Mark Maras

No need to crank it up. Pull the tops and pistons being careful not to damage them. Next, with the choke off, view the fuel level in the nozzle holes. if you can't see the fuel level in the holes, that's OK for now. Next, pull the choke lever all the way back. Next, look at the tops of the nozzles again. The nozzles will be down from the previous, no choke position. You should be able to see the fuel levels now. Verify the fuel levels are similar in both carbs.

We want to be sure the fuel level is BELOW the top of the nozzles. No matter where the mixture screws are set. No fuel above the nozzle tops is the goal. The nozzle top is the part with the little hole in it. Pull the domes, pistons and take a look at that little hole in the center of the nozzle. Now pull the choke on and take another look. You'll see that the nozzle has been pulled down. If you couldn't see the fuel level in the little hole before pulling on the choke, you should be able to see it now. While the choke is on (nozzles pulled down), it's a good time to compare the fuel level in both carbs to see if they're the same.

I want to verify that the fuel in NOT puddling on top of the nozzles at 3.7 turns down. You'll also be able to verify that the float level in both carbs is the same. I still can't figure out if the sputtering when it's cold is caused by a lean or a rich mixture. At first I thought it was getting too much choke but if the the engine runs worse when you ease off the choke a little bit, the mixture is too lean.

No need to readjust yet. I'd just like to know where the fuel level is to use as a reference point and to be sure the fuel level isn't too high and puddling on top of the nozzles. Also, I'd still like to know if easing the choke off a little when the engine is sputtering and cold makes the engine run better or worse?

@jalexquijano When it's cold, sputtering and the choke is all the way on, what happens when you ease the choke off a little bit. I'd expect that the sputtering would go away or get better and the engine would run a little better if the mixture is rich-normal. If the engine wants to die, especially under a load, the mixture is too lean and we need to take a look at your choke cable adjustments. The cables may not be pulling the nozzles all the way down, which is full choke. At some point in time I still want you to pull the domes and pistons so we can verify the fuel height in the nozzles.

Drive it. You'll get to experience wiper lift.

That's going to be the quietest Z on the face of the earth.

@jalexquijano Ditto what Cliff said. Swap #4 plug with any other plug and see if #4 cylinder fouls with a different plug. BTW, I'd like to see a little Never-Seize thread lube on those plug threads.

Could be #6 was tested first. I retest the first cylinder when I'm done and usually find the C. R. has gone up a bit.

It acts as a sound deadener as well as a carpet pad. Necessary? No, but it will get louder in the cabin without it. A lot of owners are replacing it with Dyna_Mat or the equivalent.

Google "Jute matting 240Z", you'll find lots of options.

It depends on the outside and engine temp. I give it full choke to start it when the engine is cold. When the engine starts to run a bit rough and it starts blubbering or stuttering, I EASE the choke off UNTIL the engine sounds better. I adjust the choke to the off position as the engine warms up. If I push the choke off too soon, the engine will almost stall. You can start driving the car anytime after a minute or so to be sure the oil is circulating. Just keep the revs reasonably low. VOE speaking when I say, you can and will spin a rod bearing if you high rev one of these engines when it's cold. Be patient and let the engine warm before you start having fun. A possible help may be to adjust the choke as follows. Full choke at start. 3/4 choke when the temp gauge starts to move. 1/2 choke when the temp gauge is 1/2 way to normal operating temp. 1/4 choke when the temp gauge is 3/4 to operating temp. If the engine starts to run rough, give it a bit less choke. If the engine balks and almost stalls, give it a little more choke. Don't be afraid to experiment with choke settings while you're driving it. You won't wear out the nozzles, I promise. I can't help but wonder if a valve adjustment would even out those compression ratios. Might be something to think about when we're done with the carbs.

A scene like that is what triggers my impulsive side. I wouldn't have been able to stop myself.

@jalexquijano Sure give it a try. Be sure to report back. Inquiring minds, want to know.

@jalexquijano 12 btdc sounds about right. Do you have a timing light? If so, it would be interesting, as well as easy, to advance it a couple of degrees and see if the carb backfire goes away or gets better.

If there was no damage during the rebuild, it should be just a matter of trying all the piston-dome combinations.

Did they slide freely before the rebuild? Use a Sharpie pen on the piston. You might also try leaving the 4 screws slightly loose and gradually tighten them as the piston is being slid up and down. Did you try switching the pistons too?

@jalexquijano Actually, I'm glad to hear that you have to drive it slow until it warms up. It's a good habit to get into. Two questions. Does it backfire thru the carbs at full choke and how hard are you pushing the engine when it backfires?

@jalexquijanoNever heard of that one. Keep in mind that every time the choke is pulled and pushed it lowers and raises the nozzles just like adjusting them with the mixture screws. In my case, 20+ years of pulling the choke on and adjusting the same set of SU's never wore the nozzles out. If he was that concerned about nozzles why didn't he just lube the outside of the nozzles. My guess is, he was afraid that you'd get the mixture set correctly before he did. What other gems of information has he passed on to you. I feel the need to be entertained. How's it running?

I'm sure you meant PUSHING the choke off.

You're right, Fred was cool.

Excellent. The coughing you're describing is a too rich mixture caused by too much choke. You've figured out that the choke can and should be adjusted to suit the conditions, ambient and engine temperatures. While I'm thinking about it, should you ever want to see if the fuel mixture is too lean (when the engine is warmed up) gradually pull the choke while you're SLOWLY accelerating. If you feel the power increase the mixture needs to be a bit richer. Drive it for a few days or until a new question arises. Oh yeah, have you driven by your old mechanic's shop yet?

There are a few things that would cause dark plugs at 2 1/2 turns. Nozzle?s are sticking in the down (choke on) position. You can manually push up the nozzles with the choke off to see if they're sticking. They should be all the way up with the choke off. Choke cable?s mis-adjusted and not allowing the nozzles?s to go all the up. Readjust choke cable clamps at carbs. Float?s adjusted too high. Easy way to tell is remove the tops and pistons. With the choke off, there should be no fuel above the small hole in the nozzle that the needle fits into. The fuel level should be below the hole. I've also seen nozzles stick down because of the little flat bar linkage that attaches to the nozzle on the bottom. If both mating surface on the ends of the linkage aren't exactly parallel to the other mating surfaces, the linkage will bind. It's easy to fix with a slight twist after careful scrutiny.

Earth to Cliff. Earth to Cliff. Come in please.

@jalexquijano "choke on for 10 to 12 mins", did you leave it pulled all the way on until the engine was warmed up (presumably temp at half) or did you gradually push the choke forward as the engine warmed?