260z stalls at idle after warm up. Carbs?

[Rill Cosby]

Hello, kind folks of CZC, long time lurker here! Normally I can figure out my issues through the vast information already here but this is the first problem I came across that I feel the need to post and get some more direct answers.

The car cold starts every single time and runs from anywhere to 30 seconds to 2 minutes. The RPMs bounce from around 750 to 1,000 until it puts up a fight to stay alive and then stalls out. Very occasionally white vapors will pour out the carbs trying to crank her right back up. After that I will have to wait about 5 minutes for it to crank back up. Now the problem first arose with some bucking on the highway, normally around in the 2,000 to 3,000 RPM range. Then it slowly came to the state that it is in now.

Little bit about the car.

1974 260z. Stock 2.6L with Ztherapy carbs. Electric fuel pump. 77,xxx miles. Daily Driver. Owned about a year.

I started the search for engine harmony in the ignition. Pulled plug wires until I came across #1 to see the tip of the plug and plug wire was rusted completely. I then replaced with NGK plugs and wires along with cap and rotor thinking that had to be the problem. Same symptoms afterwards.

I then started to believe I became victim of the timing chain jump myth but after checking TDC with marks on the rotor, cam, and piston everything seemed to be in line. Timing set to 8 degrees at 750 RPM's the few times I had it running well enough to do so.

Valves were adjusted tight a couple months ago. Moved on to a dry compression test and the results seemed consistent enough to rule out any issues there.

1. 160

2. 160

3. 165

4. 175

5. 170

6. 170

I then moved over to carbs and disassembled the majority of the body and cleaned out what I could do the best of my ability. I came across a crack in the top portion of one of the floats. Would that possibly cause flooding issues after a certain period of time of running and cause the white vapors from the carbs?

I redone the fuel tank and cleaned out the lines when I first bought the car so I believe all is good there. Clear fuel filter installed so I do see clean amounts of fuel flow going through it. Been running the same fuel delivery setup for months. Brake master vac always had a leak in it but spraying carb cleaner around other lines had no effect on RPMs.

I grew a terrible stubble of facial hair to profusely scratch while trying to think of a solution to no avail. I feel like the problem is so minuscule and I may feel like a dummy afterwards but better a dummy that has a running car than none. I'm losing days at work, patience, and money at parts to trying to get it going again. Sorry for all the reading material. Any input appreciated guys!

[Rill Cosby]

Probably could have been put in a more appropriate thread category..

[Rill Cosby]

Things I forgot to mention before and a little more information.

The engine does experience dieseling after cutting off the ignition with the key every now and then. The fan spins in a counterclockwise direction and white smoke emits from the carbs.

In my first post I was worried about the carbs maybe overflowing due to a crack in the float but forgot to mention that no fuel is coming out the carbs mouth. A friend mentioned the idea of one of the needle and seats maybe not sealing and fuel being let into the intake manifold. I checked for fouled plugs and smelled for any fuel on them without finding any problem there. It's being ran and has been ran on 89 octane since I've had it.

After some stalling I checked the fuel bowl levels on each, both even, but they seem to only fill about a 1/4 of the bowl up no matter how I adjust the float levels. Levels are same after ran and after stalling.

I measured a consistent 5 p.s.i right before the fuel rail with a fuel pressure gauge.

I will do a better measurement of vacuum readings tomorrow. Still can't find any hints of leaks with starter fluid.

A picture of the distributor rotor, of what I assume is right for TDC, for ghits and siggles.

Ignition coil was replaced with no change in performance because I like wasting money.

[SteveJ]

You measured 5 PSI? I think you need to dial that down a bit. By the way, do you need to use the choke to start the car?

[Rill Cosby]

I never had to use the choke since she starts up right away when left alone for a bit. The choke doesn't seem to help at all when trying to start her back up after she stalls out.

[SteveJ]

The stock mechanical pump is designed to have about 3.4 to 4.3 PSI (values rounded). (See 1972 FSM, EF-3.) Can you dial your pump down some or install a fuel pressure regulator?

[Rill Cosby]

Man, forgetting my FSM has been the biggest mistake of this whole entire situation. Don't rub it in haha.

It's worth a shot. I might not be able to get my hands on a regulator today but I'll dig around for the stock pumps and try that. It's tough to think that too much fuel pressure could be the issue.

[SteveJ]

It is impossible to forget the FSM as long as you have an internet connection available. It will ALWAYS be with you. If you don't understand what I'm saying, read my signature.

By the way, even if the fuel pressure isn't the direct cause of the problem, it is best to minimize the number of variables at play by having as many conditions as possible set at the factory specifications.

Edited September 16, 2013 by SteveJ

[Rill Cosby]

You're absolutely right. I should have stated before that it's hard for be to believe that fuel pressure could be AN issue, instead of THE issue. I wasn't able to get my hands on a regulator today but I will try for tomorrow.

I've seen the FSM on XenonS30 before and this is the first time it actually opened in WinRar for me so thank you for that.

Any other variables to maybe look into while I'm out wrenching on it tonight?

[SteveJ]

Check to make sure the oil doesn't smell like gas. Also, I can't remember, what does the FSM say about the symptoms of a plugged PCV valve?

[Rill Cosby]

No smells of gas in oil. I couldn't locate anything in the FSM about symptoms of a plugged PCV valve, just some information on how to diagnose if it is working properly. Which does seem like it is working fine.

Tonight I blocked off the fuel line, popped open the float bowl lids and just spoon fed the bowls with fuel just to see what it would do. Same symptoms, runs fine until it warms up.

Steady 17 in Hg through the intake manifold where it would normally connect to the brake booster line.

About 12-13 in Hg through the PCV valve.

[rob240z]

Could your chokes/choke be stuck open?