I'm going to ramble here - it's part of deducing the problem - got to cover all the possibilities.
The laws of physics govern how or if an engine runs - it takes air and fuel - together and in the right amounts - control the air fuel mixture and you control the speed the engine runs at. For the engine to accelerate, it has to get more air mixed with the correct amount of fuel - where is this source of air? A vacuum leak may increase the engine speed a little, but without mixing fuel at the point of the leak, the engine will also surge and usually spark knock because the leak causes the fuel mixture to become lean.
When looking in the carb, you saw the slide rise as the engine speed increased. This is normal - technically, the operation of the carb is this: the butterfly valve is the primary control of the air/fuel mixture allowed into the combustion chamber. The slide and slide needle is a secondary control of the air/fuel mixture. When the engine is running at a constant speed, the vacuum controlled slide rises to a position dependent on the velocity of air through the carb. As an example, let's say the butterfly valve is half open and the slide is also half open - if the throttle is opened gradually, the slide also rises gradually as the engine speed increases. As the slide rises, more air is allowed into the engine and the needle in the slide is withdrawn from the needle jet which meters more fuel being mixed with the increased volume of air which is now traveling at a higher velocity. However, if the throttle butterfly is opened suddenly from the 1/2 position to the wide open position, the vacuum is reduced and the velocity of the air passing through the venturi slows - the slide drops, partially blocking the air flow and reducing the amount of fuel mixed with the air. The engine does not slow down because the primary air control is open and the slide has a stop set at about the 1/4 open position. With no load on the engine and the primary air control wide open, the engine will rev to near maximum RPM. If you want to test this - remove the spring from the slide and replace it with a piece of material that will prevent the slide from rising. Start the engine and open the throttle slowly to find out how high the engine will rev. When the engine fails to increase RPM, from that point and on changes in throttle position will not affect engine speed. NOTE: when shutting the throttle from wide open to the normal idle setting in this exercise the engine may stumble or die due to the fuel air mixture being overly rich upon the sudden closure if the primary air control. If you close the throttle slowly, it should return to normal idle.
Now, the engine will not accelerate if it is only given more air or more fuel - it has to get a proper ratio of air to fuel. If it is only given more of one or the other the performance will suffer and the engine will either increase it's speed only slightly or die.
Therefore, where is the engine getting the increased air/fuel and why? Answer these questions and the engine will perform as designed. Cam timing, spark timing, compression, grade and quality of fuel, jetting, air density and engine temperature only affect idle speed a few thousand RPM at most. The properly performing spark plug has a less than 1% effect on engine idle speed, acceleration or maximum RPM. 98% of idle speed is controlled by the throttle.
Definition of throttle; Definition of throttle - transitive verb -Definition of throttle - transitive verb
1 a (1) : to compress the throat of : CHOKE
(2) : to kill by such action
b : to prevent or check expression or activity of : SUPPRESS
2 a : to decrease the flow of (something, such as steam or fuel to an engine) by a valve
b : to regulate and especially to reduce the speed of (something, such as an engine) by such means
c : to vary the thrust of (a rocket engine) during flight
I did experience 'run-away' acceleration on an old Kawasaki 2 stroke motorcycle. It was a heat related issue similar to what you have described. The engine was idling normally - already hot from 1/2 an hour of riding on a 90 degree day - sitting at a stoplight, suddenly the engine started to accelerate. I immediately hit the kill switch shutting off spark to the plug - it continued to run (dieseling) - I turned the throttle wide open - no change. I locked both brakes and let the clutch out slowly to kill the engine. After the bike cooled for about 20 minutes, it started and ran normally again. 2 stroke gasoline engines were derived from diesel engine design, so dieseling is not so uncommon.
In a 4 stroke engine, runaway acceleration is always attributed to throttle position failure - I could find no documented cases of runaway acceleration in gasoline engines where the carburetor or throttle body was not the culprit.
Now I suppose if you had a fuel that was supplying oxygen as well as hydrogen (water) (it's the hydrogen in gasoline that burns - the rest of whatever comprises gasoline ends up being pollution). Alcohol absorbs water - have you drained the fuel tank and tried a fresh fill of non-ethanol 91 octane or higher gasoline?
Perhaps under ideal conditions a blend of water contaminated alcohol and gasoline in a hot engine........
If you were willing - I would like to examine one of your carbs. If you are conducive to this idea, let me know. I'll PM you with an address and instructions. I'll even pay the shipping.
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