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  #31 (permalink)  
Old 12-14-2012, 12:38 PM
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Quote:
Originally Posted by ap72 View Post
how do you think those holes get in the jets that you buy out of a catalog? And I've been drilling jets since I built my first engine- often the local parts house doesn't have the jet I want so you get the next smallest one they do carry and bust out your drill bits. MOST carb shops drill their own air bleeds and those are even smaller holes in smaller brass inserts.
Ummm, I believe the correct way to punch a hole in a jet is to broach it, not drill it.
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  #32 (permalink)  
Old 12-14-2012, 05:41 PM
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Originally Posted by gearheadslife View Post
air speed through the venturi = better fuel atomization..
less fuel needed for same power/rpm as it burns better(completely)

why would the air speed be any different based only on throttle blade size?
I think you are missing my point,Im not saying either carb is better,especially based on throttle blade size alone.

If I have a carb of any size at a fixed height and I need 100 cfm of air,the air would travel at or very close to the same speed in all carbs,at least when passing the throttle blades.

other factors combined are going to make the MPG change,not blade size
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  #33 (permalink)  
Old 12-14-2012, 05:45 PM
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Jet sizes can be up sized with a drill bit.There used to be special bits sold for that purpose.trouble with drilling is the machine marks left in the hole were not consistent enough to meter fuel flow,or sometimes the increase was huge in comparison to the size difference due to rifling in the hole.
some people drilled the jets from both ends to prevent rifling.or buy jets that are broached,,,
Thanks tech for the use of your vocabulary
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  #34 (permalink)  
Old 12-14-2012, 11:21 PM
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Quote:
Originally Posted by ap72 View Post
how do you think those holes get in the jets that you buy out of a catalog? And I've been drilling jets since I built my first engine- often the local parts house doesn't have the jet I want so you get the next smallest one they do carry and bust out your drill bits. MOST carb shops drill their own air bleeds and those are even smaller holes in smaller brass inserts.

This **** really isn't rocket science.

and adding weight is not that hard with some epoxy and scrap lead, just like in the pinewood derby days.

I know a LOT of people prefer to buy their parts out of a catalog and pay someone else to do the work, and there's nothing wrong with that, I buy parts too. But there's also nothing wrong with fixing your own stuff.
You are nothing but a poser and YOU would say to me something like YOU build and try to make it sound like I prefer to buy stuff?????

You are a total jerkoff of the highest order and you have no business even posting 90% of the time- you have no engine experience all you are is a ****ing book worm.

Last edited by cobalt327; 12-14-2012 at 11:26 PM.
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  #35 (permalink)  
Old 12-15-2012, 06:19 AM
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Quote:
Originally Posted by vinniekq2 View Post
why would the air speed be any different based only on throttle blade size?
I think you are missing my point,Im not saying either carb is better,especially based on throttle blade size alone.

If I have a carb of any size at a fixed height and I need 100 cfm of air,the air would travel at or very close to the same speed in all carbs,at least when passing the throttle blades.

other factors combined are going to make the MPG change,not blade size
I don't know<<(sarcastic) why does it slow when you use bigger ports in heads???
same reason..
why water spray farthar when you put your finger over hose end... the pressure didn't change.. just the size of the hole..
why does ehaust speed slow when it gets to the collector...

Last edited by gearheadslife; 12-15-2012 at 06:26 AM.
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  #36 (permalink)  
Old 12-16-2012, 04:31 PM
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Jets varying in flow rate such that jets with two numbers apart may flow the same, ie a 70 jet that flows on the high end of the tolerance may flow the same as a 72 jet that flows on the low side of the tolerance. You can buy sets of flow tested jets but they cost alot more.
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  #37 (permalink)  
Old 12-17-2012, 01:46 AM
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The flow through a jet is determined by the surface finish, the length of the orifice, the entrance and exit angles and their finish, and the size of the orifice. When a jet is drilled oversize, all of these things are changed except the angles of exit/entrance, but even the lengths of the angles are changed.

The number stamped on a jet (Rochester, Holley, etc.) is an indication of the actual flow, NOT the orifice diameter. The manufacturers flow the jets to see what number they get stamped on them. The same orifice diameter jet may be stamped w/different numbers. This tells you they do not flow the same, even though the orifice diameter is the same. Subtle differences in the above parameters account for the different flows.

The difference between the correct and too rich/lean jet sizes may only be 0.002”-0.004”. There are no readily available drill bits that are that close in size to one another. So at best, the changes made to jets by hand drilling are going to be in steps decided by the availability of drill bits. Because drill bits are often available in 1/64” increments (the small numbered bits- which have a finer increment between bits- are too small for drilling the average carb metering jet), this will be on the order of a 0.0156” change between drill bits- which is HUGE (~40%) change in metering area when in the 0.070” jet orifice range.

It is for these reasons there is no practical way for a person to casually drill a jet and have any way of knowing what the flow is going to be. Drilling out jets is a holdover from the dark days when guys were struggling to make “high performance” engines live, and if they got within 10% of ‘right’ they were at the top of the heap. Nowadays, thankfully, we all (should) know better and leave the drilling of jets and other dubious practices to the desperate, the uneducated, and the poverty-stricken.
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