jets/drill bit sizes
- Thread starter holeshot1
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mideastrider
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- Jul 8, 2006
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I don't know but it would be interesting to find out. Because I too have often thought about this. We drilled jets all the time when I raced go-karts. But we ran methanol for fuel and jets of that size where not available. Unlike motorcycle carbs the stock jets where all you could get so we drilled them.
Mikuni hex head jets are numbered by bulk flow, while Mikuni round head jets and Keihn jets are numbered by inner diameter in mm.
The flow difference between a Mikuni 175 hex jet and a 180 is about 6cc ~3%) . Keihn jets are numbered by jet id, and the difference between a 178 and a 180 is about .0008" (I'll let those so inclined do the math to determine the flow difference) .
Good jets can vary as much as 5% cheap jets can be closer to 10% variance even when marked with the same jet number.
It's been my experience that quality jets are marked pretty accurately in terms of the physical dimensions, and the differences in bulk flow can just as often come from the manufacturing process. With flow rates this small it doesn't take much in the way of an internal flaw to effect the variances we are talking about. Not to mention the fact that it is pretty difficult for most of us to accurately measure a hole this small.
Drilling jets is a tough road unless you have a ton of time to test every single jet you modify and document it's characteristics.
The flow difference between a Mikuni 175 hex jet and a 180 is about 6cc ~3%) . Keihn jets are numbered by jet id, and the difference between a 178 and a 180 is about .0008" (I'll let those so inclined do the math to determine the flow difference) .
Good jets can vary as much as 5% cheap jets can be closer to 10% variance even when marked with the same jet number.
It's been my experience that quality jets are marked pretty accurately in terms of the physical dimensions, and the differences in bulk flow can just as often come from the manufacturing process. With flow rates this small it doesn't take much in the way of an internal flaw to effect the variances we are talking about. Not to mention the fact that it is pretty difficult for most of us to accurately measure a hole this small.
Drilling jets is a tough road unless you have a ton of time to test every single jet you modify and document it's characteristics.
...not to mention the scenario of the guy buying your bike being very frustrated when he puts what he thinks is the next larger jet in there, only to seize the motor because it is actually smaller than your drilled jet...unless for some reason jets are unavailable, I would just buy them. Mains are only $3.00 for genuine Mikuni.
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- #5
Still would be nice to know
especially in a pinch after doing motor work or just for general reference....must be someone out there with a chart or something....must have something when they make them??? searched around but never thought I wouldn't find it on the net ??? Thanks
especially in a pinch after doing motor work or just for general reference....must be someone out there with a chart or something....must have something when they make them??? searched around but never thought I wouldn't find it on the net ??? Thanks
The Keihn and Mikuni tuning manuals have loads of general reference information on jets.
SFO
LIFETIME SPONSOR
- Feb 16, 2001
- 2,001
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I would start this endeavor by buying a set of gauge pins.
Gauging with drill bits is an open ended loop. Accurate hole sizing requires a reamer, drills are not able to hold a .0005" tolerance, IMHO. Also, factory jets vary the chamfers and lead in bores on their jets, based on the basic information Rich posted above.
Flow is not fully a function of jet bore, even though you may be able to gauge jet ID within a .001".
That being said I have drilled jets to save my life at the track when trying to find the right throttle response based on conditions.
Gauging with drill bits is an open ended loop. Accurate hole sizing requires a reamer, drills are not able to hold a .0005" tolerance, IMHO. Also, factory jets vary the chamfers and lead in bores on their jets, based on the basic information Rich posted above.
Flow is not fully a function of jet bore, even though you may be able to gauge jet ID within a .001".
That being said I have drilled jets to save my life at the track when trying to find the right throttle response based on conditions.
right on. Just because I care, I did look around a bit for more jet info, found all kinds of numbers on the needles and needle jets but nothing on the mains.
I have used a set of what I think are torch tip drills (the kind that store in the handle of the mandrel) to mess with jets, but still there is a big jump between sizes.
At least you are working on a single cylinder bike...try juggling brass on a triple or four cylinder. $100 goes by in a blurr and it still doesn't run quite right...especially when you get into needle jets and throttle slides
I have used a set of what I think are torch tip drills (the kind that store in the handle of the mandrel) to mess with jets, but still there is a big jump between sizes.
At least you are working on a single cylinder bike...try juggling brass on a triple or four cylinder. $100 goes by in a blurr and it still doesn't run quite right...especially when you get into needle jets and throttle slides
- Dec 19, 2005
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An old timer pro engine tuner told me that drilling your own jets used to be common practice back in the 70's. You could buy a kit at an auto parts store which had a few brass jets and an assortment of drills along with a handle that had a chuck on the end of it. Drilling jets was done strictly by hand, i.e. you twirl that contraption with your fingers. If you made the hole too big, then you would heat up the brass jet and fill it with a drop of solder, and start over again.
It occurs to me that you don't really care about jet I.D. as much as you care about actual flow rate. So couldn't you just measure flow rate of your custom-drilled jet by comparing it to the rate you measure thru a known "reference" jet?
For example, fill a container with 1 liter of fluid (e.g. water) and let it drip till empty thru a known-sized jet and measure the time it took. Repeat the same with your custom-drilled jet, and calculate flow ratio from there. Flow rate depends on "head" of fluid (i.e. volume and height above orifice), as well as on viscosity, so you will have to hold those 2 things as consistent as possible. Recalibrating with a known jet every time should eliminate the viscosity variable, and using a large volume of fluid should improve "head" accuracy.
A more convenient variation on the same theme is to build a "fluid divider" as follows. Connect a reference jet to the left side of an upside-down Y fitting, and a custom jet to the right side. The "common" side of Y goes to a source container of fluid. Place 2 containers to capture flow thru the jets. If both jets have the same flow rate, then both "capture" containers will contain exactly the same amount of fluid, i.e. 1/2 of what was in the "source" container. So by weighing what you capture from reference jet and comparing to the weight of fluid captured from the custom jet you can once again determine flow ratio.
It occurs to me that you don't really care about jet I.D. as much as you care about actual flow rate. So couldn't you just measure flow rate of your custom-drilled jet by comparing it to the rate you measure thru a known "reference" jet?
For example, fill a container with 1 liter of fluid (e.g. water) and let it drip till empty thru a known-sized jet and measure the time it took. Repeat the same with your custom-drilled jet, and calculate flow ratio from there. Flow rate depends on "head" of fluid (i.e. volume and height above orifice), as well as on viscosity, so you will have to hold those 2 things as consistent as possible. Recalibrating with a known jet every time should eliminate the viscosity variable, and using a large volume of fluid should improve "head" accuracy.
A more convenient variation on the same theme is to build a "fluid divider" as follows. Connect a reference jet to the left side of an upside-down Y fitting, and a custom jet to the right side. The "common" side of Y goes to a source container of fluid. Place 2 containers to capture flow thru the jets. If both jets have the same flow rate, then both "capture" containers will contain exactly the same amount of fluid, i.e. 1/2 of what was in the "source" container. So by weighing what you capture from reference jet and comparing to the weight of fluid captured from the custom jet you can once again determine flow ratio.