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engine stummbling when hot outside


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since i live where the summers are real hot (108 degrees or more) it seems to make my engine stummble and shake when its time to accelerate from sitting for about 30 seconds to about 2 minuets. this only happens during the day time, doesnt happen in the morning or at night when the outside temp is cooler or no as hot. im assuming the problem is my carb is either dumping too much gas in from built up pressure from the heat or its evaporating quickly. i could be wrong but does anyone have any ideas whats going on?

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The carb is borderline lean. The high temp leans it out further.


The carburetor adjustment should be done w:ith the stock Datsun air cleaner on and working correctly. It regulates air temp to 122 degrees at idle, so you get the same performance summer or winter. And when you floor it, it opens up to cold air. Best of both worlds.


I'd you are running an aftermarket air cleaner, adjust the idle mixture when its over 100 degrees.

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You are also running on very hot under hood air. The stock air cleaner has a snorkel and  cool air supply from in front of the rad. It also has an idle compensator that bleeds air into the intake to compensate for an abnormal over rich condition at high idle temperatures.


Get your stock air cleaner and cut out a weber pattern and install it. Sealik  on here did this. Looks stock and bigger better air filter. Even the valve cover vent connects back up and there is fresh cool air.

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I was under the impression hot underhood temps tends to make a carb engine go rich.  High temp, gasoline evaporates easier.  Hot air is less dense. 

A carb does not measure air density, only the speed of the air going through it.


We are spoiled by the modern fuel injected car engine we have now.  Before the engine even starts, it has measured the barometric pressure, engine coolant temperature, air temperature, and a whole lot of other factors.  As soon as it can, the oxygen sensor is telling the computer to make adjustments to the injector timing to make the engine run better.


A carburetor is just a pipe, with air being sucked through, with a pinhole, that the moving air sucks some gasoline into.


Now that is is comfortable temps in Oregon, summer weather, t-shirt kind of weather, my Datsun runs pretty good, once it is warm.  But even now, I try to let it warm up for a minute or two, before I start driving it easy, and hopefully down hill, at first.  I have a 521 with a manual choke.  My truck stumbles a little when cold, and sometimes after a longer run down the freeway, at the bottom of the exit ramp, it has a stumble, or even dies when I try to move again at the stop light. 


You can tune a carb to run pretty good, but the engine is just going to have a little more "charactor" that a modern fuel injected engine, that is constantly correcting itself.  And if the atmospheric conditions change, the carb could need adjustment again.  Maybe that is why Datsun recommended a minor tune up every 3,000 miles, and a major tune up every 12,000 miles on 521 trucks.  I do not know service intervals on 720 trucks.

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