Author Topic: carb question  (Read 5137 times)

Tim175

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carb question
« on: July 21, 2009, 11:33:49 PM »
this is mainly for glen since he knows so much about carbs but if anyone else can help that would be awesome. on my 302 i have the holley street avenger carb i believe its the 670 cfm model but not 100% sure i need to run the #'s. but my question is: is that too big for my engine because when i accelerate too much it bogs down and stutters then goes as if its over carbureted or something. thanks if you can help

Soaring

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Re: carb question
« Reply #1 on: July 22, 2009, 02:17:42 AM »
If that is a stock 4V 302, then yes it is way too much carb.  You should be  running somewhere between 480 and 520 CFM. 

Tim175

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Re: carb question
« Reply #2 on: July 22, 2009, 11:25:04 AM »
woah alright then. anything i can do to make it not be too big other than stroke it or something? I didn't put it on the car the people that i bought it from had gotten the carb and the Edelbrock intake as a set off of ebay and stuck it on there, so i had no idea that it would be too big.

Soaring

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Re: carb question
« Reply #3 on: July 22, 2009, 03:03:08 PM »
What I would do would be to sell the carb on Ebay and get another one.  That would be your cheapest fix. A lot of inexperienced people think that bigger is better when it comes to carburetors.  Nothing could be further from the truth unless you have the engine set up to handle it. 

Jeff73Mach1

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Re: carb question
« Reply #4 on: August 25, 2009, 10:59:49 AM »
you can rejet the carb and change the accelerator pump to match it to your engine.  Also with a slightly oversized carb you can also put a 1 inch spacer between the carb and the manifold and it will help a little. 

I agree that is too much carb for most 302's.  I ran a 600 CFM Carter AFB (Now what is sold as an edelbrock) on mine and with just minor tuning and the spacer plate I had smooth throttle response and no bogging or flat spots. 

Soaring

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Re: carb question
« Reply #5 on: August 25, 2009, 05:54:48 PM »
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. 

Tim175

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Re: carb question
« Reply #6 on: August 25, 2009, 06:27:44 PM »
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

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Re: carb question
« Reply #7 on: August 25, 2009, 07:01:12 PM »
There ya go......

Jeff73Mach1

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Re: carb question
« Reply #8 on: August 25, 2009, 08:08:31 PM »
Changing out parts in that carb to try to make it flow less gas is Southern Engineering 101 IMO. 

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.

FASTBACK65GT

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Re: carb question
« Reply #9 on: August 25, 2009, 10:22:38 PM »
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

Tim175

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Re: carb question
« Reply #10 on: August 26, 2009, 12:06:03 AM »
Yeah its a wonderful carb isnt it? Ive enjoyed it so far, and im sure you will too.

 

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