QuestionHi.
Thanks you for voluteering your time and expertise to help novices such as myself. I appreciate it.
I am rebuilding a 78 GS400 Suzuki that sat for 12 years. I installed new points and coils and I rebuilt the carbs.
It finally fired up, and the engine will idle normally for a little while. But once it idles for a while, and gets warmed up, it will all of a sudden start revving up to 4 or 5000 rpms, stay there for a little while, and then drop back down to normal idle. The throttle responds well at first, but doesn't return promptly to idle, just stays at a high rpm then drops slowly back down. THe throttle is closing normally, I can feel that it is closing just fine.
I replaced all the rubber in the carbs but the diaphrams after soaking them in carb cleaner. They looked pretty good. I didn't have to remove any air jets or anything which would require re-adjustment. I don't have an air filter on it yet (I know, bad idea... but I'm just trying to make the thing go right now). The choke is operating normally, it is not stuck on. I replaced the plugs, battery, bulbs, etc.
Can you help me?
Thanks
Jeremy
AnswerHi Jeremy,
I suspect an intake air leak to be causing the erratic idle. Unsynchronized carbs have similar symptoms. Check for air leaks around the carburetors and intake manifolds and synchronize carbs.
To check for leaks: Use carb clear spray, WD40, or brake cleaner and spray around suspect areas. If leak is found, engine will bogg down or rev high depending on which spray product used.
Review the carb info files below. Email me if you want additional carb related info files: mshively1@woh.rr.com
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).
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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 whatever the leak demands. Another symptom is engine starts, idles and runs well when cold, but stalls when warmed.
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.
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What does "balancing" mean and why do I need to do it to my bike?
Balancing keeps each carburetor working the same to deliver the same amount and mixture of gas and air to each cylinder. We determine how "hard" each cylinder is working by measuring the vacuum at each of the carburetors which is provided by the downward motion of the cylinders on the first of the 4 strokes (the opposite of the compression that takes place in the cylinders on the 2nd stroke). There is a pressure "drop" that occurs as you pass by the butterfly throttle valve (the flat circular piece that pivots about its center when you move the throttle grip). The throttle valve will never close all the way - or else the engine would die without another source of air. What balancing does is to make sure that each throttle valve on each carb is allowing the same pressure drop to occur, therefore "sucking" the same amount of gas up through the jets.
• OK -now how do I do this?
Each bike is a little different, so I'll try to be general, but keep it specific enough so that you have a clue as to what I'm talking about.
The first thing you'll need to do is to print this page out (unless you have a service manual that will step you through the process). Before you gather your tools - you will need a set of VACUUM GAUGES. There are basically two different types - the dial type and the mercury type. Sure, you can get good and bad mercury-type gauges, but unless you're planning on really going into business for yourself, the cheaper mercury-type gauges are probably your best bet. You can order them from a number of sources including Dennis Kirk and J.C. Whitney in Chicago - ask for a motorcycle catalog (312)431-6102. When you order the gauges, tell the supplier what kind of motorcycle you have and ask them if you need any special adapters to use the gauge. The adaptors are sold separately and cost less than $15.
• Now that you've got your balancing "sticks" let's get down to it
o Warm the engine to operating temperatures. Adjust the idle speed. Shut the engine off and put it on its center stand.
o Disconnect the vacuum line (the small hose) to your fuel tap.
o Take the gas tank off. If you can leave the gas lines connected and still take the tank off, do so. If not, you'll have to find another container to supply gas to the carbs. I've heard of using inverted plastic bottles with a tapered cap. Small moto-cross MC gas tanks work well. Motion-Pro markets a variety of carburetor servicing tools, including plastic auxiliary fuel tanks. Connect the fuel line to the fuel supply and fill the container. If using a make-shift type container, invert it and make a small vent hole in the bottom. Raise the fuel supply above the carbs. Keep a fuel rated fire extinguisher on hand.
o There should be one small rubber vacuum tube connected to one of the "nipples" that are located on the rubber carb intake manifolds (usually on #2 carb). Disconnect this small vacuum hose and set it aside (the other end was connected to the fuel tap). Remove the other vacuum caps on each carb intake manifold nipples or rubber vacuum ports.
o Now you have 4 little brass tubes or small pipes sticking out from each intake manifold. Connect the vacuum gauge hoses to each of the carb vacuum ports. Make sure the hoses seal and there are no vacuum leaks.
o Start the engine. Do not rev the engine above 3,000 rpm. Opening the throttle doesn't create a big vacuum, but if engine rpm's are high, closing the throttle will create a momentary high vacuum in the venturi that may draw Mercury into the combustion chambers. That is a REALLY BAD THING. Mercury vapor is extremely toxic and exposure to it in sufficient quantities and for a certain time has shown to cause bain dramage. So..... don't slam the throttle closed when the gauges are hooked up and with the engine running.
• Ok, gauges hooked up, engine warm and running......
•
o Set the idle speed to specs. Idle speeds may raise and lower while synchronizing. They may already be low and hardly readable or very high. You'll notice idle change with extreme adjustments.
o Adjust the fuel/air mixture screws to obtain the highest idle speed. These screws are located on the back, near float bowls of most carbs. These screws are precision made with a fine point. FYI: seating them tightly will damage them-use care. Turn each screw both directions until minimum and maximum idles are obtained. Then set each screw in the middle of these settings. It's hard to tell exactly, but manage.
o Readjust idle speed to specs if necessary with the idle speed adjuster.
o Look at the gauges. The quantity (usually measured in millimeters or inches of mercury) of vacuum is not nearly as important as whether all of the carbs are pulling the same vacuum.
o To synchronize them, we adjust the position of the throttle valves in each of the carbs. If you don't have a service manual to show you where the adjusting screws and locknuts are located, open and close the throttle a few times while you are looking at the carb assembly. Look between each of the carbs. There will be moving linkages with a screw adjuster and lock nut. These are the adjusting screws.
o If you have a 2 cylinder bike, you've only got ONE screw adjuster to deal with. Loosen the locknut and try turning the screw in either direction until you see that the mercury levels are about the same. Motion-Pro markets a combo screw drive/lock nut tool for simplifying this task.
o If you have a 4 cylinder bike, you will have THREE screw adjusters. Adjuster screws located between carbs 1 & 2 adjust only these carbs. Same for carbs 3 & 4. The center adjuster adjusts the carbs ½ and ¾ together Adjust each bank of carbs, then adjust the two banks together by adjusting the center adjuster. Open and slowly close the throttle a few times. Each time bring rpm's over 3,000 rpm's.
o Recheck Mercury Levels and repeat the procedure until each carb is within about a quarter to half inch of the others, or within two gauge lines of another.
• When done, shut the engine off and remove the vacuum gauges. Reassemble everything that you took off. If your carbs were out of balance, you will notice the difference in power and/or gas mileage.
