Drilling Main Jets?
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Fleet Mechanic
Joined: Apr 2004
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From: Willamette Valley, Oregon
Drilling Main Jets?
Im having a difficult time finding Motorcraft main jets, in the "old days" we used to drill out jets. I have one spare pair of jets, they are #54s just like what was in this unit (in a 302 with a rebuilt , new to me, carb) I drilled the #54s out with #52 bit, it seems to be somewhat better. First of all, can anyone tell me what # jets should be in a 302, 2bbl, ???I was told has mild cam (not sure if its true).
Second, anyone have a conversion chart, or just a good handle on what drill size = what jet size? if a #54 jet is the same as a #54 bit, (and it is) then when I drilled it with a #52 drill bit which is bigger, did I make it a #52? or a #56? ....is a #52 jet smaller than a #54? I am thinking about drilling one more size to a #50 bit, is this crazy too big? or is it reasonable that it may need this much fuel, it still seems starved, float level is good, vacuum is good, and drilling to a #52 did help a but... HELP!!
Second, anyone have a conversion chart, or just a good handle on what drill size = what jet size? if a #54 jet is the same as a #54 bit, (and it is) then when I drilled it with a #52 drill bit which is bigger, did I make it a #52? or a #56? ....is a #52 jet smaller than a #54? I am thinking about drilling one more size to a #50 bit, is this crazy too big? or is it reasonable that it may need this much fuel, it still seems starved, float level is good, vacuum is good, and drilling to a #52 did help a but... HELP!!
New User
Joined: May 2004
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The bigger the number on the jet the more fuel you get. As far as the drill bits iam not sure, lol stupid me. When does it seemed starved. All the time or during accellration or what? I would think that no bigger than a #60 would be pletty of fuel for a 302. Are you running headers and good mufflers to let the engine breath better then maybe you could stand a bit more fuel. Dont know probally just have to keep expermenting. I have bought jets for holly 600cfm carbs and put them in Ford 2-barrells before, so you might check the local parts store for them.
Posting Guru
Joined: Jul 2003
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From: Colville, Washington
Every book I have seen on carbs, recommends NOT drilling jets because the jets are sized by FLOW not diameter of hole. The jet number is larger as the hole increases, but they are not directly correlated. I agree with LMM on buying some Holley jets, start with about 56 or so and experiment from there.
Rabbit-Hole Researcher
Joined: Feb 2004
Posts: 466
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From: Middle of Ohio
What is difference between Holly and Motorcraft jet size/number
What is the difference between Holly and Motorcraft jet size/number?
I understand that they use different systems. Is there a conversion chart, or even a formula? Somewhere someone said that one was based on drill bit size (true?) and the other was based on flow/cross section.
Little cornfused...
Kevin
I understand that they use different systems. Is there a conversion chart, or even a formula? Somewhere someone said that one was based on drill bit size (true?) and the other was based on flow/cross section.
Little cornfused...
Kevin
Posting Legend
Joined: Sep 2001
Posts: 30,255
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The jet sizes are based on flow. They can not be drilled out and flow correctly for a given "size". The tiny things like the profile of the edge of the hole on both entry and exit affects flow greatly. You can't control those things with a drill bit.
!["Beemer Nut"'s Avatar](data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 50 50'><circle class='avatar-circle-default' fill='%23bc5285' cx='25px' cy='25px' r='20px'></circle><text class='avatar-text-default' x='49%' y='53%' fill='white' text-anchor='middle' alignment-baseline='middle'>"</text></svg>)
Post Fiend
Joined: Dec 2003
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From: "Islander"
I ran a wide band F/A gauge ended up between two sizes of jets, one number too lean, other too rich what I used was small tapered clock bushing reamers. Ream, take notes and test drive.
Drilling out a jet can result in less flow as the length of the restriction is increased due to the taper at each end of the jet.
By shortening the restriction length alone flow increases. You can have the same restriction size for more than one jet number.
I recall on Holley jets the flow increase was 3% for every jet number increase.
Drilling out a jet can result in less flow as the length of the restriction is increased due to the taper at each end of the jet.
By shortening the restriction length alone flow increases. You can have the same restriction size for more than one jet number.
I recall on Holley jets the flow increase was 3% for every jet number increase.
Last edited by "Beemer Nut"; Dec 25, 2006 at 09:37 PM.
Tuned
Joined: Jun 2006
Posts: 363
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From: Graham WA
Don't want to hijack this thread. Simple question: What is thestandard size jet in a holley 600 cfm 4150/4160?
I want to go a size or two larger.
I want to go a size or two larger.
Last edited by E30tdf; Jan 11, 2007 at 11:42 AM. Reason: Finish
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Tuned
Joined: Jul 2003
Posts: 455
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From: Berkeley
btw, these guys have motorcraft jets. i've ordered some from them in the past to play with various sizes...
http://www.carburetion.com/ford2v.htm
http://www.carburetion.com/ford2v.htm
Tuned
Joined: Jun 2006
Posts: 363
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From: Graham WA
When you re-jet a holley, do you replace both the primary and secondary or just the primary. Do you need new gaskets for the metering plate and float bowl each time you go in or can the old gaskets be reused? My carb is a 1 yr old 4150.
Last edited by E30tdf; Jan 16, 2007 at 12:02 PM. Reason: clarity
Hotshot
Joined: Dec 2003
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From: iowa
Originally Posted by E30tdf
When you re-jet a holley, do you replace both the primary and secondary or just the primary. Do you need new gaskets for the metering plate and float bowl each time you go in or can the old gaskets be reused? My carb is a 1 yr old 4150.
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