I have been fighting this for over a year, and now I must surrender to the Z Gods on this board. I am at a loss of what to do next.

What I am running?

1971 240Z with 71000 miles.

Stock engine with MSA header and 2.5" exhaust

Z therapy rebuilt carbs (about 1000 miles ago installed), ran great when bolted up

Advance distributors Rebuild OEM dizzy recurved to Eurpoean specs (static timing at idle 13 Deg BTDC, at 3000 rpm I am 32 deg BTDC) I do run a vacuum advance on top of that.

New NGK resistor style plugs, but car had problem with new NGK non resistor plugs

New Pertonix installed in dizzy, and new flamethrower coil.

I have rebalanced the carbs and I am currently almost at 3 turns from the top on the mixture nuts

All the fuel lines and filters are new

I installed a new fuel pump and I am getting 3.6 to 4 psi at all rpm ranges

Running Taylor pro 8mm wires.

Car starts immediately even after sitting for a whle and has a rock solid idle at 750 rpm. Engine barely moves. Valves are quiet and in good adjustment as it was dodone 2000 miles ago.

Car has super quick throttle response and is a complete joy to drive normally with ZERO issues ever. However, once you get it up to the 4500 rpm range I start getting pops from the exhaust. I get these whether I ease slowly up to 4500 rpm or I nail it, something always happens at 4500 rpm and up. Seems I have had this issue since I owned the car in some form of severity or another. When I first bought the car, it would simply NOT rev past 4500 rpm at all. Now I can get it to rev to 6500 rpm thanks to the Z therapy carbs, but now it is popping pretty bad on the top end.

My most recent change was to put the timing to where it is right now (13 BTDC) and it definitely improved, as this is the setting the distributer guru told me to run. Go figure. Now the car pops in first, and a little in second, but will pull to 6500 rpm rather smoothly in second. I do not have a road safe enough to get it to 5000 in third near my house.

I keep thinking this is fuel related and not timing related. I am not sure if my M/R is right or not. Popping through the exhaust means to me it is too rich.

Before I adjust the carbs again, I would love to get more opinions on this.

Edited November 21, 201113 yr by Zedyone_kenobi

.......at this point it is too difficult to review all 15 pages to see if this has been tested but have you put in all new spark plugs to see if that could have the fix you are looking for. I got my Z out today - had a little stumble going on - changed the plugs and running and gunning great now.................just a thought.

BGM, You are the man!

I don't know how you find this stuff, but you are the Jedi Master of topic appropriate pics.

Yup, 2 new sets of NGK's.

UPDATE:

I performed several vacuum related tests yesterday. More data, for those who are still with me.

I Tee'ed into my manifold vacuum near the power brake tap. Warmed up I am solid steady at 17 in. I fiddled with the timing a bit so that may be why I am off from the 19 or 20 I had last time. When I drove the car, I saw nothing unusual with the vacuum readings as I moved along. Very steady all of the time, and responding to my inputs form the throttle as you would expect. When the popping happened, the vacuum was steady and the needle did not jump around.

So I pulled it into the garage and sprayed carb cleaner around the gaskets, and no change was noticed in RPM

Lastly I moved the SM needles flush with the groove in the bottom of the piston (although I know the FSM says otherwise) This make the car super rich at idle and it really struggled to hold a 500 rpm idle with so much fuel going through her. But the problem still happened.

the LAST THING TO try with the carbs is to weigh down the pistons more so it is harder for them to rise as suggested before.

I may try to hook up my GoPro camera under the hood with a flash light to see what the pistons are doing if I get sporty.

I truly think my car is not getting the fuel it needs. THis is the reason: I got her up to 5000 rpm and she was popping like mad. I held her there at 5000 rpm still popping and with the ARF at the mid to high 16's. I yanked down the choke all the way this time and she went to 12.5 and took off. THat would seem to indicate that the needles are NOT rising ENOUGH!

I have a sneaky suspicion that this is a far more mechanical issue than just carbs. But I am about ready to crush this problem with my wallet and a phone call to Datsun Spirit!!!!

This tells me that the needle profile is not correct for your engine. I'd try different needles. If the piston rises more, you will make the mixture leaner because of a diminished vacuum signal. Run a little experiment with stock needles, placing the shoulder at varying heights and recording what happens.

I can try that as well. It will have to wait for my next day off...Maybe Saturday.

Leak down test is on hold until I can get a new air compressor. Mine went kaput

Lastly I moved the SM needles flush with the groove in the bottom of the piston (although I know the FSM says otherwise) This make the car super rich at idle and it really struggled to hold a 500 rpm idle with so much fuel going through her. But the problem still happened.

I would think that with the needles moved flush with the bottom of the piston that you would correspondingly return the seats to just two turns. As your needle shoulders were beyond the base of the piston, your seats were corresponding lower, 3.5 turns out if I remember correctly? These move hand in hand to provide the correct fuel ratio at idle.

the LAST THING TO try with the carbs is to weigh down the pistons more so it is harder for them to rise as suggested before.

This seems to be the opposite of what you really want to do.

I truly think my car is not getting the fuel it needs. THis is the reason: I got her up to 5000 rpm and she was popping like mad. I held her there at 5000 rpm still popping and with the ARF at the mid to high 16's. I yanked down the choke all the way this time and she went to 12.5 and took off. THat would seem to indicate that the needles are NOT rising ENOUGH!

Given this last bit of information, you do not have a fuel delivery problem; you have a fuel ratio problem. I will propose the KD needles once again. As you can see in the pic, they allow more fuel at the higher RPM's, more than the SM's which don't seem to be doing the trick. KD needles would be a pretty cheap item to check.

Best of luck,

Rich

PS: This is easily my favorite thread on this site.

Edited March 15, 201212 yr by motorman7

Adding weight to the suction pistons should enrich the mixture. It may seem counter-intuitive, but it should enrich the because the velocity through the venturi will be higher, providing a stronger vacuum signal to pull more fuel despite have a slightly smaller opening between the jet and needle. SU offers different stiffness springs for the suction piston for this same purpose. The mixture will be affected at all throttle openings except idle when the pistons all the way down. It seems that this has become somewhat unnecessary now as Zedyone has accomplished the same thing by pulling the choke out while driving.

I agree that the KD needle could solve the problem. The stock needles could be modified as an alternate. The trick will be determining the correct point on the needle to reduce the diameter, and by how much.

Adding weight to the suction pistons should enrich the mixture. It may seem counter-intuitive, but it should enrich the because the velocity through the venturi will be higher, providing a stronger vacuum signal to pull more fuel despite have a slightly smaller opening between the jet and needle.

That statement helps a lot. Was trying to figure out Leon's statement (which is your same point) and this clarifies it.

Thanks! (such the educational thread )

I am willing to try the KD NEEDLES but where on earth would you get them?

Another question is if SM needles are rich enough to feed an L28 why on earth can't my baby L24 get enough gas

As with most complex systems with many variables it may just be a tolerance build up. Maybe not.

Motorman it is good to here you enjoy this thread. I often wonder if people get tired of me and this problem.

I will look for some KD needles

I tried to find stiffer SU springs

Ztherapy said there is only 1 spring used for the Z

He did not have any to sell me.

I was on the hunt for springs but found none. Mine look like cad plated springs, so I have no idea what they are.

I would order a complete set of springs if I knew where to order

if SM needles are rich enough to feed an L28 why on earth can't my baby L24 get enough gas?

I'm stuck at the same question and I'm not endorsing the KD needles thing. ZT put your carbs on their car and they said they were fine. Why would your car need so much more fuel than theirs? Also, about your piston springs. Why would you, just you, and no other Z owner need to run stiffer springs in their round tops? Why? Why? Why? I'm just not buying it.

So what about your results with grabbing a handful of choke while it's popping and having the problem go away? The results clearly indicate that your car runs better with more fuel being supplied, but does that absolutely guarantee that the root problem is in the fuel system? By that, I mean... Is it conceivable that the problem is ignition on the hairy edge of not working and the reason the problem goes away with a handful of choke is that the richer mixture burns so much easier than a leaner one? Ignition isn't my stong suit...

Leon? Conceivable?