Pants above 9k... any ideas?

[Gonzohop]

Hi all

Finally got the old girl run in and have started giving her a good thrashing but alas all is not well...

Starts fine.

No excess smoke.

Runs great up to 9k.

Above 9k splutters badly on full throttle. Can get to redline by dropping throttle to about three quarters.

Over 80mph no matter what I do with the throttle she coughs and splutters badly from 9k to redline.

Any ideas?

BTW - even running like this she embarrassed a ZX-10R & B-KING around some twisties.... oh how I laughed....

[JanBros]

jetting is too lean I'd say.

by shutting the throttle a bit, you raise the vacuum in the carb sucking up more fuel and making the mixture richer.

[Gonzohop]

Thanks JanBros. Is this just a case of playing with the air screws or am I looking at new jets?

[ScottaKR]

If the carbs are set up as per the manual I'd think you should be pretty safe myself, as they're set a bit rich from the factory to leave a safety margin and avoid detonation.
Seems to me it's more likely to be weak spark under load. I'd check the usual things, plugs, coil, and stator, any of which could be letting you down.

[Gonzohop]

Thanks ScottaKR - I'll check the carbs are at the factory settings first then go through the usual suspects.

[JanBros]

Thanks JanBros. Is this just a case of playing with the air screws or am I looking at new jets?

bigger main jets.

first check the colour off your plug's

[Cexley]

Check for air leaks which will make it run lean. Coked up exhausts have a similar affect on top end. Float heights set wrong, causing bowls to drain out on full throttle. Check KIPS valves set-up correctly?

[Gonzohop]

Thanks everyone. I'll keep you posted on the solution...

[KR-1R]

.
.
[head on block here]try Smaller main jets (too rich) [fight, fight!, fight!!] U need to check what you have first !
?? by shutting the throttle a bit, (at WOT), you go from solely running on the Mainjet to back onto needle and main (needle restricting flow of fuel somewhat) ??
because up to 3/4 throttle, the needle and other jets were working OK... was not until running solely on the mainjet that mixture went wrong.

BUT as JAN said closing the throttle down speeds up the velocity of the air flowing through the intake and thus the static pressure drops and the difference between this low pressure and the higher atmosphere will force fuel through the jetting circuit/components that are working at that particular throttle opening (Bernoulli).

IF you are running genuine Keihin jets (generics might not be rated precisely) you will be on a 130 or 135's mainjet - IF this is what you have now, then your problem is elsewhere! (fuel blockage, clogged plug/electrical missfire at high rpm/load...)

You recon theres nothing to improve problem above 80mph (so is better at slow speeds at WOT?) - so problem seems load related (wind drag at speed) - maybe electrical coil/plugs/flat battery (you cant expect to charge a flat battery in those conditions and drive the plugs too)

(1st half of diagram generic for Keihins includes FCR etc)
lowest chart/words, specifically for PWK


but other things like...
broken or misalligned KIPS spools might do this also. (they can be snapped on removal of tight spark plugs (righthand cylinder). If the bike will reach redline cleanly in lower gears/less than half throttle - then it will be carb jetting, not KIPS (and not ruling out electrical).

As mentioned before a carb may appear to be quite clean but until you blow it out with compressed air and/or ultrasonically clean you wont be sure. (A bike I bought, ran Okay, but was far from decent, had all the correct carb jet settings, appeared clean, but wasnt running good until carbs were swapped for a good SPARE set)

[TwoStroke Institute]

A blocked air corrector or over oiled air filter?

[Gonzohop]

Wow - thanks for all the info! I'll get her apart in the next few days and have a tinker.

Will report back soon....

[Bertie_Mollie]

Coked up exhausts have a similar affect on top end.

+1

I had very similar symptoms and found the pipes were REALLY coked up. If you have 1S pipes take the cans off and have a look at the carbon build up down the stinger. I had about 3mm thick i.e. 6mm off the diameter

I didn't seriously think this could be the answer and was a bit of a last effort of desperation. I'd already tried all of the things you would otherwise look at first .

Got to be worth a look .

[KR-1R]

.
.
and someone else here (racing) had found their brother hadnt screwed a mainjet in properly and it had fallen out - similar top end problem

[Gonzohop]

Progress so far:

Needles were at position 2, not 4 as standard so put to 4 and the midrange was loads better. Still spluttering above 9k.

Tried position 5 - worse all round.

Went back to 4 and tried 140 main jet. Felt horrible all the way through so changed needle to 2 - just felt slower.

Went back to 135 jet and needle at 4. Noticed carb sliders were slightly out of line with each other - about 3mm off. Lined them up and now she pulls very well but still struggling above 9k although not nearly as bad as before...

Think I might tackle exhausts next...

[Gonzohop]

And the winner is....

Coked up exhausts have a similar affect on top end.

&

Coked up exhausts have a similar affect on top end.

+1

I had very similar symptoms and found the pipes were REALLY coked up. If you have 1S pipes take the cans off and have a look at the carbon build up down the stinger. I had about 3mm thick i.e. 6mm off the diameter

I didn't seriously think this could be the answer and was a bit of a last effort of desperation. I'd already tried all of the things you would otherwise look at first .

Got to be worth a look .

Yup, 'twas the zorsts. Got some KR1-S pipes (thank you reservoircogs), stuck them on, and she sings like a good 'un now. Yee hah!

Thanks for all the advice everyone - very much appreciated. =D>