Tach goes crazy @5000rpm
 

Hey all, long time lurker on the forums. I've gotten great information I've used with a huge amount of success on my bike from here, but I now have to post.
I haven't seen any other thread detailing a problem I'm currently having.

My 96 F3 seems to have what i can only describe as an exhaust leak.
That's what it sounds like anyway.
I've poured over every inch of the intake/exhaust system and found nothing. I've cleaned and replaced gaskets.

The bike performs flawless at lower speeds, but as I increase the speed, and the RPM's, the tach starts going nuts.
At about 5000rpm's, (around 60mph or so in 5th), the tach just goes crazy.
It starts jumping up and down between like 4k and 6k. And doesn't settle unless I slow down.

Once I get her around 3-4k, it's rock sold and perfectly responsive.
It idles smoothly.

I'm replacing the CCT this weekend, but I really don't think that's the problem (doesn't seem logical anyway). But I know that's another issue, so I might as well knock it out.

Anyone else had these RPM problems?

so the tach goes crazy; but the real question is: does the motor go crazy?

can you hear the bike going from 4k to 6k back and forth?

I don't think so.
As in, from everyone who's listened to it it sounds consistent and smooth.

To my ear however, it doesn't sound right.
It's not smooth in its sound to me.
Kinda like the aforementioned exhaust leak sound that it seems to be making (but I'm almost 100% certain it's not intake/exhaust related).

My best guess is that sound I'm hearing is in fact the engine missing in its timing. It's cutting out, for that 100th of a second or whatever, before the next combustion. And each time it misses, the tach tries to reflect that by dropping off, only to ramp right back up again with the next firing.

Or, it could be a broken tach, but I have no indication why it would be broken, since it performs so properly at lower speeds, and idle.

well if your motor is really going from 4k to 6k and you are sure then its obv. not the tach going nuts its your engine that is going nuts.


i bet it is fuel and carb related. Pretty damn sure.

clean em!


but if your engine sounds normal when the tach goes nuts then its that tach buddy, its electrical

That makes sense.
I hadn't considered a carb/fuel problem.

The engine only seems to sound normal to other people.
No matter if they ride along side me, ride it themselves, or listen to it at idle.
But I still maintain the engine isn't as smooth as it was, or should be.

I'm sure if I can track down this mysterious sound that sounds-like-an-exhaust-leak-but-I-know-isn't I would find the underlying problem.

And taking a look at this carb may be the way to go.

Hey, thanks for bouncing these ideas around jpanside.
I have a whole new course of action now. :)

the first month i had my f3 it did the same thing, sometimes it would even just kill. then my headlight went out (wtf right). i took off the the front cowl and changed my headlight, well it still didnt come on. a buddy and i then checked all the wires in the dash and kill switch area, and some were loose or not even connected. basically what im sayin is check the wires around your dash could be loose causing it to go haywire like mine did.

That's a weird problem, had a BMW K series that did that with the speedo, but not related to motor worries. Usually things like that are the easiest to fix, but its finding the problem that sucks.
It does sound like a detonation problem, & the fact that your tacho goes weird kind of makes me think its a electrical problem. The tach works off a sensor, so any fault that effects the plugs firing could also effect the tach, so Xander071584 suggestion would be the 1st thing I would be looking in to.

My reluctance to check the electronics isn't necessarily out of a lack of belief the problem could lie there. Only in that it's my absolute weakest skill with any motor vehicle. lol

So I'm falling back to my strengths first. Engine stuff, intake/exhaust, timing, etc.
Only when I have to finally admit it's an electrical problem will I grab my wizard's hat, draw a magical circle around my bike and prepare to deal with this sorcery known as Electrical Wiring.

But I do appreciate the suggestion. Having a plan B, C & D on what to check next can only be a good thing.

So if the bike is running good and at a constant speed and rpm and the tach jumps around while the motor holds the same speed and rpm this is NOT a carb / fuel problem !

The last time I had a tach jump around like crazy on a CBR600F series bike the Voltage Regulator / Rectifier was on it's way out...

IMHO would pull the bike apart and start cleaning and applying die electric grease to all quick disconnect electrical connections on the bike.

I would bet either a connector is loose or is starting to rot / corrode from oxidiation.

The newest F3 is 10 years old now and if no one has serviced the wiring harness on the bike you might have a corrosion problem.

This fix takes time because you have to strip the bike, but IMHO is worth the peice of mind knowing the electrical system is in good working condition.


Exhaust leak?

Have you had the header off or does the bike have a aftermarket header?

Possibly someone might not have removed one or more of the old copper O-Ring exhaust gaskets at the cylinder and your leak is there.

Possibly you have no gasket on one of the cylinders or you have a doubled up exhaust gasket and it's leaking there.

I would pull the header and check the gaskets, it's a cheap fix to se if the problem is the gaskets.

Good Luck!

SIRR1