QuestionHi Mark - I rebuilt all 4 carbs on my girlfriends 82 V45 Magna 750 and it runs great now except that it will occasionally stay revved up very high (about 3000 rpms) and then come down very slowly and then die, but only sometimes. You can then restart the bike and it will idle just fine at just a touch under 1000 rpms. I soaked and blew out all passageways in the carbs, all linkages are working smoothely and the butterflies snap back smoothely and quickly. Is this a symptom of out of synch carbs or what do you think it could be? Thanks! George
AnswerHi George,
The problem you described is consistant with carburetors that are out of synchronization, an air leak, or routing of a cable.
Review the following information files.
Respectfully,
Mark Shively
Carb Cleaning 101
By M. Shively
The elements of internal combustion engines are: correct fuel/air ratio, spark at right time, adequate cylinder compression.
There are many passageways and openings to check and clean. All are important in function and when obstructed or not working properly, have subtle to radical effects on engine performance. Vacuum leaks and carburetor synchronization also have effects on performance and should be inspected and adjusted following the below procedures.
Carb Cleaning 101
Warning: Remove all rubber parts before you begin. These parts usually include vacuum diaphragms, needle valves, orings, hoses, and other parts. Spray cleaners will damage these parts. Do not disassemble individual carbs from the carb bracket.
Air & Fuel Passageways: Trace and learn individual fuel and air circuits from beginning to end. Machines can only drill straight through the cast passageways. To change direction, another angled passageway must be drilled. The union is plugged with a brass or bronze bead. Inspect and clean each passageway with spray cleaner, brushes/pipe cleaners/etc, and compressed air. Remove any discoloration and debris. Look for spray cleaner to exit from one or more passageways.
Jet Cleaning: Inspect jets by holding to light and look through them. You should see an unobstructed round hole. Clean the jets with one or more of the following: jet cleaning wires, soak solutions, carb spray cleaners and compressed air. Re-inspect jets after cleaning and install when clear of obstructions. Some main jets have paper-like gaskets. Most have metal spacers between the jet and the emulsion tube. Some screw directly into a brass emulsion tube which is machined for a 7mm wrench at its float chamber exposed base.
Inlet Fuel Valve: Inspect the needle valve & spring. Press down the tiny metal rod that protrudes from the butt or float end of the needle valve. The spring should move freely and return the rod to its location. Check the needle valve's seat area for a groove or other wear. It should appear highly polished. Some needle valve seats are rubber and wear may not be visible. Inspect the needle valve jet seat. You can clean the jet seat with Q-tips and semi-chrome polish if necessary.
Carb Body Castings: Blow air through the atmospheric vent holes located on the dome of each float bowl chamber. Air should exit via hoses or brass nipples. Inspect the emulsion tubes and passageways (cast towers that jets thread into) for discoloration and debris. Clean interior emulsion towers with a soft bristle gun cleaning brush. Clean the Venturi's (main carb bore).
Needle Jets & Jet Needles: Clean the needle jets, jet needles, and passageway or tower that needle jet screws into. Clean the emulsion tube (pipe between needle jet and main jet) (Main Jet may screw into emulsion tube). Jet needles are part of the throttle slides. See below…
Throttle Slides: There are several types of throttle slides: Mechanical linkage, vacuum, diaphragm, and cable. Disassembling the jet needle from the slide is not always required for cleaning. If you have vacuum piston type throttle slides (large diameter solid metal slide), avoid cleaning the lubrication from sides and caps. If piston type check cap vents and passageways with air. Clean if necessary and re-lube. If you have rubber vacuum throttle diaphragms, inspect for dry-rot, defects, and tears by gently stretching rubber away from center. Do this until all areas around diaphragm have been inspected. Replace any defective part as described above. Clean carb body areas around diaphragm including air passageways and air jets. Diaphragms have a locator loop or tab fabricated into their sealing edge. Observe this locator upon reassembly. Avoid pinching the diaphragm when reinstalling caps.
Fuel Screws: Fuel screws have sharp tapered ends. Carefully turn one fuel screw in while counting the turns until it seats lightly. Warning: These screws are very easily damaged if over tightened into their seats. Record amount of "turns-in" and remove the fuel screw, spring, washer, and oring. The fuel screw is part of the enrichment (choke) circuit...clean passageways as described above. When carbs are assembled, spray low PSI compressed air into diaphragm air vents located at intake side of carbs. Throttle slides should rise, then fall when air is removed. Lightly lube external moving linkages. Reinstall carbs and follow through with carburetor synchronization.
Throttle Cables: Lubricate cables periodically. If cables are disconnected from carbs or removed for replacement, etc . . . remember cable routing and ensure proper reinstallation routing. Avoid bread-tying, sharp bends, and pinching cables. Adjust cables so throttle grip has about 5mm of play or throttle slides or butterfly valves may not open completely (full throttle)(wide full open).
Float Bowls: Inspect float bowls for sediment, gum or varnish, crystallization, and defects. Clean all pipes, tubes, passageways, and embedded jets with cleaners and compressed air. Remove and clean the drain screw and area. Inspect bowl gasket and replace if necessary. Clean and inspect overflow pipes and tubes, look for vertical cracks.
Floats: There are several types of float materials: plastic, brass, black composite, tin, and others. Handle floats carefully. Avoid bending, twisting, denting, or other means of mishandling. Most floats are adjustable by bending a small metal tab near the float axle end. Do not change the float adjuster tab unless tuning fuel service levels. Clean metal floats by soaking or spray cleaners, if necessary. Replace other type floats if cleaning is necessary. Clean the float axle or pin.
Synchronization: This is a fine adjustment performed usually and preferably with the carbs installed and the engine running. The unusual part is performed with gauged wire with the carbs on the work bench. Carburetor synchronizing balances Venturi vacuum at the exhaust side of each carburetor, resulting with smooth idling and optimized performance at all throttle openings. Synchronization is checked using a set of gauges which are either air vacuum type or liquid mercury type. The gauges are connected to vacuum ports on the intake manifolds via nipple tubes or if sealed with screws, sync gauge adapters will be needed. With the engine running at temperature, and with a fan or means of forced convection aimed onto the engine, the carbs fuel screws and idle are adjusted, then the synchronization is adjusted via adjustment screws on the carbs. A reserve fuel tank is recommended for convenience of accessing carbs during this procedure. See gauge instructions and repair manuals for detailed use of synchronization gauges.
Notes: While carbs are apart, record the jet sizes. Look for a very small number imprinted on the body of the jets. It should be the same number for all. If you have dial or veneer calipers, measure and record float heights. Perform measurements with floats just touching needle valves, though not depressing the needle valve rods. Replace fuel and vacuum hoses. Be sure to use fuel rated hose for fuel. Install or replace in-line fuel filters. It's a good time to remove and clean interior petcock fuel filters. Inspect carb manifolds for dry-rotting, inspect all clamps and air ducts. Inspect, clean, lube, and/or replace air filter(s).
Carb Maintenance & Synchronization
Experiencing rough idle, galloping, backfire, and uneven acceleration? This is a good indication you need the carbs synchronized. The procedure is fairly similar for all multi carb engines including Goldwings, V-max and a great many more. The idea is to get all the individual cylinder vacuums to match. I'll use the standard inline four as a demo type. First thing pull the seat & fuel tank so you can get at the carb adjustments and vacuum connections. You'll need an auxiliary fuel supply as the tuning will take enough time to drain the carbs dry. Here's a neat little tank that will do the job with minor cash outlay.
Fill the auxiliary tank about 1/2 full. Slip the hose over the spout tip. There's no need to vent the tank as the sides are soft and will "inhale" as fuel is used. Hook up the tank to one of the fuel lines (some bikes have (2) lines that feed all the carbs, one is standard the other is reserve. Using a 1/4" drill bit or short Phillips screwdriver, plug the remaining lines so fuel doesn't spray all over. Lay the tank on the frame or faring just so gravity can do its thing. Start with the idle fuel mixture settings. Check the service manual for how many turns out the mixture screws are supposed to be set. (Some mixture screws are non-adjustable. Skip this step and go to synchronization). Start engine & allow it to warm up. Place a fan or furnace blower in front of the engine to keep from overheating. (Mixture screw tips are precision machined points and may be easily damaged if tightened into seats). One screw at a time, turn mixture screws in until they seat, counting number of “turns in” as you go. Record this number for later reference if needed. Once seated, back them out to factory recommendations. Turn the screw on #1 carb out until you notice a roughening idle. Go back to the original setting and turn in until the idle roughens again. Set the screw at the mid point of these two settings. Repeat with other mixture screws. Some carbs use a fuel mixture screw and some use an air bleed screw. The procedure is the same for both. On the fuel mixture screw type, turning screws clockwise is leaner; counter clockwise is richer. The air bleed type is just the opposite.
Carburetor Synchronization:
To sync the carbs you will need a gauge set or one of the more popular carb stick synchronizers. These can be purchased through "Dennis Kirk" 1-800-328-9280 or www.JCWhitney.com, (Keyword: Carburetor Synchronizers). Most tuners prefer the mercury carb sticks as they are more reliable in the long term and don't require any needle flutter calibration. Hose adapters are required for some motorcycles. They are not included with the tool and are a separate purchase. They are listed with the sync tools. I recommend the purchase of a sync adjuster screw tool. This tool works the locknuts and adjuster screws.
Support the sync tool on a lever or handle bar grip and connected to the respective carbs. Keep the hoses clear of engine's hot surfaces. For tuning Yamahas, go to the "YICS" section for the tuning procedure.
With fan on in a well ventilated room or area, aux fuel connected, idle mixture set, and carburetor synchronizer tool connected, and engine warmed to normal operating temperatures, set the idle speed to about 1000 RPM and watch the sync gauges. Pick a carb and set the vacuum of the others to match this one. Pick one that is not the highest or lowest, but in the middle of the readings. Some carburetors have one non-adjustable carb that the others are adjusted to. Check the service manual or call your local dealer if you are not sure if your carbs are this type.
The majority carbs have adjusting screws between the carbs. (On older Kawasaki KZ type carbs, adjusting screws are located under the carburetor top cover caps.) Adjusting screws have locknuts to maintain adjustments. You may notice your adjustments change slightly when tightening locknuts. Compensate for this accordingly as you work. The adjuster found between carbs 1 & 2 balances those two carbs. The adjuster found between carbs 3 & 4 balances those two carbs. The adjuster found between carbs 2 & 3 (or center) balances the left and right carb sets (1-2 & 3-4) to each other. On older Goldwings, screws are between carbs as above, but center adjuster is on the left side. The order in which the carbs are adjusted doesn't matter as long as they all come out balanced in the end. When adjusted as level as possible, you're finished.
Sync adjuster screws may have a drop of red or yellow paint to mark or seal the factory sync settings. While synchronizing, open and close the throttle slowly to avoid drawing mercury into the engine.
Vacuum Leak Testing:
An engine needs the elements to run: Adequate compression, spark at exact time, and correct air-fuel mixture.
Compression is the first test. Spark is the 2nd test. Fuel is the 3rd test. Each test has a routine order to follow, so you are not guessing.
Vacuum Leaks:
A vac leak is un-metered air allowed in error to get by the intake. This affects carbs and fuel injection equally. You do not want a vacuum leak on any engine ever, but they are very common. Sometimes you might not know it if the leak is small, and other times an engine won't start at all. So symptoms run a broad range of what a vac leak can do.
Some symptoms are a sudden lean condition, which can result in loss of power instantly and a matching increase in fuel consumption.
This might be found to be true if heat cracked a vac line to the petcocks while you were riding, or the line just lifted off. Other types of symptoms cause what is known as "Hunting" which is idle RPM that will not stay correct. The idle goes up to a given range maybe even to 2,200 rpm, and then will drop to 600 RPM and go right back up as if a demon has the grips. The bike might go to 2,200 RPM and stall forcing you to restart. Lesser leaks might effect idle, and what was correct yesterday suddenly is high today. Turning down the over all Throttle linkage screw will work to lower idle sometimes but is a misadjusting when you should not do that. If the leak becomes worse, the idle will do what ever the leak demands.
The leak leans out the correct mix of 14% to 17% fuel to air and makes the mix undeterminable, ALWAYS lean. On bikes each carb can have leaks, and manifold mount for any carb can have vac leaks. Any throttle plate shaft can leak on either end. Any vac lines can leak on either end. And any test port can have a bad cap, and also leak.
Most bikes don't have vac operated accessories, with the one exception of vac operated petcocks. A vac operated petcocks will say, PRI = prime, ON/Run, AND RES = reserve. There is NO OFF setting. Also the petcock will have 2 lines each. One line is for fuel and the other is a vacuum line telling the petcock the engine is MAKING vacuum, and to turn on the petcock diaphragm to pull open the on/off valve with in the petcock. The way an internal combustion engine works creates vacuum. I have never seen any bike with a vac pump. In my experience vac lines in general do not deal with heat and weather well. They crack, split, and become brittle, and should be replaced once a year. Same goes for gravity feed fuel lines.
To locate a vac leak you need a can of WD-40 which is probably the best thing you can use WD-40 for.
Also you can use WD-40 to test whether or not idle mix is right. This chemical beats ether hands down for use as an engine starter as well, and will not cause engine damage in moderate amounts. WD-40 makes what you can't see and probably what you can't hear findable.
You need to listen to know. SO to tell if idle mix is right, spritz a shot right at the intake with a running engine, and listen. Does the idle go up? Or, does the idle go down? If things are correct the engine has all the fuel it wants and the idle will drop, as the engine wants no more.
If the idle goes up you are lean.
If you have 4 carbs and all go down but one, then that one is lean. Why it is lean remains a question. Maybe the setting is wrong, and the fuel screw is out too far. [Often a book setting will say 2, or 2 1/2 turns out. That is a place where a fresh built engine should run to start and IS NOT always the best mix for any given cylinder]. Or maybe you have a vac leak...and so adding fuel in the form of WD-40 causes the idle to jump to who knows what, and that depends on the unmetered air. It is possible for a bike to run on 2 cylinderss out of 4, and have the two dead cylinders fire up above idle speeds as the engine approaches mid range RPM.
So finding leaks becomes a bit of hit and miss, as you spritz about the carbs after an initial shot into the carbs. Each time you spritz you must listen, so with an air cooled bike you might want a fan on the engine. Places to spritz are the manifolds looking for loose clamps, throttle shaft ends, and any vac line ends and components vacuum operated. On injected bikes any Throttle body lines, and injector bases, also any vac operated components as you find them. Often times vacuum leaks are misdiagnosed as clogged carbs, and bad plugs, wires, pick ups coils and more.
