A few SU tuning questions

[Murph]

When my 260Z (with 240Z SU's) was last tuned it had a vacuum leak on the balance tube, so I'd say it was tuned around that problem.

When I fixed that leak, it was running ritch as hell.

Now, I've never tuned the SU's before but have read plenty about how to do it. Anyhow, as a quick fix I leaned both carbs off by half a turn which helped quite a bit. Still runs quite nicely other than hesitating a bit when cold (any ideas why?).

Anyhow, I had another play with it today and found the the rear carbs mixture adjustment is two and a half turns on the ritch side of stoichometric. (I assume it will be the stoichometric mixture, where you will get the highest idle speed and lifting the piston wont alter engine speed?)

So...you'd gather from this that it's still running really really ritch? All the guides say to back the mixture off 1 flat (1/6th of a turn) fom the max idle position.

I was going to have a go at doing the full tune up procedure the other day, however I don't have a unisyn. Instead I decided to try the hose in ear trick, however for the life of me I couldn't hear any difference between the two. Of course that could just mean they were properly balanced? What is the differnce in the sound that I should be listening for? Is is the pitch, volume or something else?

I suppose I'm mainly just in need of some general advice on tuning these things.

Thanks,

Justin

[seerex]

I think your best bet and investment is the z therapy video on su's or the z tuneup guide <which I got > , the tune up guide showed everything on tuning them and tearing them down for yearly cleaning , check ebay for unisync but they are worth the $$ make it much easier , good luck

[Murph]

Ultimately I'll buy the vid etc and a unisyn etc, but I simply can't afford it atm. I can't afford to get it tuned by someone either.

I've been asking around to see if someone has a unisyn around here that I can borrow.....but nothing yet.

One main question. Can people confirm for me from their personal experience, how far ritcher than stoichometric they run their carbs? Is it one "flat" as all the tuning guides say?

Oooo....another question.

If I were to have it tuned a little too lean, is it just going to mean a loss of performance, or will it start pinging and kill itself?

When my mechanic last tuned it, he said it needed to be tuned fairly ritch in the bottom end to keep the right amount of fuel to it in the top end. Is this normal? This's my main concern really. If I'm tuning for mixture at idle, how do I know it's getting the fuel it needs in the top end?

Thanks,

Justin

[zeiss150]

Hey Justin, The Ztherapy video is AWSOME!!!!!! I was having problems with my Z and I bought the video (they have it on DVD now) and pow, it runs great. But let me tell you for a quick fix,

1. set your valves (hot, 12thou ex, 10thou, in). this is a must.

2. set your timeing, get it as advanced as posible with out "pinging" under load, about 8 degrees BTDS(I think).

3. screw up your mixture adjusting nut ( under the carb) till it stops. Then screw it down 2 1/2 turns down on each carb. this is just a starting point as recomended by Ztherapy. If your running to rich turn up nut euqal amounts on each cab.

4. Adjust idle screw (its the one by the butterfly valve) WITH A FOLWMETER Untill they are both balanced. first at idle the flow meter should read about "6" then at 2500 rpms meter should read "25".

4a. the flow meter I used is from Motorsport www.zcarparts.com for $27.95 part # 60-9976. Its a great meter, I have used the other meter that is cheeper and belive me it's worth the 5 bucks more. I don't belive that you can truly tune your SU's with out it. If you want to call in your order in

(800)633-6331 ask for Sal he's a great guy and he know's his stuff.

5. adjust the carb balance screw so that both pistons rise at the same rate when throtteling up. kinda look at them to see that they go up at the same rate.

6. test your new tuned carbs buy doing 0-60 in 7 sec.

I hope that this was helpfull and dont forget the Ztherapy DVD its called Just SU's www.ztherapy.com

let us know if any of this helps!

Later Matt-

[Murph]

Thanks Matt.

That's a great help. I'll have to give that a whirl when I get my hands on a flow meter. Alternatively I'm thinking of just making somekind of flow meter myself.

I just had a look and as is the mixture nut is a bit over 2 turns out.

Another question. Say, if I go an adjust my mixtures now, with out changing the balance, am I likely to have any problems, or stuff something up? I would have imagined it would most likely be ok as long as I keep the mixture fairly conservative?

[seerex]

Datto check you pms , sent you some info might help

[Murph]

Oh righty.

Just went a turned out the mixture nuts out 2 turns while cold, then fired it up. Pushing up on the piston lifting pin, the rear carb was at stoich, and the front one was a bit ritch. I turned each out another quater turn which had them both running ritch.

Took it for a drive and it felt quite good. Pulls quite strongly though the rev range although maybe a little doughy at low revs in higher gears.

I left the rear mixture as is, and leaned the front one of a quater turn at which point it's still running ritch. The car seems to drive quite fine like this.

Would I guess correctly that the balance (idle screws) is out if they're getting different mixtures with the same number of turns on the mixture nut?

Btw the other balance screw (so they open at the same time) is set correctly, and I adjusted the vavles last weekend. havent' looked at the timing yet.

[my_mad_z]

Justin,

here's my 2c.....

I had British Su's, so I'm not sure if what I've experienced applies to the 240z su's; when you screw in the mixture nut, they don't actually screw in the same distance.... I learn't this the hard way as I would always have one carb running ritcher than the other when I tuned them exactly the same. Had me stumpped for a while until I decided to remove the piston and investigate. When you screw the mixture nut the whole way in, you'll notice that they sit at different levels, this is visible with piston removed looking inside the carb.

I screw them up until they are both flush with the surface (looking inside carb with piston removed). Once flush, I screw 2.5 turns out.... from there, assuming you've checked tappets, timing, and dwell, I usually turn the mixture screw a 1/4 turn at a time (either clockwise or anti clockwise) till the engine hits a nice note and runs smooth. I tend to give it a sharp rev every now and again if it's been idling for too long while adjusting.

Lifting the pistons is usually a good way to check if the mixture is right. Check the resistance of the pistons as well, make sure they are the same, if not, check the damper oil.

I've never used the flow meter to balance the carbs, always did it by ear.... should hear the same sound coming from each carb; it's pretty obvious when they're out of balance, one will suck harder than the other and there is a noticble difference in sound.

[Murph]

Excellent! Well that clears that problem up then. Tomorrow I'll pull the carbs open and visually inspect the needle height, then take it from there.

Thanks for all the advice everyone!

Justin

[zeiss150]

Hey Justin, Sounds like your doing well. The other post is right about the screw's not moving the same. that is covered in the video. While you have the round tops off check your needle lenths to make sure they are the same. You can adjust the idle screws with out a flow meter but I was suprised how suttle the adjustments were, just an eighth of a turn makes quite a differance and I couldn't hear a differance, in the sucking sound but you could see it with a meter. good luck Justin.

Matt-

[Murph]

I'd say my best bet with balancing them, will be to playwith the idle screw adjustment on one of them so I can learn how the sound changes/what difference I am listening for.

I'll post up tomorrow with how it goes.

[beandip]

datto go to the Technical articles and look for SUs adjustment