Tachometer fluctuations

[andrewsw]

Here's another one of those annoying things that's cropped up. On highway cruising in top gear (maybe also in 4th and 5th) my tachometer fluctuates independently of engine speed. Typically this happens between 6k and 8k.

It's sort of like the tach is losing and then regaining signal causing it to drop and then rebound. It seems to go away up above 8k, but not reliably so. EIther way, it gets less severe the farther away from about 8k I go. Down below 6k it's non existent. I"ll do some testing today on my ride and see if I can narrow it down.

Where does the tach get it's signal? (I know, follow the wires you lazy bum). Could this be a sign of some deeper problem, like a CDI failing? It doesn't seem to be effecting the engine (no misses or stumbling to indicate that I'm losing spark, for example) though at those speeds, it's kind of hard to tell.

A

[gsxr400 racer]

pick up coil -cdi-coil-tac

[andrewsw]

hmmm... so it could be a cdi problem?

I tested a bit today... it's most noticeable in 5th & 6th, but also shows up in 4th. Highway speeds.... in 4th, I hopped on it a bit and it did it up around 11k as well, but was jumping rapidly all over the place up there... weird.

A

[Herr Tod]

I've seen that on my bike as well sometimes. I don't know if it was the cause but it happened after I crashed the bike pretty hard. It only happens a few times a year. I think it might have something to do with the coil wires leaking to the frame or something. I haven't had any ignition problems though, the bike runs fine up to 15000 RPM.

[interfuse]

I had the same thing happening when the 12v wires going to the coils were breaking. The tach would sometimes drop for a second and the come back. Eventually the wire broke and I was running on 2 cylinders on the highway. To fix the problem I cut the wires back and crimped on spade connectors.

http://forums.banditalley.net/index.php?topic=8845.0

[PitterB4]

So, it's running OK when this happens?  I can't imagine it's anything other than the lead to the tach itself then, right?  Mine was doing that when like Mike said, the leads to the coils were making intermittent contact... BUT, the tach was the least of my problems.  The B4 doesn't make a very good B2!

[andrewsw]

Seems to be running fine, though as I said, it's most prevalent at highway speeds... which would make an intermittent mis-fire a little hard to notice.

But yes, seems to be okay (other than the crappy speed limiter kicking in...)

Maybe the tach lead is just getting dodgy in the wind and vibration of those higher speeds...

A

[interfuse]

Breaking 12v wires going to the coils will be noticed only at highway speeds -- that's when the wind shakes the crap out of them. I remember the needle dropping and returning quickly without any effect on performance. Eventually the wire broke and problem became no longer intermittent. Idle the bike and jiggle the wires and see if you can make the tach drop, if you can, cut the wire back and replace the spade connector...

Since your tach is cutting out, it'll be the coil on the right hand side...

[andrewsw]

Good idea. thanks

A

[gsxr400 racer]

Hey wiggle the spark plug lead while your at it, while the bike is running. your 

[andrewsw]

well, all sorts of wire wiggling produced no results... just another one to keep an eye on I guess...