvement on mine. It has been that way for years now without burning out a coil.

2. make sure the photosensor is "phased" according to the instructions. That is when the rotor is pointing to a plug wire on the cap (at the advance you have set), the interruptor should be just breaking the optical beam. So in this order - set the initial advance, phase the photosensor, check the initial advance. If you change the initial advance, adjust the phasing again.

BTW, I run 18 deg initial advance. Always have, even on a stock carbed motor.

[Murph]

Hmm interesting.

1. Might give that a try. I know I have a pretty large voltage drop between the alternator/battery and the coil (and the alternator is pretty shagged), so I might get that sorted first. Low voltage = high current, so keeping the ballast resistor for current limiting might not be a bad thing.

2. Phasing could be an issue. I have to turn the dizzy pretty much to one extreme to get the timing down into the sub 10 region. I had trouble simply finding a good position for the optical sensor as that offset "screw/cam" that you used to adjust the points gap is in the road. Maybe I'll have to pull that out.