LeonV

Well there's your problem! Down-draft carbs don't belong on an inline engine, especially one with high-performance in mind. Some will be drawn to contradict me because their's runs great, but that setup is just inherently inferior to proper side-drafts. Slap on some SUs (or triples) and be done with it. People make the 390cfm "work" but that 600cfm Holley is probably no where near jetted correctly for your engine. Oh, and there is a rev limiter on the L24, it'll stop revving once you see a piston flying through your hood! An SU-equipped L24 will rev to 8000rpm just fine, just ask Greg Ira.

What I'm thinking is happening, is that your horn is shorted. When you turn the wheel, the copper contact that comes from the steering column touches the contact ring on the steering wheel and sounds the horn. I'd look at that copper contact first and see if it's touching the ring at all times. The horn circuit is very simple and there are a lot of good posts about it on this site, if you search.

It's conceivable, and I've been thinking all along that it's ignition-related. However, Zed has done quite a few tests to eliminate most variables in that scenario. The only other ignition stuff I can think of is poor grounds, but I think that's been covered as well. We should note that the spark plugs are grounded through the block, so the block must be well-grounded. Otherwise, you're robbing power from the spark. With that said, it really sounded like there was not enough advance at part-throttle but it seems as though adding vac advance didn't help, either. At this point, I would do what I can to alleviate the issue. It doesn't matter so much as to why this or that, but what can I do to make it run right. Maybe Zed's engine is not similar to other engines because there are some unknown modifications? Who knows! What I would be doing, with all other options exhausted, is seeing if I can make any adjustments that make it run like I want. I will repeat, though, take a look at your valve lash! Although no one seems to understand the importance of this, valve lash affects valve timing. If you have a tight or loose valve(s), you are putting in more/less air-fuel mixture into the cylinder, the effects of which are most prominent at part-throttle! Uneven mixture distribution can lead to misfires in certain cylinders, the effects of which would be drowned out by the other "good" (compression and valve lash) cylinders. Basically, you may have 4 cylinders firing evenly, but the other 2 are misfiring, giving you problems. An engine with the same setup, but 6 "good" cylinders will not have a problem. This really seems like one of those extensive, time-consuming, head-bashing problems that is being caused by some little, minor adjustment out of tolerance.

This tells me that the needle profile is not correct for your engine. I'd try different needles. If the piston rises more, you will make the mixture leaner because of a diminished vacuum signal. Run a little experiment with stock needles, placing the shoulder at varying heights and recording what happens.

I like using cardboard (or even better, laminated cardboard), it lets you slide around!

Thank you, Jeff! You've saved me from typing this. Stock brakes are just fine, people love to get caught up in "upgrades". I will even venture to say that you'll be worse off with the 4X4 calipers than stock because of added cost, unsprung weight and they may not perform much different than stock. People will say, "it feels so much better" but I haven't seen any supporting data. A bigger caliper will give you more pad surface area, which will tend to have less fade than stock, but this is not noticeable until you hit the track. In Jeff's case, and in the case of many ITS Z-racers, stock has been adequate even out on the racetrack.

Nope, no discernible difference between the two, in that respect. I completely agree.

Ouch, I hate to see something like that happen, especially to a Z in such great shape! Hopefully, you'll have some insurance money and you can buy back what's left and use it in your next Z.

True, but unfortunately I don't have the time to make the trek, and I think my fiancee would kill me at this point.

Damn, I'd pick that up in a heartbeat if it were closer!

Spot on! You'd have to try different needle profiles in order to tailor the carb to a certain engine. This is when you realize that triples are more user friendly in this regard. Take old jets out, put new jets in. Drive. Repeat. Idle, accel, and main circuits can be tuned separately. Time for triples!

That's a parts car.

Booyah! I spend too much time on this stuff...

It opens with Microsoft Excel. I don't have access to it right at this moment, so I can't give you any numbers. I think that distributor has 17 degrees of mechanical (typical ZX unit) and a really large amount of vac advance (30 degrees?).

Now THAT is how you do a restoration! Nice work.

Make sure the valves are closed, chock the wheels, engage the hand brake and put the car in 1st.

You can find the information in Walter's spreadsheet: http://www.classiczcars.com/forums/showthread.php?34192-280ZX-Distributor-Advance-Curve-Reference

Yoshi, I'll have to take a look and see if I can dig up a heat shield. I can't guarantee it though, my garage is a mess! I can come by and have your car running the same day, we can improvise on a throttle return if you just need the car moving. I commute to Petaluma so I drive through SF everyday. I can stop by and give a few pointers or an extra hand sometime, just let me know if/when and we'll try to schedule it.

At this point, I would do a cold valve adjustment. Get it warm. Take off the valve cover again and check hot lash. After you've checked/adjusted the valves hot, you can perform the leakdown test. Find TDC on other cylinders by looking at the cam lobes.

Well, most people that run without vac advance do so when there are no provisions for a vac advance, e.g. when switching to triple Webers. In any case, load sensing spark advance (vac advance in this case) is a good thing. What you have going on sounds like a bad combination of fueling and spark timing. Sounds like you were leaning out at partial throttle and high rpm, while your spark timing did not advance enough to burn the leaner mixture on time. I don't run a vac advance on my triples because there isn't a provision for one, but my triples also run rich. A richer mixture will burn quicker than a lean one, thus the spark doesn't need to be as advanced in order to light-off the mix at the right time. However, clearly this is not good for efficiency. Sounds like your new carbs are set richer and you can test with vac advance connected. I think a properly working vacuum advance will cure your problem.

I'd say it's more like paying for the ice cream and having the ice cream man drop it as he hands it to you!

I think you said that popping occurs at 4500rpm and only partial throttle, correct? If this is the case, your distributor was probably not providing enough timing advance to light off the more dispersed, leaner mixture. I recommend swapping in a distributor with a functioning vacuum advance. Have you still not checked valve lash?

There isn't really a "stock" with the DGVs since they never came on the car, but were an aftermarket bolt-on. IMO, I wouldn't spend any money or even much time on them, I would either get round-tops or triples. Downdrafts don't make much sense on an inline engine. Good luck and enjoy the drive!

Weld the holes shut, grind down the weld, prep and paint.

A heim-jointed LCA is the better solution, with regard to LCA misalignment.