Two quick questions......

[mally002]

I'm not sure if this wonderful heat wave has anything to do with my question, but here goes.

1. After driving the 240 today in 96 degree heat I stopped for about 45 minutes and when I returned it took about 6-8 attempts to get the car started.....It would just keep turning over, hit a few times, then stall out...This has never happened before.

2. Since it has been very warm my car will not run smooth unless I keep it fully choked...Even after it's warmed up if you begin to push the choke back in the engine runs really rough, like it's missing and hesitating.

I'm a rookie on mechanics so any help would be appreaciated.

Thanks,

Randy

[xray]

Sounds to me like what most people describe when they get vapor lock. I couldn't stomach the thought of driving in 100+ degree heat without air, so I've not yet experienced the phenomenon myself.

As a SWAG to the second question, if it's vapor lock causing the first situation, driving with the choke on may allow for a higher proportion of liquid gas to make it into the carbs which is why it ran when only choked. I forget--do you have an electric pump at the tank or only the mechanical?

[mally002]

Hmmmmm,

I'm not sure on the pump I'll have to go look, but thinking it's mechanical. I appreaciate the input Steve.

[hls30.com]

1) Sure sounds like vapor lock to me-but

2) usually it clears up shortly after starting(until the next rest). Maybe you have two temperature related things happening-is your tach running stable or jumping around(suspecting primary ignition)?

Will

[mally002]

Tach appears to be fine, very stable.

Once I got it started it was great, it ran smooth unless I pushed the choke back in, it would hesitate and stumble and then as soon as I pulled back on the choke it ran great......I ran it the rest of the day with full choke and no problems. It seems weird that after 20 minutes of driving pushing in the choke causes rough,stumbling engine.

After doing some searching it does seem like my starting problem was VL....but the car had sat for 45 min.

I'll check on the pump Monday.

Thanks guys.

[xray]

IIRC, that the car sat for 45 minutes contributes to the VL phenomenon, as underhood temps continue to rise with (a) hot ambient temps and ( hot engine temps not being ventilated by the fan or outside air, causing some of the fuel in the rail to vaporize and pose as a blockage. As a test, some propose popping the hood to allow engine heat to dissipate, then attempting to start the car. Putting an electric fuel pump on helps in that there is added mechanical force to overcome the vapor blockage (may be why I've not experienced it as I put a new one on during the resto).

Good luck, and stay cool (102 here again today--yuk!)

[sblake01]

Also could be that his mechanical fuel pump isn't providing enough pressure, (i.e. malfunctinoing), to overcome the vapor lock because, as Will said, it should clear up shortly after starting. Check your fuel pressure.

[Carl Beck]

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Nissan's Definition Of Terms:

To end confusion, the terms "vapor lock", "percolation" and "vacuum break" are defined here.

VAPOR LOCK:

Vapor lock causes a LEANER than normal air-fuel mixture. It affects the suction side of the mechanical fuel pump. The gasoline vapor-liquid ratio in the fuel line becomes greater than the fuel pump is able to handle. The fuel pump sends less fuel to the carburetor, causing a lean fuel-air mixture.

PERCOLATION:

Percolation causes a RICHER than normal air-fuel mixture. It affects the carburetor float bowl, and the high pressure fuel line (the outlet side of the fuel pump), and may cause hot starting problems. Percolation can come about in several ways:

1. Heat soak-back into the fuel pump and high pressure fuel line can raise the line pressure enough to unseat the float bowl needle valve. This condition is known as "after-fill".

2. Float bowl evaporation may lower the float level while there is residual pressure in the fuel line. This pressure then forces an excess amout of fuel into the float bowl.

3. Fuel evaporates in the float bowl, and the resulting vapor pressure may not vent out quickly enough. The high pressure developed in the float bowl forces liquid fuel into the intake manifold.

VACUUM BREAK (73 carb.s...cjb)

When the choke is closed (except for the small air gap) and the engine starts, the vacuum created by the engine starting will try to completely shut the choke valve. This vacuum is broken by the vacuum break diaphragm, which holds the choke open a small amount. The vacuum break diaphragm operates by manifold vacuum and is connected to the choke valve by linkage.

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hummm.. Just FYI

FWIW,

Carl B.

[Ttiger]

The '73 240 and early 260 had a bad "hot soak" problem which is why all the fuel lines got wrapped in that astro-foil stuff as part of a "fix" Nissan put together. You have to hold the gas pedal about 3/4 while you crank it when the motor is hot and it's hot outside. Once it starts, it will take a few seconds to clear out and then should be good-to-go.

You might also check the float levels in the carbs. to see if the're off

[7277]

i know this is a little out there, BUT.... sometimes my fuel nozzles (the black 'Y' tube)stick in the 'down' (fully choked) position. when choke cables are pushed in the car runs like C-R-A-P...due to flooding. perhaps the excessive engine heat caused some sticking down there. when you get the car running pop the hood (choke cables pushed forward after warm up) and check that the nozzles are up tight against the bottom of the mixture knob...