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carb question

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CFM or CFPM stands for cubic feet per minute of liquid that flows across any given point, and unless you have add ons that will efficiently burn the gas that is being fed to that engine, it will not only waste gas, it will cause wash out.  Changing out parts in that carb to try to make it flow less gas is Southern Engineering 101 IMO.  Either heat up that engine, or sell that big carb and buy the one meant for that engine, which is a Motorcraft 4100 if it is a 4V and a Motorcraft 2100 if it is a 2V.  You should already have a 1" spacer under the carb.  If not, get one.  The phenolic spacer is the best mainly because it is not aluminum and will not allow the intake heat to cause vapor lock. 

Yeah, well now the 670 cfm street avenger is sitting on top of a 351c bored .030 over so it should be just fine ;D

There ya go......


--- Quote from: Soaring on August 25, 2009, 05:54:48 PM ---Changing out parts in that carb to try to make it flow less gas is Southern Engineering 101 IMO. 

--- End quote ---

Well I am going to respectfully disagree.  Changing jets and accelerator pumps is not an effort to make the carb flow less, it is an effort to calibrate the carb to the engine's needs.  A carb will never draw more air than the engine can suck through it.  If a normally aspirated engine can only use 500 cfm it will only draw 500 cfm.  Jetting adjusts the fuel flow for the needs and demands of the engine.  Changing an accelerator pump is again the same adjustment to make the part  in tune with the engine's capability or need.

The addition of the spacer, as you noted is to keep the carb cool, but the increased intake length allows the air to move faster and allows an engine to utilize more of the carb's capacity.  It increases the air flow velocity to compensate for the less restrictive bore of the larger carb.  Note I am talking velocity not total flow.  Total flow will not increase as the engine is only drawing air in as fast as the pistons can suck it through the carb.  Increased velocity helps maintain fuel atomization.  Excessively oversized carbs allow fuel to drop out and burn poorly.

The autolite 4100's are really wonderful carbs, but the stock carb on that engine was a 4300 spreadbore and had flat spots and hesitation issues.

I know this is all moot for Tim, but I would not want someone reading this to accept your statement at face value.  I tend to agree with the proposition that an overly large carb is going to hurt performance, but an undersized carb is also going to limit the engine's performance.  670 CFM is large for a 302 and may be large enough that it can't be tuned to run well.  But if someone had the same set up, taking the time to dial in the jets and accelerator pump makes more sense to me than just changing the carb.  I think we should strive to be good mechanics in how we approach problems and that means diagnose the problem and repair it, not assume and swap parts.

I'm southern and I would as soon you found another term to use to disparage the next comment of mine with which you might disagree.

I just put the 670cfm street avenger on my 289 last night (.030 over, roller rockers, cam, headers, etc.) and baby "HANG ON!"  Its so new I can't tell you too much, but so far it runs VERY smooth, no hesitation, and its fast.   Timing is way up there at 40.
I'm happy! ;D


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