Stop and Stall

[drolston]

What might be the cause of engine stalling at a stop light. Idle mixture too lean? Too rich? Sticky intake valve? When acting normal I have a smooth idle at about 650 rpm. Doesn't happen all the time, but takes spells of stalling. Thank goodness for manual throttle control.

[aussie merc]

how long since you did major tune up including carb rebuild some times if the float level drops when you stop the fuel will inertia away from the jets a momentary lean out and it will stall works in your favor at times when you accelerate you get the fuel pushing against the jets and o momentary rich up just a thought as carbs are so sensitive to float level and fuel pressure and is often overlooked

[chap52]

My first suspect would also be float level. Chap

[Flathead Fever]

Stopping sloshes the fuel around in the bowl and if the level is off it can stall. Its a good chance that is the problem but before you pull the carb top off I would start with pulling the air cleaner and giving the carburetor a little spray of carb cleaner or propane. If the idle comes up it is too lean, if the idle goes down it is too rich. "Gently" turn the Air fuel mixture screws in counting the turns just to see if they were both even. While they are turned in paint a little white dot on the head of the mixture screws so you can keep track of how many turns out they are so you can keep both screws the same. Open them back up to where they originally were making sure they are both even. Then give the carb a small spray of the carb cleaner or propane. If the rpm drops its lean. Turn them both out a 1/4 turn at a time. Each time spraying a mist of carb spray over the carburetor. When you have the mixture adjusted correctly spraying with the carb cleaner, starting fluid or propane will not change the rpm. If the idle goes down the idle was already too rich and the extra spray fuel is more than it can handle. Do the opposite and turn the screws in a 1/4 turn at a time to lean out the mixture. Once you have them set where the idle does not change or barely changes with the spray turn the screws out another 1/8 to 1/4 turn. This makes the idle air fuel ratio slightly rich to compensate for atmospheric changes, temperature and other things that effect the idle. Computer controlled cars can make air fuel adjustment to compensate for different situations but with a carburetor the best we can do is set the mixture "slightly" to the rich side.

Also before you start adjusting the air fuel mixture take the propane bottle (less of a mess than carb spray and it won't damage your engine paint) and flow a little propane directed at your butterfly shaft and carb base gasket. If they are leaking the rpm will come up. If the shaft is just leaking a little and you can still adjust the air fuel mixture with the screws I wouldn't worry about it. The correct thing to do would be bringing it back to the original fit. It depends on what your monthly restoration budget is. While your there make sure the carb mounting nuts are tight (not gorilla tight) just nice and snug.

If the idle air fuel mixture adjustment does not cure your problem then I would suspect the float level. Normally it would be checked first when your rebuilding the carb. If it is your problem, after you correct the problem your going to need to adjust your air fuel mixture so I have included the best way to do that. Always do your tuneups in this order. Set the point gap first. Next set your base timing. The very last thing you do is adjust the idle air fuel ratio. The reason why is the point gap changes the timing so it has to be done first. Timing does not effect the points dwell but it does effect the air fuel ratio so it is done second. Everything effects the idle air fuel ratio so it is always the very last adjustment you make.

If you have one of those little chrome hot rod air cleaners. Install it with the engine running to see if it has any effect on the rpm. If it causes the rpm to drop the air cleaner is too small and it is restricting the air flow. If it restricts it at idle imagine how much restriction there is at higher rpms. I had it happen to me when I installed one on a '34 1 1/2 ton truck. It did not have an cleaner when I got it and I wanted to go for a drive. I grabbed a little chrome 4" air cleaner off the shelf. The engine was running and I stuck the air cleaner (new) on and the idle dropped. I thought, that's no good at all. Not all of those little air filters flow the same. I think they are all bad idea on a single carb. Multiple carbs with multiple air filters is probably okay.

This is an OTC fuel enrichment kit that screws onto a small propane bottle. It has a little button you push to turn the flow on and off. And a knob to adjust how much flow you want. They are really neat because you take that hose and run it around the base of a carb or along an intake gasket. Its gets into a lot tight of places on modern cars. You even squirt propane inside the engine block if you suspect an intake gasket is leaking in the lifter valley. If you spray it in the oil fill cap and the idle comes up the intake gasket is leaking. First you have to remove the PCV valve to make sure the propane does bot get sucked out of the block. By law in CA, every smog repair shop must have on of these. when you rebuild a carb they want you to adjust air fuel mixture to the lean side. You adjust the mixture screws to the highest idle rpm which would be the ideal air fuel mixture but then you screw it up by turning the screws in to where the idle drops 50 rpm. Then you check it with the enrichment tool. A squirt of propane should bring the rpm up 50. Adjusted like this they run rough. We had Dodge K-Cars (JUNK!) in the fleet. If they were adjusted like the State wanted the engine would die at Idle when the AC compressor kicked on. Before the dyno testing started you had to set them like that to pass the idle portion of the smog test. Legally you have to leave it adjusted like that. The next day the driver would complain that the engine was stalling at idle. Then you turned the screws back out.

Propane tool kit on eBay. Look for a used ones at swap meets. Or make one of your own up with a hose on it. Ttat little push button on the brass "T" is handy. It keeps you from wasting propane.

https://www.ebay.com/itm/Otc-7148-Fu...wAAOSwPEFbkRo2

That's everything I know about adjusting air fuel mixtures.

[deuce_roadster]

My guess is the float level is too high.

[drolston]

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Center carb (97) was apart for another problem recently. Thoroughly cleaned and the float is set at exactly .5". Checked the idle mixture and found both jets set just a bit lean. I will give it a road test this week.