Question about Yamaha XJ 900 F Motorcycles

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1985 Yamaha Virago 1000 (not XJ 900 F) Put stage 1 kit in about 3 years ago and have made about 30 adjustments with no luck. Motors idles fine. Upon acceleration between Off idle and 2500 rpm motor stumbles/misses. After that (2500 rpm)the acceleration is great.Have raised and lowered needles, opened and restricted air cleaner, changed jets. Little to no change in all accounts. I am at my whits end and have contacted Dynojet before and was told to work with air cleaner. That made no difference on the stumble/miss.

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  • Yamaha Master
  • 41,663 Answers

Hi, Rdrudolph1 if you have changed your fuel delivery system, air filter size or flow rate, mufflers or exhaust system or a significant change in altitude your carburetors need re-tuning and if your fuel system (gas tank, filters, fuel valve and carburetor) is contaminated with ethanol sludge, varnish, rust, dirt, water etc. or your bike has been sitting for months or years without running these components must be "PROPERLY" cleaned and reassembled "CORRECTLY" before any adjustments can be made. Tuning your carburetor is fairly simple once you understand the basic principals. You engine is a simple airbox sucking air in and blowing it out, it is finely tuned at the factory for maximum performance once you upset that delicate balance by changing air filters, camshafts or exhaust systems your performance may go down the and the engine may run poorly, you need to compensate the air-fuel mixture in the carburetor in order for the engine to run smoothly and at peak performance. If you are running multi carburetors you need to sync them first and make sure your air cleaner element is clean and dry for paper elements or lightly oiled for foam and meshed elements and properly installed. Here is how and where you compensate trouble: "TIP" if your engine "BOGS" your not getting enough fuel.
1. Closed to 1/8 throttle is managed by the air screw and pilot/slow jet.
2. 1/8 to 1/4 throttle is managed by the air-screw, pilot/slow jet, and throttle slide.
3. 1/4 to 1/2 throttle is managed by the throttle slide and jet needle.
4. 1/2 to 3/4 throttle is managed by the jet needle, needle jet, main jet, and air jet.
5. 3/4 to wide open throttle is managed by the main jet and air jet.
6. Wide open throttle is managed by the main jet.
If you are running lean, spark plug electrode color is white, engine runs hot and feels like it is starving for fuel you need to go up on the jet size or move the c-clip down one notch. If you are running rich, spark plug color is black or dark gray, the engine runs cool, and bogs down when accelerating you need to go down on jet size or move the c-clip up one notch. When your carburetor is properly tuned for maximum performance your spark plug electrode will be a light tan color like coffee with cream. If you prefer fuel economy over performance you can go down on main jet sizes until a satisfactory level of lower performance is acceptable versus MPH, your spark plug color will be whiter and your engine will run warmer. These tuning adjustments will only make improvements if your intake and exhaust system have no air leaks or sealing issues and the entire electrical system is in proper working order and you have no mechanical issues.
For more information about your issue and valuable "FREE" downloads that you will need please click on the blue links below. Good luck and have a wonderful day.
ViragoTechForum com Yamaha Virago XV Forums That Rock
Carburator Theory and Tuning
Yamaha 1986 XV1000 Service Manual
OEM parts for Yamaha
Yamaha 1989 XV1000 Owner Manual

Posted on Jun 05, 2017

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6 Suggested Answers

6ya6ya
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Posted on Jan 02, 2017

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mrgreenz
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SOURCE: 1998 Cavalier 2.4 L stumbles and misses at 2000-2500 RPM

Vacuume Leaks can cause this problems. you have to do a through check of vacuum hoses and the EGR valve. Your best bet can be to have your vehicle scanned at an autozone store and get a code. The EGR has to be closed at idle, then as the engine warms up or is accelerated, the EGR opens. That is if your car does not have an electronic EGR . The electronic EGRs do not use vacuum to operate. They are controlled by the PCM (power train control Module).

Posted on Nov 20, 2008

SOURCE: 98 Cavalier 2.4L engine stumbles at 2000-2500 rpm

Could be a short in the wiring for the O2 sensor.   I currently have the same issue only my car finally threw a code after two days of it acting up.  Code 106.  Changed the MAP sensor and it now throws 108, MAP sensor high voltage circuit.  Ah the joys of cold weather and broke down vehicles.  But I know that you wont always get a code with a short.  Let me know if you fixed the problem.

Posted on Jan 08, 2009

  • 66 Answers

SOURCE: low power stumble at idle to low rpm

try placing the reed spacer between .reed assembly & cylinder, also try an adjustable ignition stator plate, set at +4, k & n air filter, open air box.replace T.O.R.S. eliminator system w/ a cap & cable, and idle air screw install kit, also a pipe tuned for more torque(bottom -end/mid range) blasters dont have a lot of power on bottom end and the engine needs to rev to hit power band.cylinder porting,240 bigbore kits, & +4mm stroker crank assemblies are good performance options.

Posted on Feb 23, 2009

  • 72 Answers

SOURCE: my 01 929 idles like its missing and stumbles on

check your battery cables to see if they are tight and then check your gasline filter . Drain your carb. bowls .While you are at it check your air filter too.Or your carbs. might need to be synchronized

Posted on Mar 20, 2009

  • 21 Answers

SOURCE: Stumbles and backfires on acceleration

Check the distributor shaft. They have a history of developing finite cracks that causes the condition your describing. Let me know if I can help you with any other info.

Posted on Jun 01, 2009

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