QuestionXV1100 Virago 1995 with Vance and Hines Classic II exhaust.
No rejetting or Tuning has been done. Symptom, Riding in top gear at approx 4000rpm, Down throttle to approx 3000 rpm engine unloaded. When opening throttle again it hesitates. (Fuel consumption seems slightly High)
Engine rev's and accelerates normally when going up through gears.
Unknown if problems existed with standard exhaust.
I have a K&N filter and Dynojet Stage 1 kit on order. Will these solve the hesitation and improve Fuel Consumption isssues
AnswerHi Dan,
Q: "...No rejetting or Tuning has been done. Symptom, Riding in top gear at approx 4000rpm, Down throttle to approx 3000 rpm engine unloaded. When opening throttle again it hesitates. (Fuel consumption seems slightly High)
Engine rev's and accelerates normally when going up through gears. Unknown if problems existed with standard exhaust.
I have a K&N filter and Dynojet Stage 1 kit on order. Will these solve the hesitation and improve Fuel Consumption isssues?"
A: Installing less restrictive pipes, less restrictive air filters, and rejetting to compensate for these new performance features will likely increase fuel consumption.
On the contrary, if the engine was running poorly the new parts may improve consumption a tad.
As for the hesitation, that remains a carburetor air-fuel mixture tuning issue. The jet kit and filters may eliminate the problem. You'll have to wait and see.
Learn about reading spark plug appearance to know what corrections to make with the air-fuel mixtures. This is not an easy task.
Visit the following websites for more tech assistance:
www.viragotech.com
www.bikebandit.com (to view parts microfiche)
www.yamaha-motor.com (parts michrofiche)
www.motorcycleusa.com (join the Message Boards and ask other Virago owners abnout their tuning experiences)
When describeing carburetor or performance related problems, use throttle opening positions instead of engine RPM's. Carburetor fuel and air circuits (jets) are tuned with respect to throttle opening positions. Each circuit has a range of operation which is directly related to a specific throttle opening.
I return tech files by email. There was no email address with your question as requested and explained in my instructions. I've pasted a couple of tech files below for your convenience.
Respectfully,
Mark Shively
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.
See carb cleaning information below. See microfiche parts, check parts prices, and order 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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Drop-In Jet Kits
By Mark Shively
With respect to the engineers who designed these products, my experience with jet kits are they usually require additional tuning to get the correct air-fuel mixtures. Jet kits are engineered to make power improvements for the intended model application.
Many of my customers buy these kits, drop them in per instructions and expect a well running machine. Why doesn't every engine with these kits installed perform as expected or proclaimed? The reasons may be many and include: environmental variables, interpretation of instructions, skill level of installer, available tools, condition of the engine, and more. If it were that easy, you would not likely be reading this.
It should be known that factory motorcycle engineers do an outstanding job designing engines to operate under a variety of elements and variables. Think about it, a stock motorcycle must and does runs very well in a variety of climates and other elemental conditions. If not, their products wouldn't sell well. Dealership service departments would be overwhelmed with unsatisfied new motorcycle owners.
When we make changes to the stock air intake, carburetion, and exhaust, we unknowingly de-tune the engine in most cases. Who cares when the result is that desirable new exhaust sound? Most riders would not notice that their engine was de-tuned unless the problem was an obvious change from what they were used to hearing, feeling, or seeing.
What I'm trying to explain is that making changes to the OEM design may require on-going tuning. For example, at the Daytona International Speedway, I would have to tune the Yamaha TZ250's carbs (Lectron carbs) for the morning practice air temperatures. By noon the temps have changed and the engine requires new air-fuel mixture tuning to produce optimum performance. Tuning changes are required constantly as the environmental elements change and as engine components are changed or modified.
The same can be said for a street motorcycle engine, though not as important to tune as often as a roadrace engine. In any case, the air-fuel mixtures must be very close or performance problems will likely be noticed.
If you're about install a jet kit: Review the instructions in advance of the installation. When ready, follow installation instructions carefully. Work patiently and be prepared for the task. Expect to perform additional tuning to dial-in the carburetors.
If you've already installed a jet kit and are experiencing problems with performance or installation: Review the instructions and review your work step-by-step. Verify each step is properly installed. Having a second person verify your work can be helpful. Review the illustration below and learn about tuning each range of carburetion. Make necessary tuning adjustments one at a time to know what effect each change makes. Doing otherwise is fruitless causing you to waste time un-doing and changing things. If all fails, have a local tech perform the work for you.
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Most motorcycle carburetor circuits are governed by throttle position and not by engine speed. There are five main metering systems inside most motorcycle carburetors. These metering circuits overlap each other and they are:
* pilot circuit
* throttle valve
* needle jet and jet needle
* main jet
* choke circuit
The pilot circuit has two adjustable parts (fig 2) the pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.
The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.
The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes. The size is determined by how much is cut away from the backside of it (fig 3). The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).
The jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle. The jet needle is a long tapered rod that controls how much fuel can be drawn into the carburetor venturi. Thinner taper, means a richer mixture. The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Jet needles have grooves cut into the top. A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner, (fig 4). If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer.
The needle jet is where the jet needle slides into. The inside diameter of the needle jet affects the jet needle. The needle jet and jet needle work together to control the fuel flow between the 1/8 thru 3/4 range. Most of the tuning for this range is done to the jet needle, and not the needle jet.
The main jet controls fuel flow from 3/4 thru full throttle, (fig 5). Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). The higher the number on the main jet means the more fuel that can flow through it and the richer the mixture.
The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start. The choke system will add fuel to the engine to compensate for the fuel that is stuck to the cylinder walls. Once the engine is warmed up, condensation is not a problem, and the choke is not needed.
The air/fuel mixture must be changed to meet the demands of the needs of the engine. The ideal air/fuel ratio is 14.7 grams of air to 1 gram of fuel. This ideal ratio is only achieved for a very short period while the engine is running. Due to the incomplete vaporization of fuel at slow speeds or the additional fuel required at high speeds, the actual operational air/fuel ratio is usually richer. Figure 6 shows the actual air/fuel ratio for any given throttle opening.
Carburetor Jetting Troubleshooting
Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly, (fig 7). It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.
