Question about 1978 Yamaha XS 650 SE
Yamaha 650 engine
You either have cracked intake boots/ manifolds or the idle circuit in the carb is plugged with stuff. Spray some brake cleaner around the intake manifolds (air box in place) and see if the idle picks up. If nothing happens pull the air box, fire it up and put your hand over the carb(s) to try to **** it through....rev it up...cover carb with hand...repeat. Next you're removing carbs and cleaning
Posted on Dec 17, 2008
under the left side of the gas tank you will see i black knob/screw at the end of what looks like a hose. the knob faces the rear of bike. that adjusts the idle. its real easy to find and the only adjustment you can make to the carbs.
Posted on Apr 02, 2009
Fuel filter is located near the fuel shutoff valve within three inches. It is inline between the shutoff valve and the fuel pump (towards tank).
The fuel circuit is: Tank/Fuel Pump/Fuel Filter/Fuel Shut-off Valve/Carbs.
You first step is to check for fuel delivery to the filter. Make sure the bike is cool and no iginition sources are nearby. Disconnect the inlet line at the filter and turn key on and crank engine (to trigger fuel pump.) Do not try and start at this time (choke off).
If fuel is adequate to the filter, check the filter by blowing through it, it should easily pass air. Replace filter if questionable.
Next reconnect the fuel lines and remove float bowl drain plugs in the back of the carbs (2 per carb). If with key on and cranking fuel pours out of these ports then you probably have clogged fuel jets. If not then the float bowl needles / pipes could be plugged with varnish from dried fuel. You can try spraying carb cleaner into the fuel inlet to clear the varnish and then retest. Careful not to spray carb cleaner on yourself (wrap straw with a rag).
If you have good fuel flow to the carbs and bowls you probably have varnish plugging the jets. You should remove the carbs and clean them thoroughly.
Hope this helps,
Posted on May 18, 2009
You should check the valve clearances, camshafts timing and make a compression test on the engine too.
One or more of your inlet valves is/are either not closing entirely or they close but their stem is damage and it doesn't seal off the combustion chamber properly.
Posted on Jun 28, 2009
just had that same prob and we cleaned the carb and works great but the rpm's go up and down from 1 to 3 u can ride but very unsafe why could this be?
Posted on Mar 30, 2010
weak mixture,dirty fuel,air leaks on induction manifolds,jets blocked,fuel tap filter chocked,float needles sticking,fuel level in carb incorrect,thottles not opening together,air leaks at exhaust pipe joints,
Posted on Jul 04, 2010
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