Jump to content

IGNORED

Fuel pouring out the front of the carbs


Recommended Posts

To set the scene: early ‘74 260z with four screw round tops and a recent l24 swap. 
 

ive been chasing little issues trying to get this l24 that I swapped in, to run. 
i drove the car with these round top SUs on the l26 with the current float level and mixture adjustment. 
Since putting the l24 in and trying to start it, fuel has started to pour out of the front of both carburetors after I stop cranking. 
 

I previously adjusted the float level and recently checked the float level, they are within spec. 
I ran and drove with the carbs set up this way and didn’t notice fuel leaking from the carbs before, and I’m guessing that it would’ve started a fire if it had been leaking previously, because the fuel does land on the exhaust. 
I looked at the floats action in pressing the shut off valve and they appeared to press the little button in, I’ve heard that that could be a problem. 
If it makes a difference, I put the mechanical fuel pump from the l26 onto the l24. 
 

Ive gotten the engine to start running in this configuration but it quickly gets flooded and won’t start again. 
Im just trying to see if there’s anything I can try before just having the carbs rebuilt. So if anyone has tips as to how to fix the fuel leak, please let me know!

Link to post
Share on other sites

There are filters at the banjo for the SU carbs, steel mesh. My setup would flood because there was crud that would enter through that filter and jam the float needle. Inspect them for debris.

Edited by heyitsrama
  • Like 1
Link to post
Share on other sites

I would put a cheap fuel pressure gauge right out of the fuel filter before it enters the metal hardline. You may have too much fuel pressure? I think 3psi is about the max.

Lowes has cheap gauges in the swimming pool supply section, fyi. Good luck. You want that fixed asap. While you're at Lowe's get a fire extinguisher. They have a metal rechargeable that fits perfect in the tool bends behind the seats.

Link to post
Share on other sites

I'm having a hard time reconciling these two statements:

On 6/24/2021 at 11:39 PM, Cooper260z said:

fuel has started to pour out of the front of both carburetors after I stop cranking. 

I ... checked the float level, they are within spec. 

About the only way to get fuel to pour out of the carb mouth is for the float bowls to overflow. So if the float levels are withing spec, I'm having trouble correlating that to fuel coming out of the front of the carbs.

Are you saying that you have checked the bowl levels right WHEN the problem is occurring and they are in spec? Even while there is gas pouring out of the carb mouths?

  • Like 2
Link to post
Share on other sites

I wonder if his nozzles are stuck down or the choke wires are in a bind somehow. Would that cause the overflow?

I know when I got my first 240 I would leave the choke lever pulled back after cranking and running it for a few minutes leaving the nozzles dropped down. My upstairs apartment smelt like gas really bad. Girlfriend wasn't happy with the smell and I was glad to be alive with my pilot light water heater downstairs where the fuel was. LOL

Edit for clarity of dumbassedness. 

The choke lever bracket was broken all to hell so I tried not to use it too much until I got the metal holder from Mr Singer. Best $$ spent on a 240 BTW.

 

Edited by siteunseen
Link to post
Share on other sites
Posted (edited)

I haven’t gotten out there to check the screen at the banjo bolt yet. 
 

@Captain Obvious I set the floats to .55” I think that’s what the Ztherapy video said to do. That’s why I believe they are within spec. Should I try to set them at less then .55” so that they shut off sooner?

 

@siteunseen I thought that the nozzles might be getting stuck but I did give them a little push upwards after having the choke on, to make sure they were set all the way home, so I don’t think they’re stuck. 

Edited by Cooper260z
Link to post
Share on other sites
3 hours ago, Cooper260z said:

 I set the floats to .55” I think that’s what the Ztherapy video said to do. That’s why I believe they are within spec. Should I try to set them at less then .55” so that they shut off sooner?

No. You should assume that your .dry 55" measurement got you (hopefully) close to your target, and do a REAL fuel level test using the clear tube method.

The mechanical .55 is just to get you in the ballpark. Sometimes it works straight out of the box without subsequent adjustments, and sometimes it doesn't. You need to verify the level on the car with fuel in the bowl, not dry on the bench.

Link to post
Share on other sites
3 hours ago, ETI4K said:

Any chance the fuel pump pressure is overcoming the float needle spring/float force?

That will show up on a clear tube wet test if he uses the fuel pump to fill the bowls.

  • Like 1
Link to post
Share on other sites

Ah, okay. My bad. It’s been a while since I watched the video. 
 

I’ll look into testing them with fuel then and try that next time I get the chance. 

Link to post
Share on other sites

Yeah, it unfortunately sounds like the bowl level is going too high for some reason. Either the fuel pump is forcing the valve open, or the setting was always ballpark and never precise. The wet set method is a bit of a pain, but it's the only way to be sure.

If you want to confirm the theory, you can unscrew the air vent nipple on the top of the float bowls and see what happens. Nothing is supposed to happen, but I suspect you're going to have fuel come pushing out of those vents. And the only way that happens is if the bowl level is too high.

Link to post
Share on other sites

That DVD method doesn't work with the newer style needle valves. You can't turn them up side down and set the distance then install them. They'll be way off. The newer needle valves are too sensitive for the weight of the floats. You have to turn them right side up and let the float hang down, blow through the fuel inlet while raising the float until it shuts off, no more air gets through. When you get there that's the .55" measurement.

Like Captain says that'll get you in the ballpark but the clear tube will get you a real idea of where the fuel levels are.

1972 Float Adjustment ... - Page 3 - Carburetor Central - The Classic Zcar  Club

Link to post
Share on other sites
On 6/26/2021 at 5:45 PM, siteunseen said:

The carbed cars don't have fuel pressure regulators like the efi ones do. 

I have a regulator and assumed it was on all 260z's. I attached a fuel pressure gauge and was told to keep the fuel pressure below 4.5 psi. I guess what I'm saying is to check your pressure especially if you have an electric fuel pump and mechanical pump both running.

  • Like 1
Link to post
Share on other sites
4 hours ago, Jeff Berk said:

I have a regulator and assumed it was on all 260z's. I attached a fuel pressure gauge and was told to keep the fuel pressure below 4.5 psi. I guess what I'm saying is to check your pressure especially if you have an electric fuel pump and mechanical pump both running.

I always forget about the 260s and my lack of knowledge about them. I don't think I've ever even seen one in the "wild". Just at ZCON and on the internet. 

  • Like 1
Link to post
Share on other sites
4 hours ago, Jeff Berk said:

I have a regulator and assumed it was on all 260z's. I attached a fuel pressure gauge and was told to keep the fuel pressure below 4.5 psi. I guess what I'm saying is to check your pressure especially if you have an electric fuel pump and mechanical pump both running.

As @Captain Obvious has mentioned in the past the "fuel pressure regulator" in a stock 240Z or 260Z is the orifice on the return line of the fuel rail. The 280Z was the first with a stock FPR.

  • Like 1
Link to post
Share on other sites

If I remember correctly, there's a reducer (restricto) in the return line. Nothing there to go bad but it could become plugged or corroded. I believe C.O. is the one who looked into this previously.

  • Thanks 1
Link to post
Share on other sites

Yes, there is an "on-purpose" restriction on the end of the return side of the fuel rail right where it transitions to the rubber hose (over by the fuel pump). It's a swaged down tip brazed onto the end of the return leg. 

I've got all kinds of pics of all kinds of carb stuff, and I can't find a single detailed pic of that orifice restrictor.

So, all I've got is this:
orifice location.jpg

I'll take a look to see if I have any 240/260 fuel rail stuff around here. Not sure I do, but I'll look.

  • Like 1
Link to post
Share on other sites
18 hours ago, Captain Obvious said:

Yes, there is an "on-purpose" restriction on the end of the return side of the fuel rail right where it transitions to the rubber hose (over by the fuel pump). It's a swaged down tip brazed onto the end of the return leg. 

I've got all kinds of pics of all kinds of carb stuff, and I can't find a single detailed pic of that orifice restrictor.

So, all I've got is this:
orifice location.jpg

I'll take a look to see if I have any 240/260 fuel rail stuff around here. Not sure I do, but I'll look.

You pointed it out, I found it then. Now I understand. Thanks.

Screenshot_20210630-152034_Gallery.jpg

 

Link to post
Share on other sites

Yup! There it is! Thanks for the pics!

So that little hole partially blocking the return line of the rail is what provides the fuel pressure to the carbs. If that return line wasn't necked down like that, the pressure in the rail would be pretty much zero and the carbs wouldn't get any fuel. That orifice provides the restriction which generates the pressure in the rail.

Link to post
Share on other sites

I wonder how much pressure the original electric pumps for the carb'ed cars generated.  The system started with the mechanical pump that relieves pressure at the pump itself through internal valving.  They hit a pressure and just don't push anymore.  What did Nissan specify for the pressure of the factory electric pump?

Link to post
Share on other sites

According to the 74 FSM (page EF-9), the electric fuel pump should deliver 1400 cc (85 cu in) in one minute or less. And also on the same page, it lists the pressure spec at 4.6 psi.

No idea if that's what they actually did, but that's the spec.

  • Like 1
  • Thanks 1
Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...

Important Information

By using this site, you agree to our Privacy Policy and Guidelines. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.