Need suggestions for troubleshooting odd problem

[Sci85]

Hey guys,

So I took the 673 baby kat out for a quick ride today with the warm weather. After some fairly steady state riding at 4-6K rpm, the bike begin to not rev with throttle. It doesn't die but barely revs with throttle and begins to loose speed unless I hold the throttle open. No studdering just no power/rev. When i come to a stop, it will die when it's doing this. Now, I did notice that if I pull the clutch in and rev the $hit out of it, it recovers and runs ok for a bit. Then it may or may not act up again. There's not much ryhme or reason. Sometimes it will just recover for no apparent reason which can be a little scary as the power suddenly comes on. Also, if I stay off the throttle (closed) for 10 seconds or so, it seems to recover. It seems like a fuel or air flow issue. Almost like maybe the slides are stuck but I know that all 4 slides would not stick at the same time. Or another way to describe it is like someone stuck a sock in the carbs. Maybe coils going bad? I'm baffled...

Valve clearance is good. Carbs cleaned over a year ago. I have jetted it and it runs good with a small flat spot in the midrange that I haven't bothered to work out. I am running the twin k&n air filters and a mack pipe but with a medium sized baffle in it. The K&N's are not quite like pods but they are open element. Tank has a new OEM petcock assembly and flows fine. It has done this off and on for the past year but only happens very rarely. Temperature doesn't seem to affect it. The only thing that seems common is that it crops up when running steady throttle for awhile.

Oh, one other thing. I did make my own homemade gas cap gasket last summer as the original was toast. Not sure how that might affect things if it's not sealing good but I suspect it might be an area to explore. I just need to know how to test it.

So that's it as best I can describe it. I'm looking for ideas on how or what procedures I can try to at least narrow it down. I am thinking fuel, air or electrical.

Thanks for all the usual experience and help guys.

[Steve]

Next time it happens, stop, pull your plugs and take a look.

They will tell you if they are running rich or lean, to rule out carburetion issues.

.

[rwnielsen]

Does that bike have a vacuum operated petcock? If so, I'd check that connection / line. If not, well..... never mind

It sounds like fuel starvation to me

[Nessism]

A few things that come to mind...

Petcock not flowing properly

Gas cap not venting properly

Battery voltage issue

[Sci85]

Good ideas guys. I've checked the petcock and it flows fine on prime. As mentioned it is new but how can I test the vacuum or the run normal position?

Good or idea on the plugs. I'll bring a wrench with me next ride.

If the gas cap gasket isn't working properly, how does that affect fuel flow and how can I test that theory?

Thanks again.

[GSX1000E]

A vacuum pump or simply sucking on the vacuum line in the run position should show results.
As for the gasket causing any problems, I doubt that as it is vented through the key slot. Take it apart and you will see the float and vent. It could be stuck inside that. (rusty)

[Missile99]

Any luck figuring it out Sci?

[chef1366]

Sounds like you're dropping a coil

[robbiem1961]

You can try the cap venting issue by riding it with the filler cap off, or by borrowing one you know to be good, electrical components either work or they dont, ignition coils can be fine when they're cold and fail when warm, but they just stop working!

[Sci85]

Any luck figuring it out Sci?

Just to close this out, yes I finally did figure it out. It was a bad electrical connection to the 2/3 coil and so it must have been cutting out. I cut out the connector and replaced it and now she runs like a dream again. Some day I probably should build a whole new harness. This one has seen it's best days...

[posplayr]

Sounds like you're dropping a coil

Good diagnosis

[chef1366]

Good diagnosis

Thanks Jim
Everyone gets lucky sometimes.