Halp - Krapasake does it again..

[Captain Starfish]

Since I bought this krapasake it's been a faithful commuter, even though it did have a few things to tidy up. I've done the head stem bearings, cleaned up the levers, sorted out some evil vibrations coming from an unsupported exhaust, swapped out dead headlight relays and corroded indicators, got rid of the shitty dynabeads and it's reasonably ok now. Still gotta re-oil the forks and sort out the suspension which is a bit spongy, but that can wait.

My next item to deal with is this: regardless of gear, at about 100km/h (seems to be effort/throttle related) it starts hesitating or missing. It's not cutting out, and winding on the throttle gives me clean running and acceleration. It's only an issue when I'm trying to just maintain speed.

This is related to one of the air intakes being covered over when I bought the bike. No difference at low speeds but, again, when I get to that kinda 100km/h point and remove the cover the same hesitation/missing thing happens: a lot worse.

On the basis of these two things (more air makes it worse, more throttle makes it better) I'm guessing at fuel starvation. I have only the basicest grasp of how carbies work - butterfly controls airflow, venturi through the throat pulls fuel through in a mist, there's a float bowl with a valve same as in a dunny that gives a constant level/head of pressure for the venturi to work with. Jets? I know they control the fuel delivery rate as a function of airflow, but that's about it. I don't know where the idle/main jets are and when the main jets get kicked in or how the idle mixture screw on my whipper snipper motor works or any of that shit.

So - carby brains - are these symptoms ringing a bell? Should I be tearing down/cleaning/rebuilding, re-jetting, balancing carbs, checking float levels or just running some spitfire or similar through the next tank of fuel?

[Nero Diablo]

Do you know if they are CV carbs or a direct action? CV carbs use the engines vaccum to lift a slide and a needle to control the amount of fuel/air going to the motor the throttle cable acts directly on a butterfly that only controls the vaccum. Direct action carbs the throttle controls the slide directly and often an accelerator pump. Typically the average inline four has CV's. Basic stuff have you looked at the condition of the intake boots and the like? Needles and slides do wear out eventually but they usally lead to running very rich rather than fuel starvation issues. I'd go a basic carb balance and a bit of wonder snot/fuel cleaner of your choice through the fuel system and see if you make a dent in the problem if not you might need to pull things down and give them a going over. I'd also stay away from putting the cheapest swill that is passing itself off as petrol even if its only a commuter bike.

The other issue is has someone messed with it and not adjusted the carbs to suit (exhaust/intake) or put a "jet kit" which often comes from the states and are not suitable for the tune of the local models market.

[jmoore]

fishfingers,

the old owner didnt play around with the PAIR valve or similar did they?

[Captain Starfish]

FIIK. Not even sure this model had one - it's the in-betweenies model, 636 motor but with carbies and the older twin RAM intake frame. It does have a fairly fat exhaust system, not sure if that causes a huge difference with these.

[19andrew59]

Does it have an enrichener (choke)?
If it does, try bringing that on slowly while you hold your steady 100k.
If it improves the situation or makes it worse, you may need to be looking at needle positioning.
As stated earlier, intake boots, balance, etc.

[Captain Starfish]

Good point on the choke, thanks Andrew. Sorry, Nero - missed your post earlier. I'm guessing the easiest way to check is RTFM or alternately look to see if the throttle cam connects to anything other than the butterflies? I generally run 95 and the servo I use never caused issues with other bikes.

Time to lay my hands on some 4mm tubing and retic fittings, last time I threw together a balancer was maybe 10 years ago

[Gryphen]

You might need to carry out the airbox mod as I did on the ZX7R... As there was an issue for float bowl pressure at speed etc


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[Rich]

Time to lay my hands on some 4mm tubing and retic fittings, last time I threw together a balancer was maybe 10 years ago

Wanna borrow a proper one?

[speed3]

Or you could just leave the tape in place. If it aint broke don't fix it.

[Captain Starfish]

Nah, the tape makes it better but doesn't solve it.

Rich - that would be nice if effort(pickup and return) < effort(bunnings)

Gryphen you are scaring me now.

Will try the choke check, and work out next step from there.

Thanks guys!

[speed3]

This will fix it.

[Gryphen]

Gryphen you are scaring me now.

Airbox mod is UBER easy, relates only to the older airboxs (ie 97 era) on normally aspirated engines, Kwaka had a convoluted method of maintaining airbox air-pressure etc, easiest thing was to remove some of their valving associated with the airbox and blocking off a couple of holes in the airbox itself.

I also fitted a system that provided the even amount of airpressure to the float bowls (cost about $150) for the kit.

[Captain Starfish]

Interesting.

Doesn't happen until the bike's been running at that speed for a couple of minutes. Open the throttle and back off: comes back pretty much straight away. Choke: goes away and stays away for a minute or so, then creeps back.

Hmmm.

Running a little lean for some reason, maybe?

[jmoore]

if you need some bungs to cover up airbox holes butterfish, theres a whole box of loose end caps here which happen to fit airbox fittings quite well.

Out of curiousity, does it do the same thing at the same speed, but in a different gear? Aka different revs but still holding the throttle constant?

[Captain Starfish]

Bungs? Meh.



Yeah, it's doing it in 5th or 6th, a little slower uphill, a little faster downhill - seems to be effort and/or throttle position related more than anything else.

[Gryphen]

Some basic talk on the zx6r airbox mod... Air box mod for the zx 636's : Kawasaki ZX Forums: Kawasaki Ninja Forum

Airbox mod? - ZX6R Forum

and one called kleen air mod...

08 ZX6R kleen air mod?