Captain Obvious

So there's no spider webs in any of the holes on the intake face of the carbs? No gasket sealer goo smeared in the holes? When you're running with no air cleaner, the gaskets between the carbs and the air cleaner are gone too, right? Nothing on the intake face of the carbs except aluminum. What about suction piston springs? Any chance you're forgetting to put them in? I know... I just gotta ask. Did the springs come with the carbs from ZT? When you're popping at 5K, what happens if you just hold it there? If it just keeps popping and never smoothes out then we should be able to rule out damper issues (wrong oil, not enough oil, check valve not working)... Really sounds like a lean midrange needle tuning issue, but there's just no way that should be the case with the SM's. Have you considered switching to the flat tops? I'd be happy to guide you on a journey to the dark side.

I'm not surprised that your car runs OK with the needles the way you've set them. It's not a huge difference, and as beermanpete says, you can compensate for it with nozzle adjustment. It's a little less than one turn on the nozzle knob to make it essentially the same as setting them at the bottom of the groove. However, the correct way to set them is to put the shoulder flush with the bottom of the piston. That manual that Pete posted is pretty blurry, but a similar picture appears other literature as well. They're all set the same. I'll see what I can dig up.

The purpose of that canister is to capture and store gasoline vapors that evaporate from your gas tank while the car is parked. The vapors are stored in that can, and the next time the car is started, those stored vapors are sucked into the engine and burned. As a temporary fix until you find yourself a new can, you should be able to cap the line that comes from the throttle body and leave the nipple on the carbon canister hang open. Done that way, the engine will still suck a tiny bit of the vapors, but not as much as it should. So the plastic nipple that snapped off... If it's the one that goes to the same "T" as the distributor, then that's the one that goes to the plastic CAP on the canister, right? If that's the case, then you should be able to just replace the cap, not the whole can.

So the wideband showed exactly what it sounded like. At the same RPM, you're 16 lean at part throttle, but flat 14 at WOT. At the same RPM... Does the same thing with the stock needles. Maybe even a little worse. That makes sense, but there's no way you should be lean with the SM's OR the stock needles. It's not fuel delivery because you're clearly not running out of fuel. It's not your "typical fixed" vacuum leak because if it was bad enough to cause that much of a problem at 5000RPM part throttle, then it would simply ruin your idle. So... What can look like an RPM variable or throttle position variable air leak? - I think I see a cap on the port vacuum signal on the front carb... If that cap or the nipple it is connected to is leaking, you wouldn't have a vacuum leak at idle, but you would have one as soon as you opened the throttle. That leak would be "fixed size", but wouldn't affect idle. Would make your WOT numbers higher as well, but the SM's might be making up for it due to the fact that the SM's are way richer at the tip. Or, what could cause the suction piston to raise too much, too soon? - Maybe some restriction on the atmospheric vent side of the suction piston? Something in the way of the vent holes to the air cleaner? What are you doing for an air cleaner? I don't see anything in your pics. Also, I see that your bowl vents are not connected to anything. Is it always like that, or is that just for the pics? What do you connect your cam cover vent to? Anything? BTW - Have you talked to Z Therapy about this? I love a good mystery, but it's got to have an good ending! All of the literature says that shoulder of the shank should be flush with the bottom face of the piston, not flush with the bottom of the venturi groove. That means you should be able to see like .035 worth of shank.

I have verified the low pressure with a mechanical gauge. Other than that, I haven't looked into the details at all. At this point, I'm hoping that it's an oil pump. The other alternatives are much more unpleasant.

You can get a replacement anywhere used parts are sold. You could bypass it if you do it right, but there's no performance cost and there are eco-benefits. I'm a tree hugger.

Gotcha. I was assuming after reading that paper that since the pressure peak to achieve MBT usually ended up around 15 degrees or so after TDC that you couldn't possibly knock if the wave occurred that late. Thanks again for all the education and the willingness to help the ignition challenged. So back to my original question, I don't see any significant advantage to changing the Z's original port source to a manifold source. Once you're off idle, the port source is the same as a manifold source anyway, and I might be gaining some idle stability with the port source. I will also have lower idle NOx with the port source. Sound about right?

Nissans newer injectors are side fed and mount into the fuel rails in such a way that you can pull them individually. Great idea. However...:mad: [RANT] So they go through all that effort to design injectors that can be pulled individually without messing with any other part of the fuel rail, and what do they do on the 90-96 300ZX??? They put that big stinkin' upper plenum on top and make it impossible to get the freakin' injectors out without pulling ten million vacuum lines, sixty throttle body coolant lines, fifty-nine thousand electrical connectors, all the DIS coils, all the PCV plumbing.........:stupid: And once you can even GET to the injectors, they are so snug into the fuel rails that it takes a space shuttle worth of upwards force to the them OUT of the rails. [/RANT] OK, I feel better now. The bottom line is... If think the 280 is a pain, try a 90-96 300ZX!

Genius! As for the so-so ultrasonic cleaning results... Did you hold the injector open while it was in the ultrasonic bath? Do the early Z's have filter screens in them? It's been many moon since I messed with 280 injectors.

I haven't determined the build month of my 77, and I don't know what previous owners did to it. I have no reason to believe that it's not stock, but who knows? One of the reasons I was asking about all this is because I've got low oil pressure on my 77, and I know that a leaking spray bar could be one of the reasons for it, especially after seeing just how leaky my rickety original spray bar was on my 74. Anyway, unless a previous owner put an internally oiled cam into my spray bar head and didn't cap off the journal holes, that's not my problem. Haven't looked into it yet. C'mon spring!

I understood that, and I think he did a great job of explaining MBT as well. It's just such a shame to spend all that time and effort only to have something like that happen at that end. Grades are grades. I suspect he ran out of time or contracted senioritis else he would have fixed the issue and run the test again. Yes, and he made mention of that in the paper as well. Something I forgot to ask you about earlier. I was asking about using a knock sensor and always advancing until right before knock, and you said: I now understand how you can over-advance beyond MBT if you haven't yet reached the knock limit but have reached MBT, but why would a knock limit stop you before MBT? Is that because of false triggering? Seems from that paper that the peak pressure is pretty short lived, and I'm not sure how you could possibly knock before you have even reached MBT. What am I missing?

Thanks Leon, that's good stuff there. That thesis paper was definitely deeper than I needed to go, but I made it through. Good to know that there are smart people out there still working on trying to eeek out more usable power from the same amount of gasoline. I'm surprised this topic so quickly included calculus. Two things in that paper that really stood out to me... The first was that there's actually (what I consider) a pretty wide timing window while still attaining 99.6% of max torque. Seems you can miss the perfect spark timing point by +/- 2 degrees without making that much of a difference in the final outcome. The second thing that hit me was that his original purpose for the whole paper was an attempt to use the ionization current as an indicator of the perfect spark timing point, and it seems that there was an experimentation error that prevented him from actually making that conclusion. Maybe I'm misunderstanding the paper (which is quite possible), but my interpretation is "I tried to this, but there was something wrong with my test setup which made my experiment results unusable". Seems unfortunate to spend that much effort on the subject, only to have that let-down conclusion at the end.

Steve, that all makes perfect sense to me. Thanks for that. PA separates their inspections into "safety" and "emissions" and I've had my state safety inspection license for years. I never bothered to get my emissions cert, and I've been kicking myself over it ever since. Now the prices have gone way up on the training classes, and I can't justify the cost for "hobby" use. Also, these days I suspect there's a whole lot less theory being taught in the classes, and a whole lot more of "Where to find the OBD II port", and "How to change an O2 sensor". :sick: I should have got my emissions cert years ago back when they were teaching stuff like what we're talking about here.

Thanks for the additional info. Looking at that chart I see what you mean about the HC. Almost flat with respect to timing while NOx continues to climb as timing is advanced. So our idle timing is on that chart, but once the mechanical and vacuum advance mechanisms kick in at higher RPM's we quickly end up off the right side of that chart. Do the NOx continue to climb, and do the unburned HC still stay pretty much flat? That would explain why EGR only kicks in above idle. Well that, and it would probably completely ruin your idle stability if it was active at idle. So throwing emissions to the wind and timing for power only... You want your timing to be as far advanced as possible just before you start to ping, right? That would ensure that you're eeeking out the most energy from every charge. How effective are knock sensors? Do they pick up each and every knock, or is more of a "average" thing? If I bolted a knock sensor to the center of my L6 and then went out for a drive with my base timing set five degrees higher than normal, what do you think the sensor pick up?

The drop test I was referring to is done with the pistons and covers off the car. Here's a link to the video that Blue posted in the SU section: What you're talking about (pistons stopping 10% above full down with the carbs fully assembled with springs installed) shouldn't happen. I'm sure you already knew that... So, when this sticking thing happens, does it only happen with the damper valve installed, or does it still happen with the stalk completely removed If you've got some misalignment with the piston against the carb bore, that could be made worse when the engine is running and manifold vacuum is pulling that piston in towards the engine. Can you see signs of rubbing anywhere on the suction piston? How about on the inside of the suction chamber dome? Any scratches?

No, no... Go on with the finer points. I'm cool with that. Why do the exhaust temps go up? Because the exhaust valve opens before the burn has completed and you're dumping still burning gasses into the exhaust manifold? And about those unburned HC? Why don't you just give the burn more time to complete in the cylinder by advancing the timing? That way you could get the energy out of it too instead of just wasting it in the exhaust manifold? I'm assuming there's a reason this doesn't work, but I just have to ask. Before I asked here, I did do a little digging and turned up this document about ignition timing: http://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf This guy doesn't like port advance at all. He talks about it on page four: This makes sense to me, but seems to require an air pump in order to work (which we have - or used to have). But then the question becomes: If you're not running an air pump, does port advance do anything for the unburned HC?

Sounds like the filters were a sight to behold. The flat top filters I've messed with have been clean. Only because everything was flowing around them instead of through them. But they were clean. So now that the immediate crisis has been averted, how do your plugs look? And have you figured out the correct yoga position to view the sight glasses?

Our Z's use ported vacuum for timing advance. So what's the theory difference between port based vacuum advance and simple manifold vacuum advance? I'm not a timing guy, but I got the basics: Manifold vacuum is highest at idle, lowest at WOT, and in-between when in-between. Port vacuum on the other hand, is zero at idle, highest just above idle, and the same as manifold vacuum everywhere else. I guess the question is "why"? Why do they eliminate the vacuum advance at idle? What's the advantages? Normally, the vacuum source is used as sort of an engine load sense device. In other words, when the engine load is low (like at light cruise) the mixture is lean and you need more time for burn, but when the engine load is high (like at WOT) the rich mixture takes less time for burn. No problem. But following that concept... Wouldn't the load be lowest at idle? I've done some web searching and have seen some theories that draw a link to emissions reductions and port vacuum, but I'm a simple follower and can't tell established truth from speculation. So, we got any timing experts in the house?

I know, I know... I'm just starved for entertainment. :classic: So this piston sticking thing. That's new information isn't it? You're talking about the suction pistons in the carbs, right? If the suction pistons are sticking, that'll screw everything up! That's one of the carb's major organs! There's not a lot of contact between the suction piston and the rest of the carb. In fact, other than the spring, the only thing that is ever supposed to make contact is the polished steel tube in the center that you fill with damper oil. The needle is never supposed to touch the nozzle, and the outer rim of the suction piston is never supposed to touch the inside of the round topped piston housing. Have you run the "thumb over the damper hole gravity drop test" to see if the pistons are about the same front to back? Maybe you swapped the two of them somewhere along the way. Blue posted a good link to a youtube demo sometime back... I think it was over in the SU specific section. If you're rubbing the needle on the nozzle with enough force to get that piston to stick, then you've probably ruined the needle or the nozzle or even both by now. I've measured a couple needle and nozzles, and it's amazing how good something can look and how far off spec it can measure.

Yeah, I consider that excessive. Maybe I'm just blessed with a fast starting car, but that's definitely too long where I come from. Your MGA sounds more like my Z. So have you checked your float bowl levels yet? I'm assuming your garage hasn't reeked like gasoline for the past five days...

Man! What a disappointment... I saw that you had updated the thread, and I was expecting results!! :ogre: Haha! The suspense is killing me!! It's like a Z equivalent of a reality show!

Haha! Excellent! Sure sounds like smoking gun to me. In the meantime, you have bought yourself a stay of execution and now have some time to practice with that swivel mirror and flashlight, right? You still need to verify that the float levels are on target while running. And once you figure out what you're looking for and how to do it, it will be so easy that it will be the first thing you'll check if you ever have issues in the future. Out of curiosity, when you rebuilt the power valve the second time you had the carbs off, what did you find inside. How bad were they? Were the old sealing washers OK, or had they turned to goo? Boat Anchors = I'm officially out of the closet.

I pulled the oil cap on my 77 as suggested above, and I do not have a spray bar. My lobe wasn't in a position where I could confirm an oil hole for internally oiled, but I gotta assume that it's internally oiled. I would have already wiped lobes if it wasn't. mgood, Thanks for the pic. So assuming both cars are stock, then they changed somewhere between 76 and 77. I've been on a spray bar kick lately, and the small tube on yours that sprays the two rearmost lobes looks like it's angled inwards toward the cam. If that's the case, then the bar probably isn't sealing well against the rear journal. You might want to give it a good going over to make sure everything is OK. I recently completely reconstructed mine on my 74 because it was falling apart.

Yes, I'm positive your flat tops (are supposed to) have filter screens in them. I don't have any pics handy, but they're not hard to find. On the cooling fan side of the carbs, there are two brass hex head plugs. If you replaced the float valves, that means you had the lower plug out in order to pull the float pin. There's another similar plug almost directly above the float pin plug, and behind it is supposed to be a filter screen. The filter is not inside the main fuel input, but is about halfway down the carb body between the main fuel input at the top and the float pin plug near the bottom. That said, in my experience, every one of those filter screens that I've come across was bent and mangled by a previous owner's rough handling, and probably wasn't sealing at the ends. In other words, just because it's in there, doesn't mean it's doing anything. And by the way... No, replacement filters do not come in the rebuild kits, which might be why they don't call it out on the diagrams. As for the float bowl sight glasses... An inspection mirror with an adjustable head in your left hand. A strong flashlight with a tight beam in your right hand. Leaning over the driver's side fender ever so slightly, you can get a pretty good look at the sight glasses. Took me a while to figure out the correct mirror, light, and eyeball positions, but now that I've got it, it takes me ten seconds to check both carbs. If you still can't get a good line of sight, pull the air cleaner cover and backing plate off and remove the air tube that goes from the bottom of the front carb to the bottom of the rear carb. With that stuff off, it should be "almost easy". I don't believe the problem is getting fuel to the carb, and your float levels are probably OK, but you have to start there to rule out the obvious. (I'm Captain Obvious) As Pete has already alluded to, it might not be carbs at all. You mentioned earlier that you think the front three plugs were wet. Can you run the engine for a few minutes and then pull them again for a fresh look? Also, can you describe the order of events leading up to now? It was running fine, you were out for a drive and all of a sudden out of the blue it just started acting up? Or it ran fine one day, and the next day now it doesn't? Was it running when the problem started? That's what I've got. :disappoin

The list of fuel related things you've tried looks pretty good, and should rule out fuel delivery issues to the carbs themselves. However, just because fuel delivery to the carbs is OK, it doesn't mean that fuel delivery to the float bowls is OK. Couple of quick easy things to check: Have you verified that your float levels are correct? I'm not talking about on the bench. I'm talking about checking through the sight glasses with the engine running. Did you check the fuel filter screens that are located in the carbs themselves? Maybe they are partially plugged and will pass enough fuel for idle, but not to move the car. If you haven't checked them, it's easy. You can pull them out of the carb bodies without having to take the carbs off the car. Let's start there because after those two things, it's gonna get more complicated.