|
|
 |
Some older carburetors use jets no longer available, or in sizes not useful for the engine's state of tune.
Some have fuel or air circuits that were simply drilled passages with no adjustment or replaceable jet.
Improvements
If you drill the original jet out, its possible to get another size (but not exactly what you had) by soldering the jet closed and re-drilling it, but the chances of getting the exact flow rate on two different jets isnt very good, perhaps within 5%. Try not to use this on dual carburetors.
In some cases, new main jets will not be easily available in useful sizes for your obsolete carburetor. However, some other model or brand may have the same thread size, permitting a foreign jet to be used without modifying the carburetor.
The head thickness of the jet, and the exact position of any countersink or bevel affects the flow rate, and also the effect of any metering rod passing through it. A lower (thinner) jet may be adjusted slightly higher to match the original with a thin washer, &c. A thicker jet can have some material removed from the underside of the head. |
Common jets |
Mikuni 4/042 | 200 | 220 | 230 | 250 | 270 | 290 | 320 |
Mikuni N100.604 | 130 | 137.5 | 140 | 145 | 150 | 155 | 160 |
Jet ID | .0492 | .052 | .0531 | .055 | .057 | .059 | .0615 |
Mikuni 4/042 | 330 | 360 | 380 | 420 | 500 | 540 | 580 |
Mikuni N100.604 | 162.5 | 167.5 | 172.5 | 180 | | | |
Jet ID | .0649 | .067 | .070 | .0492 | .076 | .0795 | .082 | |
Jets, even for the same brand carburetor, also have different countersunk angles at top or bottom. These should all match, as they affect the flow rate. There may also be a relief or bevel under the head that affects seating.
There are a limited number of threads used to mount jets, and may be the same between not only models but brands (with the above exceptions). Some carburetors used metric thread, including 7mm Χ .75mm pitch.
The common Holley 122-XX jet is Ό-32 NS.
The S&S (motorcycle) 72-XX jet is 5/16-24 NF.
In addition, Bill Jenkins suggested use of very fine wire draped through a jet (and looped around a screw &c. for retention) as a miniature metering rod. Its cross-sectional area is deducted from the jet area, leaning (if a fuel supply) or enriching (air supply) it proportionate to a comparison of the squares. E.g., an .060 jet obstructed by a .020 wire is the square root of (.060^2 - .020^2), or .0565. This is very useful for quick modification of the PVCR in a Holley which still uses the original metering block.
Here are some common jets.
If you prefer to have more accurate control over jet size, the existing jet hole in the body casting can be re-threaded to accept a Mikuni main jet. The common Mikuni # 4/042 (6mm hex-head type, 5mm ΄ .9mm fine thread) is easy to use since theyre available in closely-spaced increments and sized by flow capacity (rated in CC per minute) rather than diameter, so 10% increased richness is easy to calculate: add the same percentage to the existing jet size (if now 200, go to 220, &c.).
The Mikuni N100.604 (8mm large, round slotted-head type, 5mm ΄ .8mm coarser M5 thread) or N102.221 (6mm small, round slotted-head type, M5 thread) are shorter and will have better clearance to the float, but are sized by aperture (not fuel flow), so a direct comparison of sizes is not valid.
Common jets |
Drill No | 54 | 53 | 52 | 51 | 50 | 49 | 48 | 47 | 46 | 45 | 44 | 43 | 42 |
ID, Inch | .055 | .0595 | .0635 | .067 | .070 | .073 | .076 | .0785 | .081 | .082 | .086 | .089 | .0935 |
Jet size tables
Table 1: S&S or Holley jet enlargement for air correction hole
Drill No 5/32 20 19 18 11/64 17 16 15 14
ID, Inches .1563 .161 .166 .1695 .1719 .173 .177 .180 .182
Area, Inches .0192 .1600 .5294 .4696 .0232 .9806 .0624 .7150 .9384
Drill No 13 3/16 12 11 10 9 5mm 8 7
ID, Inches .185 .1875 .189 .191 .1935 .196 .1969 .199 .201
Area, Inches .0269 .0276 .0281 .0287 .0294 .0302 .0304 .0311 .0317
Drill No 13/64 6 5 4 3 2 1 M-74B Ό
ID, Inches .2031 .204 .2055 .209 .213 .221 .228 .235 .250
Area, Inches .0324 .0327 .0332 .0343 .0356 .0384 .0408 .0434 .0491
Table 2: Useful air correction metering restrictions
Wire Type Diameter Wire Area
Paper clip (medium) .048 .000 In.2
Coat hangar .090 .002 In.2
Welding rod .125 .006 In.2
|
|
