Flat spot between 5K and 7K

[chrismeister]

Having replaced my manifold on the ZX6 after it started to sound like a Panzer tank due a to a massive hole, I seem to have acquired a range between 5 and 7K where there is noticeably less power.

If i accelerate hard from low revs it feels like there's a lot of torque, then once it hits 5K it's like it's hitting a wall and takes a lot longer to get to 7K where it takes off again all the way towards the red.

It's got to be something to do with the manifold, although i would have expected better performance all round, or more top end or more bottom, not a flat spot in the middle?

I have a stubby can, could that be an issue with midrange?

Help appreciated.
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[temeluchus]

sounds like a lean spot- try raising your needles a notch or two.
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[chrismeister]

Thanks, reading more on the issue points at adjusting the carb needles to adjust the fuel mixture as you say.

Thinking about it, when i pull away, the revs drop a lot more than they used to and to get a fast start i have to have the revs noticeably higher than before.


Anything past 7K is fine.

Could someone confirm that it is a lean mixture that usually causes low to mid range bogginess?

If it is this i'll raise the needles a notch and take it for a spin.
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[temeluchus]

it can be lean or rich. if it is rich it will smell a bit ripe and petrolly.

a look at a spark plug can also be an indicator.
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[chrismeister]

I'll have a sniff around tomorrow and see what's occuring
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[chrismeister]

Also, as a side note, the previous owner said it had been dyno jetted....

I assume this would be good for taking advantage of the full stainless system i have now. Pretty sure this is a fueling adjustment i need to make.

Any pointers on how do do this would be great.
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[herulach]

Are they OEM replacements or pattern ones?

I bet if you com;pare them you'll find either the pipes are a different length to the collector, or are missing crossover pipes (assuming it has them as stock)
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[chrismeister]

More forum trawling... I read in a post that if past 7K the bike carrys off how it should do then that's an indicator that the main jet does not need to be adjusted.

It is running rich in the mid range according to this site: Which looks very useful! https://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

It does feel like someones turned the choke on maximum in that range.

Could someone give me a quick guid on adjusting the needle height please?
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[temeluchus]

with the needle vertical, pointy end down as it sits in the slide, moving the clip up towards the top of the needle makes the needle sit lower in the carb which is leaner. moving the clip down the needle making the needle sit higher in th carb is richer.
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[Billing]

You can also add a .5mm washer to alter the height if you find you can't get it sorted by moving the clip or you don't have adjustable needles
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[chrismeister]

Brilliant, I will give it all a go when i get time, most probably after work this afternoon.

Will drop the needles in a tiny bit and see how they do.

Pulling away fast it bogs for a second or two before tearing off so hopefully the low end overfueling will be fixed by adjusting the needles.

Thanks for all the help, will report back.
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[chrismeister]

I have been putting this off a lot but everytime i get on the bike it's a niggle.

I took the airbox apart and found all sorts of crud in there, cleaning the filter out has made it a bit better but i really need to adjust the needles.

When i took the tank and airbox off, i could see the carbs, they've got caps on them with 4 screws holding them on, I need to take the caps off to get to the needles and floats don't I? I won't have to take the whole carb off that way?
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[temeluchus]

Take the plastic caps off the top, you will see the diaphragms and slides.

These will come out and you can then get at the needles.
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[chrismeister]

Nice one, cheers.
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[chrismeister]

i got the caps off of the carbs, took the spring out, then the diaphragms.

There was a plastic thing that the spring sat on, on top of the needle, took that out and at the bottom was the needle, but no way of adjusting it at all?

It was just a tapered needle with a flat end, i need the midrange to be leaner so i need the needle to sit lower in the carb, it looks almost impossible to do with what's there? Unless you can get needles with different taper profiles?

To test i put a 2mm washer on each needle and ran - extremely boggy and un-ridable so i definitely need to get the needle to sit lower.

Is there a way of adjusting the fuel - air mixture without touching the needle? I did notice an allen bolt on the back of each carb?

Any ideas?
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[jjdugen]

No movable clips means they are standard needles NOT Dynojet.
You could try removing the airfilter for a test run as this will help lean out the mixture to prove the point.
It might be that you will have to go to smaller main jets or do the sensible thing and fit a stock silencer.
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[chrismeister]

Damn, that's annoying!

I'll try that in that case.

Would a standard can not make it run richer, i thought race can type exhausts made it leaner?

I like the look of the MTC GP, would that make it leaner?
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[temeluchus]

take the silencer off and go for a spin hehe
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Some shite cruiser. Now with guns and FREEDOM!

[chrismeister]

Might get pulled if I do that ahah
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[chrismeister]

OK I moved the exhaust further down the link pipe kinda thing so that it's closer to the end, making the total length of the pipe longer.

It seems to have got rid of the flat spot in the revs and it doesnt bog down at low revs/pulling away fast like it did before so I guess the stubby made it rich.

I have another problem now though, and one I think is related to me taking the airbox and carb caps off.

If im on the motorway, i'll cruise at 6/7K, after about 20 seconds the bike starts to splutter a little bit, like someones put the choke on full and gets really boggy. If i shift down a few gears and accelerate it goes, until I cruise again.

Would a gap in the airbox / top of the carbs cause this? It's almost like it's losing air pressure or something and half throttle because it's fine when you're on WOT. If so I'll make sure it's all sealed perfectly.

I just want to get my bike running well again! (Self inflicted I know)
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