su adjustment

[racer88]

Hi everyone

i have 4 screw round top su's on my 260 with 280 engine. I have read everything i can find on su carbs and how to adjust them. I have rebuilt them,checked to make sure there is no throttle bushing play. Set the float level by backing out the nozzles 10 turns set the needle height by lossening the srew and setting the piston down onto the fully raised nozzle made sure the nozzles return to normal position when choke is released. My problem is if I raise the nozzle with the mixture nut on the front carb all the way up with the engine running it runs smoother. If I do that on the rear carb it starts to run rough. The rear carb likes the nozzle about 1 3/4 turns down on the mixture nut. the front carb likes all the way up. anybody have any ideas about possible thing that could cause this. The cars seems to idle way down to 700rpm rev's nice and crisp but has some popping in the muffler when cold

Thanks

Bob

[Bruce Palmer]

Set the float level by backing the nozzle out 10 turns?? Not following that.

When they were rebuilt did you replace needles and nozzles?

Well, anyway, our JUST SUs DVD should show you what's up with these carbs and how to rectify issues.

[Arne]

I'm with Bruce. You don't "set the float level" by turning the mixture adjuster nuts. Sounds like the real float level (set at the float inside the float bowls) are not the same in the two carbs.

[JonnyRock]

I'll second the 'Just SU's' DVD. Great stuff, in fact I'm going to start swapping my old carbs for my new Ztherapy set tomorrow!

[racer88]

hi

Some of the tech articles that I read (datsun z garage) says that if you back out the mixture nut 10 turns that places the top of the nozzle at the 23 mm mark and if you adjust the float until the fuel is level with the top of the nozzle viewed with the piston and needle removed that will be the correct level in the bowl. Do you know of any place in Canada that might sell the Ztherapy dvd as I would be happy to purchase it but shipping across the border is a real pain for a small purchase. Failing that is there a better way to ensure that the fuel level is correct.

Thanks again

Bob

[racer88]

Sorry, forgot to answer your question about needles and nozzles. I did use the ones that came with the carbs. The needles had no markings on them (even under bright light and magnifying glass). P.S. I did set the mixture nuts back up to the proper setting after I was done attempting to set the float

Edited March 18, 2010 by racer88
left something out

[Arne]

To verify my levels, I generally put a clear vinyl tube on the bottom float nipple (instead of the normal tube to the bottom nozzle). Then I remove the power to the coil and crank the engine over a couple of times to fill the float bowl. (If you have an electric pump, just hot wire it for a couple of seconds.) You will now be able to see how high the fuel sits in the float bowl under operating conditions. The fuel should be 23-25 mm below the top of the float bowl when measured this way.

[racer88]

Thanks Arne. That sound much easier. I will give it a try tomorrow

Bob

[Arne]

I'm betting you'll find that the one is way too high. Once you determine that, you'll need to adjust it at the float pivot and needle valve inside the float bowl.

[Bruce Palmer]

We mail DVDs to Canada all the time. They may be getting hosed for fees but we don't hear any repercussions.

[racer88]

Thanks bruce I will get one ordered up this week. I can rebuild rochester 2 barrels and holley 4412's in my sleep but the su's despite their simplicity are a new animal to me. I like the look and the 70's feel of the su's and for a medium performing street zed they should be great

Bob

[racer88]

Hi

Sorry it has taken so long to respond to all the advice about su adjustments. Had a family emerg. that took all time away from the datsun. Since I last posted I borrowed a uni-sync and started the adjustment procedure from the beginning. What a difference that made. The z idles as low as 600 rpm is crisp in the throttle response. Half choke and it starts right up. Now that I have a good base line I am going to start playing with different damper oils and springs. I have a friend with a small machine shop to change the profile of the needles. Thanks again for all the help. This sight is great for pointing newbies in the right direction

Bob