Any ideas ? 3-5K stumble...

[scootertrash]

Anyone have any ideas? I have a 1999 ZX9R with an intermittent problem. The engine stumbles between 3-5K RPMs, like it is mis-firing or not getting enough gas...basically just running rough. It is fine at idle, and smooths out if I raise the RPMs. The problem is it happens infrequently, with the following pattern:

- Up until yesterday, it was only happening on cool mornings, but not always. It would typically happen if I was riding it into work and it was 40-45 degrees (F) out. The bike would be running fine for the first 20 minutes, then start stumbling through the 3-5K range. Sometimes it would stop after 10 minutes, and sometimes it would be like that the rest of the way to work. It always ran fine all the way home. I don't think it ever happened when it was warm out. (above 60 degrees)
- It happened yesterday at the track day. It would run great for about 6 laps, then start to stumble when I got on the gas coming out of a corner. Once I got the RPMs up, it would run good, but it was unnerving coming out of a corner and have the power come on choppy in the rain...it made for a few tense moments. It didn't do it every session, only about half of them.

I bought the bike used 3 years ago with 3000 miles on it. The only real change it had from stock was a D&D "carbon fiber" slip on can on the exhaust. I don't know if it was ever jetted or not. When I started having the problem a year ago, I changed the spark plugs and fuel filter. The air filter is clean and the vacuum hoses all look fine. I did a valve adjustment over the winter. I plan to do a carb sync very soon, but don't expect it to fix the problem, as the problem is intermittent. The bike is always garaged and well maintained.

Anyone have a guess ?

[TheIglu]

Coils maybe? CDI? Maybe the condensation inside the CDI box in the cooler weather. Just a guess.

[KevinB]

It was telling you to keep it going faster.

[scootertrash]

Originally posted by KevinB
It was telling you to keep it going faster.

TheIglu...that is a possibility...I'll have to double check the wiring in that area.

[R7]

I think what you need is to get rid of it and get a nice new Yamaha

[Rye]

If you haven't cracked the carbs open Gordo, it is likely overdue for at least the 3 years you've owned it. With a slip-on (even a near baffleless D&D) it shouldn't do much more than a stock setup. Although I think there are gains to be had (even in stock form) with a jet kit. It would be an easy maintenance step to clean them. If you don't want to go to crazy rejetting, then you can try turning the mixture screws out 1/4 turns at a time up to 1/2 turn total. This may help richen up the mixture a bit for that 3k bobble. It has a bigger impact off idle, but will richen along the whole curve. I believe the screws on those cv's are plugged with little circular aluminum plugs. They can be drilled (better yet dremeled) out very easily. The factory doesn't want salami's like you and me messing around with stuff like that for emissions sake. Go get Jay's sticks and sync those bad boys too. If it can wait 'til next race weekend, bring the bike with you and we can take a look...I have a shop manual for that year 9r.

[Stoneman]

I'd look at the spark plug wires for starters. If it's only happening when it's cool/wet out, I'm wondering if some condensation may be having an effect that would otherwise go unnoticed...

Second guess: The stator. I had a similar sounding problem years ago on my RF900R. They/we chased it around for damned near a whole summer before someone traced it to that...

FWIW-It seems like a helluva more than a 'stumble'. Sounded to me like it was cutting out. Which is why I suggested checking the above mentioned items...

[scootertrash]

Originally posted by R7
I think what you need is to get rid of it and get a nice Yamaha

You know, that R7 would make a nice track day bike for me...

[R7]

And you could have that R7 in your very own garage if you'd like?

[stoinkythepig]

Carb icing would be my guess.

[Stoneman]

Originally posted by stoinkythepig
Carb icing would be my guess.

...knob...

[legalspeed]

*cough cough* ZX10r...

[twrayinma]

Originally posted by legalspeed
*cough cough* ZX10r...

NOOOOO!
you'll kill yourself on a beast like that!

[stoinkythepig]

Originally posted by Stoneman
...knob...

Hey, I was serious. You can get carb icing in well above freezing temps due to the cooling effect of the pressure drop and subsequent water vapor release just downstream of the the throttle plates. Ever look at the top of an airplane wing when taking off on a humid day? When the plane rotates and the wings have a steep angle of attack, you can see huge clouds form above the wings. Same idea here on a smaller scale. It can cause localized icing on the bores of the carb which, even in small amounts will affect the airflow and fuel delivery/mixing at certain throttle openings. The pressure drop and water vapor release happen all the time; the icing is most likely to occur on cool raw days. It's really common in England, so much so that many bikes destined for there have carb heaters.

[scootertrash]

You could be onto something Stoinky...
It happened again this morning on the way to work. It was 40* out and pretty foggy. About 20 minutes into the ride it started acting up. If I hold the throttle constant, it runs fine. As soon as it twist it slightly, it starts to sputter. If I pull in the clutch and race the engine, it helps, but doesn't usually totally clear it up. Over the next 15 minutes or normal riding, the problem slowly went away and the bike was running perfect by the time I got to work. Seems to me it's a carb issue...wrong fuel delivery or maybe it is icing, as it only happens when it's cold out. (mid-40* or less)

[scootertrash]

Oh...and Rye...I may take you up on your offer to help me sync the carbs next race weekend. I had planned to do it myself, but I wouldn't mind having your experience there to help me out.
I am not sure if it's worth me tearing into the carbs to clean them or not. The fact they it runs so well when it's warmer out implies that cleaning them won't help much

[Rye]

With pleasure Gordo.

[Stoneman]

I need a refresher too!!!

Lemme know if/when ya wanna clean the carbs, Gordon. I'll come up & give ya a hand...