'76 280Z Question on dist/manifold vacuum, dist timing, fuel pressure, brake booster

[Zed Head]

I just realized/remembered something about the 1976 year and vacuum advance - you only get vacuum advance in 4th gear. It is controlled by a solenoid which is activated by a switch on the transmission. Unless it's been bypassed.

So fixing the vac advance will only matter in 4th gear. One of those odd Datsun things they did for a few years for emissions..

Sorry to throw a damper on the potential for improvement with a fixed distributor. The car still needs drivin' though. Get the gas flowing through the injectors, hot oil washing the crud off the valve stems, warm up the valve seals so they seal better...it will add up to smooth things out. Climb some hills, put some load on it, rev it up. Bill Clinton was long ago.

[Jennys280Z]

I just realized/remembered something about the 1976 year and vacuum advance - you only get vacuum advance in 4th gear. It is controlled by a solenoid which is activated by a switch on the transmission. Unless it's been bypassed.

So fixing the vac advance will only matter in 4th gear. One of those odd Datsun things they did for a few years for emissions..

Sorry to throw a damper on the potential for improvement with a fixed distributor. The car still needs drivin' though. Get the gas flowing through the injectors, hot oil washing the crud off the valve stems, warm up the valve seals so they seal better...it will add up to smooth things out. Climb some hills, put some load on it, rev it up. Bill Clinton was long ago.

Yup it was a 4th gear-only deal. My car is set up like a California car for whatever reason. Theoretically I should be full-time vacuum advanced but I'm unfortunately full-time vacuum retarded. :ogre:

On the first sunny day after I throw my tuneup parts on, I'll drive it like that.

Howeverrrrrrrr.....I have my taillight assemblies removed and disassembled at the moment so actually I'm also waiting on getting things like taillights and license plates on the car. The "chrome" around the taillights was coming off and looking bad so I took everything off the back of the car. I found two petrified hornets nests behind the rear panels between the taillights that were anywhere from 15-35 years old. And probably took a couple pounds of dirt off my car in the process of cleaning everything.

[Jennys280Z]

I wanted to acknowledge what Zed said, that the projection of the tip is a function of the heat rating of the plug. The pictures show the difference between a heat rating of 5 (old plugs) and a 7 (new plugs).

I agree it doesn't advance the timing of the spark per se, but the longer nose would combust a bit "ahead" of the air/fuel, not in time but in displacement.

And another thought I'm having is that if you're right about just driving it, then maybe I'd be better off with my old hotter plugs back in.

[FastWoman]

Wow, I've only been away from my computer for a day (increasingly dealing with real estate issues), and you're bubbling over with frustration! I have to agree, though, that your car needs to stretch its legs a bit. That will improve the condition/running of the engine, and perhaps more importantly, that will give its mechanic the emotional re-charge she needs to continue forward!

I don't know whether to nod in agreement with Eric about your not having a lean-running condition. It's true that you're exhaust is popping. However, you might get exhaust popping if you have a bad misfiring situation and are blowing raw gas and excess air through the exhaust from a non-working cylinder. I once blew out one of the cats on an Expedition that way. Otherwise the mixture was spot-on. You might also be getting carbon on your plugs from partial burns. My plugs looked a lot like yours even when I was running extraordinarily lean. So I wouldn't close the book on a possible lean issue YET. I do think the best way to determine the ideal fuel mix is to tweak the AFM vane with your finger and see which direction of tweak, if any, makes the engine run faster and with more vacuum. It's true that that tells you more about idle than running under load, but it's quite difficult to take measurements under load, unless you're on a dyno. Still, this approach got me quite close on my car. FAIW, my idle mix screw was/is adjusted 5 full turns open from full-closed. Anyway, this approach will tell you the proper mix irrespective of engine misfiring.

So your afterfiring could be caused by a rich mix, but it could also be caused by an ignition failure. If I were you, I'd get all the ignition issues squared away first, and then worry about the mix. That includes the distributor, and proper timing, of course. I'd table replacing the coil or ignition module for now. I replaced both and saw some (but limited) improvement, but in truth, the problem was an extremely lean mix, and if I had straightened out the mix, the coil and IM might have been good enough.

After you get the distributor squared away, let's re-tweak the AFM vane (with your finger) to see what that tells us. IMO that's far better than pulling a hose or jumpering a couple of electrical contacts, because it's easy to overshoot with those methods. When your mix is right, the RPMs will only decrease when you nudge the AFM vane to either direction.

Anyway, go for a drive! Enjoy your Z! Recharge, refresh, and return. Stay on track, and all will eventually be well.

Edited January 25, 2011 by FastWoman

[cozye]

Cozye, it's big news that I'm not running lean....then why the 17in of vacuum and 1100RPM idle at full enrichment?

I guess this invokes Zed Head's larger point, which is, I was obsessing over a vacuum gauge

Well it could be that it's reading 17" because you are at 1100 rpm. At 2000 rpm it's probably reading 20 or 21. If you are going to read the vacuum gauge, do so at 800 rpm (idle) only after the car is completely hot and the air bypass (fast idle) is definitely closed.

By the look of one of those plugs I'm guessing you have a slight miss. All of this could boil down to the cap/rotor and plugs. The fix could be that easy.

valve adjust 16,000 miles ago is new information (or I missed it). Have you done a compression test that I forgot about ? If you have, where the cylinders all close to each other in the results ? If so, you can skip the valve adjust for now.

Get those tune up parts on there and drive it. I agree with Sarah.

Edited January 25, 2011 by cozye

[Jennys280Z]

Well it could be that it's reading 17" because you are at 1100 rpm. At 2000 rpm it's probably reading 20 or 21. If you are going to read the vacuum gauge, do so at 800 rpm (idle) only after the car is completely hot and the air bypass (fast idle) is definitely closed.

By the look of one of those plugs I'm guessing you have a slight miss. All of this could boil down to the cap/rotor and plugs. The fix could be that easy.

valve adjust 16,000 miles ago is new information (or I missed it). Have you done a compression test that I forgot about ? If you have, where the cylinders all close to each other in the results ? If so, you can skip the valve adjust for now.

Get those tune up parts on there and drive it. I agree with Sarah.

cozye that's great advice about the vacuum. But then...putting vacuum aside, does my motor merely idling so much faster indicate that I was too lean? All we did with the TPS was enrichen the mix. It seems I contributed enormously to improving my idle but doing nothing for actual driving performance, huh?

I think I mentioned the valves in the long post about the history of the car. Maybe I'm wrong. But yup if I can trust the repair invoice it was done at at 125,000+ miles. So then I'll do the compression test first.

Also I did just what you did, cozye. I got my tune up parts at MSA and my clutch stuff at RockAuto.

[Jennys280Z]

Wow, I've only been away from my computer for a day (increasingly dealing with real estate issues), and you're bubbling over with frustration! I have to agree, though, that your car needs to stretch its legs a bit. That will improve the condition/running of the engine, and perhaps more importantly, that will give its mechanic the emotional re-charge she needs to continue forward!

I don't know whether to nod in agreement with Eric about your not having a lean-running condition. It's true that you're exhaust is popping. However, you might get exhaust popping if you have a bad misfiring situation and are blowing raw gas and excess air through the exhaust from a non-working cylinder. I once blew out one of the cats on an Expedition that way. Otherwise the mixture was spot-on. You might also be getting carbon on your plugs from partial burns. My plugs looked a lot like yours even when I was running extraordinarily lean. So I wouldn't close the book on a possible lean issue YET. I do think the best way to determine the ideal fuel mix is to tweak the AFM vane with your finger and see which direction of tweak, if any, makes the engine run faster and with more vacuum. It's true that that tells you more about idle than running under load, but it's quite difficult to take measurements under load, unless you're on a dyno. Still, this approach got me quite close on my car. FAIW, my idle mix screw was/is adjusted 5 full turns open from full-closed. Anyway, this approach will tell you the proper mix irrespective of engine misfiring.

So your afterfiring could be caused by a rich mix, but it could also be caused by an ignition failure. If I were you, I'd get all the ignition issues squared away first, and then worry about the mix. That includes the distributor, and proper timing, of course. I'd table replacing the coil or ignition module for now. I replaced both and saw some (but limited) improvement, but in truth, the problem was an extremely lean mix, and if I had straightened out the mix, the coil and IM might have been good enough.

After you get the distributor squared away, let's re-tweak the AFM vane (with your finger) to see what that tells us. IMO that's far better than pulling a hose or jumpering a couple of electrical contacts, because it's easy to overshoot with those methods. When your mix is right, the RPMs will only decrease when you nudge the AFM vane to either direction.

Anyway, go for a drive! Enjoy your Z! Recharge, refresh, and return. Stay on track, and all will eventually be well.

Thank you sweetie I'm glad you're here. I'll follow all of that good advice!

I wish I had a dyno that would be so nice...or at least a boyfriend who worked at a place that had one

The symptom I'm having here (the major one) I don't think is mixture related. Because on driving the car leaner, the way it is now, and both rich and super rich, the "problem" existed across all of these mixture conditions.

I've learned so much here in the past months...so cool!

And oh yeah how is your new car??

[cozye]

cozye that's great advice about the vacuum. But then...putting vacuum aside, does my motor merely idling so much faster indicate that I was too lean? All we did with the TPS was enrichen the mix. It seems I contributed enormously to improving my idle but doing nothing for actual driving performance, huh?

I think I mentioned the valves in the long post about the history of the car. Maybe I'm wrong. But yup if I can trust the repair invoice it was done at at 125,000+ miles. So then I'll do the compression test first.

Also I did just what you did, cozye. I got my tune up parts at MSA and my clutch stuff at RockAuto.

What happens when you adjust the idle to 800 or 850 ? Typically I would say that if you add fuel and it makes it idle faster, you are either running lean or you have a vacuum/intake leak. that doesn't necessarily apply though. Why is your car idling so high? Are you sure that your air regulator is closing all the way after the car is warm? If the air regulator is for sure closing all the way, adjust your idle down to 800-850 after its hot.

Then put the tune up parts on, then go drive it for like 30 minutes at least, maybe even on the highway for a bit. then bring it home and see how it idles, where the timing is, play with the AFM a little if you want. I think you need to start paying attention to how it actually drives, rather than how it idles. tuning for idle can be misleading. Sure you want it to idle good, but get a good idea on how it's running the rest of the time. It's more data. A car can run great, and idle poorly due to a simple problem like a bad distributor cap, or plug wire, or leaking injector for example.

[FastWoman]

FAIW, Eric, I had some trouble adjusting my idle down all the way too, once I corrected a few problems with my intake. I actually had to adjust the throttle stop screw just a bit to give myself the needed adjustment range with the idle screw. It was VERY small adjustment!

Jenny, I'm sure the valves are good at 15k miles. That said, the guy who last adjusted my valves before I bought my Z was a certified Nissan mechanic, and two of the valves were TIGHT, almost to the extent of not allowing complete valve closure. That tells me they weren't properly adjusted in the first place. Just saying!

You may think your problems are more ignition than mix, and they might be; however, a bad mixture problem can feel like an ignition problem, and vice versa. I really beat my head against a wall chasing down ignition problems that maybe didn't exist, before I finally corrected the mixture problem.

Our new car -- It's not registered yet! We're trying to locate some other paperwork we need to take in to the DMV, so as to kill two birds with the same stone. So the little Miata has been sitting there in the garage. It's not been a good time to drive, though, so it's just as well. Besides, we're busy with real estate stuff. No time for having fun.

[Jennys280Z]

Jenny, I'm sure the valves are good at 15k miles. That said, the guy who last adjusted my valves before I bought my Z was a certified Nissan mechanic, and two of the valves were TIGHT, almost to the extent of not allowing complete valve closure. That tells me they weren't properly adjusted in the first place. Just saying!

You may think your problems are more ignition than mix, and they might be; however, a bad mixture problem can feel like an ignition problem, and vice versa. I really beat my head against a wall chasing down ignition problems that maybe didn't exist, before I finally corrected the mixture problem.

Our new car -- It's not registered yet! We're trying to locate some other paperwork we need to take in to the DMV, so as to kill two birds with the same stone. So the little Miata has been sitting there in the garage. It's not been a good time to drive, though, so it's just as well. Besides, we're busy with real estate stuff. No time for having fun.

My mixture probably isn't ideal right now, but I bet that's true for the majority of us (but not you ). A few things I can't get around are how it began running badly in the space of one or two drives. I probably made the mistake of starting it and just idling it in the cold a few times during its downfall. Based on road tests the problem is independent of mixture. It feels like a miss. I'm not experienced enough to use the right adjectives with the word "miss" so I don't know if its regular, irregular, random or what. Would a mixture problem come out of the blue? I was blaming the AFM for a time there, but that passed all the electrical and mechanical tests with flying colors. Whatever it is, it's hit the performance of this car to where it's only 75% what it should be, mixture held constant.

Anyway, focusing just on vacuum for a moment, my car may be getting between a quarter and a half inch less vacuum now than it was before moving the AFM wheel three teeth CCW. Maybe I should have left it alone.

My distributor is becoming toasty toast. It's grimy and gross inside all dirty and sticky looking. No vac advance when my car is likely "tuned" to run full-time vac advance. Just saying...

Do you think a new dist will have more static advance than this one or do I need to adjust something underneath the dist?

I hope you won't stop coming here when you start working on your other car. And yup I'm waiting for a warm day over 50F to go for a fun drive.

Edited January 27, 2011 by Jennys280Z

[Jennys280Z]

What happens when you adjust the idle to 800 or 850 ? Typically I would say that if you add fuel and it makes it idle faster, you are either running lean or you have a vacuum/intake leak. that doesn't necessarily apply though. Why is your car idling so high? Are you sure that your air regulator is closing all the way after the car is warm? If the air regulator is for sure closing all the way, adjust your idle down to 800-850 after its hot.

Then put the tune up parts on, then go drive it for like 30 minutes at least, maybe even on the highway for a bit. then bring it home and see how it idles, where the timing is, play with the AFM a little if you want. I think you need to start paying attention to how it actually drives, rather than how it idles. tuning for idle can be misleading. Sure you want it to idle good, but get a good idea on how it's running the rest of the time. It's more data. A car can run great, and idle poorly due to a simple problem like a bad distributor cap, or plug wire, or leaking injector for example.

I've lowered my idle speed at the screw a few times as I've done work to my car we've talked about on this thread. So my idle is kinda rough but I'm idling right now at 770-830. It was 1100 and pouring dark smoke with the TPS jumped to full enrichment.

Remember I idled my car several times in the cold with a bad thermostat to boot. I wonder what havoc that could have caused that may have resulted in my running problem?

I'll follow your advice in your 2nd paragraph Cozye thank you.

[Zed Head]

Would a mixture problem come out of the blue? I was blaming the AFM for a time there, but that passed all the electrical and mechanical tests with flying colors.

Actually, as I recall, your AFM did not pass the testing with flying colors. But you did some work on it, and put it back on your car and said that it ran great now so the problem was fixed, even though the resistance readings were out-of-spec..

I remember because the resistance numbers on your AFM were the same as mine had been when I got my car, and my engine ran terribly until I got a rebuilt AFM. I remember being surprised that mine tested the same as yours but mine would not run the engine well at part-throttle at all, but yours did.

I also remember because the impression was that I had led you on a wild-goose chase, which I had not intended to do. So your AFM might be working fine, but the test resulkts of it were not "in-spec.", at least at that time.

Just trying to help you keep things clear. You have a lot of things going on. My car ran terribly with a bad AFM but yours might be fine.