Tuning Puzzle

[Jetaway]

The car: 1972 240Z w/ a 280ZX electronic distributor.

I had, perhaps still have, an odd problem with the rear carb. The car would be running fine but our of the blue would lose the ability to accelerate at a rate faster than a bus without going into a spasm of misfiring. It would accelerate and get up to speed, but it was ugly getting there to say the least. Then, mysteriously, it would be fine.

Eventually I figured out that the rear carb would lose its damping -- despite having oil in it. After convincing myself, that yes, there was oil and more importantly, being able to reproduce the problem at home, I pulled the carb and dissembled the vacuum chamber.

To be honest, I didn't see anything that could explain the behavior. I did clean the hell out of it and it is possible that gunk had clogged up one of the holes in the dome, so perhaps that caused the loss of damping. Put it back together, put it back on the car and balanced the airflow at idle (650 -- 700 rpm) and higher speed (~2700 rpm). Test drive was a bit doggy and some misfirings, but I got rid of most of the issues by richening up the mixture (I start purposely lean, so this is not a surprise). But not all of it -- power seems to drop off around 5000 with a hint of misfires also.

I have three thoughts: One, a month ago I replaced the 280ZX distributor that came with the car with a rebuilt unit due to wobble in the shaft. The original's vacuum advance didn't work and so I set the timing around it. The new has a vacuum advance which I have hooked up. So, I'm thinking that it might simply be a timing problem and I should play around with the timing a bit.

Two: When I had the carb dissembled I saw that the needle was apparently not correctly set in the piston. My repair manuals and the FSM all indicate that it should be set so that the shoulder is flush with the piston while on mine it was sticking out a bit. So I reset the needle and tested per instructions. It seems to work, centered in the nozzle, but perhaps something went wrong.

Three: This may be the oddest. As I wrote earlier, the airflow is balanced between the carbs both at idle and at higher speed. But -- if I disable the rear carb at idle by lifting the piston, the engine picks up speed and runs just as smoothly. Doing the same to the front produces the expected result, a drop in idle speed and rough, choppy running. If I repeat the test at higher speeds, say mid 2000s, the result is as expected, disabling the front or rear carb causes the engine to run very roughly and slower. Is this suggestive of a problem with the float level? I checked it a couple of years back and don't know why it would change, but I've seen odder things with my Z over the years.

Any help appreciated.

Chris

[240260280z]

Good point about float level in the last section (three).... you know that may be why the lift trick does not give the same results at idle vs high rpm. From the results, the front carb would be richer at idle and have higher fuel.

How do the plugs #2 and #5 compare after 5 min of idle and after 5 min of spirited driving?

For your first section, I am thinking the new distributor may have issues transitioning. (possibly the breaker plate has broken). Maybe stick the old one on and compare has a quick check then watch your timing advance and return while hand-reving the engine.

The problem could also be too much mechanical and vacuum advance in transition. Checking the timing while hand-reving should show what is happening.

If you have two ZX distributors, take both apart and build one with the best parts from both. The fresh grease will smooth things out.

[240260280z]

I forgot to mention that you should set the needle height to be the same on both carbs.

[Jetaway]

Thanks, Blue.

I returned the old distributor for the core charge (between the wobbly shaft and my GM HEI conversion I didn't figure to have much use for it) so I can't do a comparison. I do remember thinking that the vacuum advance seemed to give an ungodly amount of advance when I installed it, so I'll take a look at it again.

But first -- on to checking the float level.

And a question: In your second post you referred to "needle height." Am I correct in assuming you mean that the needle in the front carb is set in the correct position with the shoulder even with the piston? I had wanted to get the rear carb installed and the car retuned before cleaning the front carb, but if I can't get it running good today, I'll stop futzing with it and go ahead and dismantle the front carb.

Plug reads are a bit of a problem -- with 200,000+ miles on the engine there is too much variation in wall wear and top-end clearances to be able to make a very definitive diagnosis with the plugs.

Chris

[240260280z]

I should have said. "Needle depth in the piston" rather than needle height.

When tuning, I am programmed to pull out the piston, flip them over and look at the needle and shoulder sticking up in the air from the piston thus my use of height.

The correct method for setting the needle depth in the piston is to have the shoulder of the needle flush with the base of the piston.... not flush with the trough machined out of the piston. It is important for this to be the same on both carbs as well as the same fuel level if you want more precision when balancing at idle.

The carbs have a lot of self-healing properties so having different needle depths in the pistons can be compensated by having different jet heights... but to set the correct jet height you have to read the plugs or measure A/F with an 02 from one carb at a time (at different exhaust manifold runners or at exhaust pipe with only one carb at a time in operation.)

For balancing the correct jet heights, I recommend you drive then check plugs 2 and 5. Adjust jet heights to match plug colour...this woks on old and new motors.

[Jetaway]

I checked the rear carb fuel level and it was as I remember setting it, 21 mm below the outside top of the float chamber. I'm not entirely sure if measuring from the top outside is what the ubiquitous "float level drawing" means by 23 mm. I then take 2 mm off of that because of the different setting for rear carbs on 72s.

Before taking him out for a run, I brought the mixture nuts all the way to the bottom then moved them a half turn up. In the coolish morning air, that seemed to solve the missing problem though I _think_ it might be running out of power in the high 5000s. Could be my imagination, however. I want to give it a run this afternoon with expected highs in the mid-90s, but for some unexpected reason, we have a bit of cloud cover, keeping the temp down. It doesn't smell rich, but I'll be keeping my eye on fuel consumption. In addition, it did nothing to address my puzzle over why disabling the rear carb improves running at idle speed.

Upon returning from my run I went over the timing. I've been running at 10 degrees static when between 500 and 600 rpm and vacuum disconnected. Because of the non-stock distributor and so much variation in ZX distributors I attempted to ascertain the contribution of centrifugal and vacuum advances. I first estimated and marked with paint 40 degrees BTDC on the crank.

First thing I discovered was the most I could retard the timing was to 6 or 7 degrees BTDC. I then obtained the following measures, as closely as I could estimate:

Centrifugal Only: 25 Total Advance

Centrifugal and Vacuum: 60 Total Advance.

Using 6 degrees as the base, that works out to 19 degrees of centrifugal and (60 - 19 - 6 = 35) for vacuum. Looking at the "Datsun Z Car Distributor Timing Curves" chart (sorry, I don't remember who made it) and assuming that; a) it hadn't been rebuilt with American-spec 240Z or 260Z springs and that: these are real seat of the pants estimates, I'm guessing that I have a "red zone" distributor with 17 degrees of centrifugal and 25 or 30 degrees of vacuum advance.

If I go back to my setting of 10 degrees static and assume the maximum vacuum advance module of 30 degrees, I have a theoretical peak of 57 BTDC. That sounds like it could be trouble but: a) at least so far I haven't detected any symptoms of pre-ignition (which I assume would be similar to very early ignition) and; I had to work the throttle to get the big vacuum advance figure and I wonder if under a real world load it hits the maximum vacuum advance at the same time as the centrifugal has fully kicked in. OTOH, it hasn't had a real stress test with these settings and parts -- he hasn't been out doing 70 - 75 mph in near or in triple digit heat for a couple of hours followed by a delightful slog over city streets.

So, as always, any suggestions are welcome. In particular, any ideas on why disabling the rear carb appears to improve running at idle speed and, in addition, what should I be watching for regarding engine timing. For obvious reasons, I prefer to be aggressive, but what would tell me that I've gone to far advanced?

Chris

[Jetaway]

The correct method for setting the needle depth in the piston is to have the shoulder of the needle flush with the base of the piston.... not flush with the trough machined out of the piston. It is important for this to be the same on both carbs as well as the same fuel level if you want more precision when balancing at idle.

Ahhhh, crap! When I set the needle flush I set it with the trough, not the base. So, it was right before. Dang it!

If I didn't want to pull the needle again, would the following do as a work-around? I figure I managed to set the needle too far into the piston by a guesstimated 1.5 mm. Assuming that is correct, would I be able to compensate by "leaning" (i.e. raising or turning clockwise when looked at from underneath) that carb out by 1 and 1/2 turns of the mixture knob?

Chris

[240260280z]

Yup, that is how it works but it is not how it is designed.

If one carb had needle in and the other with needle out but both jet screws were turned out the same amount, the carb with more fuel would do most of the work. This is why you should adjust the jet height by reading the plugs if you don't have the O2 wideband sniffer. This matches the jet height to the fuel burn...which is different depending on needle location.

[madkaw]

As far as the dizzy - go with what you can get away with. I think there should be a number stamped on the pull rod of the vacuum advance. Double that number to get advance. My ZX only had 17 degrees vacuum.

I didn't run vacuum, just static at 19 + mechanical 17=36 total.

The vacuum advance units seem to be trouble in a lot of these old units.

[Jetaway]

I was able to give the Z a stress run yesterday. Pulled nicely, if a bit sluggishly, w/o pinging up a stiff foothill grade in 4th from 2800 rpm. A new problem, went from slight to worse upon my return, however. While no symptoms while moving, the idle went from a slight chop to very rough on my return.

Pulled the plugs, #6 was wet, all the others looked pretty good. Pulled the cap and discovered that the rotor had been busy carving out paths through the contacts. Gave the distributor shaft a wiggle and it did -- probably more wiggle than in the old distributor which I replaced less than 1000 miles ago!

So, the immediate task will be to pull the distributor, take it back to the store, and get a replacement for the "rebuilt" that I bought less than two months ago.

Madkaw -- You're running what I did before I replaced the distributor (the old vacuum advance never worked on the original). I was happy with it, you are happy with it, so when I get the new distributor I think I'll just ignore the vacuum advance and go with aggressive static and let the centrifugal do its thing my itself.

Chris