What's going on??!!

[OzJavelin]

1978 GS750 (basically stock with aftermarket twin mufflers on stock header pipes) .. Misfiring at both idle and higher RPM .. Only appears to be from RH exhaust. However, if I remove the vacuum line from the fuel petcock (attached to carb#3) and let it draw air, then the misfire disappears immediately .. Block it off with my finger and it starts again?? I've pulled the carbs and cleaned them (didn't find much) but no difference.

What could be going on here affecting both the idle and main circuit which would either need more air, or less(?!) vacuum.

Help ..

[Octain]

Sounds like the carbs need cleaned, really clean.
Did I mention, clean?...

http://www.thegsresources.com/gs_carbrebuild.htm

[7981GS]

What condition are your ignition Points and Condensers in?

Daniel

[OzJavelin]

What condition are your ignition Points and Condensers in?

Daniel

Points aren't great and I'll change them (and condensors) when I find a set. But I'm confused how mucking about with a vacuum line affects this?

[Octain]

Points aren't great and I'll change them (and condensors) when I find a set. But I'm confused how mucking about with a vacuum line affects this?

Hmm, bet it wont do it if ya clean the carbs.

[OzJavelin]

Hmm, bet it wont do it if ya clean the carbs.

OK .. OK .. I'll remove them *again* and clean them *again*. I just want to understand what gunk can affect both idle and main circuits?

[Big T]

Points aren't great and I'll change them (and condensors) when I find a set. But I'm confused how mucking about with a vacuum line affects this?

That is a point of confusion. It shouldn't have anything to do with the bike misfiring.

Removing the vacuum line makes the carbs run leaner, which, if anything, should cause more of a misfire

But, take off your points cover tonight and start the bike.

Is there sparking at the points? If yes, there's your misfire.

Order yourself up a Dyna S if possible and never mess with points again

[Octain]

That is a point of confusion. It shouldn't have anything to do with the bike misfiring.

Perhaps he's mistaking misfiring for sluggish running conditions.
My money's still with the carbs.

OzJavelin, You've been around enough to know how temperamental these carbs can be and how important "clean" means.
Buy a can of dip and do it right.

[tom203]

That is a point of confusion. It shouldn't have anything to do with the bike misfiring.

Removing the vacuum line makes the carbs run leaner, which, if anything, should cause more of a misfire

But, take off your points cover tonight and start the bike.

Is there sparking at the points? If yes, there's your misfire.

Order yourself up a Dyna S if possible and never mess with points again

I agree. disconnect #3 plug, does it still "misfire " as you block and unblock the vacuum line?

[OzJavelin]

Perhaps he's mistaking misfiring for sluggish running conditions.
My money's still with the carbs.

Nah .. it's a misfire/backfire/popping-thru-exhaust. I'll try and video it ..

OzJavelin, You've been around enough to know how temperamental these carbs can be and how important "clean" means.
Buy a can of dip and do it right.

Never has this drama with my GS550. All my Kawaka's have CVs, which again I've never have this drama .. and I'll pulled and cleaned those lots (unfortunately!)

[Big T]

OzJavelin, You've been around enough to know how temperamental these carbs can be and how important "clean" means.

There's nothing temperamental about VM carbs

I've cleaned mine twice in 32 years

[OzJavelin]

This is it ..

http://s27.photobucket.com/albums/c168/OzJavelin/gs-misfire2.mp4

Hope you can see it. Basically it starts with it idling with carb#3 vacuum line connected; you should hear the popping from RHS exhaust (only). Then I disconnect the line and rev it up and it appears fine .. then reconnect it and repeat and it's a popping again ..

BTW: Before this video I accidentally pulled the fuel line .. DOH!!

[KEITH KRAUSE]

This is it ..

http://s27.photobucket.com/albums/c168/OzJavelin/gs-misfire2.mp4

Hope you can see it. Basically it starts with it idling with carb#3 vacuum line connected; you should hear the popping from RHS exhaust (only). Then I disconnect the line and rev it up and it appears fine .. then reconnect it and repeat and it's a popping again ..

BTW: Before this video I accidentally pulled the fuel line .. DOH!!

Not saying I'm right, just speculation...
Cylinder 3 and/or 4 could have a rich condition resulting in right side exhaust "popping/missing"....OR...the spark is compromised/poor timing that's causing poor combustion.
Introducing an intake leak to number 3 cylinder adds more air and helps combustion somewhat.
Check the battery for good condition/voltage. Check the coils for proper primary and secondary voltage. Check plug leads for condition (no cracking/arcing) and check plugs condition, gapping and correct heat range/type. If you still have points, I'd look in there (and it needs checking anyway as part of basic tuning.) Be sure the points are clean/not pitted, contact/close correctly (no gaps), set dwell and then set timing. Double check for good bluish spark. Test.
If all above is correct and it still pops/misses, then check plug color and tell us what they look like after general cruising around town for a few miles after fully warmed up.

[OzJavelin]

Thanks for the suggestions. I pulled the carbs and clean again .. And no significant improvement. I've tried pulling the #3 plug lead when idling and there is no real changing in idle.
I'm going to install an DynaS ignition so I'll start again with the basics.

[OzJavelin]

Thanks for the suggestions. I pulled the carbs and clean again .. And no significant improvement. I've tried pulling the #3 plug lead when idling and there is no real changing in idle.
I'm going to install an DynaS ignition so I'll start again with the basics.

Install Dyna-S .. no difference
Have noticed it seems to go away when warm ..