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What's going on??!!

  • Thread starter Thread starter OzJavelin
  • Start date Start date
O

OzJavelin

Guest
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 ..
 
What condition are your ignition Points and Condensers in?

Daniel
 
7981GS said:
What condition are your ignition Points and Condensers in?

Daniel
Click to expand...
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 said:
Hmm, bet it wont do it if ya clean the carbs.
Click to expand...
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?
 
OzJavelin said:
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?
Click to expand...

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
 
Big T said:
That is a point of confusion. It shouldn't have anything to do with the bike misfiring.
Click to expand...

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.
 
Big T said:
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
Click to expand...
I agree. disconnect #3 plug, does it still "misfire " as you block and unblock the vacuum line?
 
Perhaps he's mistaking misfiring for sluggish running conditions.
My money's still with the carbs.
Click to expand...
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.
Click to expand...
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!)
 
Octain said:
OzJavelin, You've been around enough to know how temperamental these carbs can be and how important "clean" means.
Click to expand...

There's nothing temperamental about VM carbs

I've cleaned mine twice in 32 years
 
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!!
 
OzJavelin said:
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!!
Click to expand...
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.
 
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 said:
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.
:(
Click to expand...
Install Dyna-S .. no difference :(
Have noticed it seems to go away when warm ..
 
OzJavelin said:
I've tried pulling the #3 plug lead when idling and there is no real changing in idle.:(
Click to expand...

When you pull the #3 plug lead there should be a drop in idle speed and the motor running more roughly. If there is no change or little change in idle condition when the plug lead is pulled I would suggest there is a problem with #3 cylinder. To test pull one of the other spark plug leads and see if the idle alters much. If it does then #3 cylinder is not doing as much work as the other cylinders.

If so check valve clearances, do a compression test, etc. have you checked the coil voltage, it should be around 11.6v.

If compression is low compared to the other cylinders, you need to find out why.
 
As Don said, pulling the plug lead off a healthy firing #3 with good combustion would obviously effect how the bike runs. That it has little effect tells you it isn't firing well at all.
It's also very common/typical that as the engine warms up fully the firing can improve. Heat helps a poor combusting cylinder operate better. The apparent weak spark at #3 can't provide decent combustion under cold conditions inside the engine. Before it warms up you'll typically hear some missing and moderate backfiring from that cylinder. The backfiring is mainly created by raw fuel collecting on the header walls until it combusts all at once and you hear the "bang".
Unless you can see something obvious in your electrical inspection I'd suggest a compression test if you can. Even with good spark and good carburetion you'll have poor combustion if the compression is not within factory standards. If compression is OK then double check all spark related items and then focus on the carb at #3.

By the way, small amounts of oil entering the combustion area will also semi-foul the plug and cause what you're experiencing. Worn valve guide seals will allow oil to enter at different quantities depending on what position the valve was in when last turned off. The leaking will worsen as the engine becomes cold and tolerances widen. As I said, depending on valve position when last turned off, the amount of oil entering will vary but certainly effect cold combusion and as it worsens effect combusion all the time. At cold start up, especially if it's been off a longer time, that oil's just laying in there.
 
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