Hello everyone! About a year ago I bought a 99 ZX6R overall looking and working great,however since after the winter I noticed a hesitation when taking off,only below 2500 RPM or in the same rep range when revving it on idle.This happens on a warmed up engine,if I try to rev it cold it stalls instantly,but if I choke it and leave the choke on just a bit so the bike stays at about 2000 RPM,it will rev just fine all the way to redline with no hesitation, and this made me think it is a fuel delivery problem.I have done the following while trying to solve my problem"

-clean & sync the carbs twice.
-changed spark plugs and coils
-cleaned the air filter
-checked for cracks and vacuum leaks
-compression check: cyl#1,2,3 188psi/13 bars,cyl#4 203psi/14bars

the problem seems to be on the 3rd carburator, because while 1,2,4 have the mixture screws on 3 turns, I had to turn the screw on #3 to about 8 turns!! to get rid of the hesitation completely at cold and when warmed up.This improvisation gave me problems starting the bike when hot.

the bike has 20k miles and is completely stock as far as i know, the only mod that I added was a slip-on exhaust and iridium plugs,but I remember it doing the same on the stock exhaust. could it be a clogged pilot circuit even though the carbs were cleaned with carb cleaner and air compressor?

Any suggestions? this is driving me completely insane because I have to wait 15 mins to be able to take off on a cold engine decently on a sunny day, and of course taking off from a stop. The bike pulls hard with no problems when above 3000 RPM but it is a nightmare below.

 

hmm, that is a fairly large gap on the compression test in my book. (5 psig difference). Sounds like the valves may not have been checked and adjusted. first impression especially since you listed items that you have done already. get that timing and firing a bit better and you may be golden.

Welcome to KF and keep us posted on your findings.

 

Hello and thank you for the input! I was in fact thinking of doing a valve clearence check once I get the suitable tools for it in the near future,however I slightly turned away from it when I saw that turning the mixture screw on the third carb (cyl #3) which adjusts fuel on the bike and not air, got rid of the problem, I was thinking of checking the float heights on the carbs,maybe they are out of spec and causing fuel starvation at low RPM. What do you think about this?

P.S when I first noticed this problem I did a water test on the pipes and cyl #3 was not evaporating water as fast as the others, and the bike sounded rough on idle till it warmed up.

 

When you stated this:

I thought that the floats were already checked and adjusted to the correct levels. That is normally done at the same time.

Do the fuel height check/adjustment. One less thing to worry about. Have everything done so that when the valve check tools arrive, you will be ready.

 

I had the sync done in a shop so I don't know if they checked the floats but just to be sure I removed the carbs and went through them again myself,floats seemed within spec,diaphragms in cood condition,all passages and jets seemed clear,I have adjustable needles (as per EU model of the bike) so I moved the shim down one notch and gonna try it this way. Another thing I noticed is that at idle it has a feint smell of unburnt fuel,but mostly the smells comes when I pull the throttle and it does the hesitation,like the spark is not strong enough, I will try with standard plugs and see what happens today. Maybe iridium plugs are not that good with this carbed bike and it's cold blood )

EDIT: this seems sort of like a rich at idle lean after condition,because of the smell of gas at idle but last time I did a plug test by turning the bike off on a cruise,the plugs were white-ish. what is however weird is that 2.5 turns on the mixture screw shouldn't be making it rich, but turning it up and giving it more fuel gets rid of the problem somehow, hence why I'm going for the spark plugs again

UPDATE: Didn't get to swap plugs with standard ones.I installed the carbs back in,the needle (shim) seem to affect mid range in a very good way. The hesitation is nearly gone when the engine is warmed up! however I let it sit till about 1 temp line in the gauge and the hesitation is there but it wont make the engine stall...it revs till about 3 k and I can hear alot of air being sucked during the hesitation. I pulled the choke in just a bit after the free play so it didn't affect RPM but I was able to rev it like FI bike from 1100 RPM to about 7k.

What I don't understand is why is it doing this...the choke gives it more fuel and closes air right? so it appears to be like there is too much air getting in and not enough fuel, I just can't figure out where is the problem. Somehow it is tied considering the problem goes away when warmed up and the air is not so dense in a hot engine as it is when cold.This kinda points to vacuum leak...somewhere

 

How did you check for cracks and vacuum leaks? A visual check only? Or have you verified with either or WD40 spray around the boots and gaskets while running? (a light mist is all it takes!) listen to the idle and see if it ramps up or not.

 

I checked visualy but just now I bought a start helping spray to check, did a small test but the engine was hot so no noticeable increase in RPMs since the engine runs ok when it's hot,gotta wait for it cool down where the problem is and check it

 

No vacuum leaks found, this is driving me crazy, this beats me, how can the bike have a feint smell of unburnt fuel in the exhaust,but running lean at the same time. (judging by the plugs)

EDIT: I think I found the problem.On the third carburetor the pilot air screw body was cracked in two sucking more air in, thats why giving it more fuel by turning the mixture screw made it run better at low rpm. I made an improvisation and it runs better,no more gas smell from the exhaust,but I will eventually have to replace the carbs.