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Whatn is the MA4-SPA plugged opening used for?

hcccs

hcccs

Well Known Member
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This is my spare Marvel Schebler MA4-SPA carb. I have the same on my RV-4. The plugged hole at "A" is where the carburetor temperature sensor is located. What is the plugged opening at "B" used for?
My manifold pressure sensor is connected to cylinder no 4 but on my previous homebuild with a MA3-SPA it was connected to the carburetor at "A". What is the difference between having MAP sensor connected to the cylinder and the carb?
 
correct on A. B is, I believe, a remnant hole from final drilling/machining. The plug just covers the hole that was necessary to drill from the outside. IIRC, it is where the main vertical fuel channel turns to horizontal to feed the opening in the venturi. You don't want a MAP sensor feed in A, as it is before the throttle blade. Need to drill a hole closer to the base that is on the plenum side of throttle blade. A plenum based MAP location will give more stable readings. The port near the intake valve sees fluctuations due to the velocity changes when the valve opens and closes.
 
Last edited:
lr172 said:
correct on A. B is, I believe, a remnant hole from final drilling/machining. The plug just covers the hole that was necessary to drill from the outside. IIRC, it is where the main vertical fuel channel turns to horizontal to feed the opening in the venturi. You don't want a MAP sensor feed in A, as it is before the throttle blade. Need to drill a hole closer to the base that is on the plenum side of throttle blade. A plenum based MAP location will give more stable readings. The port near the intake valve sees fluctuations due to the velocity changes when the valve opens and closes.
Click to expand...

Yes, you're right. The hole must be upstreams of the valve to give any reading.
 
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