Barefootdan's 280z Build

[Patcon]

IIRC there are two pieces of metal that spin closely to each other. If the gap between them isn't clean or has corrosion then it will read high across the range

[Yarb]

Are you still running the same trans & diff combination or has something changed?

[Barefootdan]

Yep, its still the 4 speed and original diff from when I got it. Its possible the previous owner swapped something out as I don't think it was the original motor to begin with. 

Yesterday I softened up the suspension further and took it out for another cruise. I'm learning to trust it more and more each time.

It is definitely a conversation piece! First time getting gas under its own power and already had a talk with a gentlemen who owned one in his college days. 

 

[Yarb]

Double the speed seems more than possibly having the incorrect speedo gear. Any chance the speedo itself has gone south on you?

[Barefootdan]

I picked up an AEM wideband to help with the tuning of the Mikuni's. Before I start blindly doing my own process, curious to see what the order of operations should be. My thought is idle, cruising, then WOT pulls. Thoughts?

[inline6]

Do nothing until you set, and confirm that you have set the fuel level in the float chambers properly.  Any changes to float level alter all the jets that you land on using your wide band 02.  If you have incorrect fuel level, and you correct it after your tuning with the wideband 02, you will surely have to tune for jetting again.  

Mikunis are very sensitive to fuel pressure also.  Look up the recommended value range in psi.  I believe it is 3.5 psi.  Use a quality fuel pressure regulator to ensure steady and regulated pressure.

If I recall correctly, idle and cruise are going to be running off of the same jets - these are the pilot jets.  Setting idle mixture screws appropriately is critical to the selection of those pilot jets.  Adjusting the amount that the idle mixture screws are turned in or out will cause differences with any given pilot jets.  Set them at 1 and a 1/4 out... to  1 and 1/2 out from full in.  Do not go further out than that.  And leave them alone after doing so.  Later, you can tweak them between that 1.25 to 1.5 to fine tune AFR at idle, but set that thought aside for now.  

Also, be very careful when bottoming them out.  Note (write down or draw) screw head slot alignment... or "clocking", for each screw on each bore.  That way, you won't have to repeatedly bottom them out in the carburetors because you forgot where you were while turning them.  The idle adjustment screws are fragile, and easily damaged.  The screws should have perfectly shaped tapers, not any wear marks or indentations in them where the taper seats against the carburetor bodies.  

Next, adjust the throttle stop, idle speed screws.  You need the throttle flaps cracked open enough to get the car to idle.  If you have a lumpy cam, do yourself a favor and set idle to at least 1200 rpm for initial idle and cruise tuning.

Start the car.  Adjust throttle stop screws to get the car to idle at 1200 rpm.  Check air flow across the carbs.  Sync at idle speed by adjusting the throttle stop screws per carb.  I won't go into ignition timing here, but make sure your ignition time is set properly - static and dynamic.

Once the car is idling at the desired rpm and carbs are in sync, swap pilot jets until you get 14 to 15.5 for the air fuel ratio.  You can tweak idle mixture screws between the 1.25 and 1.5 range and watch what that does to AFR at idle.  

From here the car should be drivable. 

Next, you need to determine appropriate mains and air correction jets as well as accelerator pump jets.  When determining all of those, keep in mind they all kind of are interrelated.  If you are super fat/rich on mains, the accel pump will be impacted as well.  Messing with air correction before getting the mains close is a waste of time.  Same with the accel pump jets.  Oh, and use the middle hole on the pump lever rods.  Don't mess with the other holes in the rod.

I recommend avoiding wide open throttle for a while.  Instead, I believe the Mikuni manual indicates that the main circuits don't come into play until throttle openings exceed 20 percent.  I recommend backing out the throttle stop (on the floor of the car) to give you max 80% throttle - maybe try to fab something to give you near 60 percent.  When cruising, ease to your 60 percent throttle (pedal against your floor stop modification - allowing you to hit a consistent throttle position repeatedly for tuning) and record runs (if your wide band has that ability) for examining after the fact.  Get 60 percent throttle and record or observe AFR from around 3k through 5k rev range (at 60% throttle).  You are looking for steady state of 12.8 to 13 or so AFR while the main circuit is engaged (60 percent throttle should guarantee that).  

After you get the target AFR for 60 percent throttle, extend the rev range to whatever you max engine revs are, but use the same throttle opening percent.  If AFR drops (richer) as highest revs are approached, go bigger on air correctors.  Again, I find that the mains and air correctors are interrelated.  For example, if you change the air correctors a large amount, it may well change your "baseline" you had with the mains that were achieving the target AFR range.  Keep this in mind if you find you are having to make large changes to air correction to get where you need to be AFR-wise at the last 1000 rpm of the range.  

Do not drill Mikuni jets.  They are sized due to their flow.  It is possible that the orifices will differ in size for a specified jet number.  Do not alter.  Again, they are sized for flow, so if the orifice is bigger on one of same numbered jet, it will flow the same as o,ne of the same number but with a smaller orifice.

[Barefootdan]

Thanks @inline6 This is super helpful. In post #295 you can see the approach I took which is a bit different. First note being the idle mixture screws were turned out 2-2.5 turns which is double your recommendation. I will need to see where I landed at and if I need to change my pilots. Is there a reason why 2 turns out is too much?

I was able to get my AFR gauge temporarily installed enough to check my idle AFR. I'm glad I picked this up because I was LEAN. At idle I was in the 17s. It makes sense now, but I assumed that I would be rich just because I smelt fuel. But looking back now, running lean can have poor combustion where not all the fuel is vaporized. Wasn't able to drive the car since its been raining all day so that'll be for another day. In the meantime I'll check my idle mixture screws and adjust my pilots if needed.

[inline6]

On 5/12/2023 at 10:20 AM, Barefootdan said:

First note being the idle mixture screws were turned out 2-2.5 turns which is double your recommendation. I will need to see where I landed at and if I need to change my pilots. Is there a reason why 2 turns out is too much?

I will tell you what I think based on my reading and re-reading of the Mikuni service manual, and based on some comments from others when I was doing Mikuni jetting with a wide band just over 10 years ago.  Essentially, backing the idle mixture screws (pilot screws) out too far causes the pilot circuit to cease to provide fuel and air mixture too early.  

First, know that the pilot circuit is a separate circuit from the main circuit.  And the pilot circuit supplies fuel not only for the pilot outlet (tiny hole just below the pilot screw... BUT ALSO the bypass (group of outlet holes just at, and on the other side of the edge of the brass throttle plate).  According the service manual, when the throttle plate is "shut" (held open only by the throttle stop screw), manifold vacuum pulls fuel and air mixture (per the pilot jet and air jet) into the carburetor bore through the pilot outlet hole.  The car idles from this mixture. 

The bypass holes are on the other side of the throttle plate.  With the throttle plate in basically a closed position, the bypass holes are seeing basically atmospheric pressure (way less than the pilot outlet which is seeing negative pressure from engine operation at idle).  In fact, the Mikuni manual says that with the throttle plate in this position, the bypass holes actually supply air into pilot circuit!  So, air from the pilot air jets as well as air from the bypass holes is entering into the pilot circuit, mixing with the fuel that is there also.

As the accelerator is actuated slowly, the throttle plate opens, and the first of the bypass holes now also becomes like the pilot outlet hole on the manifold side of the throttle plate.  Negative manifold pressure acts on that first bypass hole.  At that point the air that was traveling into the bypass and mixing with the fuel and air mixture in the pilot circuit stops flowing into the circuit.  Instead, the negative pressure pulling on the first bypass hole, now pulls fuel and air mixture into the carburetor bore just like it has been happening at the pilot outlet all along.  

More throttle opening and another bypass hole adds more fuel and air mixture, as negative pressure pulls on that one as well.  With more throttle opening, yet another bypass hole comes into play.  At 20% throttle opening, the negative pressure has dissipated, and there is no longer "pull" on the pilot outlet, nor the bypass holes.  It is here in the throttle opening that the mains are designed to take over.  

Now... 

Clearly, one can set the pilot screws at 2.5 turns out and get the car to idle well with a corresponding pilot jet with throttle plates on the throttle screw stops (barely cracked).  However, when the throttle is opened more, in the 5-20% zone, the bypass holes are uncovered as discussed above.  And evidently, if the pilot screw is pulled back to far from the pilot hole, then the bypass holes do not supply fuel and air mixture as they should.  I think, with the pilot screws backed out too far, the negative pressure being applied from the manifold side of the throttle plate is "not enough" to pull fuel and air mixture through the length of the pilot circuit pipe that spans the distance between the pilot outlet and the bypass holes.  

So, what happens?  Many people have experienced a lean spot as they open the throttle from idle to that 20%.  Running the pilot screws at the specification (per the Mikuni manual: 1.0 turns out for 40 phh and 1 1/8 turns out for 44 phh), allows the negative pressure to pull fuel and air mixture out of those bypass holes to give better drivability up to 20% throttle opening, and therefore, helps to make the transition to the mains more seamless. 

From all of this, my advice is to go a size (or two) higher on the pilot jet and set those pilot screws at 1 and 1/8 out (for the 44 phh).  Remember to set float level properly and get the fuel pressure right first.

 

 

 

 

Edited May 15, 2023 by inline6

[Barefootdan]

Wow its been 5 months since my last update! I have been slow to work on the Z as I have moved to a new home in Phoenix and it has been a very hot summer. Lately I have been able to get some time to work on the interior. 

I was never satisfied with the molded carpet kit I originally installed. Probably user error since it was my first kit but it felt very bulky and didn't lay nice. I eventually reached out to Chester and Herod for a carpet set. The gentleman I spoke to was very nice and had my carpet shipped out the following week. This is a loop pile style that I think fits the Z much better than the cut pile. Fitment was stellar and I had no issues installing and gluing down the individual pieces. While everything was out of the car, I decided to finally address my seats and ordered up a set of Sparco Sprints with mounts and OMP harnesses. I am looking into taking the Z to a NASA track day so this made my seat style an easy choice to meet safety requirements. Overall I am very happy with how the interior turned out. It is a comfortable place to be.

 

One piece I completely forgot to pick up was the door sill covers. I recall my original covers being glued down vinyl. And I played around with the idea of ordering new ones or making my own. But I decided to paint the sills first to see how it looked, then decide on what to do. I am pleased with the satin black look. It blends in just fine and I think the pressed circles provide a nice "race" look as well.

 

Lastly, I was able to pick up a close ratio 5 speed transmission out of an '82 zx from a friend who is doing a small block chevy swap. It was a direct swap and the extra 5th feels great. Rowing through the gears without bogging feels GREAT.

Thats all for now, happy drives ahead!

[Barefootdan]

Finally made time to take the Z to the track. What an exciting day. Made it 30 minutes to the track, four 20 minute sessions of track driving, and 30 minutes of driving home without any issues!! I have a list of new tasks to dial in since the first shakedown, but looking forward to spending more time driving the Z before summer hits! 

[Barefootdan]

Life is always full of surprises. Unfortunately for me, that means the Rotsun is up for sale. Definitely not a restoration build, so please take that into thought. But alot of hard work and help from many members here has lead to a beautifully fun car to drive on the weekends. Feel free to ask any questions! I have posted an ad in the forum's classifieds for pricing details.