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Too Rich when hot


240ZMan

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I have an ongoing issue that I have run out of ideas trying to fix. I have a 73 240 with 72 carbs. The symptom is the following: when the engine compartment heats up enough, the engine begins to stumble at idle and sometimes eventually stalls. The rest of the time all is fine. For example, if I drive around the block a few times and then park in the garage and let it idle, and the air temp is above 60 degrees or so, after several minutes the idle will begin to slow down below 500 rpm and inconsistently change speed up and down. The exhaust will begin to smoke a bit as well. If I open the hood and let it idle within a few minutes all is well as things cool.

Today I tried an experiment when it began to stumble. I gradually turned the mixture screws in until they were all the way closed. Most (not all) of the stumble went away and the smoking was reduced. My conclusion is that something is causing my idle mixture to go too rich when the heat under the hood rises. Of course, with the mixture screws all the way in it didn't want to drive worth a darn.

Here are some of the things I've tried with no change:

- new ZTherapy carb bodies, nozzles, hoses, float chamber needle and seats.

- rebuilt the rest of the carbs per their tape.

- bought another set of used carbs, rebuilt them, same issue.

- checked fuel pressure when problem occurs. With mechanical only, pressure is 3-4 psi. With the electric only (I bypassed the mechanical), pressure is ~2.5 psi.

- replaced fuel filter.

- electronic iginition timing is solid at 12 btdc at idle.

- bypassed hot water to intake manifold.

- I can blow back through the fuel line from the gas tank without too much effort.

- I removed the return fuel line and ran into a cup to make sure there was good flow and the fuel in the rail wasn't getting too hot.

- The PO looks to have had all the fuel lines around the gas tank replaced as the cloth around them looks brand new and they are supple.

- checked to make sure the nozzles are all the way up when the choke is off.

What really confuses me is that both sets of carbs exhibit the problem. The only common element is that the 2nd set of carbs didn't have floats so I swapped them over. Today I tried raising and then lowering the levels almost 1/4" but it made no difference.

Given that it's winter and we don't have too many warm days this is more of an iconvenience (and annoying!). But when the spring comes I know this is going to become a major issue when driving in stop and start traffic.

Any suggestions?

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Is your fuel pump ok? check the pressure. also, is your fan clutch working? if all else fails, get an electric fuel pump. that will keep the pressure high so the fuel won't boil in the rail or carb. also try taking off the blower from a 280zx and mount it so it keeps the hot air away from the carbs.

just some more suggestions

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Increasing the fuel pressure will make the mixture fater(richer).

240ZMan are you sure the rich mix is caused by the idle. Not the other way around? Low idle causes poor burn of mixture. Resulting in High HC's, which can be interpeted as rich. A Rich mix would be High CO. Of course this would require a

n exhaust gas probe. CO burns the eye's and throat. Z's always smell a little nasty at the tailpipe. I did smogs for the states Referee program and I could never tell with a Z by smell. On the gas analyzer they run high CO and then Higher HC's. What do the plugs look like? Hows the ignition system? Are you running your PCV system? Check the function of your PCV valve. Pull it out and shake it about does it rattle? Is it gummed up? If so spray some carb cleaner in it to clean it out. Very intresting. Hope some of this gets you closer to the end solution. :devious:

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Originally posted by Sonic

Is your fuel pump ok? check the pressure. also, is your fan clutch working? if all else fails, get an electric fuel pump. that will keep the pressure high so the fuel won't boil in the rail or carb. also try taking off the blower from a 280zx and mount it so it keeps the hot air away from the carbs.

just some more suggestions

I've tried running both the mechanical and electric together, separately, and even bypassed the mechanical just to run the electric alone. There was no difference I could notice. I did mount a fuel pressure gauge in the engine compartment for a week or so and found that fuel pressure wasn't any differenct when the stumbling occured.

I also tried running the engine with the return fuel line into a cup and the flow seemed reasonable. Also, when the tank is full, at idle I can hear the fuel returning into the tank. At this point I've concluded that my fuel delivery system is ok, given I'm out of ideas of what to check next.

You mentioned a "blower" from a 280zx. I'm not familiar with that. Can you describe it? Not sure from the sound of it that it would solve the underlying issue, but it might give me another clue.

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Originally posted by frank13

Increasing the fuel pressure will make the mixture fater(richer).

240ZMan are you sure the rich mix is caused by the idle. Not the other way around? Low idle causes poor burn of mixture. Resulting in High HC's, which can be interpeted as rich. A Rich mix would be High CO. Of course this would require a

n exhaust gas probe. CO burns the eye's and throat. Z's always smell a little nasty at the tailpipe. I did smogs for the states Referee program and I could never tell with a Z by smell. On the gas analyzer they run high CO and then Higher HC's. What do the plugs look like? Hows the ignition system? Are you running your PCV system? Check the function of your PCV valve. Pull it out and shake it about does it rattle? Is it gummed up? If so spray some carb cleaner in it to clean it out. Very intresting. Hope some of this gets you closer to the end solution. :devious:

Frank13, thanks for the suggestions. You made an interesting point regarding the cause and effect of the rich mixture at idle vs. engine speed. Not sure if this matters, but when the engine compartment doesn't have the extra heat in it, the idle speed is rock solid at around 700 rpm and very smooth. As the heat builds up it starts to stumble intermittently, and gradually slows to 500 rpm or less. FWIW, before I put the rebuilt carb bodies on, my throttle shafts were leaking and under these conditions the idle would slow so much that it would eventually stall.

I have confirmed that the pvc valve is loose and appears to flow well. I have a new hose on it as well.

The spark plugs look just fine. They have that nice tan color and are even across all 6. Like I said, it's only at idle, and only when the heat builds up in the engine compartment.

I have a feeling I've overlooked something, or made a bad assumption somewhere, but as I've been chasing this since last summer, I'm having a hard time bringing a fresh perspective to it.

Any other thoughts? Thanks in advance!

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Originally posted by gramercyjam

Heat shields between the intake/exhaust still installed?

If you mean the one between the manifolds and the carbs, then yes. Of course, it's for a 73 and I have 72 carbs, but I didn't think that would matter.

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  • 1 month later...

I have this problem with my triple carbs! They heat up, then things go wrong!

A very simple test is, when things are hot and the idle suffers, poor some tap water all over your intake manifold - as the water cools things down, you might notice your idle returns to normal.

If it DOES return to normal, you know that HEAT is the problem.

In my case, it is caused by the intake manifold sitting very close to the exhaust headers (in one case touching!). With no heat shield in place, the heat simply transfers from my exhaust to my intake manifold and heats things up.

Even if you've got a heat shield, if my suggested test reveals that heat is the problem, you should try a heat-resistant coating on your exhaust headers (e.g. "Jet Hot" coating, or heat-wrap material).

The 280zx "blower" that was suggested is simply a fan that will mount to your engine quite nicely, and blow away the hot air. Not sure whether it would be sufficient in my case, but perhaps in yours?

Good luck!

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Moyest, sorry to hear you are having a similar problem. But I'm not sure ours are the same issue. My engine is basically a stock 73 with round top carbs. I have the stock heat shield in place as well.

I have been playing around some more and have discovered something interesting. I have both a mechanical and electric fuel pump. I put a switch in the cockpit to control the electric because it's rather loud and most of the time it's not needed.

As a test, I bypassed the mechanical pump and ran only off the electric. I've found that if I let the car idle long enough to buld up heat in the engine compartment, the idle will begin to stumble. The engine acts as if the mixture is suddenly very very rich. What's interesting is that if I turn off the fuel pump, within 30 seconds the idle is back to normal! If I turn the pump back on, the stumble returns almost immediately. I have a fuel pressure gauge temporarily installed and can see that the pressure stays around 2-3 psi with the pump running.

Today I put a pressure regulator in but it didn't make any difference regarldless of where I set it. I've double checked my float levels and even swapped a different set of floats as well as needle/seats from a spare set of carbs I have. Still no difference.

At this point I am completely stumped. How can the warming of the carbs cause them to go rich, and then recover by turning off the fuel pump?

Any thoughts would be greatly appreciated! :stupid:

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Originally posted by Sonic try taking off the blower from a 280zx and mount it so it keeps the hot air away from the carbs.

Does that blower really do anything. I have a 78 280Z and a 79 810. They are both fuel injected and they both have no blower. In fact from 75-78 there was no blower on the Zs. The 810s and Maximas never had them. I think that they are just one of many unnecessary gadgets from the 280ZX. IMHO it's just another useless item that draws current from your electical system. I'd look elsewhere for a solution.

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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:

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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.

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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.

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