Honda 400ex Carburetor Float Adjustment

The Carburetor Shop LLC

  1. 2005 Honda 400ex Carburetor
  2. Honda 400ex Carburetor Float Adjustment Chart
  3. Honda 400ex Carburetor Float Adjustment Tool

If you get to this point, the carburetor probably needs to be removed, and rebuilt. The sales pitch - if the carb needs rebuilding, we can probably help you with a rebuilding kit. FLOODING (OVERFLOWING) Fuel overflowing from the carburetor may be caused by a number of issues: (1) Float/fuel valve incorrectly adjusted. I have a 02 Honda trxex and it doesn't seem to stop leaking gas from the hose coming from the carb. I've rebuilt the carb, replaced the float. Buy OEM Parts for Honda ATV Carburetor Diagram. Honda TRXEX A - SPORTRAX EX Carburetor Diagram. Catalog Honda ATV. I took the carb apart, new main jet, new needle (set to 3rd ring from top). Shop online for OEM CARBURETOR parts that fit your 2005 Honda TRX400EX A, search all our OEM Parts or call at 1-866-842-0086.

Carburetor Carb, Carburetor Fits for Honda 400 400EX Sportrax 400X ATV Carb Assembly. New Carburetor for Honda Sportrax 400 TRX400 TRX400X TRX400EX Carb Replaces 16100-HN1-A43. 4.2 out of 5 stars 45. Get it as soon as Fri, Jan 22. FREE Shipping by Amazon. I took the carb off just to see what changes were made. Here's what I have, White bro's E series slip on, #155 keihin main jet, #42 keihin pilot jet, stock needle jet (3rd groove down), pilot screw 3 turns out, (3) 1-1/2' holes in air-box, K&N filter with pre-filter.

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Accelerator pumps Bog

Fuel Leak by throttle shaft Fuel economy

Hesitation MarvelAdjustments Stumble Throttle body gaskets Tripowertuning

Accelerator pumps

Often the accelerator pumpgets the blame for other problems. It is very easy to test the function of theaccelerator pump. Start the engine, and warm to normal operating temperature.Shut off the engine. Remove the air cleaner. The choke butterfly should befully open, as the engine is warm. Observe the pump jet in the carburetor, andwith your hand, work the carburetor throttle to the wide-open position. Youshould observe a healthy squirt of fuel from the pump jet. A single barrelcarburetor will normally squirt a single stream; while a two or four barrelcarburetor will normally squirt 2 streams. If you see the stream(s) of fuel,the pump is working. It is important to start the engine prior to doing thistest. With modern gasoline, it is quite possible the carburetor will becompletely dry prior to starting. If there is no gasoline in the bowl, the pumpwill not work; and this would give a false result.

Bog, Hesitation, Stumble

This paragraph applies toan instantaneous bog, hesitation, or stumble upon acceleration. Constanthesitation is covered under “surging”. This paragraph also applies torelatively stock engines with the original carburetor. We will discuss twotypes of bog: the first is bog when the vehicle is accelerated from a stop; thesecond is bog when the vehicle is accelerated from cruise. Bog from a stop isvirtually always (and generally erroneously) diagnosed as a faulty acceleratorpump (see the section on “accelerator pumps” for testing). Most moderncarburetors are designed to function with roughly 0.020 (20 thousanths)clearance between the center of the throttle plate edge, and the throttle bodyat a point equidistant from the throttle shaft bearing areas. This clearanceallows for maximum velocity of idle air past the idle ports. Exceptions to thisare GM carburetors with the idle speed air screw, and end carburetors ontripower. Setting the idle for the highest vacuum idle reading will result intoo little clearance of the throttle plate; forcing too much of the idlemixture through the lower idle port and too little through the idle transferslot. This will cause a phenomena called “puddling” where little droplets ofgasoline adhere to the intake manifold runners. When the throttle is opened,there is now sufficient velocity of air to sweep all these droplets into thecylinders, creating a mixture which is too rich to burn, hence the bog. As soonas the overrich mixture is pumped out the tailpipe, and a normal mixture isingested by the cylinders, the bog disappears. A defective advance mechanismcan also cause bog; as can a defective accelerator pump. If bog exists onlyfrom an idle, not when accelerating from a constant speed, the idle adjustmentis probably the culprit.

Bog from a cruise RPM maybe caused by a defective advance mechanism, but on 4 barrel carburetors isoften caused by the secondary side opening too soon. Most original equipment 4barrel carburetors have “on-demand” secondaries (I use this term rather thanvacuum, as some early 4 barrels used vacuum to accuate the secondary, whilemost 4 barrels from about 1960 up used either spring tension or weights tocontrol the secondary). The Carter AFB uses weights, and therefore never goesout of adjustment. Other 4 barrel carburetors such as the Carter AVS, CarterTQ, Rochester 4GC, and Rochester Q-Jet have a tensioned secondary spring. Asthe spring fatigues, the air valve will open too soon, creating aninstantaneous lean condition, and a bog. These units, when rebuilt, shouldvirtually always have the tension spring replaced, and adjusted to factoryspecifications. A defective accelerator pump will rarely cause bog from cruise.


Many mechanics have been conditioned to ask for a float each time theyrebuild a carburetor, due to the reasonable price of modern, mass-producedfloats, and the propensity of nitrophyl (foam) floats to absorb gasoline aftertime. In dealing with older, NON-CURRENT-PRODUCTION brass floats, neither ofthe above are true, and a mechanic should attempt to 'save' the float if at allpossible.

The first step is to clean the float and inspect it for obvious damage.Small dings and dents are quite common, even in unused floats, and occurredwhen the manufacturer shipped the floats in bulk. Major dents (generally causedby water freezing in the carburetor) are not generally repairable. If one canhear liquid sloshing around inside the float, skip to the next paragraph. Ifthe float looks to be reasonably damage-free, it should be tested. Testing isaccomplished by grasping the float arm with a pair of needle-nose pliers, andsubmerging the float in very hot water. The hot water will pressurize the airinside the float, and a leaky float will blow a stream of bubbles.

If the float should need repair, it is important to understand how the floatwas originally produced. Virtually all brass float pontoons (the floating part)are composed of two pieces (a few are more) of brass soldered together. Thepieces differ in the seam area, as one piece has a male seam and the other afemale seam. One float piece will also have a small hole for temperatureequilization. This hole will be covered by a small drop of solder, and will beas far from the seam as possible. The manufacturer would solder the two piecestogether, allow the float to cool completely, AND THEN close the equilizationhole. Soldering MUST be done using a soldering 'iron'. Repair should not beattempted using either a torch, or a soldering gun. If you plan on disregardingthis advice, read the next paragraph first! The following procedure works forus (no, we will not repair your float unless we restore the entire carburetor):First, if liquid is present inside the float, find the hole, and remove theliquid by placing the hole down inside the hot water. The pressure will forcethe liquid from the float. If the float has much liquid, it may be necessary toremove the float from the hot water, allow the float to cool, and repeat thehot water dip. Once the liquid has been removed, and the leak has been marked,open the equilization hole by removing the solder. Solder the leak closed usingas little solder as possible. A small piece of tape over the equilization holewill allow the hot water test to be preformed. If there are no leaks, removethe tape, and ALLOW THE FLOAT TO COOL COMPLETELY before closing theequilization hole. A final test, and you have 'saved' a valuable float.

Automatic Chokes

An area of the carburetorgenerally misunderstood is the function of the automatic choke. Automaticchokes use a bimetallic coil to close the choke plate, and vacuum to open thechoke plate. It is important to understand that the bimetallic coil does NOTopen the choke. Automatic chokes are of two types: integral, and divorced (alsocalled remote). The integral choke is an integral part of the carburetor. Thedivorced choke resides on the manifold (divorced or remote from the carburetor)and has an operating rod from the choke to the carburetor. In general,carburetors with divorced chokes use a separate choke-pulloff to open thechoke. Integral chokes have a piston inside the choke housing. In general, thebimetallic coil rotates when cold to close the choke. As the bimetallic coil isheated, it relaxes, and the choke is pulled open by vacuum. In the case of theintegral choke, there will be a tiny vacuum passage from the throttle area(vacuum source) up to the choke housing where vacuum is exerted on the piston.If this tiny passageway is clogged (often), no vacuum is applied to the piston,and the choke does not fully open. A problem with divorced chokes is the use ofan incorrect thickness carburetor to manifold gasket when the carburetor isrebuilt. This will change the required length of the choke operating rod, andmay result in the choke either not closing, or not fully opening.

Setting an automatic chokeis quite simple, even if an aftermarket choke is used. For integral chokes,loosen the retaining screws such that the choke will rotate freely. Adjust thechoke such that the choke plate just touches closed at 68 degrees F. (65~70degrees is close enough). Tighten the retaining screws. For the divorced choke,the same setting applies, but bend the operating rod to set the choke plate.


Many of the less expensive carburetors from the beginning upthrough about 1940 were originally equipped with floats made from cork. Most ofthe floats were coated with orange shellac, and then the finish was baked,creating a finish fairly impervious to the gasoline of the day. A few of the manufacturersdid not coat their floats, and used a cork material that seemed to work fairlywell with the gasoline then being sold.

The gasoline of today cuts orange shellac like a hot knifein butter, and also will permeate the natural cork material!

This poses a severe problem for the restorer. It is noteconomically feasible to attempt to mass produce brass floats to replace thecork floats. Also, the company producing the poly-nitrofill foam floats hasbeen most un-cooperative unless orders of very large magnitude are placed withthem. We are currently machining float pontoons from this substance, to be usedwith the original float arm.

For those who are independently wealthy, individual brassfloats can be made. This also may be a solution for a retired machinist withaccess to a good machine shop. This is a very time-intensive remedy, expensiveif one must pay for the time.

For the rest of us, it becomes imperative to attempt to usea replacement cork (or foam) float, and seal the cork (or foam) against thepermeation of the gasoline. The procedure we at The Carburetor Shop arecurrently using is as follows;







This procedure seems to be working with the current mixtureof gasoline.

If anyone comes up with a better procedure, we wouldcertainly wish to be informed!!!

Honda 400ex Carburetor Float Adjustment


Thissection is not designed to turn a novice into a professional mechanic, ratherto suggest to the novice a step by step approach to troubleshooting.

Enginenot starting (engine you have PERSONALLY heard run, but has been not startedfor some amount of time).

(1)Compression test (3 percent)

(2)Ignition test (90 + percent)

(3)Fuel delivery system test (less than 1 percent)

(4)Carburetion test (3 percent)

Enginenot starting (engine you have never personally heard run)

(1)Compression test (3 percent)

(2)Valve timing test (less than 1 percent)

(3)Ignition test (90 + percent)

(4)Fuel delivery system test (less than 1 percent)

2004 honda 400ex carburetor

(5)Carburetion test (3 percent)

Soif the probability of failure is 90 percent with ignition, why should I do a compressiontest first? The tests are numbered in the order of if the one above is faulty,nothing you do in a subsequent test will fix the problem. If you have badcompression, adding a new carburetor, and/or completely replacing the entireignition system will result ONLY in a thinning of your wallet!

Compressiontest - mycompression ratio is a number, and the compression gauge reads pounds, whatgives?????

Thereare mathematical formulae to determine the number from the compression ratio,but they get complicated because of factors such as altitude, etc.; so best tosimply research the suggested maximum cranking values for your engine. If youcannot find your engine, find an engine with an equivalent compression ratio.

Whatyou really are looking for are 3 things: (A) the average should be at least 70percent of the suggested cranking value, (B) no cylinder should deviate fromthe average by more than 15 percent (10 percent would be better), and (C) thevalue read on the first revolution of the engine is at least 80 percent of thefinal reading for that cylinder.

Valvetiming test - verylow probability of failure, but if you bought someone else's project, you donot know their capabilities, or why they abandoned the project. The distributorCOULD be off 180 degrees (remember in a 4-stroke engine the crankshaft makes 2revolutions for each revolution of the camshaft) OR a timing chain COULD havejumped a tooth. Testing for these items may save you a ton of time down theroad.

Ignitiontest - somany possibilities here. If you have access to a diagnostic oscilloscope - USEIT! If not, you are looking for a really good spark at each plug at the propertime. Research how to check the various components in the ignition system. Andwhen you finally set the points gap (also known as the dwell angle) and thetiming, SET THE DWELL FIRST! If you set the timing before setting the points(dwell), changing the point gap (dwell) WILL change the timing.

Fueldelivery system test - basically, you are checking to make certain the fuel delivery system isdelivering fuel to the carburetor. Possible items to fail - (A) the sock a.k.a.sending unit filter can clog in the tank, (B) a fuel line may be filled withdirt-daubers, (C) a fuel line fitting may have cracked, allowing air to enterthe system, (D) the flexible hose from the hard line on the frame to the fuelpump may be collapsed, (E) the fuel pump may be defective, the fuel filter maybe clogged.

Ifyou get to this point, the carburetor probably needs to be removed, andrebuilt.

Honda 400ex Carburetor Float Adjustment

Thesales pitch - ifthe carb needs rebuilding, we can probably help you with a rebuilding kit.


Honda 400ex Carburetor Float Adjustment

Fueloverflowing from the carburetor may be caused by a number of issues:

(1) Float/fuel valve incorrectly adjusted

(2) Defective fuel valve

(3) Defective float

Honda 400ex Carburetor Float Adjustment

(4) Excessive fuel pressure

(A) incorrect or defective fuelpump

(B) clogged or missing tank vent

(5) Cracked housing

Discussingin order:

(1) Make certain that you have the floatadjusted to the manufacturer’s original specifications, and make sure you knowHOW the manufacturer measured the adjustment. Some manufacturers specify thedistance from the float to the casting without the gasket, some with the gasket,some the distance to the fuel level in the bowl at a specified pressure, andRochester was probably the most creative, often measuring to a “dimple” on thefloat itself or the float seam. If you buy an aftermarket rebuilding kit, NEVERrely on the generic specification sheets which come in the kits. Check yourfactory shop manual or the carburetor manufacturer’s manual.

(2) If the fuel valve is defective,replace it.

(3) If the float is brass, check thislink: Brassfloats , if a material other than brass,and there is any doubt, replace the float.

(4) A fuel pressure gauge placed right atthe carburetor will confirm/deny the presence of too much pressure. However,one should consider the clogged or missing vent. The fuel tank MUST be vented,or no fuel can exit the tank. Many older vehicles were vented through thegasoline cap. Vented caps were obsoleted because of smog emissions regulations.If the tank vent is clogged or missing, normal ambient temperature change willcreate either a positive pressure or negative pressure (vacuum) on the tank.Some tanks are located close to the exhaust. As the engine warms the hotexhaust heats the fuel in the tank, and can create excessive pressure in thetank.

(5) Cracked carburetor housings are quiterare; in more than 50 years, I have only seen a hand-full, but the issue CANexist, ESPECIALLY if the “mechanic” feels the necessity of using some form ofTeflon (either tape or paste) on a tapered fuel fitting. Teflon is a fabulouslubricant, and will allow anyone to apply excessive torque to the fittingthreads and crack the casting.

Concerned about the price of gasoline? Beyond our control;however most can make their vehicle use fuel more efficiently, in many casesMUCH more efficiently. All of the following will help your vehicle to use lessfuel per mile traveled.

(1)Clean out the trunk. Weight uses fuel.

(2)Air up the tires, and check the air in the tires periodically.As a general rule, the manufacturer listed tire pressures which will give a“soft ride”. Talk to professionals at your local tire shop to see what theyrecommend. Do NOT exceed the pressure listed as maximum pressure on the tiresidewall. Lower pressures create more rolling resistance. Higher pressures, inaddition to being more fuel-efficient, tend to improve vehicle control, ANDprolong the life of the tire. The figure of a 2 percent reduction for eachpound underinflation should be sufficient incentive to monitor ones tires.

2005 Honda 400ex Carburetor

(3)Turn off the cruise control! A good driver should average 10to 20 percent better fuel economy than the cruise control unit.

(4)Pay attention to the condition of the vehicle. A dirty vehiclehas more “drag” in the air; a brake disc or drum which is dragging or a frontend out of alignment causes more rolling resistance. A clean, waxed vehiclereduces drag.

(5)Keep your vehicle’s drivetrain in good condition. Tune theengine, check the transmission and final drive at the recommended intervals inyour owners/operators manual. Spark plug wires are an often-overlooked culprit.(Of course, if you need a carburetor rebuilding kit, we would be pleased tohelp).

(6)Adjust your driving habits. Everyone knows that full throttleacceleration wastes fuel, so we will not discuss this. However, anticipating astop sign and gradual slowing (traffic permitting), will save both fuel andextend the life of your brakes.

(7)Adjust your driving cycles. Take a little time and think;idling in traffic wastes fuel: can I change my route and avoid signals or stopsigns? How about trips to the store; can I pay the water bill, go to the postoffice, and then come by the grocery store instead of making 3 separate trips?

(8)Avoid the use of ethanol (if possible). Ethanol has lessenergy and will thus deliver worse fuel economy. It may also require carburetorre-calibration.

(9)Carpool – the most efficient vehicle is the one unused in yourgarage.

Buying a new vehicle? On the same vehicle, a manualtransmission will result in 10 to 25 percent better fuel economy. However,until the rest of the buying population learns this fact, expect to take abeating when you trade in your vehicle. In fact, a good used second vehiclewith a manual transmission might just pay for itself over your existingvehicle.


A common complaint today is fuel dripping out of thethrottle body by the throttle shaft AFTER the engine is switched off. While anumber of issues may cause this problem, by far the most common issue is thevolatility of modern fuel. Mechanical fuel pumps have a check valve whichprevents fuel from moving back to the fuel tank. The problem is as follows:

(1)After the engine is switched off, heat from the engine heatsthe fuel in the fuel line.

(2)The expanding fuel (increased volatility) creates pressure inthe fuel line from the pump to the carburetor.

(3)The check valve prevents the fuel backing up through the fuelpump.

(4)The pressure increases to a point the float/fuel valvecombination in the carburetor cannot withstand the pressure.

(5)An amount of fuel (usually from a teaspoon to a couple oftablespoons) flows into the fuel bowl of the carburetor.

(6)This raises the fuel level in the bowl above the maindischarge nozzle(s).

(7)Fuel flows through the main discharge nozzle(s) and drips ontothe throttle plate(s) which is/are closed, and exits out beside the throttleshaft(s) dripping onto the intake.

Possible solutions:

(1)IF POSSIBLE, AVOID ETHANOL LACED FUEL! Sometimes you can buyreal gasoline at a marina

(2)Buy the lowest octane name-brand fuel that does not ping ordetonate in your engine (the higher grades often have more ethanol)

(3)Install a “vapor return line” (take a look at return linesused on many factory air-conditioned cars)

(4)Learn to live with the issue.


Difficult starting a vehicle that has been allowed to sitfor a number of days (that will then start well the rest of the day) is oftencaused by modern fuel. Modern fuel begins to vaporize (evaporate) at a muchlower temperature than fuel before the 1970’s. Once the engine is shut off, thefuel in the carburetor bowl begins to evaporate through the bowl vent. If thereis no fuel in the carburetor, the engine will not start. Pumping the footfeedduring this time simply prolongs the agony, as the accelerator pump will pumpthe fuel into the engine, but in amounts insufficient for starting. If you havethis problem, try priming the carburetor by using an eyedropper and filling thecarburetor bowl through the bowl vent prior to cranking the engine. If you donot wish to prime the engine, crank the engine for 15 to 20 seconds WITHOUTPUMPING. Then stop cranking, pump the footfeed 3 or 4 times, release it, andthen reattempt to start the engine. Priming eliminates excessive wear on thestarter. Another possible solution is the installation of an electric fuelpump. If an electric pump is installed, check local, state, and federal lawsabout wiring; and pick a pump with pressure not exceeding that of the originalpump.

Honda 400ex Carburetor Float Adjustment Chart


Difficult starting of a hot vehicle from 5 minutes to anhour after the engine has been operated, can be caused by the volatility ofmodern fuel. If you have this problem; try using the following method to startthe engine: DON’T touch the footfeed (VERY important). Crank the engine overfrom three to 5 seconds (different vehicles will respond to different times);and then GENTLY (so as not to activate the accelerator pump) press the footfeedapproximately 1/3 of its travel. The engine should start, and may run rough.Run the engine at a high idle for about 10 seconds. This issue is caused byvolatility of modern fuel. Once the engine has been shut off, the gasoline isheated by the latent heat of the engine, and percolates the fuel from the bowlinto the throttle area, forming a mixture that is too rich to fire. If you pushthe footfeed to the floor (as has been the traditional method of “unloading” aflooded engine) the gasoline continues to flow into the engine (again due tothe volatility). By not touching the footfeed, you do not open the throttle plates,and the engine will pump the overrich mixture out of the tailpipe. Once theoverrich mixture has been alleviated, gently opening the throttle will allowthe engine to start.


Beginning with the 1957 carburetors, Rochester started usingthrottle body gaskets (the gasket between the throttle body and the bowlassembly)that have slots in thesealing surface, thus not making a complete seal of the two castings ON CERTAINCARBURETORS ONLY! Not all carburetors use them. This is only one of the reasonswe want a tag number when we supply a kit, as our kits are manufactured (by us)to the original Rochester bill-of-material. The correct gasket will be in thekit. The slots were provided to allow pressure in the venturi area to be bled tothe outside of the carburetor during hot city driving thus helping to preventstalling during hot city driving. For all you die-hards (or hardheads) – thereis no vacuum leak! These slots are above the throttle plates!! Are you still adie-hard?Here is a link to areproduction of the original Rochester bulletin introducing the slottedgaskets.ROCHESTER BULLETIN

TRIPOWER TUNING TIPS Download wheel of fortune slots.

Honda 400ex Carburetor Float Adjustment Tool

Tripower was used by General Motors on Cadillac (1958-1960); Chevrolet(1958-1961); Oldsmobile (1957, 1958, and 1966); and Pontiac 1957-1966). Moreoften than not, there are more than one tripower per year and make fordifferent engine/transmission configurations. The information below is general.The factory shop manual is an excellent resource when working on thesecarburetors.
Get the correct parts!!! With many generic “one kit fits all”, repair kits onthe market; it is difficult for the novice to know what to purchase. Componentsthat one might not consider which can cause issues are: fuel valves,accelerator pumps, gaskets, and power valves. Discussing these components:
Fuel valves - I am aware of 4 styles of fuel valves that are being sold: (A)the conventional pointed fuel valve (our second favorite type); (B) thealuminum plunger with a neopreme disk inserted in the plunger that seals on ainverted flare seat (our favorite, but unfortunately, the manufacturer is nowout of business and no new complete units are available); (C) the 2 ball valve(these tend to hold pressure well, but we have had issues for full fuel flow inhigh performance applications, and also have had these flood profusely onvehicles not driven daily – we will not use these valves); and (D) an imitationof the valve (B) where a wafer containing the neopreme disc is placed betweenthe seat and the plunger (we have seen the wafer get stuck causing profuseflooding, we will not use this valve). If, when redoing a setup containingvalve (B), we can include new neopreme discs in our kits. Since the neopremedisc is the only wear item, replacing this disc and cleaning the plunger andseat will restore the unit. Others may have differing opinions of the variousvalves.
Accelerator pumps – in the good old days, accelerator pumps were made fromleather. Somewhere along the way it was determined that accelerator pumps couldbe made much cheaper with neopreme, rather than leather skirts. BE (beforeethanol) the neopreme pump would last maybe 3~5 years, while leather will lastindefinitely. Neopreme pumps used with ethanol will fail rather quickly, whilethe leather pump will still last indefinitely. If at all possible, purchasekits with leather pumps. In fact, if your old accelerator pump is leather, trysoaking it in light machine oil rather than replacing it with a modern neopremepump.
Gaskets – during the 1957 model year, Rochester began using a slotted throttlebody to bowl gasket ON SOME MODELS ONLY! For the carburetor to functionproperly, it is imperative that the PROPER throttle body gasket is used. Usinga solid gasket on a carburetor designed for the slotted gasket WILL result inhot idle issues.
Power valves – Rochester used a number of different calibrations and twodifferent plunger lengths for power valves on tripower carbs along. Using theincorrect valve will create mixture-timing issues.
OK, you rebuilt the carbs using correct parts to stock specs and now you areready to install and tune the carburetors. Unless you are a carburetionspecialist, install the center carburetor ONLY and install blockoff plates toblock off the end carburetors. If you are a carburetion specialist, you alreadyknew that, and I didn’t need to tell you.
Adjust the idle. Using a vacuum gauge and setting for the highest vacuum cancause hesitation (see the paragraph on BOG). You cannot adjust the idle unlessthe engine is fully warm. If you blocked off the intake crossover, this couldmean 30 minutes or more. When the engine is warm enough to properly set theidle, the choke butterfly will be in the vertical or wide-open position. It isimportant to understand the idle circuit to properly adjust the idle. Contraryto popular belief, the idle mixture control screws DO NOT adjust the mixture.The mixture delivered by the carburetor is controlled by the idle tubes(gasoline jets), and the idle air bleeds (air jets) in the carburetor. The idlemixture control screws control the VOLUME of the preset mixture. An analogywould be a shower where you first set the temperature and then adjust thepressure. In this analogy the temperature (mixture) would be preset in thecarburetor, and the pressure (volume) is set by the mixture control screws. ForBEST results, the clearance from the throttle plates to the throttle body willbe about 0.020 (20 thousandths) at idle. If the tripower is being used on otherthan the stock engine (455 instead of a 389, or a very radical cam), it may benecessary to modify the idle circuit. There are two common possibilities in themodification of the idle circuit (if others are needed, your engine is tooradical for the scope of this discussion). REMEMBER BEFORE MAKING ANYMODIFICATIONS THAT THEY PROBABLY ARE PERMANENT!!!
Idle modifications - the two common modifications are: enrichening the fuelmixture and increasing the idle air supply. Enrichening the fuel mixture MAY benecessary when using ethanol or if the engine has been built to a slightlyhigher tune, or headers have been added. Increasing the idle air supply MAY benecessary if the engine has been built much more radical than stock or if thedisplacement has been significantly increased. The goal of either modificationis a steady idle with the mixture screws from ¾ turn to 1 ½ turn from fullyseated, and about 0.020-inch clearance from the throttle plates to the throttlebore. The idle mixture control screws in these carburetors are the pre-smogshort taper. 1 and ½ turns from lightly seated, and the valves are WIDE OPEN.
To enrich the idle mixture, one must first measure the inside diameter of theidle tubes. One can then drill these tubes oversize. We recommend NO MORE than0.005-inch increase in the diameter. The first attempt may be made at plus0.002 inch. If this is not sufficient, then subsequent attempts should be madein 0.001-inch increments not to exceed 0.005 inch total.
To increase the idle air supply, one may drill small holes in each of thethrottle plates of the center carburetor. If one observes the throttle plateswhile attached to the throttle shaft, the throttle plate will appear as twohemispheres. For best results, the holes should be drilled in the center of thehemisphere AWAY from the idle mixture control screws. One should start with a0.060-inch hole (60 thousandths). If necessary, the holes may be increased insize, not to exceed 0.125 inch (125 thousandths). This modification does notchange the idle mixture, rather this modification is done to control theclearance of the throttle plate to throttle bore. This clearance is importantto minimize or eliminate bog from a stop.
Once the idle circuit has been tuned it is time to direct attention to the mainmetering circuit. IF A MORE RADICAL CAM HAS BEEN INSTALLED, a vacuum gaugeshould be connected, and a reading of idle vacuum obtained. If the idle vacuumis less than 12 inches Hg. then it will be necessary to install a weaker springon the power-valve actuating-valve. A kit with a number of different calibratedsprings is available from The Carburetor Shop LLC. The purpose of changing the springis to allow the power valve to remain closed at high vacuum cruise and open atW.O.T. Once the power valve is operating properly, one can calibrate the mainjetting of the center carburetor. This is best done with one of the portableair fuel ratio meters. Both the main jets and power valve should be calibratedon the center carburetor.
Once the idle, main metering, and power circuits have been calibrated on thecenter carburetor, one may install the two end carbs and tune them for W.O.T.Again, this is best accomplished with the use of an air fuel ratio meter.
Different applications will have different desires for air/fuel (power oreconomy). These setting will be left to the tuner; but I would highly suggestconsultation with one’s engine builder for suggested ratios.
The sales pitch: repair kits with the proper gaskets, leather acceleratorpumps, etc., as well as additional jets, power valves, vacuum spring kits, andother parts may be obtained through The Carburetor Shop LLC.