Hey Z (Zed) people! I've been working on my Z for over 3 years now and for over 3 years now I've had smelly exaust. I've read just about every thread on how to fix this problem, from body seals and tail light gaskets, to sealing up the header and exaust system, to adjusting the carbs and nothing ever worked. I decided that it was just a flaw of the SU design and that it could not be fixed... wrong!

What I've learned is that the SU's are super simple but they are also very complex.

It turns out that the smelly exaust seemed to come on at idle (provided I had the carbs tuned right... otherwise it would be constent and make the car almost undriveable) I decided to try one last time to fix it and this is what I learned...

I thought it was the float level being set to high and leaking gas into the cab at idle so I check the carbs and the rear was a little to high so I adjusted it. Well I adjusted it wrong and now the problem was worse, now gas was spilling out of the carb:stupid: so I adjusted it correctly and the smelly exaust was still there .... SHOCKING:cry:

I decided to try a needle change, I was using SM needles which are recomended by Ztherapy, and cursed by many members of this site. I had my old N54 needles that were stock with the L24 (they always gave me smelly exaust too), but I thought I would give them another try.

And here is the magic... It seems to me that the needle position in the piston will give you smelly exaust. the higher up the needle sits in the piston the smelly'er it is. the lower it sits, no smell. The fine line is just how low to set the needle. To low and you loose performance... to high and back to smelly. I tried the Ztherapy needle setting system. You know, where you loosly set the needle in the piston and then drop it down on the nozzle and that sets the perfect needle depth... guess what, that $hit works... with a minor adjust ment that is. I found that if I slide the needle up just a bit from the droped piston set point I got better performance AND SMELL FREE EXAUST.

I thought I would share this info with all the SU loving members of this site that continually complain (and rightfully so) about smelly exaust. I'm sure many of you already new how to fix this problem and this is no brainer stuff , but, I never saw a thread about how to get rid of the smelly exaust other than someone saying "all you have to do is tune your carbs". This is just my personal anicdote and it may not work for everyone but I hope it helps people pin down there smelly exaust problem

Just thought you should know.

Rock on Z people, Matt-

I'm assuming you then had to lean the mixture nuts (raise the nozzles) to set the a/f before/after idle adjustment. That in affect lowers the fuel level in the nozzle. Ideally, the nozzle should be level with the bridge and the fuel bowl level just flea's hair below that. Maybe your method helps induce better atomization at idle with the nozzle above the bridge. My exhaust problems are accentuated during deceleration after a hard run where the system is loaded with fuel. Again, your setup may reduce excess fuel being pulled into the engine during decel? I wonder if simply setting the fuel bowl level slightly lower and having the nozzles equal with the bridge would have the same affect? Any one else want to comment?

I set the nozzles 2.5 turns down. I did the "drop set" with the nozzles turned all the way up. I don't think that leaning out the rear float bowl did anything because I was still smelly after I fixed that problem.

My guess is that getting the needle positioned right in the piston makes it put out less fuel on idle.

I know that the needles have a diameter change every 16th of an inch or so and they all start off with the same diameter for idle... so... if that needle is set to high in the piston then the needle will deliver more fuel at idle. This is just my theory... of course I have no proof or statistics to back that up ... but it sounds good doesn't it.

I also used my air flow meter to set the idle so it was nice and smooth. Each carb came in at a 9 on the meter, giving me an 800 idle RPM.

Matt-

All those needles are tapered so the OD is infinitely variable along the length.

Not all needles are the same at idle position.

Raising or lowering the needle in the piston would be akin to changing needles. Air/fuel is controlled by the OD of the needle and the ID of the nozzle orifice at a GIVEN PISTON POSITION which is controlled by the volume of air drawn into the engine at any RPM.

Hey Bruce, Thanks for the reply!

My statement that all the needles have the same idle diameter was based off of a spread sheet with about 70 + needle diameters and they almost all seemed to have the same idle, but Im sure there are a bunch that are different.

I think I was just setting the needle to high in the piston all these years, so no matter how I tuned the carb I was to rich for the given RPM, basicaly like having a richer needle.

I did give props to Ztherapy for the "piston drop" way of setting the needles.

Matt-