QuestionMy son has a VT250 Honda with CV carbs. Bike wouldn' start, took carbs down and cleaned them, then traced fault to split hose on vacuum fuel cut-off. Bike now runs beautifully except that revs only drop slowly when throttle is shut off quickly. viz. revs will not drop between gear c hanges etc. Throttles are closing properly & needle pistons return to closed position properly. Idle is good otherwise. Any clues? Been working with cars nearly 40 years but bikes are a new thing for me! Trying to save uni student son a few bucks! Any help much appreciated Thanks. Dick.
AnswerHi Dick,
I suspect the problem is either an engine speed (idle) adjustment or carburetors are out of synchronization.
Respectfully,
Mark Shively
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Bad Gas
By Mark Shively
Gasoline goes bad with time and in as little as 3-4 weeks. This effect is known as varnishing. Jets and passageways within carbs become obstructed when varnishing occurs.
Liquid gasoline changes chemically into a gel like substance. Advanced stages of varnishing results with the solid gel changing into a crystal powder substance. Interior carb surfaces are etched in the process and may require carb replacement.
The choke and pilot circuits with most motorcycle carburetors share passageways. When pilot jets become obstructed, the choke circuit compensates and allows engine to start and idle with choke, but stalls without choke.
Review carburetor cleaning information below. View microfiche parts, check part prices, and order service (repair) manuals here: www.bikebandit.com
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, o’rings, 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 each Venturi (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 o'ring. 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 by spraying cleaner and wiping clean. Other material type floats may require replacement if cleaning is necessary. Inspect the needle valve (float valve) and seat. Check needle valve’s spring loaded pin. It should depress and return smoothly and without resistance. Check the needle valve’s tip for a worn groove. Replace needle valve and seat if either symptom exists. These parts wear together and must be replaced as a set.
Choke Plungers: It is common for Mikuni slide carburetors to have indented or hardened choke plunger pads. If the pads are worn, indented or hardened with age, then the idle of your bike will vary wildly as the pads no longer seal well.
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. Verify that numbers are the same for all jets on models with in-line cylinders. A few transverse-4 models and V-engines, the inner and outer carbs use some different size jets and it's important to not mix them up. If you have dial or verneer 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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On most multi-cylinder 4 stroke motorcycles the carburetors will eventually get “out of synchronization.” The reasons for carbs to do this can vary, but the results are the same: less power, bogging at various rpm’s, and poor performance.
We hope this tip on how to synchronize your carburetors helps. Keep in mind bad jetting,
damaged or “open” exhaust, and electrical problems can cause these problems, too. You
should get professional advice on weather or not synchronizing the carburetors will work for you. I have found a major difference in bikes I have synchronized.
First, you must acquire an auxiliary gasoline tank. This is a tank that can feed gas to your
carbs while your tank is off the bike. (Note: a antifreeze overflow tank from a water cooled
bike or car works well. Most junk yards and bike shops have these on hand). Motion-Pro makes one for about $40 that has a petcock and fuel line.
Next, you must acquire a set of synchronizing gauges or sticks. The type I recommend
are the mercury filled type. They can be purchased from Motion-Pro or J.C.Whitney for about $70 with adaptors.
There are two basic types of carbs. The two types are Standard and CV (constant
velocity). Standard carbs will have a small cap on the top that must be removed for you to
synchronize them. The CV carbs, usually on most all twins and most 4 cylinder bikes after ‘79, are easier to synchronize due to the fact that the adjustments are external.
The concept of synchronizing the carburetors is to make the carb work at its most efficient level
with the vacuum available to it. One carburetor out of synchronization can drag down the rest of the cylinders on power. What you are going to do is hook up a device (synchronizer tool) to
each of your carburetor’s vacuum tubes and adjust the carburetors until they are all working on the same vacuum level or volume.
Here are the steps.
1- Take off the tank to expose the carbs.
2- Hook up test tank. DO NOT FILL YET.
3- Find the carb or manifold vacuum hook ups. These vary from bike to bike.
Some are on the intake boot (on the head) and are actually tubes with vacuum caps that
you can remove. Others are a Phillips screw that can be removed from the head or front of
carb to install the tubes that come with most sync gauges. And still others have the
vacuum hook ups on the top of the carb. Most manuals will show this location.
4- Hook up your synchronizer to the bike by installing one hose per cylinder and set the
gauges on a stable surface or hung safely.
5-If you have an older bike with standard carbs, remove the small caps at the top of each
carb (if the cap is larger than 2” x 2” you may have CV carbs and this is not necessary).
This will expose the carb throat adjustment.
5b- If you have CV carbs look between each of the carbs for a screw with a spring under
it. These are the butterfly valve adjusters. Most 4 cylinder bikes have only 3 adjuster screws. One of the
middle carbs is non-adjustable. This is the carb used as a gauge (standard) for the rest of your carbs.
You will be adjusting the other carbs to match this carb in vacuum level. Also make sure
you have the proper tools to get to all the adjustments.
6- Next you need to find the screws to adjust the mixture. They are also in various
locations and are sometimes covered with a aluminum (anti-tamper) plug to hide them. Make sure you
have the tools to move the screw for the mixture. You may remove tamper plugs by carefully drilling them. They usually pop out with the drill bit just as the bit breaks through the bottom of the plug.
7- Fill the test tank with good fuel.
8- Start the bike, but do not rev the engine. Revving the engine may cause mercury to be drawn into the cylinders during de-acceleration. You may even want to warm the engine before starting. Set the idle to factory specs.
9- While the bike is running at the appropriate rpm adjust the mercury levels or needle gauges, depending on which type tool is used, so that all are equal or as close to equal as possible. Some variations is expected and allowable.
10- After step 9, recheck engine idle and adjust if necessary.
11- Adjust all the mixture screws to their highest idle. This will be a very small difference
in rpm but if you listen hard you will hear it. Back them down to their lowest idle. Find the center of these two positions and set the mixture screws there.
12- Now re-adjust the main idle to the normal level for your bike.
Now you should have a synchronized set of carbs that will make your bike more powerful
and smoother than before. If you have any questions about this procedure or where to find
the adjusters on your bike, visit: www.bikebandit.com, click “OEM Parts” and view microfiche parts on-line.
