As some of you all know i built an intake kanifold for 2 SU carburetors awhile back
It ran great but i decided to do some redesign and temporarily fitted the stock holley webber 5200 2hich ran like crap..
Ive been doing some research and did some mods on the linkage as I described in another post.
With the SU carbs the engine ran easily up to 6000 plus rpm and had great torque but gas mileage suffered.
With the 5200 it hardly got up to 4500 and ran really rough and strained at 4000 plus.
The carb was a "rebuilt" one fitted by the previous owner
I checked the jets and discovered that when the carb was "rebuilt" the primary and secondary main and air bleed jets were both swapped.
To.complicate things there are 3 different measuring systems used to designate jet s8zes for the 5200, flow rate, micron and thousands inch hile diameters.
Here are some jet size comparisons i found
103 cc/min=105 microns, or 41.34 thou
128 cc/min=110 microns, or 43.31 thou*
152 cc/min=115 microns, or 45.28 thou*
178 cc/min=120 microns, or 47.24 thou*
201 cc/min=125 microns, or 49.21 thou*
225 cc/min=130 microns, or 51.18 thou
2 51 cc/min=135 microns, or 53.15 thou
275 cc/min=140 microns, or 55.12 thou
298 cc/min=145 microns, or 57.09 thou
325 cc/min=150 microns, or 59.06 thou*
346 cc/min=155 microns, or 61.02 thou
375 cc/min=160 microns, or 62.99 thou
400 cc/min=165 microns, or 64.96 thou*
425 cc/min=170 microns, or 66.93 thou*
450 cc/min=175 microns, or 68.90 thou*
475 cc/min=180 microns, or 70.87 thou
525 cc/min=185 microns, or 72.83 thou*
Fuel flow is controlled by a combination 9f the main jets in the bowl.and air bleeds or compensators.
main jets in the bowl meter gas to venturies and gas and diluted by air bleeds or "compensators" at 5he top to lean mixture.
Bigger mains = rich
Bigger bleeds = lean.
Basic theory
Run primaries with large mains large bleeds
Secondaries smaller jets smaller bleeds
Sounds backwards but thats how they work.
Sp3cs from an international harvestor website with 4 and 6 cyl engines.
4 cylinder
Primary Main Jet 140
Secondary Main Jet 140
Primary Air Corrector Jet 170
Secondary Air Corrector Jet 160
Primary Idle Jet 060
Secondary Idle Jet 050
Pump Jet 050
6 cylinder
Primary Main Jet 160
Secondary Main Jet 160
Primary Air Corrector Jet 190
Secondary Air Corrector Jet 180
Primary Idle Jet 075
Secondary Idle Jet 065
Pump Jet 060
Since The carb is supposed to have about 140 micron main jets in the primary and secondary and 170-160 micron jets in the air bleeds.
Mine were way small and had a small one in primay side and large in secondary.
Well i researched sources for jets and got really confused and couldnt make any sense of the sellers and supposedly the jets cost like $8 apiece or $32 for a set which may or may not work.
SOOO...
All the "experts" on webbers say that you shouldnt drill the jets which i sorta agree with but since i couldnt find jets so i decided to drill.
I also installed a cheep narrrow band fuel air meter..
Air fuel gauge doesnt have numbers but looks like an oil pressure gauge with a rotating needle. If you assume that the scale went from 0 to 100 Supposedly "optimum" mix would be at 50 max lean at 0 and max rich was at 100
I sawpped the primary and secondary jets and Performance improved dramatically with jets in proper holes but Gas mileage was about 10-12 20 highway and gauge stayed at 85, sometimes dropped to 60
So i decided to enlarge the primary air bleeds to.lean the primary mix.
I got a set of "metric" drills from amazone and found that in the set there were about 5 s8zed from .060 to .070
So i selected a middle sized on probably about .065 or so for the primary air bleed and drilled.
Well it turns out that the enlarged hile was too big and the mix was too lean, car bucked and bogged.
Gauge read 85 at idle but dropped to about 20-30 at hard acceperation indicating lean.
Hmm how to reduce the size of the air bleed.. cant make the hole smaller. but you can obstruct it to reduce the flow.
So.. i cut about an inch of 24 gauge wire from an ethernet cable, stripped the insulation, bent a piece into a "U" shape and stuck it into the primary air bleed see pix.
Car ran strong but gauge indicated rich once more, always around 60 -80 just a little rich.
I then used a single wire inserted into the primary air bleed and now car runs good not great but gauge reads 85 at idelllle and cruise, 30-40 at hard acceleration in secondaries, pulls to .6000 good good but not great torque so I suppose the mixture is about right.
See attached photo.
Will.check the mileage and update.
Bottom line.. air bleed.size seems very critical.since the addition of a thin wire can affect the mixture a lot..
But good news is that if you need to play with the jetting just drill the bleeds a bit oversixlze and add some wires to fich up the mixture till you get the performance you want.
A lot cheeper than buying jets at $32 a pop.
Btw..I also noticed that the booster venturies were l99se so i removed them applied superglue to them reinstalled andmpeened them in.