Tech - Rejetting ZX6R for optimum power

[BikerBoy]

Hey all,

Was wondering whether anyone had had a rejet after installed a bolt on and high flow filter, and whether they'd noticed big power gains.

Also whether installed new jets can iron out the acceleration and the 'stumble' at around 5000 rpm on most sports bikes.

My 96 ZX6R is at the shop today, being rejet and tuned, should there be a noticable difference when i get it back??

Most people seem to recommend rejetting if you have a race can and high flow filter.

And that 'dead spot' at 5000 rpm is ass, hopefully it'll be ironed out.

Cheers

[christianzx6]

Hey,
I got a 2000 model zx6r, I built the bike up as a project and installed a highflow race filter, and a yoshi system that was already on the bike.
Rejetting for me was essential, as after completion of the project, the bike was running really lean, and bucking under acceleration. Basically it ran like shit. I balanced the carbs, factory pro jet kit, ignition advance rotor +4deg and now it runs crazy awesome. My jets are 162.5's and 160's, so pretty big. Before it had 140 somethings in them, basically way lean. So i noticed a MASSIVE improvement, even during the jetting process, each time i went larger i got so much more up top, until my hands started to shake on return from each run, because of the adrenaline dump (what can i say, i was new to 600's). I then ironed out flatspots in the midrange with needle adjustments. So i tuned for max top end, then went onto each subsequent circuit after that. Turned out float heights were sweet at factory's recommended level.

So if ur midrange is weak, i'd be adjusting needle positions to see what happens. Tell the people u got tuning it why u took it in, make sure they fix the problem that bugs you most. Keep us posted on your experiences.

[BikerBoy]

Wow, sounds like a huge improvement Christian!

I spoke to my mech again today, asking him to tune for bottom end / midrange power, hardly ever ride past 12,000rpm, normally ride around 5000, right on that dead spot. But its being ironed out.

Its turning out to be an expensive excersise though! i needed a new front tire as well.

I've just asked him to have a look at the chain and sprockets, if they need changing i might go a (-1) front?? i think its minus to get more torque and less top end.

Should be like riding a new bike when its done! hopefully anyways! i'll get it back either tomorrow night or saturday, i'll keep ya posted.

[Technik]

interesting mods! in terms of the actual re-jetting process, how much does it usually cost?

do you need to buy a re-jet kit prior to taking it to your mechanic?

bikerboy: i'm in the similar situation for my zx-6r - i hardly rev it past 12k. maximise the midrange would be most useful for everyday street riding.

[BikerBoy]

actually sorry its a +1 Front sprocket to get more bottom end torque not -1.

The costs are a little hefty though..

Generally for rejetting and carb synchronisation it ends up being a little expensive because your paying for about 4 hours of labour for the mech to take everything apart, test, retune, put back together.
maybe round $400

New sprockets and chain would probably cost about $350 fitted

The guys i'm dealing with seem good and sound like they really know their shit. BUT i don't have the bike back yet so i don't want to give credit out just yet... if i find problems later on.

carb sycn should be part of a minor service with most mechs, i wouldn't bother rejetting unless you have an aftermarket pipe and high flow air filter

[MickLC]

actually sorry its a +1 Front sprocket to get more bottom end torque not -1....

Actually you were right the first time, -1 will give you better acceleration but less top speed. If you aren't sure, think about the old 12 speed treadly and how it reacted when you changed gears at the front sprocket and the rear sprocket. The bike's the same, you just have to get the spanners out to do it.

[Technik]

Ok thanks. Perhaps I should rephrase my question - is it easy to tune the carb yourself if you have the rejet kits?

Well I got a remus pipe on my ZX636, other than that - everything else is completely stock. I thinking to replace the stock filter with a K&N at my next service.

[MickLC]

Ok thanks. Perhaps I should rephrase my question - is it easy to tune the carb yourself if you have the rejet kits?
....

You'll be relying on a seat-of-the-pants dyno and a plug-chop fuel/air ratio, as opposed to a proper dyno with an accurate fuel/air meter, but it can be done.

[BikerBoy]

found this awesome link that shows you exactly how to shim your needles within the carbs..

Which should generally iron out flat spots at around 5000 rpm.

looks easy enough... but personally i get an experienced mechanic to do it.

http://www.gstwins.com/zx6r/zx6r_shim.htm


z

[mike-s]

also have a look on here for threads started by me talking about tuning carbs. my biggest problem was it was a total slut to get the carbies out to DO the tweaking.

i ended up using OEM main jets and the aftermarket spring, needles and slider mods from my dynojet kit. There are other kits out there that dont run the bike as disgustingly rich as the dynojet kit did my bike.

[Stereo]

I had a 98 CBR600F3, this is the last model before fuel injection... added a K&N filter and after market pipe.... and it started popping when rolling off the throttle.... and backfiring when rolling back on.... There was already an improvement in performance, but it had a flatspot after adding the pipe...

I bougth the Dynojet kit and got it installed....

The guy who did it got it completely wrong, I took it back and it still sucked........ took it to a mate of mine who got the instructions from the net, adjusted it to the recommended specs for my bike and WHAM perfect...

I wouldnt have a clue how many HP I gained, but it sure seemed a lot quicker, specially through the midrange.... which isnt reflected in pure HP.

[BikerBoy]

damn... lots of stories of good outcomes and a fair amount of not so good outcomes..

I pick my ride up tomorrow morning, i have to leave my right ball there too after all the costs.

better be good!

[Technik]

so how did it go, Bikerboy? do you notice any improvement at all?

[BikerBoy]

Got my bike back on saturday, feels awesome, flat spot at 5000 RPM is GONE!!

now instead of always riding a gear taller to avoid the flat spot i can actually ride in the correct gear (which is one lower).

power comes on steadily from go till about 3 then power band all the way to 8500 where i hit second power band till red.

Its hard to measure whether or not you've gained without a Dyno reading, but i'd say i've perhaps gained about 5% power especially through the mid-high range.

on a slight negative, it does feel like its now using more fuel, because when i feel engine braking slightly more than before..

was almost a disaster when i picked it up though, they replaced my fuel rod (???) which was cracked so emptied most of my fuel out. I had a 25km + ride home (and they didn't tell me the bike had about a litre of fuel in it).. i hadn't noticed untill 2 mins down the road as i (luckily) followed my girlfriend into a servo for her to get fuel, my bike konks out just as i pull in... now i pissed because i think the mech fucked up my bike and i'm swearing like a trooper at the servo.. untill i fill up with 16 litres of fuel (in the 17litre tank! ) luckily it was just that i'd run completely dry!!

all in all - i'd definitely recommend a rejet - if you have an aftermarket pipe and filter and you hate that dead spot at 5000RPM that all sports bikes have.

And when i get my next new bike, (if i keep it all stock standard) - i'd get a carb shim - again to eliminate the dead spot.

Safe riding all!
BB