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 testing small engine carbs. 
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Joined: Tue Aug 28, 2012 5:04 pm
Posts: 12
Location: Georgetown TX
when flowing small engine carbs (like 24mm mikuni or tillotson style carbs that flow between 70 to 95 CFM I have been using PVC plumbing fixtures that plug into the bore adapter. Thinking about how calibrating the bench you shouldn't use the bore adapter should I be making a carb testing fixture that bolts on to the base instead of the bore adapter?

Sun Apr 17, 2016 11:27 am

Joined: Fri Jan 08, 2010 10:36 pm
Posts: 1171
Location: Maryland
I test Harley carbs on a fixture that has length to support laminar flow and that bolts directly to the bench top. My testing has proven this is repeatable for the testing i do of the entire intake tract on a ported head. Example the head may flow 300 CFM on the intake side at 28" and 650 lift with just a radius, but with intake and carb i may get 275 CFM if i take a reading of the DP of the intake between the valve ad the carb i will get a number say 17" h20. I i now take the carb off the intake setup and place it on my carb test fixture set the test pressure to the measured I/E 17" the carb will flow 275CFM. The point i am making here is that there is a pressure drop of some sort across the intake valve that the carb sees and it has a drastic affect on performance. In other words it shows just how important flowing the whole system is. In restricted class racing i would think the key is to reduce the pressure drop across the valve thus exposing the fixed size carb to a greater DP thus more flow.



Sun Apr 17, 2016 8:27 pm

Joined: Tue Aug 28, 2012 5:04 pm
Posts: 12
Location: Georgetown TX
thanks! I was concerned that using a carb adapter on the bore adapter would affect the reading due to vena contrata effect. (pardon my spelling) I will make a adapter like you did on the base.

Mon Apr 18, 2016 1:47 pm
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