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CaptainMajestic

Flat top carbs not sending fuel to engine

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Hi Everyone,

Completely stumped on this one. So my engine was running completely fine since it's first start about a month ago and then suddenly it just stopped working all together. I have spark, I have fuel pressure, and there is fuel in the carb bowls. I figured it's an issue with the carbs so I pulled them and cleaned them up + did the basic rebuild, but still no luck. 

  • Pretty sure I set the floats correctly, center shutoff to 0.5" from the machined surface, and power valve pickup 1/32", but when the bowls fill up it goes well above the dot in the viewing window. is this normal?
  • I tried pulling the power valve to service it, but couldnt get it to crack open. Any tips here?
  • Is there any way I can test the functionality of the carb other than trying to start the car? Just want to verify it is mixing fuel with air.

Thanks!

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2 hours ago, CaptainMajestic said:

Hi Everyone,

Completely stumped on this one. So my engine was running completely fine since it's first start about a month ago and then suddenly it just stopped working all together. I have spark, I have fuel pressure, and there is fuel in the carb bowls.

Try starting fluid through the carbs before getting too crazy.  Confirm that it is actually a fuel problem.  I've had weak spark that wouldn't start on gasoline but would on starting fluid.

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1 hour ago, Zed Head said:

Try starting fluid through the carbs before getting too crazy.  Confirm that it is actually a fuel problem.  I've had weak spark that wouldn't start on gasoline but would on starting fluid.

Ah yes, forgot to mention I've already tried this. Engine starts up fine for a few seconds and then dies. That's why I think it's the carbs

 

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3 hours ago, CaptainMajestic said:

then suddenly it just stopped working all together.

That's a good clue then.  Here's another clarifier - were you driving it when it "stopped working" or do you mean it just wouldn't start one day?  Any chance you got some bad gas?

Did you follow the proper starting procedure?  Maybe your choke cable came disconnected and you've lost choke.

Only posting to get some carb time.  Don't really know much.

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It just wouldnt start one day. It's pretty hot here in austin so I normally just choke it 1/4 of the way, but choking at 25% 50%, 75% and 100% did nothing.

what should I be looking for for bad gas? I recently pulled the tank and flushed it / did a small acetone rinse, but the gas does have a light reddish hue.

Edited by CaptainMajestic

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3 hours ago, CaptainMajestic said:

It just wouldnt start one day. It's pretty hot here in austin so I normally just choke it 1/4 of the way, but choking at 25% 50%, 75% and 100% did nothing.

what should I be looking for for bad gas? I recently pulled the tank and flushed it / did a small acetone rinse, but the gas does have a light reddish hue.

Bad gas is hard to diagnose.  It just doesn't light up like good gas.  You could drain your float bowls and refill them with known fresh gas as a test.

Did you check that when you pull the choke knob/lever that it's actually moving at the carburetor?  You might be pulling the knob but nothing really happens.

I'm just throwing out guesses that I might try if I had the car and the problem.

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1 hour ago, Zed Head said:

Did you check that when you pull the choke knob/lever that it's actually moving at the carburetor?  You might be pulling the knob but nothing really happens.

I'm just throwing out guesses that I might try if I had the car and the problem.

Going to try this tomorrow. Also, at this point anything you guys say to help I take as gospel. Thank you so much!

 

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20 hours ago, CaptainMajestic said:

 I figured it's an issue with the carbs so I pulled them and cleaned them up + did the basic rebuild, but still no luck. 

  • Pretty sure I set the floats correctly, center shutoff to 0.5" from the machined surface, and power valve pickup 1/32", but when the bowls fill up it goes well above the dot in the viewing window. is this normal?
  • I tried pulling the power valve to service it, but couldnt get it to crack open. Any tips here?
  • Is there any way I can test the functionality of the carb other than trying to start the car? Just want to verify it is mixing fuel with air.

To answer some of your questions directly... The initial mechanical setting of the float level is just to get into the ballpark. The sight glass is the real test. The desired level does depend on the carb design though. What year are you working on? 73 or 74? Can you post a pic or two of the power valves? That's another easy way to know for sure. However, without even seeing the carbs, I suspect the fuel level should be right at the dot in the middle of the window and your level is way too high.

Tips for cracking the power valve open? Not really. The gaskets hold tight and can make it difficult to separate the wafer stack. Other than brute force, I don't have any suggestions.

And testing the carb off the car? I don't have any way to do that.

If it was running great one day, and then the next day it wouldn't start at all, I'm not convinced that it was a carb problem in the beginning. But now that you've opened them up and let the magic out, there might be one now.   LOL 

Did you try starting it by holding your foot down a little on the pedal while you cranked it?

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4 hours ago, Captain Obvious said:

But now that you've opened them up and let the magic out, there might be one now.   LOL 

That's great!

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@Captain Obvious Can confirm I did not let the magic out. Sorry for the late reply, got tied up at work. Turns out @Zed Head was right on the money, I drained the tank + carbs and flushed the lines then  added new gas. Started right up! Carb rebuild definitely helped make it start quick!

Have you guys ever had issues where when the car is choked it behaves exactly as expected, but when the choke is released the car drops well below it's expected idle? It should be timed, and I thought I did the tuning procedure correctly.

 

Thanks again!

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Regarding the idle issues and settings----if you haven't used it.

260Z Carb Adjustment Manual (1).pdf

With the flat tops you need this and I would stress that you follow it exactly from beginning to end. It should resolve any issues.

(Don't worry that it is labeled 260z carb adjustment---they are essentially the same as the 73 version)

 

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