OUTBOARD CARBURETORS

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

THE OUTBOARD CARBURETOR
OUTBOARD-CARBURETOR ADJUSTMENTS
SERVICING THE OUTBOARD CARBURETOR
SERVICING OUTBOARD REED VALVES
CHECKING OUTBOARD CHOKE AND PRIMER SOLENOIDS
SYNCHRONIZING THE THROTTLE ON AN OUTBOARD MOTOR

THE OUTBOARD CARBURETOR

Single-cylinder, and two or three-cylinder outboards will normally employ just one carburetor – a single-barrel float-equipped model that possesses either: 1)an un-adjustable high-speed jet and an adjustable low or idle-speed jet; 2)un-adjustable high and low-speed jets; or 3)adjustable high and low-speed jets.

Bigger outboard motors instead harbor one or more two-barrel carburetors – each which has two jets, a high-speed jet and a low or idle-speed jet, per barrel. There are two-barrel carburetors that contain three un-adjustable jets, an idle-speed-jet, a medium-speed-jet, and a high-speed jet, per barrel instead.

What you should be aware of: the best way to affect the fuel-air fuel mixture in most outboard carburetors is to swap out one of the jets with another jet possessing a larger or smaller internal diameter:

1)If you wish to lean out the fuel-air mixture: or decrease the amount of fuel relative to air, then you need to reduce the jet size; focus on the medium and high-speed jets.
2)To enrich the fuel-air mixture: then increase the jet size, focusing your attention, again, on the medium and high-speed jets.
3)What changing an air-bleed jet will accomplish: an air-bleed jet controls the amount of air in a fuel-air mixture, and therefore functions opposite to the medium and high-speed jets mentioned above. In other words, reducing an air-bleed jet size will restrict air relative to fuel intake and actually enrich a fuel-air mixture; increasing the size of an air-bleed jet will usher in more air, and make a mixture lean.

OUTBOARD-CARBURETOR ADJUSTMENTS

Before an outboard carburetor is adjusted, the engine should be at normal operating temperature. On most outboard carburetors, only the low-speed jet can be adjusted; the high-speed jet has fixed internal dimensions that are designed to offer smooth operation over a wide array of conditions.

If an outboard carburetor does contain an adjustable high-speed jet: then it should be fine-tuned before you address operation of the low-speed jet. To adjust the high speed jet: 1)open the engine throttle to “full”; 2)if you have a tachometer to measure engine rpm during adjustment of the jet, so much the better; 3)there is no magical or uniform setting for a carburetor jet that guarantees proper engine operation; what you are looking for is the smoothest operation that can be attained without precipitously gutting(or raising) engine power. TIP: as with any carburetor adjustment, the motor should be given 20 or so seconds to adapt to the new jet setting before its suitability is judged.

For low-speed jet adjustment: the motor should be operated at roughly 750 rpm. The goal for setting a low-speed jet is as noted above, the maximum amount of engine smoothness obtainable without drastically affecting power or rpm. TIP: if a carburetor contains two adjustable jets, it is a good idea to tweak the high-speed jet again after the low-speed jet has been adjusted!

SERVICING THE OUTBOARD CARBURETOR

Perpetual flooding of an outboard carburetor: if flooding tends to occur every time the primer bulb is squeezed or the valve in the fuel line opened, the carburetor needs to be pulled from the engine and cleaned. Be aware that some carburetor disassembly will be necessary to make the cleaning successful.

Removal of most outboard carburetors is a matter of: 1)disconnecting the fuel line; 2)disconnecting the choke and throttle linkages; 3)extracting the muffler or silencing device. Certain engines will also require that you: 4)pull the recoil or electric start assembly; 5)remove the fuel pump. For specific instructions regarding the procedure, refer to your engine service manual.

What you should be aware of: it is a good idea, if you are disassembling a two-barrel outboard carburetor with multiple jets, to keep these jets separate and labeled so that no questions of positioning linger when it comes time for reassembly. Distinguishing one jet from another is often problematic, even with a magnifying glass, and the carburetor orifices that hold the jets will indicate nothing about which goes where! TIP: try locating imbedded part or size numbers, and making note of these, as a way to help you distinguish between parts.

Here are several things to watch out for when you are disassembling an outboard carburetor, in particular one that is multi-barreled:

1)Be careful with the carburetor float bowl and its components: you should exercise care when extracting the float and its needle valve – this entails being cautious not to inflict internal damage on the needle-valve seat. If it is damaged somehow, both seat and needle will need to be replaced.

2)If you are detaching a valve seat: be sure not to harm the threads in the carburetor body; if this happens, at very least the carburetor body will need replacing.

3)Try not to remove the choke and throttle plates: unless you have specific reason for doing so – such as that have sustained damage or are simply worn out. Oftentimes, the screws that bind the plates to their shafts are pinned in position to keep them tight during engine operation. Pulling the plate screws will disable these pinnings, and since re-establishing them is a monstrous pain, it is best to leave them be if you can.

Cleaning and evaluating carburetor parts after disassembly: most carburetor components can be cleaned in standard solvent. However be aware that:

1)ordinary solvents can damage the float, the needle valve, and the needle-valve seat; these components should only be cleaned in fluid engineered for that purpose. Try consulting your engine service manual for recommendations.

2)fuel-related gum and varnish deposits can be eradicated with special solvents; normally these will be safe not only for the float bowl, but the float itself, plus the needle-valve assembly.

The best way to clean a carburetor body: is to saturate its various passages with solvent or carburetor cleaner, and then flush these out with compressed air. Compressed air is also the safest means of drying carburetor parts after the cleaning. The strength or pressure of the air being used: should not exceed about 30 psi. Here are a couple of additional things to keep in mind:

1)If you do not have access to compressed air, it is better to allow parts to dry off in the sun, etc. than to pat them dry with either a cloth or paper towel; these will leave behind lint that can potentially clog miniscule carburetor passages.

2)Make certain that you clean away residue – old gasket material and traces of sealing compound – from all gasket surfaces.

Inspecting carburetor components for wear and damage: following are a few carburetor parts that should be checked, and what you should be looking for:

1)Pitting or corrosion on all parts and internal carburetor surfaces; repair technicians will generally replace a component if it contains any pitting or corrosion at all. When working on your own engine, you can make up your mind which parts require replacing and which do not. Reuse of a damaged part will invariably mean that you end up repeating the same work more quickly than you would by applying a new part.

TIP: tiny imperfections inside the float chamber and main carburetor body can often be eradicated with emery cloth stretched across a surfacing plate.

2)Nicks or abrasions on gasket surfaces; sometimes these can be addressed with emery cloth, as mentioned above, if they are relatively minor.

3)Wear and injury to the float or float arm. Cork-type floats that are oil-saturated cannot be reused; they must be replaced. If there is wear to a float-arm hinge, its movement may be compromised and therefore it should not be reused.

4)Damage or other abrasions to the needle valve or its seat; since the needle valve and its seat are designed to fit tightly and wear together, both should be replaced simultaneously.

5)Nicks and abrasions on the tapered end of the low-speed needle; anytime you unearth these, the low-speed needle must be replaced. Check your outboard service manual to determine if needle packing should also be addressed during a carburetor refurbishing.

TIP: core plugs and lead shots should be left alone in a carburetor refurbishing unless they appear to be leaking; tiny plug and lead-shot leaks can sometimes be fixed with a punch and rubber mallet by: 1)locating the flat end of the punch against the lead shot or plug; 2)striking it sharply. If this process fails to eradicate the leak, you have no choice but to replace the defective lead shot or plug.

What you should know about carburetor reassembly: it is preferable – though not absolutely essential – to utilize new gaskets and seals, even if the existing ones appear to be in good condition. This is advised because old or reused gaskets are more subject to leaks, and, if a gasket or O-ring or some other seal is destined to have leakage issues, it probably will occur quickly after an engine has been returned to service. TIP: for all of the seals and gaskets required for the rebuild of a given carburetor, look for a carburetor kit; usually these will be available from an engine manufacturer. For popular motors, they can be obtained from a variety of sources.

You should check the float position: after it and the needle valve have been installed. This will ensure that the float opens and closes the needle valve the intended amount. Some carburetor floats can be checked for position with a standard ruler, while others require a special gauge from the manufacturer. It is especially critical to check float position in a two-barrel carburetor.

If the float drop is incorrect: it must be adjusted. Often this is a simple matter of bending the float arm to alter float position within the float bowl. An engine service manual should detail the exact procedure for adjusting the float of a given carburetor model. You should also torque down the screw or screws that anchor the float bowl to prevent leaks.

SERVICING OUTBOARD REED VALVES

For a two-stroke motor to run right, its reed valves must be in good condition. Reed valves cannot be inspected or serviced, in most engines, unless the intake manifold is pulled first. Be careful: if the reed assembly must be removed, that no bending occurs to its components. Here are a few points to examine with respect to reed valves:

1)There should be no gum or petroleum deposits on the reed assembly; if deposits are found, the reed valve must be removed and cleaned.

2)The reeds must rest flat against the reed plate; if any bending or similar damage is discovered, the reeds need to be replaced. If you must replace the reeds: I recommend that you also replace the reed stops, in particular if the stops have sustained any damage. If the reed plate is worn out or otherwise deformed: it needs replacing too.

3)Employ a new gasket when you are reinstalling a reed-valve assembly. If you were forced to pull the intake manifold to access the reed valve, it is best to use a new gasket between it and the motor. Remember: to torque down intake-manifold bolts to service-manual specifications.

CHECKING OUTBOARD CHOKE AND PRIMER SOLENOIDS

Most motors equipped with electric start will possess a solenoid to operate either the choke or a primer system. These solenoids are typically reliable and maintenance free. The primary points to remember about choke or primer solenoids are that: 1)the solenoid plunger should be able to move within its housing absent any restrictions; restrictions are normally the result of dirt or debris entering the housing, so make an effort to keep engine operating and storage environments clean; 2)a solenoid plunger should not be lubricated; lubricating oil is a catalyst for any dirt that happens along.

How to check a choke or primer solenoid: you will need a multimeter that has been set to read resistance or ohms. The multimeter leads should be attached across the solenoid terminals. Resistance, in most cases, should not exceed about 5 ½ ohms. Some solenoids will boast a maximum resistance that is less than this. To be on the safe side, consult your engine service manual regarding acceptable resistance levels.

SYNCHRONIZING THE THROTTLE ON AN OUTBOARD MOTOR

The outboard throttle and magneto need to be synchronized so that the ignition spark advances automatically with rpm increases. Synchronization methods can differ from engine to engine, so check your service manual for recommendations. The following technique can be utilized on some medium-sized outboards; if it does not apply to your engine, it should provide some idea of what a throttle synchronization involves:

1)The throttle should be advanced until marks on the carburetor throttle cam and cam follower are in perfect alignment; when this happens, the throttle plate inside the carburetor should barely be opening.
2)Loosen the cam follower adjustment screw until the throttle plate is fully shut.
3)Now tighten the adjustment screw until the throttle plate shaft commences to rotate. During the procedure, be certain that the marks on the carburetor throttle cam and cam follower remain aligned. If their alignment is intact, the throttle should be synchronized.

TIP: one of the most-difficult aspects of this particular synchronization procedure is gauging exactly when the throttle plate begins to open, or when its shaft has just barely commenced to rotate. Try adhering a piece of bent electrical wire to the throttle-plate shaft opposite the cam follower – or placing it in any other location from which it can reflect throttle-plate shaft movement. The goal here is to make minute shaft movements more distinguishable to the naked eye than they would be by watching the shaft.