280zx advance curve question

[whamo]

I installed a 280zx distributor in our L24 a couple years ago and set the initial timing at 8 degrees advanced at idle. For the last few years I have noticed a buzzing at part throttle tip in that sounds like detonation. I also changed the exhaust at the same time and dismissed this as a resonance issue with the aftermarket exhaust and not detonation, dumb mistake in retrospect. I hooked up my innovate wideband o2 sensor to aid in tuning the carbs this last weekend and in the process checked the total advance with my dialback timing light. Holy crap, it showed 48 degrees of total advance in by about 4000 rpm that seems like way to much to me. I ended up backing my initial timing back to 2 retarded to get the total advance down to 39 degrees, the detonation is gone with a little richer mixture and the timing change. Does anyone know what the total advance was on the 280zx distributors? I believe ours came off a 1979 or 1980 car. Also noted that the mixture it really rich at idle (10:1) when the mixture under hard acceleration is in the right place 12.5-13.2:1. Does anyone else have experience looking at Air Fuel ratios with the SU carbs?

Edited April 8, 2009 by whamo

[doradox]

I installed a 280zx distributor in our L24 a couple years ago and set the initial timing at 8 degrees advanced at idle. For the last few years I have noticed a buzzing at part throttle tip in that sounds like detonation. I also changed the exhaust at the same time and dismissed this as a resonance issue with the aftermarket exhaust and not detonation, dumb mistake in retrospect. I hooked up my innovate wideband o2 sensor to aid in tuning the carbs this last weekend and in the process checked the total advance with my dialback timing light. Holy crap, it showed 48 degrees of total advance in by about 4000 rpm that seems like way to much to me. I ended up backing my initial timing back to 2 retarded to get the total advance down to 39 degrees, the detonation is gone with a little richer mixture and the timing change. Does anyone know what the total advance was on the 280zx distributors? I believe ours came off a 1979 or 1980 car. Also noted that the mixture it really rich at idle (10:1) when the mixture under hard acceleration is in the right place 12.5-13.2:1. Does anyone else have experience looking at Air Fuel ratios with the SU carbs?

Are you checking that with the vacuum advance disconnected? That sounds like an awful lot of centrifugal advance alone.

[whamo]

Are you checking that with the vacuum advance disconnected? That sounds like an awful lot of centrifugal advance alone.

That is total advance, vacum plus centrifugal. I should have checked with the vacum advance disconnected. The vacum advance is ported vacume so there isnt any vacum siginal at idle. Only when you crack the throttle open. I am considering changing to manifold vacum.

[sblake01]

I'm still wondering why no one uses the vacuum delay valve that was part of the original setup on a 280ZX. I'll just bet that would solve most of the problems that occur when that dist. is installed on earlier cars.

[Bruce Palmer]

If you go into the distibutor and pull the advance plate out so you can see the T-bar down inside you'll see a number stamped on it. Double that number and that will tell you how much advance is built into that distributor. There is a lot of variation in that number, application to application, and some of them are way too much advance for early engines. This can be fixed.

Any time we do an electronic distributor conversion we automatically re-curve to preclude detination. L motor pistons like having intact ring lands..

From a thread yesterday about Acar headed for the NWT it sounde like there are a couple of pretty good shops in your area + Ross Mullen 604-916-2567 Sports Imports

Edited April 9, 2009 by Bruce Palmer

[whamo]

If you go into the distibutor and pull the advance plate out so you can see the T-bar down inside you'll see a number stamped on it. Double that number and that will tell you how much advance is built into that distributor. There is a lot of variation in that number, application to application, and some of them are way too much advance for early engines. This can be fixed.

Any time we do an electronic distributor conversion we automatically re-curve to preclude detination. L motor pistons like having intact ring lands..

Thanks Bruce, when I return home I'll check that number. Are differnt T-bars available and from where? Also is it likely that the large disparity in mixture between idle and WOT is caused by worn needles or is that normal?

Edited April 9, 2009 by whamo

[Bruce Palmer]

Thanks Bruce, when I return home I'll check that number. Are differnt T-bars available and from where? Also is it likely that the large disparity in mixture to WOT be caused by worn needles or is that normal?

We shorten the slots in the T-bar with a wire gun to shorten the advance. Some will do it with JB Weld and a dremel playing with the distance 'til the advance is down where the motor will live, like mid 30 degree neighborhood.

A distibutor shop could more than likely do if for you as well. It would be a good time to check the bushing and see if the advance plate ball bearing are still there doing their job.

[whamo]

We shorten the slots in the T-bar with a wire gun to shorten the advance. Some will do it with JB Weld and a dremel playing with the distance 'til the advance is down where the motor will live, like mid 30 degree neighborhood.

A distibutor shop could more than likely do if for you as well. It would be a good time to check the bushing and see if the advance plate ball bearing are still there doing their job.

So 35 total advance in a stock l24 on premium pump gas is about the max? Any thoughts on the air fuel ratio variation from idle to WOT?

[Walter Moore]

So...

If I just ran out and randomly grabbed a distributor off of any old 280ZX that happened to be available I might have the same problem?

Do you have any advice on getting the "right" distributor to use for an upgrade on the older cars? Are there particular years that are better or worse? I have been thinking about trying to round up a ZX distributor, and I might be able to get one off of an 83. (I know about the issues with the later ignition control module, but there are instructions on this site to deal with it...)

But how do I know what advance curve I need, and which years would have the right curve?

(One more reason to stay with points....) :mad:

[Arne]

I'm still wondering why no one uses the vacuum delay valve that was part of the original setup on a 280ZX. I'll just bet that would solve most of the problems that occur when that dist. is installed on earlier cars.

Stephen, I now think that would be a good idea. Problem is that none of the many guides to installing a ZX dizzy on an L24 make mention of the delay valve. I'd never heard of the device until long after I gave up on the ZX ignition, for pretty much the same reason that Steve describes. I suspect that the lack of knowledge that they exist is the main reason no one uses them.

I installed a 280zx distributor in our L24 a couple years ago and set the initial timing at 8 degrees advanced at idle. For the last few years I have noticed a buzzing at part throttle tip in that sounds like detonation. I also changed the exhaust at the same time and dismissed this as a resonance issue with the aftermarket exhaust and not detonation, dumb mistake in retrospect. I hooked up my innovate wideband o2 sensor to aid in tuning the carbs this last weekend and in the process checked the total advance with my dialback timing light. Holy crap, it showed 48 degrees of total advance in by about 4000 rpm that seems like way to much to me. I ended up backing my initial timing back to 2 retarded to get the total advance down to 39 degrees, the detonation is gone with a little richer mixture and the timing change. Does anyone know what the total advance was on the 280zx distributors? I believe ours came off a 1979 or 1980 car.

Steve, as I mentioned above, I gave up on my ZX dizzy. I believe that mine was from an '83 automatic, but had the earlier E12-80 module. But I had to use 93 octane fuel and drop back to 5° BTDC or less to keep it from pinging under moderate load at highway speeds. I went back to the stock 240Z dizzy with a Pertronix, and am back to 12° BTDC now with no detonation, and will experiment with a bit more advance or maybe lower octane fuel shortly. I didn't do any measurement of the total advance on the ZX dizzy, but came to that same conclusion just from my testing.

Of course, not everyone has an original 240Z dizzy that's not worn out like I did, so my alternative may not work for a lot of people. But I do agree that the vaunted ZX ignition is not a totally trouble-free option in all cases. Adding the delay valve that Stephen mentions is probably a good option, if they can be easily sourced. Or re-curving the ZX dizzy is another good choice, if people can find a good place to get that done.

But considering the advancing age of even the newest of the ZX dizzies, and the number of those that by now also have worn shaft bushings, I'm wondering if the simpler option might soon be to buy a remanufactured 240Z dizzy off the shelf and stick a Pertronix in like I did. A bit more expensive than a ZX dizzy from the pick-n-pull, but perhaps a better choice in the end.

Also noted that the mixture it really rich at idle (10:1) when the mixture under hard acceleration is in the right place 12.5-13.2:1. Does anyone else have experience looking at Air Fuel ratios with the SU carbs?

I can't remember Steve, are those carbs still original? Not remanufactured? In theory, the whole approach of the SU design is to keep the ratio fairly steady through-out the rev range. If the idle mixture is way richer than under load, one thing I'd suspect is a vacuum leak at the throttle shafts. With a vacuum leak, one of two situations occurs: you can tune them either rich at idle/good at speed; or good at idle/lean at speed. If my carbs were worn, I'd want to have mine set like yours rather than lean at speed.

So it might be worth checking the throttle shafts for vacuum leaks.

[jmortensen]

Disconnecting and disabling the vacuum advance entirely will let it run on just mechanical advance. The different T bars are for 17 and 18 degrees, so if you start with an advance of about 18 that puts you square in the mid 30's at total advance. That is JUST MECHANICAL ADVANCE. The vacuum advance is IN ADDITION TO the mechanical.

I know a lot of people think that this hurts mileage or that it makes the car hard to drive at part throttle, that was not my experience. I got pretty good mileage, even with triple carbs I was pulling down low mid 20s on the highway. With SU's I once got over 30 mpg going to the track which was a long drive and I was hauling 4 tires, tools, etc, so basically it was probably like having a passenger along. This was with a .490/280 cam, MSA header and 2.5" exhaust and MSD.

Another safe bet on the ZX distributor is that if you have one and it hasn't been rebuilt there is a very high likelihood that the vacuum advance mechanism is busted. I've only seen one that wasn't. Rebuild tips: http://www.jrdemers.com/280ZX/distributor/distributor.html

I'd rather have a ZX distributor with the vacuum advance disabled than a 240Z distributor with pertronix or points. YMMV.

Edited April 9, 2009 by jmortensen

[Arne]

Another safe bet on the ZX distributor is that if you have one and it hasn't been rebuilt there is a very high likelihood that the vacuum advance mechanism is busted. I've only seen one that wasn't. Rebuild tips: http://www.jrdemers.com/280ZX/distributor/distributor.html

The one I was trying was not broken that way, I had seen that page before and checked. But that's another point to look for (in addition to worn shaft bushings) when looking at a used ZX dizzy.

I'd rather have a ZX distributor with the vacuum advance disabled than a 240Z distributor with pertronix or points. YMMV.

I agree that there are several potential advantages to a ZX dizzy over the smaller 240Z unit w/Pertronix. But my uses are mild and my L24 is stock, so the Pertronix choice was simple and works great for me.

I recall that beandip had instructions on making a simple travel limiter for the ZX advance mechanism. that may be something to look into also.