Question about 1980 Yamaha XS 400
Hi, Pjsdaddy03 engine "BOG" is mainly caused by a rich air and lean fuel condition but it can also be caused by a lean air and rich fuel condition this situation rarely occurs and is only caused by the misinformed weekend warrior that owns a toolbox.
The more you open your throttle the more vacuum you are creating in your carburetor venturi and your intake manifold. When you are operating at higher RPM any unmetered air that leaks into your system can become more obvious.
Unmetered air is air that is getting into your system after the fuel has been delivered. If you have unmetered air getting into your system between the butterfly/slide of the carburetor and the cylinder head this will create a lean condition.
All of the rubber components of the fuel system like vacuum hoses and intake manifold that you mount the carburetor to are made of rubber. If none of these components have been changed out they are more than likely highly degraded and probably cracked in places to allow unwanted-unmetered-contaminated air into the combustion chamber. Check all of your vacuum lines and vacuum plugs for carburetor synchronization. The vacuum plugs are in the head just after the rubber intake manifolds. The petcock has a vacuum line as well as part of the emission system.
1. Check the intake manifold for cracks.
2. Ensure the bands used to tighten the manifolds down on the intake are secure and have not bound up the manifold.
3. Make sure air box fittings are not warped and fit completely over the carburetor.
Your airbox is metering air and is the first step in a process of consuming air and fuel. The system requires the resistance of the air filter in order to get the proper vacuum to "SUCK" the fuel out of the float bowl and create the proper venturi effect.
Improper mounting and sealing of the airbox will create a small lean effect. This might seem like no big deal but you are inviting dust and debris in your engine that is doing slow damage by not having proper fitment. Fix it so you know it's not contributing to your issue. Pick the low hanging fruit first.
Do not go and start adjusting anything at this point. It ran fine before. There is something wrong with the assembly or a component. Do not adjust your floats. Get it back to where it was. The moment you start tweaking everything is the moment you lose OEM settings which are a must have for fine tuning and maximum performance.
Fine tuning your carburetor and multi carb syncing come at the very end following the proper procedure established by the Carburetor Gods.
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.
Carb adjustment breakdown
Idles fine but dies upon throttle
Yamaha XS360 Service Manual
OEM parts for Yamaha
Posted on May 31, 2017
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Posted on Jan 02, 2017
Posted on Mar 17, 2009
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