Can't get a rich mixture

[Rij Martin]

Alright I just bought some 1971 240z su carbs and installed them on my 1978 280z. I bought them from Holiday Florida (18') and are now on my car in Washington Utah (2792'). I have tried to tune them but I can't get past a lean mixture. I know this because when I lift the piston the engine rpms always go down instead of either soaring up or blipping up for just a moment and coming to a steady idle. I have tried driving it around how it is and it runs smooth up through 2nd gear but when I get to about mid 3rd it bogs and the car will no longer accelerate. (note this is when the car is flood) If I drive with a smooth foot I can get the car up past speeds of 100mph. I have the adjuster nuts on the bottom of the carbs all the way down and from what I have read that is supposed to give you a rich mixture, I have also played around with the mixture screws on the top and still can't achieve a rich mixture. Is there something I am doing wrong and or forgetting to do? Could the change in elevation play a larger role than just turning screws in this? Thanks in advance!

[siteunseen]

When you lift a piston it disables that carb so your only running on 3 cylinders.  Tune it so that it idles the same rpms on both when they're disabled.  I fortunately have a timing light that shows rpm so it's pretty easy to do for me.

Good luck

If I remember right it's around 500 rpms, barely running.

Edited February 20, 2017 by siteunseen

[siteunseen]

You may be running low float levels and the bowls are slow to fill.  Soft foot and you can get to 100mph.

[jonathanrussell]

+1 on what @siteunseen said regarding float bowls. The first most fundamental adjustment to make is to set the floats so that they maintain the fuel level at an appropriate height in the nozzles. There are multiple techniques that have been written about and documented on this site, in fsms, and in books, to set the floats. The ZTherapy SU carb tuning video is a pretty good place to start. There are other techniques though that, in my opinion get you to a place where you have even more accurately adjusted floats: namely setting the floats so that the fuel level hits the top of the fuel nozzles when the mixture adjuster is 10 turns down (involves removing the domes, pistons, so you can see the fuel at the top of the nozzle). Point is, I would suggest investing time into reading and learning how these carbs work if you haven't already. When your floats are adjusted correctly, you end up in the ideal situation where you can turn the mixture screws down between 2 and 2.5 turns and be very close. In addition, at this position you have adjustment range where you can turn down farther and be more rich or turn up higher and be more lean. Once the floats are adjusted, everything else gets easier in my opinion. Note that I didn't say that getting the floats really well adjusted is easy. I can do it now but it took me a long time to really figure out a way that gets it right. Once your floats are adjusted, I find that using two colortunes, one for each side, helps....along with the technique well documented where you lift each piston slightly and note what happens to idle. Also, as Site said, having something that shows you the rpms right in the engine compartment is really helpful.

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[Stanley]

Bigger motor so you need more gas. Use richer (thinner) needles or modify the needles in it now (probably N-27's, but maybe not) by putting them in a drill press and polishing with sandpaper. Go easy at first, test and measure before removing more material.  Or get modified (larger) nozzles from ZTherapy. APT sells needles etc. for British SU's. Maybe SM needles will work, with bigger nozzles. The British needles that work (more or less) in Hitachi SU's are made for bigger nozzles. APT, and also official SU sites have some literature.

At higher altitude mix needs to be leaner, because the air is thinner, especially in hot weather.