Too Rich when hot

[frank13]

on 73 and above were used in combination with the mechanical fuel pumps to combat vapor lock due to heat soaks that lead to fuel starvation and mechanical fuel pump cavitation. Which leads to a no start/hard start condition. The heat soak evaporates the fuel in the lines before the carbs had a chance to do their jobs leading to the vapor lock in the lines. The factory installed the electric pump to combat this hot start condition. This is always what I have been told and considering the function of the '73's flat top boat anchors is reasonable. Except for the fact that all HLS30's have the electrical connector at the rear harness location for the pump and fuel level sender. Also my '73 with round tops in every '71 or '72 I have had it reside in has never had a fuel starvation or heat soak condition. Ever. If you want to know for sure what the motor is doing and when it's doing it for sure, make friends with your local Datsun tech at a shop. Or the local Smog Guy who likes Z's. Then check the z on one of thier Tail pipe sniffers when cold, hot, both pumps, mechanical pump solo, then electric pump solo. Print out each samples page to compare of the dyno or tailpipe probe. This is the ONLY way you will know for sure whether the car goes rich or lean when ever you do or make it do what it's doing when it goes wrong. Good Luck! :devious:

[Zhead]

That is a cryptic comment! Can you please explain what difference the oil in the dashpots will make, and what oil is better suited to prevent the problem?

The oil in the dashpots must be SAE20 preferably, but not thicker than SAE30. Others claim that ATF is good.

This is a basic fact! so it will make a lot of difference if you put SAE40 or 50 oil.

The piston that slides in the dashpots makes the mixture of air-gas that goes to the engine. The piston is balanced by is weight, upper pressured air in the dashpots, bottom atmospheric air that comes from the air filter, plus the spring tension. All this must operate correctly for the best performance possible.

I read a letter from a Zguy in Cal. He found out that the carb's will perform better by changing oil thickness according to summer-winter.

Hope this solve your problem, good luck.

[240ZMan]

An update: This weekend I made some changes that have improved things, but not completely solved the problem.

I put longer studs on the intake manifold and switched to the thicker insulators between it and the carbs.

I also plugged the coolant lines that heat the intake manifold.

I also took the mechanical fuel pump out of the fuel path and am running entirely on the electric. I did this so that the fuel wouldn't get the extra heat added by the mechanical pump.

I also retuned the carbs to get them as lean as possible. They surge a little until everything is completely warm. I get a lean pop downshifting from high revs now and then.

The result is that today where it was about 65 and sunny (the warmest it has been since last fall), when I let it idle for long periods, the idle would periodically stumble, but not nearly as badly as it used to. Something is still causing the mixture to fluctuate more rich when hot, but it's not as bad as it was.

I have a fuel pressure gauge mounted semi-permanantly now, and pressure is staying around 3 psi consistently.

Any ideas?

BTW, I'm using Marvel Mystery oil for the dampers, but don't know how that would affect this problem.