Carb issue

[Alex240z]

Hello,

I recently got a Datsun and I’ve been trying to tune the carbs so I can finally drive it. Unfortunately the carbs have been extremely weird. 
 

I’ve been following this page https://zcarguide.com/tuning-adjusting-datsun-240z-su-carburetors/ on how to tune the carbs. After I follow all the steps to step 10 where it says “start engine” the car shoots to 2000rpm then slowly dies. I’ve checked for vacuum leaks on the balance tube, distributor, and gasket but I’ve closed everything. I don’t know if this has anything to do with this but when I use my choke I’m at 2000-2500rpm but it does keep an idle. Now quite sure what I’m missing. I’ve checked out my Haynes manual but the way they do it is different so not sure which one to exact follow. Any feedback would be great thanks. 

[siteunseen]

It seems like the float chambers are running out fuel really fast. With the ckokes on the nozzles are dropped lower adding more fuel. I would get section of clear hose and check the levels. That's the easiest way to get an idea of where you're at. Should be even with the mark at the yellow arrow. You get that hose at Lowe's or Home Depo. I think it's 1/4". Just one 2 foot piece is all you need. Cheap and easy.

Edited March 23, 2021 by siteunseen

[siteunseen]

Just remembered a quicker cheaper way to get an idea of how much or little fuel you have. 

Get one of those little red straws off a can of Wd-40 and stick it down the vent bung. It should be "bouncy". You're pushing the floats down into the fuel. If you have plenty it should raise the straw right back up after you push it down. If it stays down there's not enough.

[Alex240z]

5 hours ago, siteunseen said:

Just remembered a quicker cheaper way to get an idea of how much or little fuel you have. 

Get one of those little red straws off a can of Wd-40 and stick it down the vent bung. It should be "bouncy". You're pushing the floats down into the fuel. If you have plenty it should raise the straw right back up after you push it down. If it stays down there's not enough.

Sweet thanks for the feedback I'll check that out. If the carb is not getting enough fuel what would I have to do to adjust it? 

I think I had a smooth brain moment because I just assumed the SU carbs had damper oil and when I checked it this morning after just remembering there was none. Not sure what effects that has but my carbs would love to just spit out fuel. 

[siteunseen]

Has it ever run with those carbs, fuel tank and fuel pump? You can bypass the tank with a small gas can up front. Pull the suction hose off the fuel filter and put it in the jug. These cars are bad to get crudded up fuel tanks, mine was horrible when I first bought it. Wouldn't run for more than a few minutes so I used the jug trick and it ran fine. That's a whole other story...

The main problem that I've seen is the floats are out of adjustment and don't allow enough fuel into the bowls, starving them of gas. When you pull the choke on it drops the nozzle letting in more fuel so your's are running or idling as you said above. Here's a very simple explanation @Jughead posted as few days ago to get the floats set at 9/16" down from the bowl's lid.

I just reset the floats.  Here's the procedure I used (which I got from Steve @ Z-Therapy):

1) Removed float chamber top and held with float on bottom.

2) Blew into fuel banjo fitting while slowly lifting float and holding 9/16" spacer between top of float and bottom of float chamber cover.

3) Adjusted (bent) the float adjustment tab so that airflow thru the banjo just stops at  9/16", and air flows with any downward movement of the float.  

After doing this, I checked the levels with Float Syncs. The levels are exactly the same in both carburetors: ~ 1/2" above the top of the metal bases of the Float Synchs.

 

The base setting for 3 screw SUs is to have the adjustment nut on the bottom 2.5 turns down/out/counterclockwise what ever but anyway tighten them by turning them towards the passenger's fender, which is clockwise looking from the bottom up. Just snug them up not super tight. Then turn them backwards towards the driver's side, counterclockwise looking up from the bottom, 2 complete turns then a half way around turn. That's the ballpark and they should run like that.

The nut I'm talking about is #19 in this diagram. When you loosen it the needles drop down and lets more fuel pass through the nozzles.

Here's what I learned by and after doing it a few times it gets easy, you'll figure out what each step does and it'll all make sense.

Thank You Mr @240260280

 

Edited March 23, 2021 by siteunseen

[siteunseen]

The floats should look like this, I say a sped bump. I think that's called a metaphor? Same height on the ends but with a hump in the middle.

Most of the time people's look like a ramp instead. That won't let the valves work right and they shut off too soon, no fuel. This is bad...

You want them lower and level except the bump in the middle. Bend them down like this...

[Alex240z]

Here is the float on the rear carburetor and also the float chamber. Also forgot to mention that these are 4 screw SU. I did use the gas can before and that’s when I had it running. When I hooked it up to the tank didn’t run so well. 

[Alex240z]

I think your assumption of the floats being out of tune was correct. Seeing that the fuel only filled a small portion of the chamber. I’ll go about adjusting it and see what I get. The more and more I open this carburetor the more I feel the need to rebuild it. I’ll let you know how it goes 

[Alex240z]

On 3/23/2021 at 10:50 AM, siteunseen said:

The floats should look like this, I say a sped bump. I think that's called a metaphor? Same height on the ends but with a hump in the middle.

Most of the time people's look like a ramp instead. That won't let the valves work right and they shut off too soon, no fuel. This is bad...

You want them lower and level except the bump in the middle. Bend them down like this...

I adjusted it as you said but I’m still not getting any gas into the chamber. Pressure builds up and everything until it goes to line from the float chamber into the carb. Am I not having enough pressure in my fuel lines? I don’t think I have a vacuum leak. Only thing I didn’t plug up was the fuel tank evap into my cylinder but I doubt that’s the cause of anything. I put on another mechanical fuel pump but found out that one didn’t work well compared to the one I had. Pressure builds pretty quickly and I’ve taken apart the carb to inspect the needle and everything seemed fine. Not quite sure what’s wrong with the carb. 

[siteunseen]

Could the the "J" shaped hoses at the bottom, connecting the nozzles to the float bowls be bad? I bought one that someone had put regular fuel hose on that hardened over time and pinch not allowing proper fuel flow. I'm using my phone now so I can't be much help until in the morning. I can show you some pictures then.

Here's some threads that talk about that problem. I'll get back asap tomorrow with something better.

https://www.google.com/search?q=240z+nozzle+to+float+chamber+hose+classiczcars.com&source=lmns&bih=449&biw=320&client=ms-android-americamovil-us-revc&prmd=sivn&hl=en&sa=X&ved=2ahUKEwjO98eRnMrvAhVM6VMKHU5gCH8Q_AUoAHoECAAQAg

 

 

[Alex240z]

5 hours ago, siteunseen said:

Could the the "J" shaped hoses at the bottom, connecting the nozzles to the float bowls be bad? I bought one that someone had put regular fuel hose on that hardened over time and pinch not allowing proper fuel flow. I'm using my phone now so I can't be much help until in the morning. I can show you some pictures then.

Here's some threads that talk about that problem. I'll get back asap tomorrow with something better.

https://www.google.com/search?q=240z+nozzle+to+float+chamber+hose+classiczcars.com&source=lmns&bih=449&biw=320&client=ms-android-americamovil-us-revc&prmd=sivn&hl=en&sa=X&ved=2ahUKEwjO98eRnMrvAhVM6VMKHU5gCH8Q_AUoAHoECAAQAg

 

 

I bought new ones so they don’t cripple. Only issue it feels like is the fuel isn’t going into the carb. I can feel the fuel inside of the hose but it seems like no pressure is pushing it up. I did the straw method to test to see how much fuel is in and I saw the fuel spill off to the sides of the float chamber because my gasket is pretty much gone. Could this be preventing enough pressure to build up in the chamber to push the fuel into the carb? Do I have to cover the nozzle that connects the chamber to the air filter? I checked the jet and blew through it and the air flows through it just fine. The piston goes up nicely and falls down with a click. The floats are adjusted properly but one of the pin inside the front carb didn’t go back to the resting position but I took it apart and fixed it. Each chamber has 20mm of fuel in them. Did the drop test and it was fine. I’m just lost at what is the problem. 

Edited March 25, 2021 by Alexg280z

[Mark Maras]

 Have you checked the fuel filters inside the banjo fittings that are on the top of the float bowls? They could explain fuel in the fuel lines but not enough entering the bowls. Carbs really don't need much fuel pressure. They'll even run fine using a gravity feed.