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popping through the exhaust at 4500 rpm or so.. HELP!


Zedyone_kenobi

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if SM needles are rich enough to feed an L28 why on earth can't my baby L24 get enough gas?

I'm stuck at the same question and I'm not endorsing the KD needles thing. ZT put your carbs on their car and they said they were fine. Why would your car need so much more fuel than theirs? Also, about your piston springs. Why would you, just you, and no other Z owner need to run stiffer springs in their round tops? Why? Why? Why? I'm just not buying it.

So what about your results with grabbing a handful of choke while it's popping and having the problem go away? The results clearly indicate that your car runs better with more fuel being supplied, but does that absolutely guarantee that the root problem is in the fuel system? By that, I mean... Is it conceivable that the problem is ignition on the hairy edge of not working and the reason the problem goes away with a handful of choke is that the richer mixture burns so much easier than a leaner one? Ignition isn't my stong suit...

Leon? Conceivable?

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I'm stuck at the same question and I'm not endorsing the KD needles thing. ZT put your carbs on their car and they said they were fine. Why would your car need so much more fuel than theirs? Also, about your piston springs. Why would you, just you, and no other Z owner need to run stiffer springs in their round tops? Why? Why? Why? I'm just not buying it.

So what about your results with grabbing a handful of choke while it's popping and having the problem go away? The results clearly indicate that your car runs better with more fuel being supplied, but does that absolutely guarantee that the root problem is in the fuel system? By that, I mean... Is it conceivable that the problem is ignition on the hairy edge of not working and the reason the problem goes away with a handful of choke is that the richer mixture burns so much easier than a leaner one? Ignition isn't my stong suit...

Leon? Conceivable?

What you say makes sense. The symptom is a lean miss-fire. It is is possible that making the mixture richer is treating the symptom rather than the cause. Weak spark will show up with a lean mixure because it is harder to light, as you said. The question now becomes; what is the air/fuel mixture of another Z during the same operating conditons that are causing the problem for Zedyone? I don't have a wideband O2 sensor in my car so I cannot provide any data here. Hopefully another reader here will be able to chime in on this.

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I tried to find stiffer SU springs

Ztherapy said there is only 1 spring used for the Z

He did not have any to sell me.

I was on the hunt for springs but found none. Mine look like cad plated springs, so I have no idea what they are.

I would order a complete set of springs if I knew where to order

Try here: http://www.sucarb.co.uk/ProductCategory.aspx?ParentId=261

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What you say makes sense. The symptom is a lean miss-fire. It is is possible that making the mixture richer is treating the symptom rather than the cause. Weak spark will show up with a lean mixure because it is harder to light, as you said. The question now becomes; what is the air/fuel mixture of another Z during the same operating conditons that are causing the problem for Zedyone? I don't have a wideband O2 sensor in my car so I cannot provide any data here. Hopefully another reader here will be able to chime in on this.

I agree too, especially with consideration to Z.T. proving your carbs.

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what is the air/fuel mixture of another Z during the same operating conditons that are causing the problem for Zedyone? I don't have a wideband O2 sensor in my car so I cannot provide any data here. Hopefully another reader here will be able to chime in on this.

Zedyone is seeing high 16's when the engine starts popping. In my experience with my wideband on my Ztherapy SU running SM's this is very high (I would see 12's or 13's under similar conditions). Part of this high number is due to the popping itself. When the engine misfires a slug of unburnt air goes down the exhaust to the O2 sensor. This causes it to read a lean condition regardless of mixture. In other words, a carb producing 12AFR can read 16AFR at the sensor during a misfire. This is something I dealt with while tuning out a transition stumble on my Mikuni's. The O2 sensor would read lean during the stumble if the jetting was lean or sloppy rich. O2 sensors only read the amount of oxygen in the exhaust. During a stumble there is always extra oxygen in the exhaust.

Having said that, the fact that the application of choke makes the problem go away indicates that you are in a lean condition regardless of what the O2 sensor says.

Just may $0.02

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Guys what we are facing here, I mean with all the KD needle talk, stiffer springs in the pistons, ignition not being strong enough (which just is NOT possible folks. I have two brand new rock solid rebuilt distributors, with 2 brand new coils, 3 sets of plug wires, 2 sets of NGKs, 3 sets of caps, 2 rotors, and verified my coil is getting 13+ volts all the time. My ignition is solid. It is NOT ignition) is treating a symptom rather than the cause.

so this gets back to real issue. All these extreme fixes (aforementioned KD needles, stronger springs, 3.5 turns out, etc) are not what an essentially stock Z should need to run.

The only thing I know for sure is my piston rings are in great shape via my compression test.

I know my bearings are in good shape as I have completely normal oil pressure

I have no insight to the health of my valves, guides, seals, or cam lobes.

I truly believe that my problem in the cylinder head somewhere. I would be willing to try another set of SU's if I had some, but I am not about to buy another set from Z therapy.

I still want to try to a leak down test of the cylinder to see if I have a burnt valve or head gasket leak somewhere, but I am not sure this would be the case when the car runs PERFECTLY if kept under 4500 rpm and it runs perfectly at WOT.

Either way, I am running out of options on this one.

Steve, I do not go from 12 to 16 on my AFR instantly. I start at 10:1 at idle and under normal constant speed driving I usually settle at 12:1. If I crack the throttle I go to 13, 14, 15, (still running and pulling fine) 16, then 16.5 and the popping happens. I do not get a single pop until I go higher than 16.5.

I have priced a replacement E31 cylinder head from Zcarsource, and it will be 1400 dollars. If I do that, I may as well replace all the timing gear and water pump and oil pump. Add in gaskets and new head bolts and I am getting very close to 1700 dollars. If I am going to do all of that. I may as well pull the whole engine and rebuild it or buy a rebuilt one and rebuild mine at my convenience.

I am sorry if I am getting frustrated. I am ready to throw in the towel. My once solid conviction to fix this is getting seriously tested. I have not enjoyed driving my Z in over a year now. It is becoming a VERY good looking symbol of annoyance.

Sometimes you just have to start fresh, Rebello? Datsun Spirit?

Yes, I am talking THAT fresh.

Edited by Zedyone_kenobi
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I'm stuck at the same question and I'm not endorsing the KD needles thing. ZT put your carbs on their car and they said they were fine. Why would your car need so much more fuel than theirs? Also, about your piston springs. Why would you, just you, and no other Z owner need to run stiffer springs in their round tops? Why? Why? Why? I'm just not buying it.

So what about your results with grabbing a handful of choke while it's popping and having the problem go away? The results clearly indicate that your car runs better with more fuel being supplied, but does that absolutely guarantee that the root problem is in the fuel system? By that, I mean... Is it conceivable that the problem is ignition on the hairy edge of not working and the reason the problem goes away with a handful of choke is that the richer mixture burns so much easier than a leaner one? Ignition isn't my stong suit...

Leon? Conceivable?

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.

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With all due respect, your engine is not entirely stock. You reported in your first post of this thread that you have headers. This alone will have an affect on the air/fuel mixture. As a result an adjustment to the jetting is likely required to get the best performance from the new system. Changing the needles and/or springs in the SU carb is the equivalent to changing the main jets in a fixed venturi carb. Would you be surprised to need different main jets if you put headers on a car with a V8 and a 4-barrel carb?

The ignition is likely performing properly and is simply unable to light the overly lean mixture.

The valve adjustment, condition of the rings, valves, etc. would not cause the symptoms you report. With these types of problems you would have a rough idle and perhaps low power, not a part-throttle miss-fire.

You report that your car idles well and runs good at full-throttle. Further, you get a positive response to pulling the choke while driving which enriches the mixture. This supports the hypothesis that you need to recalibrate the carbs.

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WEll I agree Beerman, but how many of us on here are running headers on a stock L24 with absolutely zero issues what so ever. On many cars I have worked on installing headers (many SBC) was a great power adder by itself. Yes, you could get more from rejetting, but it was not night and day. And certainly NOT the difference between running like crap, and running well. The E31 exhaust is very restrictive (Rebellos words) and the stock manifold is darn near as good as a header. Nobody is making crazy power with just a header on an L24.

I have checked the valves already as well. I checked them warm, not hot or cold. I can certainly check them again. I will also check the block ground, and make some more grounds just for kicks.

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I checked them warm, not hot or cold. I can certainly check them again. I will also check the block ground, and make some more grounds just for kicks.

It would be good to double check, it's quick and easy. Check cold, drive around, check hot. If nothing helps, I'd pick a needle and start setting it at different heights, logging, and seeing the results. If anything, it's an interesting experiment!

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Thats easy for you to say!!!!

You can scratch valve adjustment off the list. I really hate trying to do them hot. I would rather take my time and do them cold. NO difference by the way... poppety pop pop

I repeated the carb spray test as well. Nothing.

What is the best way to ground the block. I was thinking the bolt that holds the tranny to the block, and then hook that up to the firewall.

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For a fast test - take it to a local hot rod shop and have it dyno'd. Have them put put a sniffer up your tailpipe (ok - no wisecracks here). That way you will know if you are lean or fat and make your adjustments to your carbs live while you are on their screen. I'm betting your lean. That will rule in our rule out air/fuel adjustments. Once you do that you will be so impressed with the exact calibration you'll go and get an air fuel gauge and install in your car to know what is going on with your carbs. Reading plugs is ok but far from what a sensor and a gauge can tell you.........................IMO.

I'm betting somebody within an hour drive of you has a dyno ready to take you car tomorrow!

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