I have been working on a better mousetrap for use on our smaller engines.
We all know they came with 1 BBL carbs from the factory. A common thing to do is drop a 4 BBL or a 2 BBL on and get it going and have fun. The performance is increased but most of the time fuel economy goes down.
I have ran 2 BBL's, 4 BBL's and even progressive 2 BBL's. The one that gave the best mileage was the progressive 2 BBL.
One thing the progressive 2 BBl lacked was full CFM at full throttle. The one I used was a Holley 350 CFM and later a Holley 500 CFM. The 2 BBl's are rated at a higher vacuum of 3 inch as compared to the 4 BBL's rated at 1.5 ". This would make my 500 be similar to a 375 CFM 4 BBL. Not enough for a healthy 250 let alone a good running 292.
To run a 450 CFM 4 BBL would be good. There are a few problems. They are expensive. And they provide too much air flow from a little foot feed needed to keep a 250 engine running at highway speed. For instance: to maintain 55 MPH with the old 1 bbl required the throttle to be open about 1/3 open. This put the carb in a state that allowed proper mixing of the air and fuel. A 4 BBl would only be operating on the primary's for 55 MPH and would only need to be opened about 1/4 or less to maintain speed. This does not allow the carb to mix the fuel/ air as well. Less efficient.
Here is the idea I came up with and am testing. Let's take a common 600 Holley vacuum secondary carb. Block off the one of the primary throttle blades and the associated air/ fuel circuits. Also the corresponding accelerator pump nozzle. Essentially make it into a 3 BBL. It would then be a 450 CFM with a small single primary for cruising and have a large secondary to kick it in the *ss.
A 750 would convert to a 562CFM , good for a healthy 292 or strong 250.
I have converted a 600 and tested it. So far it is working better than I had imagined. Still have some tuning to do with the secondary opening rate. On my wagon it pulls harder and is able to hit RPM's quicker than with the 500 2 bbl Progressive.
What are your thoughts?
