The Ford Barn

Ed in Maine · 02-17-2021, 11:15 AM

I was recently helping a club member with a carb issue (dripping) and I ended up creating a new problem, "stalling on stops." I had removed the carb, cleaned it and installed clean, flow tested jets. This is the first time I have encountered a problem after installing flow tested jets and I found the cause.

There are several things that may contribute to stalling on stops. The carb bowl level should be set at 5/8 in. – 11/16 in. and there should be no vacuum leaks at the manifold or at the carb flange. The idle mixture and the idle speed should be set to specifications.

When the Model A is brought to a stop, the throttle plate suddenly closes. All gas flow from the Main Jet and the Cap Jet ceases. The only possible source of gas to the engine is from the Idle Jet. However, in the first few seconds after taking your foot of the accelerator, the Secondary Well is nearly empty because the Cap Jet has used all the gas from the well. The Secondary Well is the only source of gas for the Idle jet and a brief moment is required to get the level in the well higher. The length of time for this potential lack of fuel supply to the engine will be a function of the sizing of the Compensator jet, the Idle Jet and how far open the GAV is. The GAV will have been set for normal operation and most likely will not be set to eliminate the lean mixture on a stop.

The sizing of the Compensator and the Idle Jets becomes important in this situation. The published flow range for the Compensator is 138 - 142 ml/min. and the Idle Jet is 44 – 48 ml/min. I have found that if the Compensator Jet selected is at 138 ml/min. and the Idle Jet is at 44 ml/min. (both at their low end ranges), the length of time of the lean mixture on a stop may be excessive and the engine will stall. In a recent experience, the jets were changed as follows: Compensator at 141 ml/min and the Idle Jet at 47 ml/min. The nuisance stalling was eliminated. I strive to size jets at their mid flow ranges but in this case, the selected jets I put into this carb brought two jets together sized at their minimums.

In my experience, I strive to set the carb jet flows for a good balance of reliability and economy as follows:
Idle Jet: 46 – 47 ml/min.
Compensator Jet: 140 – 141 ml/min.
Main Jet: 155 – 156 ml/min.
Cap Jet: 160 - 185 ml/min.

I don’t like to see the Main Jet much higher because it reduces the need to add some fuel through the GAV for best performance. I don’t like to see the Idle Jet greater than 48 ml/min. because the higher idle flow forces you to open the idle adjusting screw too far for a good idle. The amount of flow through the Cap Jet is not very important just as long as it passes all the flow coming though the Compensator and some through the GAV. The combination of flow through the Compensator and the Main Jet should be about 295 ml/min.

I felt that if some you are experiencing stalling on stops you may not have considered looking for jet problems. Ed

02-17-2021, 11:15 AM	#1
Ed in Maine Senior Member Join Date: May 2010 Location: Washington Cty., ME or Flagler Cty., FL Posts: 1,106	Engine Stalls at Stop Signs I was recently helping a club member with a carb issue (dripping) and I ended up creating a new problem, "stalling on stops." I had removed the carb, cleaned it and installed clean, flow tested jets. This is the first time I have encountered a problem after installing flow tested jets and I found the cause. There are several things that may contribute to stalling on stops. The carb bowl level should be set at 5/8 in. – 11/16 in. and there should be no vacuum leaks at the manifold or at the carb flange. The idle mixture and the idle speed should be set to specifications. When the Model A is brought to a stop, the throttle plate suddenly closes. All gas flow from the Main Jet and the Cap Jet ceases. The only possible source of gas to the engine is from the Idle Jet. However, in the first few seconds after taking your foot of the accelerator, the Secondary Well is nearly empty because the Cap Jet has used all the gas from the well. The Secondary Well is the only source of gas for the Idle jet and a brief moment is required to get the level in the well higher. The length of time for this potential lack of fuel supply to the engine will be a function of the sizing of the Compensator jet, the Idle Jet and how far open the GAV is. The GAV will have been set for normal operation and most likely will not be set to eliminate the lean mixture on a stop. The sizing of the Compensator and the Idle Jets becomes important in this situation. The published flow range for the Compensator is 138 - 142 ml/min. and the Idle Jet is 44 – 48 ml/min. I have found that if the Compensator Jet selected is at 138 ml/min. and the Idle Jet is at 44 ml/min. (both at their low end ranges), the length of time of the lean mixture on a stop may be excessive and the engine will stall. In a recent experience, the jets were changed as follows: Compensator at 141 ml/min and the Idle Jet at 47 ml/min. The nuisance stalling was eliminated. I strive to size jets at their mid flow ranges but in this case, the selected jets I put into this carb brought two jets together sized at their minimums. In my experience, I strive to set the carb jet flows for a good balance of reliability and economy as follows: Idle Jet: 46 – 47 ml/min. Compensator Jet: 140 – 141 ml/min. Main Jet: 155 – 156 ml/min. Cap Jet: 160 - 185 ml/min. I don’t like to see the Main Jet much higher because it reduces the need to add some fuel through the GAV for best performance. I don’t like to see the Idle Jet greater than 48 ml/min. because the higher idle flow forces you to open the idle adjusting screw too far for a good idle. The amount of flow through the Cap Jet is not very important just as long as it passes all the flow coming though the Compensator and some through the GAV. The combination of flow through the Compensator and the Main Jet should be about 295 ml/min. I felt that if some you are experiencing stalling on stops you may not have considered looking for jet problems. Ed