Motorcycle Repair: Suzuki GN 125-lack of power, suzuki gn 125, internal…

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Suzuki GN 125

Motorcycle Repair / Suzuki GN 125of power


Expert: Shively — 12/24/2005

I have a 2 year old Suzuki which I have had from It has has all its scheduled services but I am now having with its power output-the sounds 100% ok while and as the bike is moving but I cannot get it to go any than 35mph (downhill!) is a relatively new problem as it has been reliable until now. I no knowledge of mc repairs but I have its airfilter and this made no

I would very much any suggestions you may have.

4. Jet needle not in place.

5. Vacuum leak.

the carburetor and inspect.



Bad Gas

By Mark Shively

goes bad with time and in as as 3-4 weeks. This effect is as varnishing. Jets and passageways carbs become obstructed varnishing occurs.

Liquid changes chemically into a gel substance. Advanced stages of results with the solid gel into a crystal powder Interior carb surfaces are in the process and may require carb

The choke and pilot circuits most motorcycle carburetors passageways. When pilot become obstructed, the choke compensates and allows engine to and idle with choke, but without choke.

See carb information below. See microfiche check parts prices, and repair manuals here:

Carb Cleaning 101

By M. Shively

The of internal combustion engines correct fuel/air ratio, at right time, adequate compression.

There are many and openings to check and clean. All are in function and when obstructed or not properly, have subtle to effects on engine performance. leaks and carburetor synchronization have effects on performance and be inspected and adjusted following the procedures.

Carb Cleaning 101

Remove all rubber parts you begin. These parts include vacuum diaphragms, valves, o’rings, hoses, and parts. Spray cleaners damage these parts.

Do not disassemble individual carbs the carb bracket.

Air Fuel Trace and learn individual and air circuits from beginning to Machines can only drill through the cast passageways. To direction, another angled must be drilled. The union is with a brass or bronze Inspect and clean each with spray cleaner, cleaners/etc, and compressed air.

any discoloration and debris. Look for cleaner to exit from one or passageways.

Jet Cleaning: Inspect by holding to light and look them. You should see an unobstructed hole. Clean the jets one or more of the following: jet cleaning soak solutions, carb cleaners and compressed air. jets after cleaning and when clear of obstructions. main jets have gaskets.

Most have spacers between the jet and the emulsion Some screw directly a brass emulsion tube is machined for a 7mm wrench at its float exposed base.

Inlet Valve: Inspect the needle spring. Press down the metal rod that protrudes the … or float end of the needle The spring should move and return the rod to its location. Check the valve’s seat area for a or other wear.

It should highly polished. Some valve seats are rubber and may not be visible. Inspect the needle jet seat.

You can clean the jet seat Q-tips and semi-chrome polish if

Carb Body Castings: air through the atmospheric vent located on the dome of each bowl chamber. Air should via hoses or brass nipples. the emulsion tubes and passageways towers that jets into) for discoloration and debris. interior emulsion towers a soft bristle gun cleaning

Clean each Venturi carb bore).

Needle Jet Needles: Clean the needle jet needles, and passageway or tower needle jet screws into. the emulsion tube (pipe needle jet and main jet) Jet may screw into emulsion Jet needles are part of the throttle See below…

Throttle Slides: There are types of throttle slides: linkage, vacuum, diaphragm, and Disassembling the jet needle from the is not always required for cleaning. If you vacuum piston type slides (large diameter metal slide), avoid the lubrication from sides and If piston type check cap and passageways with air.

if necessary and re-lube. If you have vacuum throttle diaphragms, for dry-rot, defects, and tears by stretching rubber away center. Do this until all around diaphragm have inspected.

Replace any defective as described above. Clean body areas around including air passageways and air jets. have a locator loop or tab into their sealing

Observe this locator reassembly. Avoid pinching the when reinstalling caps.

Screws: Fuel screws sharp tapered ends. turn one fuel screw in counting the turns until it lightly. Warning: These are very easily damaged if tightened into their Record amount of turns-in and the fuel screw, spring, and o’ring. The fuel screw is of the enrichment (choke) circuit. passageways as described above.

carbs are assembled, spray low PSI air into diaphragm air vents at intake side of carbs. slides should rise, fall when air is removed. lube external moving

Reinstall carbs and follow with carburetor synchronization.

Cables: Lubricate cables If cables are disconnected from or removed for replacement, etc. cable routing and ensure reinstallation routing. Avoid sharp bends, and pinching

Adjust cables so throttle has about 5mm of play or throttle or butterfly valves may not open (full throttle)(wide full

Float Bowls: Inspect bowls for sediment, gum or varnish, and defects. Clean all pipes, passageways, and embedded jets cleaners and compressed air. and clean the drain screw and Inspect bowl gasket and if necessary.

Clean and inspect pipes and tubes, look for cracks.

Floats: There are several of float materials: plastic, black composite, tin, and Handle floats carefully. bending, twisting, denting, or means of mishandling. Most are adjustable by bending a small tab near the float axle Do not change the float adjuster tab tuning fuel service

Clean metal floats by or by spraying cleaner and wiping Other material type may require replacement if cleaning is Inspect the needle valve valve) and seat. Check valve’s spring loaded

It should depress and return and without resistance. Check the valve’s tip for a worn groove. needle valve and seat if symptom exists.

These wear together and must be as a set.

Synchronization: This is a adjustment performed usually and with the carbs installed and the running. The unusual part is with gauged wire the carbs on the work bench. synchronizing balances Venturi at the exhaust side of each resulting with smooth and optimized performance at all throttle

Synchronization is checked using a set of which are either air vacuum or liquid mercury type. The are connected to vacuum ports on the manifolds via nipple tubes or if with screws, sync adapters will be needed. the engine running at temperature, and a fan or means of forced convection onto the engine, the carbs screws and idle are adjusted, the synchronization is adjusted via adjustment on the carbs.

A reserve fuel is recommended for convenience of accessing during this procedure. See instructions and repair manuals for use of synchronization gauges.

Notes: carbs are apart, record the jet Look for a very small imprinted on the body of the jets. that numbers are the same for all on models with in-line A few transverse-4 models and V-engines, the and outer carbs use some size jets and it’s to not mix them up.

If you have dial or calipers, measure and record heights. Perform measurements floats just touching valves, though not depressing the valve rods. Replace and vacuum hoses. Be sure to use rated hose for fuel. or replace in-line fuel

It’s a good time to and clean interior petcock filters. Inspect carb for dry-rotting, inspect all clamps and air Inspect, clean, lube, replace air filter(s).

Poor or No Power at High Speed:


Spark plug broken, or maladjusted

Spark cap shorted or not in good contact

mixture incorrect:

Starter stuck open

Suzuki GN 125

Main jet or wrong size

Engine oil too high

Drive train

Final gear case oil too high

Air suction valve

Vacuum switch valve

Balancer mechanism malfunctioning

Leak Testing

An engine the elements to run: Adequate spark at exact time, and air-fuel mixture.

Compression is the test. Spark is the 2nd test. is the 3rd test. Each test has a order to follow, so you are not guessing.


A vac leak is un-metered air in error to get by the intake. This carbs and fuel injection You do not want a vacuum leak on any ever, but they are very Sometimes you might not know it if the is small, and other times an won’t start at all.

So run a broad range of what a vac can do.

Some symptoms are a sudden condition, which can result in of power instantly and a matching in fuel consumption.

This be found to be true if heat a vac line to the petcocks while you riding, or the line just off. Other types of cause what is known as which is idle RPM that not stay correct. The idle up to a given range maybe to 2,200 rpm, and then drop to 600 RPM and go right back up as if a has the grips.

The bike might go to RPM and stall forcing you to restart. leaks might effect and what was correct yesterday is high today. Turning the over all Throttle linkage will work to lower sometimes but is a misadjusting when you not do that.

If the leak becomes worse, the will do whatever the leak Another symptom is engine idles and runs well cold, but stalls when

The leak leans out the correct mix of 14% to 17% to air and makes the mix undeterminable, ALWAYS On bikes each carb can leaks, and manifold mount for any can have vac leaks. Any throttle shaft can leak on either

Any vac lines can leak on either And any test port can have a bad and also leak.

Most don’t have vac operated with the one exception of vac operated A vac operated petcocks will PRI = prime, ON/Run, AND RES = reserve. is NO OFF setting.

Also the petcock have 2 lines each. One is for fuel and the other is a vacuum telling the petcock the engine is vacuum, and to turn on the petcock to pull open the on/off with in the petcock. The way an internal engine works creates I have never seen any with a vac pump. In my experience vac in general do not deal with and weather well.

They split, and become brittle, and be replaced once a year. goes for gravity feed lines.

To locate a vac leak you a can of WD-40 which is probably the thing you can use WD-40 for.

you can use WD-40 to test whether or not mix is right. This chemical ether hands down for use as an starter as well, and will not engine damage in moderate WD-40 makes what you see and probably what you can’t findable.

You need to listen to SO to tell if idle mix is right, a shot right at the intake a running engine, and listen. the idle go up? Or, does the idle go

If things are correct the engine has all the it wants and the idle will as the engine wants no more.

If the goes up you are lean.

If you have 4 and all go down but one, then one is lean. Why it is lean remains a Maybe the setting is wrong, and the screw is out too far. [Often a setting will say 2, or 2 1/2 turns That is a place where a built engine should run to and IS NOT always the best mix for any given

Or maybe you have a vac leak. and so fuel in the form of WD-40 the idle to jump to who knows and that depends on the unmetered It is possible for a bike to run on 2 cylinderss out of 4, and the two … cylinders fire up idle speeds as the engine mid range RPM.

So finding becomes a bit of hit and miss, as you spritz the carbs after an initial into the carbs. Each you spritz you must listen, so an air cooled bike you might a fan on the engine. Places to spritz are the looking for loose clamps, shaft ends, and any vac line and components vacuum operated. On bikes any Throttle body and injector bases, also any vac components as you find them.

times vacuum leaks are as clogged carbs, and bad plugs, pick ups coils and more.

Suzuki GN 125
Suzuki GN 125
Suzuki GN 125
Suzuki GN 125
Suzuki GN 125
Suzuki GN 125


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