Topic: Sudden loss of power and backfire thru carbs

[brewsky]

Almost home from a 150 mile ride and my K8 starts losing power and spitting back after stopping at an overlook.
Pulled over after a couple miles and let it idle and it seemed normal at idle....smooth and exhausts even.

Back under load it's coughing and spitting and sounding like a shotgun!

Years ago I had to nurse it home on 2 cylinders due to a bad condenser, but it didn't sound like this!

Hopefully I'll find a loose points wire or the like!

[Erny]

Isn't it issue with fuel? Did you try to open tank - if that tiny hole is not clean, there can be underpressure not allowing fuel going out to fill carbs..

[Erny]

Now I see you have backfire too.. So it is then most likely another issue, ignition probably?

[brewsky]

Only thing I've done so far is to get on the web and oogle the new Z900RS Kaw and XSR900 Yamaha!

Will have a look later today.....

[DaveBarbier]

I’d check the ignition with a strobe. What kind of condensers do you have? Daiichi?

[brewsky]

I’d check the ignition with a strobe. What kind of condensers do you have? Daiichi?

That's first on the list...….honestly don't remember what condensers I last got...they have been in there a while

[calj737]

When it happens, pull over and test (carefully) the header temp on all 4 pipes. Are they all equally hot? If not, there's your clue.

[brewsky]

When it happens, pull over and test (carefully) the header temp on all 4 pipes. Are they all equally hot? If not, there's your clue.

All 4 headers passed the spit test, and the exhaust pulses felt equally hot at idle, but the miss seemed to be only under load, so will have to dig deeper.

[evanphi]

check condition of spark plugs?

[PeWe]

A plug cap that is cracked, or only loose?
If you use Dyna coils, check so all HT wires are connected to the bottom. Mine have creeped out causing shotgun or canon shots plus bad behaviour when twisting the throttle.

[brewsky]

New plugs about 500 mi ago.
Fired it up and #2 header much colder than the rest, so I changed #2 plug and got same result.
Swapped #2 & 3 leads and seemed much better and all headers hot.
Timing light shows some intermittent flash on #2 lead only, so it looks like the lead, cap, or connection between the 2

[Bodi]

What type of strobe do you have? The contact wire uses spark plug voltage, the clamp type senses spark current in the wire. The coils are dual-fire and both outputs must spark somewhere or neither will: if a strobe pickup works on one plug cable but not the other, the dead one is arcing to ground above the sensor - to the frame/tank/engine/whatever and you need new cables, or inside the coil which must then be replaced.
Note that strobe pickup coils work best in one direction and plug polarity is reversed between the two plugs on a coil. So it works best pointing opposite ways on the two cables. Some have an arrow on the pickup, on cars the plug voltage is consistent and the pickup always goes whichever way works (towards the plug or towards the distributor)... not so on our bikes.

[brewsky]

What type of strobe do you have? The contact wire uses spark plug voltage, the clamp type senses spark current in the wire. The coils are dual-fire and both outputs must spark somewhere or neither will: if a strobe pickup works on one plug cable but not the other, the dead one is arcing to ground above the sensor - to the frame/tank/engine/whatever and you need new cables, or inside the coil which must then be replaced.
Note that strobe pickup coils work best in one direction and plug polarity is reversed between the two plugs on a coil. So it works best pointing opposite ways on the two cables. Some have an arrow on the pickup, on cars the plug voltage is consistent and the pickup always goes whichever way works (towards the plug or towards the distributor)... not so on our bikes.

That's certainly food for thought.....I have an old sunpro inductive and all wires showed consistant flash except #2 (switched to #3 cylinder).....it also flashed but shows a miss. Didn't pay any attention to pickup orientation, and don't see any arrow on the pickup.
Will take a closer look with the light before digging deeper.
Haven't had a chance to work on it last couple of days.
I had convinced myself it was probably the wire to cap connection since #2 and 3 need to be manhandled to get the plugs out.

[brewsky]

Well, looks like a loose connection between the #2 wire and cap was the culprit.

It was obviously loose when I started to untwist the cap and the end of the wire showed signs of arcing, so I cut off about 1/4", screwed it back on and good to go!

I had the plugs out for valve adjust and plug change not too long ago and remember having to wrestle the long plug caps with the short wires on #2 and 3. So I probably caused the problem myself by not making sure they were on tight then.

The good part is I found the source of a new oil leak to be a rocker cap ……...loose from the last valve adjust!

Thanks for all the replies!