SUs from 72 on a 74 260z (how to tune)

[cspatter]

So I've been reading my butt off about tuning these bastards and it seems simple enough. I got the air balanced between the two carbs using the float tool thing but after it warms up a bit I'm idling at about 1100-1200. I set the mixture by raising the pistons one at a time and compensating each carb to make sure the engine did not die (almost dies but not quite). This was the second time (ever) I attempted to balance and set the mixture. I think they are balanced just fine (which was the easy part) but getting the mixture and idle right seems to be perplexing me.

 

The first time I was definitely running it too rich (valves started to stick and I could smell gas). Now I think I may have set it too lean but don't know how to tell. Without getting an O2 sensor or a color tube can anyone give me suggestions on getting the mixture right? The books I've read don't describe how my engine is behaving after making the adjustments they recommend. For instance they say the engine will rev a little when I raise one of my pistons and then settle back down if i have the mixture correct. I've adjusted the mixture through it's entire range and never had that kind of response.

 

I don't get any backfire from the carbs. The only backfire I've gotten is when i run it up and then let off the gas without shifting. I'd like that gone and maybe the two are related.

[MAG58]

Does it backfire or does it just pop through the exhaust? If you just hear that crackling sound as you let it decel in one gear, that's the car running lean. Also, you can adjust the idle mixture which is a little set screw with a spring on the back of the carb. Lean it out until you hear the engine start to run rough, then richen it 1/4-1/3 of a turn. After you get those figured out you can put a uni-syn on the carbs and see if they're balanced. Once you've finally made it to this point, tweaking the main jets will get you somewhere. Though, you may have to re-balance after you get the mains tweaked. I've had it go both ways on mine.

[cspatter]

I'd say it's popping but not sure. I've always had fuel injected cars. It is only happening when letting it decelerate in any gear (is that OK?). The spring you mention; is that the one on the bottom of the carb? That's the one I've been adjusting for mixture (has a pipe running through it with a hose connected). It's running as if it has a cam (don't know if it does though as I just bought it)

 

I just drove it during lunch and before it was completely warmed up it stuck on me at about 3k RPMs for a good couple minutes. Then it went back to normal and ran out great for the 15 minutes of driving I did. I'm getting K&N filters for it today so I hope that will help out over the messed up foam elements that are on it now.

[hughdogz]

Although I'm not an expert on tuning SU's I thought I'd throw in my $0.02 on what I've learned from watching the ZTherapy SU tuning video...

 

1) There are basically two places you can set the mixture. The "gross" setting is actually in the float bowl. Before you start tweaking the fine mixture, you should verify that the float is adjusted and working properly. You do this by removing the lid, and holding it upside-down. There should be a 3/8" space(don't quote me on that) between the bottom of the lid and the top of the float. Basically, the float needs to be parallel and the right distance away from the bottom of the lid. If it isn't, then bend the tab on the float accordingly. Adjusting the float level will adjust the fluid level at the jet nozzle (where the needle is seated).

 

2) Next, the needle in the piston needs it's height set correctly. You do this by removing the dome and the piston, and loosen the set screw so that the needle height can be adjusted. Pull the needle out say 1/4", then gently push down on the pistion so the needle will bottom out in the jet nozzle. Be sure to adjust the fine mixture screw all the way up, so the needle will not bottom out too low. Re-tighten the set screw.

 

3) Adjust the fine mixture screw (bottom of the carb that has the detents, or nubs on it, it looks like a ring) 3 1/2 turns out. This is the base fine mixture setting.

 

As Mag mentioned, you can now set the idle adjustment speed using the small screws with the springs on them and use the uni-sync tool to get a good balance around say 850 RPM. Now you can adjust the fine mixture screws 1/4 - 1/2 turn as mag mentioned.

 

4) Next you want to check the high-speed balance. Use the high-speed screw on the linkage to get the engine up to cruising speed of 3-4k RPM's. There is another screw on the linkage to adjust the high-speed balance. This is much more important than idle, since this is where you're usually at driving. Again, adjust the fine mixture screw, if necessary.

 

I've also heard of raising one of the pistons and adjusting the mixtre on the other one, but it never seems to work for me...

 

Hope this helps, -Hugh

[cspatter]

Thank you both. That was very helpful. I found that most of my troubles were from a foam filter element that was sucked into the carbs. I've now put on K&N elements in chrome housings for both inlets. I'll post a picture later today. I think I was running a little lean on Friday so I adjusted the fine tunings a little more rich and seem to be running great now. I haven't pushed it to the red line yet.

 

It's still idling a little high so I'll need to adjust it down a bit. I may be confused on the high-speed balance at 3-4K. If it is where i think it may be then the guy had lost a screw or something broke and it's now zip tied. Again I'll post pictures. I'm also going to hunt around for a good carb tuner here in GA that has a dyno I can put it on. Then I can watch an expert during the tuning process.

 

 

[Skib]

Hughs instructions should at least get it drivable. There really not that hard to tune, there pretty basic carbs. Fine tuning them takes some more skill but making them reasonably drivable is easy.