Difficulty holding part throttle.

[stephen1981]

Sorry if this has be answered before but I have searched and can't find this specific problem.

Bike is a ZXR400 L1 1991 and I've fitted new carb rubbers and new seals on the float valves so I'm fairly certain there is no air leaking in the float bowls are not overflowing.

It will start well with no choke and idles very nicely, revs pretty well too but the problem I'm having is when trying to hold the revs very low to either set off of manoeuvre at slow speeds, the revs want to increase quite quickly and it is difficult to hold them steady at say, 2000rpm. If I can get the revs to hold at 2000rpm they sometimes will rise a little without any more throttle movement.

Is this just a simple case of carbs being out of balance? They have not been balanced for some time!

[CaNsA]

Balance them and find out....

[stephen1981]

Well I've balanced them and it seems a bit better, throttle is still quite sensitive and intermittently will rev up a bit when I'm holding it steady. Very slight hang sometimes too. Bike was stood for almost a decade before I got it so I would imagine it needs a bit of use then set the carbs up again.

[stephen1981]

Just a little update on how I'm getting on with this problem:

Originally I was having trouble holding the throttle at 2 or 3 thousand rpm, when attempting to hold the revs steady they would rise to 4000rpm without any extra throttle movement. I have since then set the carb pilot screw a bit richer. The bike has a full Micron exhaust and K&N filter. The pilot screw is set to three turns out, this has improved cold starting and the revs do not hang any more. I can also now hold the revs at 2000rpm without them rising on their own. But I can't hold the revs at 3000rpm without them raising to 4000rpm.

I had been told by a bike mechanic that it's possible it could be a leak in the diaphragms in the carbs but I've checked them and they are fine. I can ride the bike as it is but would still like to get to the bottom of the problem. I'm not sure if it's a good idea to make the pilot jets even richer than they already are but it does seem to have improved things a bit, when I get time I may turn the pilots to 3.5 turns out and see how that goes.

[banner001]

anything over 3 turns out and the needle has fully retracted from the hole, so it shouldnt make a difference.

the carb rubbers are not that expensive, so if you suspect them change them and they are one potential source eliminated.

i would look at removing the pilot jets and sonnicating them, the holes inside can easily get clogged, restricting fuel flow and leaning you out...be careful though, as they are located down a tube and are prone to having the heads rounded off as they are brass...i.e. give it a good squirt of GT85 down the pilot hole in the bottom of the venturi as well as down the pilot jet itself.

[stephen1981]

Thanks for that bit of information, guess I should leave them at 3 turns out and look elsewhere for the problem. It has new rubber carb mounts as these were totally perished.

I suppose the best place to start is right from the beginning and check the carbs are properly cleaned internally.

Thanks for the advice.

[stephen1981]

Another small update.

Having taken the carbs apart and measuring the float heights, they were quite a way out, almost 5mm in some cases. They were low in the float bowl so this could be part of the problem. I'm waiting for a few other parts that I'm going to fit while the carbs are off so I'll post an update when I've tested it.

[stephen1981]

Well I'm feeling totally frustrated with the bike now!

Float levels are all set, pilot screws are 3 turns out and I still get difficulty holding the revs at 2k or 3k. Also, the revs still hang slightly and the throttle is rediculously sensitive at low revs.

To me it's still suggesting that it's lean, so air leak or blocked idle jets.

Almost had enough of it.

[banner001]

what main jets are you using?

ive got a similar setup to you and am running 105 mains across all 4 cylinders on stock kawasaki needles.

[stephen1981]

They are dynojet 96 jets with dynojet needles.

I thought the main jets and needles didn't come in to play until the throttle was opened up more?

[banner001]

about 4k rpm for them to start playing a part, but fuel vapour can always be drawn from around them. dynojet 96 is a keihin 98, even if its not related i would look at getting something with a bit bigger bore as you have a lot less restrictive air filter and exhaust.

i would pull the pilot fuel jets, the holes are quite small and get blocked easily, you will need access to a sonicator to do the job correctly. if after all that there could be a blockage internally in the carbs, this would require sonnicating the whole of the carb bank.

[stephen1981]

I had the carbs ultrasonically cleaned when I got the bike but I've had them off and sprayed carb cleaner through all passageways. I have double checked the float heights and they are all within spec. Still have the same problem.

[stephen1981]

I've given the carbs a bit more of a clean and turned the pilot screw to 2.5 turns out. I've made a video, I try to hold the revs at about 1500rpm and they rise on their own to 2000rpm then I hold the revs at 3000rpm which it seems okay with now!

In the second video I show the revs hanging, this only seems to happen when I rev it above 4000rpm.

I'm thinking maybe get some miles on it as it's been sat for about 9 years, then see how it is.

[TurkishJordo]

Hi, when the bike is up to running temp, what is it idling at? If she's idling more than 1300 you need to drop the idle, otherwise revs stick, small throttle will rev up etc