Captain Obvious

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.

Oh yeah. One other thing... CHECK THE CONDITION OF YOUR CAM SPRAY BARS!! After studying the original factory spray bar design, I believe they can be placed into one of two categories: 1) Those which have already failed, or 2) Those which are very close to failing. CHECK THE CONDITION OF YOUR CAM SPRAY BARS!!

Steve, good question. The short answer is "as tight as I could make it and still allow the tubes to slide in and out". I was concerned about vibration and play on that center mounting bracket. I thought about using some sort of flexible vibration absorbing "bushing" at that center support. Something like rubber grommets or a piece of fuel hose tubing pressed onto the tube ends that fit into a snug hole in the center bracket. However, I was worried that anything electrometric I used there would eventually fail and fall apart. And once it failed, not only would you have a very sloppy fit, but you would also have rubber chunks loose up in the valve train area. In the end, I decided that a "no play" all metal slip fit would be the best option. I figured that if there was no play, then there could be no velocity, and hence, no energy to cause wear of the surfaces. So I drilled those holes undersized and then carefully reamed them to fit the tube ends. The intention was that there would be no lateral movement but still allow for axial growth. I don't know how long it will last, but I can guarantee that it will last a whole lot longer than the :stupid: stock design!

So simple it's genius! Thanks guys! :classic:

When did they switch to the internally oiled cam and ditch the oil spray bar? I would like to know if my 280 is internal or spray bar, and I would rather not pull the valve cover right now. Is there a head number or build date? And yes, I know... There's no way to know what any previous owner did.

Thanks! I'm very pleased with the way it turned out. I was a little apprehensive about tooting my own horn, but I'm just so durn happy with the results! I tried to keep the geometry of the tubes the same as stock. To that end, the tubes are the same distance from the cam as stock. The tubes are the same height up the journals as stock, the angle that the holes spray is the same slightly downward direction as stock, and to your question... No, I did not enlarge the holes. In other words, my new bar functions the way the original one was supposed to, but probably never did. As for availability for sale, I hadn't thought about it. At the amount of time I put into the first one, there's no way anyone would want to buy them! But now that the learning curve is over and the design bugs have been worked out, the second one should take a whole lot less time, right? Haha. Maybe I need to make a second one and see how long it takes now that I sorta know what I'm doing?

Yeah, I'm sure there are other things that could cause the light to come on and I've done no investigation at all at this point. I bought the car, drove it a couple hours home, and parked it in the garage. The brakes felt fine the entire trip. The light came on when I pulled the parking brake when I stopped for gas and then never went back off when I was done. I wiggled the parking brake handle and switch there on the trans tunnel, but other than that, I've done nothing. I was just figuring that if everyone else's relay was going bad and causing this problem, then that's probably what's wrong with mine.

I pulled my valve cover to adjust my valves and discovered that my oil spray bar was in bad shape. The distribution blocks were bent a little and weren't sealing against the cam journals and the short tubes at front and rear were no longer parallel to the cam. Did a little searching and discovered that this is a very common problem. Clogged, leaking, bent, falling apart. After studying the factory design, I believe it is a misapplication of materials. The steel tubes are "soldered" into the aluminum distribution blocks at one end, and are brazed to the steel center mounting bracket at the middle. Not only does solder not stick to aluminum in the first place, but the thermal expansion rates of the steel tubes is different than the aluminum head that it's bolted to. This means that as the head grows in length when heated, the original tubes must pull out of the aluminum distribution blocks. And when the head shrinks back to room temperature length, the tubes will try to squeeze themselves back into the blocks, but cannot. This creates pressure pushing the end mounting blocks away from the center mounting point resulting in all of the stress and strain problems that occur with the original spray bar. My answer is to redesign the bar with two major improvements: 1) Seal the tubes to the distribution block in a means compatible with the materials used. In other words, soldering steel tubes to an aluminum block is not the answer. 2) There has to be some "give" in the system somewhere to account for the differences in thermal expansion of the materials used. Since the two end mounting points must be fixed, then the CENTER mounting point has to float in order to account for expansion and contraction length of the head. This is opposite of what the factory did. I also wanted to retain the same geometry of the tube with respect to the cam which means a straight bar down the side of the cam would not work because it would be farther away from the cam than the original design. My redesign project turned out very well and I figure that other Z owners might like to see what I did to fix the problems. Not taking anything away from the other solutions that are out there, but here's what I did: Pulled the tubes out of the aluminum distribution blocks and cut them free from the center support bracket with a hacksaw: Took some "billet" aluminum bar, cut off some sections, and machined them into oil distribution blocks including threaded holes where the tubes are inserted. Made some lids to seal off the oil distribution blocks and cut some gaskets to go between: Machined some threaded brass collars for the tubes and soldered them to the ends of the tubes. Solder doesn't stick to aluminum, but it sticks fine to brass and steel: Made some brass plugs for two ends of the short tubes and soldered them into place: Made a new center bracket with tight slip fit holes to accept the inboard ends of the tubes. This slip fit allows the non threaded ends of the tubes to float and account for the changes in length as things change temperature: Cut some gaskets to seal against the cam journals: New design done and ready to be installed on the engine: Once I was all done, I wanted to check the bar to make sure everything worked properly, so I pulled my spark plugs and coil wire and cranked the motor over (props to Blue for the idea) by shorting the starter solenoid terminal to the battery connection on the starter. Not the best pic because I'm doing the shorting with one hand and trying to work the camera with the other, but you can see streams of oil shooting out of the bar onto the cam lobes. I was surprised how far the oil shot out, even at cranking speed: Much better than the original design. Oil doesn't leak out anywhere you don't want it to, and it shouldn't have the same mismatched thermal issues of the stock bar. And I used "billet" aluminum, so it's got to be good, right? I've also got a bunch of other pics, so if there's a different angle that you would like to see or any questions about the project, just let me know.

My new 280 does this... Brake warning lamp stays on all the time. I'm assuming this is a common issue and this relay is where I should start my investigation?

I've also got an advantage with the flat tops in that I've got the float bowl sight glass on the side of the carbs. I checked the bowl level yesterday before I tried to start the car and it was down a little bit. Evaporation over the past two weeks I assume. Not a whole bunch.. A WAG would be 10% down? If I shut it off while full hot in the middle of Death Valley one summer afternoon and then came back two weeks later and tried to start it, I could believe that my bowls could be completely dry. But parked (in a garage), even in the middle of summer, in the Philadelphia area that was not the case. I don't think it's simply the flat tops either. I also ran round tops for a few weeks last summer, and had about the same starting performance. Only time I ever really had to crank the car even remotely as long as 30 seconds was when I just put the carbs on and the bowls were bone dry. I think a more objective test would be to pull the tube between the bowl and the nozzle before he tries to start it and measures the amount of fuel that comes out. If the volume of fuel that comes out is close to what should be in there, then the problem is somewhere else. Spraying a bunch of fuel into the carb intake before you try to start it can mask a whole bunch of other potential root causes. Checking the bowl level would rule out just the bowl level. So what I'm going to do is count my lucky stars on just how easily and readily my car starts. Maybe I'm the anomaly?

After I posted that question to you about current availability, I reread that you mentioned they were analog IC's. Unless they were some easily recognizable regulator or amp, I would consider the possibility very remote that there was a current drop in. Thanks for confirmation about counting the spark pulses. With only a relative reference, I couldn't think of any other way it could have possibly worked, but I had to ask. If I were designing such a system, I would hiccup every now and then on purpose, say every million pulses or so, and change which spark event I would spray on. You know... Wake up synchronized to 2&4, but after a million sprays, I would purposely drop one pulse, after which I would be synchronized to 3&6. Then after another million sprays I would drop another pulse and end up synchronized to 1&3. You would gradually roll around and end up spraying the same average number of times on each pair. Probably doesn't matter, but just to even things up. That's just me. Of course, that kind of thing is a lot easier to do with programmable devices.

Fine. Then I'm not stumped. I just don't have any suggestions at this time. If you were anywhere near me, I'd loan you another pair carbs to try. Sure, it would be a shotgun approach, but it would help narrow the field so much that it would be worth it.