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Author Topic:   Motorcraft 4300 carb questions
Gary Ferringer
Journeyman

Posts: 59
From: Kennerdell,Pa.
Registered: Feb 2001

posted 07-02-2001 05:51 AM     Click Here to See the Profile for Gary Ferringer   Click Here to Email Gary Ferringer     Edit/Delete Message   Reply w/Quote
What CFM are these? I have 3 of them that were off of early '70's Clevelands and was just wondering. I really never paid much attention to them before. I have always been a fan of 4100 Autolites myself. These are the square bore carbs,not the spread bores, I have 2 of the spread bores as well and I think they are 750 cfm?

Also, how do you enrich the secondaries on these if you so desire? I followed the entire fuel path from the fuel bowl, to the tip of the boosters and found no jets fixed or removable. There are 3 air bleeds in the fuel path, one at each booster, and one in the carb top right above the brass tube after it picks up fuel at the bowl. I would assume that if you would want to enrich the secondaries you would have to reduce the size of the air bleeds, or am I missing something?

Thanks for any information.

------------------
'85 Capri
Cleveland powered
carburetted
6.95 1/8th
@94 1/2 MPH
1.49 60 ft.

[This message has been edited by Gary Ferringer (edited 07-02-2001).]

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Moneymaker
Administrator

Posts: 19686
From: Lyons, IL, USA
Registered: May 99

posted 07-02-2001 10:29 PM     Click Here to See the Profile for Moneymaker   Click Here to Email Moneymaker     Edit/Delete Message   Reply w/Quote
If they are 1970 or newer they are 670 CFM. Have you dissassembled a carb yet?

------------------
Alex Denysenko
Co-Administrator and Moderator
NHRA/IHRA/SRA member
NHRA and IHRA SS/LA National Record Holder '00 & '01
Fleet of FoMoCo products
Moneymaker Bio
US Class Nationals link

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Gary Ferringer
Journeyman

Posts: 59
From: Kennerdell,Pa.
Registered: Feb 2001

posted 07-02-2001 11:11 PM     Click Here to See the Profile for Gary Ferringer   Click Here to Email Gary Ferringer     Edit/Delete Message   Reply w/Quote
Thanks for the info. Alex. I figured somewhere in the mid 6's as far as CFM goes. 1 is a '72 and the other 2 are '73's. It is my understanding that the first ones made in '67 were around 440 CFM, but I knew that the later ones were larger.

I'm not sure if I understand what you mean about whether I have disassembled a carb yet? I rebuilt my first one when I was about 8 years old. And in the mid '80's and early '90's I pretty much financed my racing by rebuilding / modifying carbs of all types , some to factory specs,and some for performance. Everything from Rochester Q - jets, and 4GC's to Carter WCFB's to Solex 4A1's (basically a takeoff on the Q-jet) off of straight 6 Mercedes Benz's from the early to mid '70's. I've disassembled a couple of them , yes.

------------------
'85 Capri
Cleveland powered
carburetted
6.95 1/8th
@94 1/2 MPH
1.49 60 ft.

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Moneymaker
Administrator

Posts: 19686
From: Lyons, IL, USA
Registered: May 99

posted 07-03-2001 12:24 PM     Click Here to See the Profile for Moneymaker   Click Here to Email Moneymaker     Edit/Delete Message   Reply w/Quote
I meant a 4300 carb.

------------------
Alex Denysenko
Co-Administrator and Moderator
NHRA/IHRA/SRA member
NHRA and IHRA SS/LA National Record Holder '00 & '01
Fleet of FoMoCo products
Moneymaker Bio
US Class Nationals link

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Gary Ferringer
Journeyman

Posts: 59
From: Kennerdell,Pa.
Registered: Feb 2001

posted 07-03-2001 07:43 PM     Click Here to See the Profile for Gary Ferringer   Click Here to Email Gary Ferringer     Edit/Delete Message   Reply w/Quote
Yes I have on many occasions. The 600 CFM 4010 series Holley that I have, I received on a trade for an early 4300 that I put together about 12 or 13 years ago. It was a good deal, that 600 was brand new out of the box, I couldn't believe that he wanted to trade.

I have been studying the fuel path pretty closely for the secondaries. It begins behind and to the left of the power valve with a hole in the fuel bowl and heads up what becomes a brass tube into a well in the carb top. This is where I encountered the first air bleed, I am assuming that this is kind of a vacuum break to keep the fuel from siphoning up this tube too hard. The fuel then heads down into a hole in the carb body right beside the brass tube, and transfers over to the secondary side through a channel between the 2 primary barrels. It then branches off into the 2 secondary fuel wells where it feeds the 2 secondary boosters as needed when the secondary butterflies open and the pressure differential is created to cause fuel flow through the boosters.

At each booster there is one air bleed that I assume forms an air assisted atomization system with the booster. I haven't determined what the brass tubes are for that come out of the bottom of the boosters, but I suspect that they provide early fuel flow to help prevent a stumble when the secondary butterflies open.

I have a junk top that I have been experimenting with. I noticed that the brass tubes in the center of the secondary boosters had a screwdriver slot in the end of them, I removed the plug in the carb top, and inserted a screwdriver in the hole, and removed one of the tubes thinking that maybe a jet was hidden behind one of these, but that was not the case.

So, how do you enrich the secondaries on these? I might not actually NEED to on the application that I am going to use these on, in fact I may find that I need to lean them out a little, so I guess that the appropriate question is how do you ALTER the fuel being provided by the secondaries on these carbs?

There is almost no information available for modifying these carburetors, so I am relying on my knowledge of how other carburetors accomplish their metering goals and making some assumptions as to how this is accomplished on these, how am I doing?

------------------
'85 Capri
Cleveland powered
carburetted
6.95 1/8th
@94 1/2 MPH
1.49 60 ft.

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