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Civilizedape

240 S/U's no start

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I have a 73' 240Z w S/U's. Right now it will run for a second with a bit of starting fluid but not otherwise. I installed a electric fuel pump in just outside the fuel tank. There are two filters one before the pump and one before the carbs. Both fuel filters don't quite fill up with fuel.  I know some fuel is getting to the carbs but could it not be enough pressure? Trapped air? I will probably try replacing the fuel pump as a stab in the dark. If that doesn't work I may have to fire my mechanic and find a competent one. 

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@Civilizedape Do your SUs have the original flat top suction chambers or round top suction chambers. The suction chamber is the most noticeable part on an SU. 73 and 74 Zs came with flat-tops but most have been changed to the round tops which most (but not all) consider to be the better carbs. If yours are the flat-tops I'll let @Zup and @Captain Obvious sort it out.

 Most of my experience is in round tops so we'll start with pressure isn't the problem. Pressure is only to get the fuel from the tank to the carbs. After that it's just a matter of supply and demand. There is another filter (banjo filter) where the fuel lines enter the float bowl. These rarely get checked. I'd also pull the tops from the float bowls and see if there's fuel in them. That would be a good time to check the float levels and the needles and seats.

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They are round tops. I rebuilt the carbs with the z therapy kit. So as far as I know everything should be working. Ill check the banjo fittings but I think I removed the screens. I have adjusted the floats once initially and a second time when one carb was spitting fuel out of the breather from the top of the float chamber. I'll pull the tops and check for fuel. 

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So I pulled the pistons and the fuel level looked low maybe 1/4" below the needle seat. I pulled the float chambers off and am trying to adjust the float and valve. In the Z Therapy video he says to set the float to .55". But he also mentions long and short eared float chambers and different levels needed. The FSM and other repair manuals give different gap measurments. What is the gap measurement that I am looking for my set up?

float bowls.jpg

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You cannot set the newer style needles like the dvd with Grose jets.  You have to hang them upside down from your picture above, actually right side up. LOL

I fooled with mine for many months before I realized this.  A lot of other guys have run into it too.  The newer needle jets are so sensitive they won't hold the weight of the floats like the dvd shows.

First off I would straighten the float tabs back into shape.  Disregard the red circle.  The tangs should look like an upside down slice of cantaloupe not like a ramp.

float.png

Then hold them right side up and blow through fuel bung while pushing the float upwards.  Have something that is .55" wide and hold it against the bottom of the lid. When you are at .55" you should not be able blow any air.  Valve is closed.  That worked for me.  I got bored last winter and did this to make sure they were right.

DSC01680.JPG

DSC01679.JPG

  

The 10mm down seems to be the easiest and most frequently used way now.  I've never done it but here's @jonathanrussell method, sounds simple huh? :love:

Regarding adjusting floats....personally I think it is the most important and tedious step in getting the carbs working right. Bending the little tabs one way and then the other in an effort to get the float set just right is insanely frustrating but worth the effort. My personal preference for setting the floats is to remove the bowls, turn down the mixture screws 10 turns, and set the fuel level so that the convex meniscus fuel bubble sits perfectly even with the top of the nozzle. Once you get this accomplished, in my experience, you are able to freely adjust the mixture down to 2.5 turns and then detail adjust using Color Tune or similar. This technique has been documented multiple times on this site and others, fyi.

 

 

Edited by siteunseen
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The method of setting float levels where you turn the mixture screws 10 turns down, remove the domes, and set the fuel level so it is even with the top of the nozzle tops has been written about many times on many sites. The site / article where I was first introduced to the method is at the link below.

https://zcarguide.com/tuning-adjusting-datsun-240z-su-carburetors/

I learned a lot from the article but 4 key things in particular. 

First, adjusting SU carbs really hinges on getting the floats set right first. If you don't have that right, then I find that everything else ends up being a compromise. BTW, some disagree that setting the floats is super important but I am a believer and kool-aid drinker on the theory. If you don't get the fuel level set right in the floats then you end up setting the mixture level significantly different than 2.5 (plus or minus .5). When you do this you end up shifting the needle zones up or down and outside of the intended range in the power band. Worse, if the float adjustment is off by enough, you can't even get the mixture adjustment screw to cause any change. Again, this is all my opinion...and is shared by some and disagreed with by others...which is fine. You will figure out what works for you.

Second, the 10 turn down method was the only method that, for me, was exact and repeatable. I just could never get the .55 hanging down while sucking (no comments Site) or clear tube set to 23mm or whatever to work for me. No doubt, it works for some. To me though all of the other methods, at the end of the day, are trying to get the fuel to a certain level in the nozzle (10 turns down), so why not measure using the goal?

Third, once you have the floats set, using a Color Tune to set the mixture works really well for me. A couple of Color Tune tips though. 1) use the Color Tune in the dark...or low light. This will let you see the colors better. 2) Clean the Color Tune often. Once the engine has been running with the Color Tune for 1 minute or so, I find that I cease to be able to see the distinctions between Red (rich), Blue (ideal), and White (lean). So, I pull the Color Tune plug and clean with carb cleaner spray often. It is a kind of tedious process but it is accurate and repeatable.

Fourth, I use the Color Tune at two rpm levels- Idle and about 3k rpms. I let idle run a little rich (red)...which makes 3k end up being about right (blue).

 

Also worth checking if you are having fuel delivery issues is the little flexible hoses that connect the float bowls and the nozzles. Make sure you use ZTherapy or similar hoses that are super flexible. And, make sure the hose is not creased...impeding flow.

I assume that your nozzles are moving up and down correctly and closing fully when choke is off and opening when choke is on.

Stay with it. You will figure it out.

EDIT:

One more thing...and many may disagree with this too. I have never had any success getting the 3 screw carbs where one float bowl lid has longer ears and you use a longer needle jet to work. This arrangement causes the float to remain angled lower in the float bowl, which I think causes it to get hung against the float bowl wall. So, if you have a carb set where one has the lid with long ears and longer needle jet, and you can't get the float level set exactly and reliably, consider locating another short ear float lid. You can use any short ear float lid from any round top carbs. I had an extra set of 3 screw carbs sitting around and scavenged the short ear lid from that set of carbs.

 

 

 

Edited by jonathanrussell
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On 7/21/2019 at 12:41 PM, jonathanrussell said:

The method of setting float levels where you turn the mixture screws 10 turns down, remove the domes, and set the fuel level so it is even with the top of the nozzle tops has been written about many times on many sites. The site / article where I was first introduced to the method is at the link below.

https://zcarguide.com/tuning-adjusting-datsun-240z-su-carburetors/

I learned a lot from the article but 4 key things in particular. 

First, adjusting SU carbs really hinges on getting the floats set right first. If you don't have that right, then I find that everything else ends up being a compromise. BTW, some disagree that setting the floats is super important but I am a believer and kool-aid drinker on the theory. If you don't get the fuel level set right in the floats then you end up setting the mixture level significantly different than 2.5 (plus or minus .5). When you do this you end up shifting the needle zones up or down and outside of the intended range in the power band. Worse, if the float adjustment is off by enough, you can't even get the mixture adjustment screw to cause any change. Again, this is all my opinion...and is shared by some and disagreed with by others...which is fine. You will figure out what works for you.

Second, the 10 turn down method was the only method that, for me, was exact and repeatable. I just could never get the .55 hanging down while sucking (no comments Site) or clear tube set to 23mm or whatever to work for me. No doubt, it works for some. To me though all of the other methods, at the end of the day, are trying to get the fuel to a certain level in the nozzle (10 turns down), so why not measure using the goal?

Third, once you have the floats set, using a Color Tune to set the mixture works really well for me. A couple of Color Tune tips though. 1) use the Color Tune in the dark...or low light. This will let you see the colors better. 2) Clean the Color Tune often. Once the engine has been running with the Color Tune for 1 minute or so, I find that I cease to be able to see the distinctions between Red (rich), Blue (ideal), and White (lean). So, I pull the Color Tune plug and clean with carb cleaner spray often. It is a kind of tedious process but it is accurate and repeatable.

Fourth, I use the Color Tune at two rpm levels- Idle and about 3k rpms. I let idle run a little rich (red)...which makes 3k end up being about right (blue).

 

Also worth checking if you are having fuel delivery issues is the little flexible hoses that connect the float bowls and the nozzles. Make sure you use ZTherapy or similar hoses that are super flexible. And, make sure the hose is not creased...impeding flow.

I assume that your nozzles are moving up and down correctly and closing fully when choke is off and opening when choke is on.

Stay with it. You will figure it out.

EDIT:

One more thing...and many may disagree with this too. I have never had any success getting the 3 screw carbs where one float bowl lid has longer ears and you use a longer needle jet to work. This arrangement causes the float to remain angled lower in the float bowl, which I think causes it to get hung against the float bowl wall. So, if you have a carb set where one has the lid with long ears and longer needle jet, and you can't get the float level set exactly and reliably, consider locating another short ear float lid. You can use any short ear float lid from any round top carbs. I had an extra set of 3 screw carbs sitting around and scavenged the short ear lid from that set of carbs.

 

 

 

So I have gotten the front carb (long ear lid) to the 10mm level but the back (short ear lid) I can't get it to the proper level with the 10 turn method. I have the float adjusted to the highest level it can reach but the fuel in the needle seat is still to low.

Having said this I can start the car and idle for a 10-20 seconds before it dies.

Edited by Civilizedape

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My struggle has always been with the long ear lid. I find it causes the float to hang too low in the bowl and hang against the wall.

What is the length if the needle valve you are using for front...long, and rear...short?

If I measure the needle valve body...not the threads and not the floating needle...just the full body...I get

18.5mm short

20.5mm long.

Any chance you are using a long needle valve in a short ear lid?

 

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14 hours ago, siteunseen said:

If your tangs are at this severe angle you'll have problems. That bent upwards tang will keep the float pushed up and no fuel will enter

Agree. 

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