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

[FastWoman]

Jenny, I'm no expert on timing, but I can tell you that my engine idles slower and less efficiently (hence lower vacuum) at the 1978 spec of 10 deg than at maybe 13 deg. I understand that's typical. However, I can also say with some certainty that the degree to which my engine bogged down at the spec setting was much greater when I was having lean running problems than after I fixed those problems. I could have easily set my advance to 10 deg, but I did give it a few extra degrees because the idle seemed happier and because I didn't feel a few more degrees would hurt anything.

I wish I could tell you something intelligent about plugs; however, it seems you already know much more about them than I do. I just run the original spec plugs, and they seem to work well.

Edited January 6, 2011 by FastWoman

[FastWoman]

I wouldn't worry too much about vacuum line diameters, as long as the hoses are secure and fit reasonably well.

Yogurt cup test: You blow, and then you hold the pressure with your mouth (not inflating further) to see how long it takes for the pressure to dissipate. Anyway it sounds like you had a successful test. If you reached a bottleneck where you couldn't blow (much) more air into the thing, it must have been pretty tight.

If you can paint your booster without voiding the warranty, I'd do it. The reman paint is usually of a rather poor quality. My reman alternator looked like your booster about a year ago and is starting to rust a bit now.

Adjusting the AFM: I wouldn't say the mechanic did anything "wrong" when he adjusted the AFM spring to alter the mixture. That seems to be an accepted approach (e.g. Atlantic Z). However, I disagree with the accepted approach. I tried it and found that it resulted in a flap tension that was way, way too loose. The potentiometer wiper would be maybe 1 cm out on the carbon trace at idle, and the flap would logically peg out at too low a flow rate. The AFM in this state would be mechanically/aerodynamically "wrong." It makes much more sense that the wiper should be very near the low end of the scale at idle, which was the case with the spring tension calibrated per the beer can method (at least after I straightened out the mix through other means).

Keeping the AFM to factory mechanical specs keeps the response properties as close as possible to design specs, and when all the other sensors are also within normal operating parameters, the final element that doesn't work is the ECU itself. Mine was delivering much too short a pulse width, as was Eric's. The solution is to proportionally lengthen the pulse across all air flow rates, temperatures, etc. Changing the resistance of the coolant temp sensor circuit seems to be the best way to execute that. It would be even better if the ECU had a calibration potentiometer just for that purpose (apart from the temp circuit), but it doesn't. Bosch/Hitachi didn't want us playing with that stuff and probably didn't really design our systems to be running properly 35 years down the road.

Anyway this mod seems to work pretty well. When the engine comes up to 180F, the running parameters are all pretty well established and uniform, and the tunings that Eric and I have done are optimized for that condition. I've found the cold enrichment isn't all that bad on my car. I did have my engine die today right after I started it, but it was rather cold and hadn't been operated in a couple of weeks (snow, salt, sand). Another turn of the key and it ran like a top. This happened one other time since I had straightened out my mixture. Other than that, each and every start has been immediate and strong, and my cold idle has been excellent. So this approach seems to compensate for engine temperature pretty reasonably, even with the extreme adjustment I've made. (My "hot" temp circuit resistance is telling the ECU that the engine temp is about 50F, as I recall.)

Anyway, I just have a different approach than the AFM spring adjusting folks -- an approach shared by others as well. But I think this approach is better.

Well, I'll be away from my computer until Sunday or Monday. It would appear I'm taking a train to go fetch a little red '94 Miata. Hopefully it's all it would seem to be from a distance. Wish me luck.

Edited January 6, 2011 by FastWoman

[Jennys280Z]

A three minute reply (when I started)! Must be cold and rainy in Oregon (where I am).

I thought that you said you were buying a new Autozone distributor back in Post #71? So I'm just suggesting being conservative until you know more about the advance curves on the new one. I don't know if you run Premium or Regular either (I'm assuming Premium?). If you give it too much static advance and you have aggressive advance curves and you ran Regular, you could get some knocking at part-throttle or wide open.

There were a wide variety of advance curves used for the different years of Z cars, and I've noticed the auto parts stores going with "close enough" for some parts (they call it SKU - Stock Keeping Unit - reduction in the business world, it saves money). Just look at the "compatibility" option on OReilly's web site.

I'm running 17 degrees on mine but I did get a bad tank of gas once and got a little bit of knocking at part throttle. But I also have a 78 mechanical curve with a 76 vacuum (Frankendistributor).

14 static would give you 34 total on a 1976 curve, so you're probably fine there. I can't tell if you're trying to get back to stock or if you're up for modifying until it runs the way you want it to. Either way is good, FW and cozye can get you there.

Edit - I actually started this before your last post...

Thanks for the info. here. Some very interesting thinking/ideas you've considered that I hadn't thought of. Yes I'm running half a tank of 93 octane gas with a can of fuel injector cleaner in it at the moment. No knocking or pinging at 14 that I could tell. From what I remember I might be closer to 13.5 but I rounded up. Still I could eyeball it back to 12.5 easy enough and keep it there. I like where it is a little better than how it ran at 10 but I do have plans to replace my distributor per the post # you referenced. I didn't know at the time if I was going to choose Autozone.

I want to meet or exceed stock performance. I'll sacrifice a little idle for a smooth power curve up through the RPM ranges. I'd like a quick, responsive car, though I'm not that aggressive of a driver and not going to race it. My motor's run very strong before, so I suspect it can do it again as it's had lots of hibernation and little exercise since 2001. (Yes I acid-cleaned the fuel tank, blew out the fuel lines and changed the fuel filter before starting it).

I see new performance distributors cost $500 and up so I'm not particularly interested in that. I might even keep the distributor I've got. If vacuum advance doesn't matter past 1500 RPMs then maybe I won't even sweat the fact that I don't any

Does it the static advance plus 20 equal the maximum total advance from my distributor, or is it not a static value of 20 plus?

Edited January 7, 2011 by Jennys280Z

[Jennys280Z]

Jenny, I'm no expert on timing, but I can tell you that my engine idles slower and less efficiently (hence lower vacuum) at the 1978 spec of 10 deg than at maybe 13 deg. I understand that's typical. However, I can also say with some certainty that the degree to which my engine bogged down at the spec setting was much greater when I was having lean running problems than after I fixed those problems. I could have easily set my advance to 10 deg, but I did give it a few extra degrees because the idle seemed happier and because I didn't feel a few more degrees would hurt anything.

I wish I could tell you something intelligent about plugs; however, it seems you already know much more about them than I do. I just run the original spec plugs, and they seem to work well.

Well I don't know as much about plugs as you do, but only know a few specific details about the plugs I'm using. My idle seemed happier at 13.5-14 as well. Revs sounded healthier than before too, and my vacuum needle was going deep into the green after those revs...I remember before I changed my thermostat I couldn't get to the high upper values like that at all.

I might eyeball it to the midway point between 10 and 15 and call it good for now.

[Jennys280Z]

I wouldn't worry too much about vacuum line diameters, as long as the hoses are secure and fit reasonably well.

Yogurt cup test: You blow, and then you hold the pressure with your mouth (not inflating further) to see how long it takes for the pressure to dissipate. Anyway it sounds like you had a successful test. If you reached a bottleneck where you couldn't blow (much) more air into the thing, it must have been pretty tight.

If you can paint your booster without voiding the warranty, I'd do it. The reman paint is usually of a rather poor quality. My reman alternator looked like your booster about a year ago and is starting to rust a bit now.

Adjusting the AFM: I wouldn't say the mechanic did anything "wrong" when he adjusted the AFM spring to alter the mixture. That seems to be an accepted approach (e.g. Atlantic Z). However, I disagree with the accepted approach. I tried it and found that it resulted in a flap tension that was way, way too loose. The potentiometer wiper would be maybe 1 cm out on the carbon trace at idle, and the flap would logically peg out at too low a flow rate. The AFM in this state would be mechanically/aerodynamically "wrong." It makes much more sense that the wiper should be very near the low end of the scale at idle, which was the case with the spring tension calibrated per the beer can method (at least after I straightened out the mix through other means).

Keeping the AFM to factory mechanical specs keeps the response properties as close as possible to design specs, and when all the other sensors are also within normal operating parameters, the final element that doesn't work is the ECU itself. Mine was delivering much too short a pulse width, as was Eric's. The solution is to proportionally lengthen the pulse across all air flow rates, temperatures, etc. Changing the resistance of the coolant temp sensor circuit seems to be the best way to execute that. It would be even better if the ECU had a calibration potentiometer just for that purpose (apart from the temp circuit), but it doesn't. Bosch/Hitachi didn't want us playing with that stuff and probably didn't really design our systems to be running properly 35 years down the road.

Anyway this mod seems to work pretty well. When the engine comes up to 180F, the running parameters are all pretty well established and uniform, and the tunings that Eric and I have done are optimized for that condition. I've found the cold enrichment isn't all that bad on my car. I did have my engine die today right after I started it, but it was rather cold and hadn't been operated in a couple of weeks (snow, salt, sand). Another turn of the key and it ran like a top. This happened one other time since I had straightened out my mixture. Other than that, each and every start has been immediate and strong, and my cold idle has been excellent. So this approach seems to compensate for engine temperature pretty reasonably, even with the extreme adjustment I've made. (My "hot" temp circuit resistance is telling the ECU that the engine temp is about 50F, as I recall.)

Anyway, I just have a different approach than the AFM spring adjusting folks -- an approach shared by others as well. But I think this approach is better.

Well, I'll be away from my computer until Sunday or Monday. It would appear I'm taking a train to go fetch a little red '94 Miata. Hopefully it's all it would seem to be from a distance. Wish me luck.

Good luck hunni!!! Wow another super cute and fun car. I like Miatas and Minis a lot. Good luck on your trip. Let me know how everything turns out.

I asked parts geek this question three times, the 3rd time was over an email and they ignored my question. So be it then, I masked the brake booster and just shot it with a thick coat of primer and it's drying now. It looks nice in this dark grey but I do have some nice looking satin black rustoleum paint that will look closer to the stock look.

I was reading yall's thread "Purs like a kitten" a few times now and I still haven't gotten all the way through it yet. It's like a treasure trove of information though. Just hearing you guys pass your thinking back and forth is really informative whether I actually do what y'all are doing or not.

[Jennys280Z]

I decided not to mess with the idle mixture screw on my AFM. It looks possible someone was in that messing with it too based on how the plug looks like it might have been off before. But, not knowing more about it, i'll leave it alone for now. But if that's something that (if molested) can be causing my cold idle problem, it's something that looks easily accessible even with the AFM installed so I can always look at it later if I want to.

[Jennys280Z]

Does O'Reilly's have a different advance than Autozone? This is an A1 Cardone remanufactured distributor at Autozone btw. It seems like for many of these parts, Cardone is the only game in town for remanufactured parts.

The A1 Cardone AFM for my Z is close to $500. Yikes.

[FastWoman]

I'm still here, Jenny -- all packed for the Amtrak now. Grand adventure!

If I were you, I'd replace the distributor, even if it might not be quite the right vacuum advance. The reason is that when these things go bad, the breaker plate can freeze and stick, so that you get a random/irratic timing. Even better, find the right distributor, per Zed's advice. I have no idea about O'Reilly parts, as we don't have that chain here.

Those refurb AFMs *are* high, aren't they! I see no reason not to buy used units and refurb them yourself. I doubt they do anything more than clean them up and adjust spring tension. They might even use beer cans full of water to do it!

[Jennys280Z]

I'm still here, Jenny -- all packed for the Amtrak now. Grand adventure!

If I were you, I'd replace the distributor, even if it might not be quite the right vacuum advance. The reason is that when these things go bad, the breaker plate can freeze and stick, so that you get a random/irratic timing. Even better, find the right distributor, per Zed's advice. I have no idea about O'Reilly parts, as we don't have that chain here.

Those refurb AFMs *are* high, aren't they! I see no reason not to buy used units and refurb them yourself. I doubt they do anything more than clean them up and adjust spring tension. They might even use beer cans full of water to do it!

Awww have some fun while you're traveling. Don't let all that red eye candy make you too impulsive in your purchase. A car is more than its paint but you know that better than I.

Now that I've printed everything about AFMs off of AtlanticZ I feel like I understand them so much better than before. Oh, like the two black wires to the thermal sensor...the ones right in front of my nose

I got 2.109 kOhms on my original AFM and 2.385 kOhms on the one I bought. The article on AtlanticZ reported a resistance closer to 2.5kOhms if I remember right.

I'll do whatever Zed Head says, then. You know I'm easy like that. NOT I hadn't even thought about the curves on these distributors. But other than A1 Cardone, I don't know of any other distributor that can be had for these cars for under $150. If I could pay $50 more for something way better, I would.

[Jennys280Z]

AFM is back on the car. Yaay.

I have to say that was harder than taking it off due to having to put that metal clip on the connector backwards. *shakes fist* I bent the end of one of my Gs on that G-clip trying to get it on, but whatever, it's on there at least. Whoever designed the AFM connector and made me put that metal clip on that connector the way I had to do it - is just evil. GRRRR

Brake booster is primed and drying. I'll apply some Satin black paint in an hour, for a topcoat to match the original and with the other black parts under the hood. Hopefully will have the brake booster installed by tomorrow night. And I hope I only have to bleed the system at the master cylinder, and I will be so happy to have great brakes again.

[Zed Head]

I'm no expert on timing curves. I just know that Datsun/Nissan tried a lot of different things over the years, probably trying to meet emissions standards and keep as much performance as they could. And a lot of those options aren't available from the auto parts stores anymore. They're not even available from the Z car focused shops either.

Before you buy a new distributor you might try taking it apart and seeing if the plastic bearing race is broken. If it's not, it is fairly easy to take the breaker plate apart, clean it up, regrease the ball bearings and put it back together. There is a thin metal circlip that holds the whole stack of parts on the breaker plate. If the bearing race is broken (you'll see a crack in it and the ball bearings will be trying to squeeze out from under the metal plate) then a new one is probably worth it. Vacuum advance will give good response at part throttle operation and better gas mileage.

If you decide to try it, take pictures or draw a diagram of what it looked like when you started! It's easy to get things swapped around and there is no real logic to how the plates and pieces go together.

Worst case, you end up with a pile of parts you can't get back together and you buy the new distributor you were going to buy anyway.

I would guess that the Cardone distributors have conservative curves just to avoid problems for their customers.

[Jennys280Z]

Hmmm...any other options? 280ZX distributor?