Author Topic: Dolmar 7900 Porting Info  (Read 8605 times)

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Offline srcarr52

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Re: Dolmar 7900 Porting Info
« Reply #180 on: September 25, 2014, 10:37:35 am »
Infinite radius does not have the traditional radiused ring to create the venturi. Instead, the main jet is placed at the smallest diameter of the gradually sweeping radius machined into the throat of the car, it Looks totally smooth.  I really don't think this new Infinite radius design provides a strong and sudden pressure drop as needed to atomize fuel well at low to mid rpm. Just speculation, but I can't see this working well at less than max rpm.  I'll see if i can find a pic or drawing

Ok, I understand what they are and I agree with you.  It's the same problem I have with the 304 and the 384 or any bored out carburetor. 

The low pressure region created by the venturi or lack there of is too weak to meter the fuel correctly.  Instead these carbs rely more on you setting the pressure of the needle (ultimate the fuel pressure under the diaphragm) correctly to change the full throttle mixture.  Everything else besides WOT and idle are pretty much worthless. 

The venturi is not only creating a low pressure region to atomize the fuel but it's also creating an area where the pressure is directly related to air flow (engine demand).  Since this area is the outlet for the fuel it changes the fuel flow based on engine demand.  No contraction (venturi) no engine demand fuel metering, well I shouldn't say no, but much less and unstable.

For instance I took a stock briggs 5hp carb (straight bore design) and modified it by boring only the inlet and exit to create a venturi, it picked up around 2hp on my dyno on a engine that produced 10hp with the stock carb and 14hp with a tilly 334.

 

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