Topic: Petcock issues???

[jefe06]

Hello all.  I just purchased a 81 cb650c.  It started right up when I purchased it , the PO did note that it was running a little rough.  I got it back home and realized the plugs were fouled.  The front brake also doesn't want to release.  I ordered and changed plugs and the bike started fine, but when I switched the petcock to on , the bike stalls out within 3 seconds.  I'll shut it off and put the petcock in the off position and after turning over a few times will fire up and run for 10-15 minutes on idle then die if I try to switch it to the on position.  There is an online filter but it looks like it's filling fine.

I'm thinking this is a fuel starvation issue, but that doesnt make sense with the bike stalling when I put the petcock in the on position.  Any help would be appreciated.

[TwoTired]

Look at the plugs again.  If they are sooting up and fouling again, you may have carbs that are overfilling.  In short, too much fuel to air mixture going to carbs, leaving conductive soot on the electrodes and draining off spark energy.

The carbs should only demand the fuel that is necessary to run.  The petcock should never deliver more than the carbs/engine should take.  In each carb, their is a fuel bowl and a float that opens a fuel inlet valve only when the carbs demand more.  If the float is stuck, maladjusted, or the valve leaks, the carbs deliver raw gas to the motor beyond what the jets deliver.  If there is rust in the tank, those bits can block the float valve open and cause it to leak/ overflow.    Adding a filter after contamination, is too late.  Also, there should be a filter already in your tank associated with the petcock, obviating a need for an added in line.  Mechanics with too many thumbs can damage these without admitting their error.

Cheers,

[jefe06]

That makes sense.  I'll take a look at the plugs and see if the float valves are not clogged/sticking.  The previous owner had the same issue, but he thought it was from a crack in the accelerator pump diaphragm which, once replaced, he said it ran perfectly and it did from what I saw.  So, that seems weird.  I doubt its the accelerator pump again and why would running the petcock on the off position allow it to run when it dies in the on position.  Is the bike just running off of what fuel is in the bowls?

[TwoTired]

A crack in the accel pump diaphragm would weaken the shot of gas when doing throttle twist, and probably cause a gas leak there.  Most would notice an engine stumble on acceleration, if not puddles of fuel.  Not much relation to the petcock setting, though turning it off could decrease the size of the puddle under the bike.

The engine runs off the fuel in the carb bowls.  It will do so until the fuel level in them lowers enough to expose the pilot jet out of the fuel supply there.  Then it won't idle and die.  If the throttle is open, as in driving about, the main will still allow the engine to run until the carb bowl is near empty, or you ask the engine to idle again.

The previous owner had the same issue, but he thought it was from a crack in the accelerator pump diaphragm which, once replaced, he said it ran perfectly and it did from what I saw.  So, that seems weird. 

I wouldn't put much credence in PO explanations, and his shotgun approach to parts replacement.  He was unhappy with the bike enough to sell it with it's remaining problems.  I would view his "fixes" with a very jaundiced eye.

I doubt its the accelerator pump again and why would running the petcock on the off position allow it to run when it dies in the on position.  Is the bike just running off of what fuel is in the bowls?

The bike totally runs off the fuel in the carb bowls.  ...until they run dry.  The petcock just allows fuel to flow from the tank reservoir.  If the carb's mechanisms don't stop that flow, the engine floods/dies.

While fixing a bike can be done with random parts replacement.  The effective outcome is just as random.  The bike operates due to logic, mechanical, and physical principles.  Meet those demands and both machine and rider are "happy".

Cheers,