zxr 750 hi

erratic revs

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28/06/2012 at 20:56

after a recent death of my bike i have stripped cleaned and balanced the carbs replaced fuel pipes filter

yet it still has erratic rise and fall on the revs till it eventually falls of to 0 and dies when i restart it the revs start at bout 2000 then drop

it will on start with full choke that needs too be nocked back quickly as the revs so high then knocked down in increments as it starts warming up

30/06/2012 at 14:45

Sounds like air leaking into the inlet tracts - not unusual after removing and refitting the carbs as they can be quite hard to seat on a 4 cylinder engine.

With the engine running, spray some carb cleaner around the inlet stubs (between carbs & engine) and see if the revs increase - if so, then it's drawing in air.

Could also be flooding caused by incorrect float height or a sticking float valve. Set the petrol tap to "prime". With the bike held vertical & level, connect a piece of clear, 3mm hose (battery breather is good) to one of the float bowl drains, hold the open end of the tube up past the carbs and open the drain screw. The fuel will rise up the tube to the same level as it is in the float bowl. You should be able to find a spec for the correct fuel height, but generally it's a couple of millimetres below the float bowl gasket joint. Repeat this for all four float bowls.

30/06/2012 at 16:36

thanx for the reply i had a suspicion that the carbs were not seatin properly due to water getting into the right inlet stub

had ensured that this time they were seated in and thorougly tightened on the clamps

  none of the inlet rubbers have been removed as bolts were seized and didnt want too risk a snap of

i will try the carb cleaner idea mid week and let ya know



Edited: 30/06/2012 at 16:37
09/07/2012 at 17:30

thanx for the tip carbs were leaking air but need too find new water coolant connections got a leak on 2 carbs has any one got a clue where i can get them

once again thanx for the tip


13/08/2012 at 08:32

sorted air leak but as soon as the carbs are nearly ballanced the bike revs drop and dies and unable too start the bike is running of balance but is very clunky the idle screw is as far as it goes have now attempted too balance several times


14/08/2012 at 22:09

To me that just sounds like it's using up all the fuel in the float bowls - i.e. there's no fuel coming through from the fuel pump.

14/08/2012 at 22:15

Or possibly the engine's over heating.

How long are you spending balancing the carbs? How are you trying to balance them? Have you also set the pilot mixture screws? How are you supplying fuel to the carbs while balancing them?

17/08/2012 at 12:05

roughly half an hour too balance on dial gauges an fed from the tank i have a smooth idle but aceeleration very poor now very hesitant and slow  but does slowly pick back up after a couple of seconds should i lift the needles another notch also it seems too heat up quicker than b4 that could be because of the throttle lag

the replacement needles i got are slightly wider than the originalsbut are the same length so should i lift them another notch too give plenty of fuel on acceleration





Edited: 17/08/2012 at 16:41
18/08/2012 at 18:59

Half an hour is a LOOOONG time to be balancing carbs - if the engine's getting too hot duiring that time it'll stop you balancing them properly, and mean any changes you make to the idle mixture settings are totally wrong.

If it's picking up poorly then it could be over-fuelling or under-fuelling, but if your gut is telling you it's lean then lifting the needles up a notch is a quick job to gauge the results.

Is it picking up poorly off idle, or just when you've got a few revs on and giving it a handful?

The other thing you should do though is check fuel height / float height - if they're out it can have a massive effect on running all the way through the throttle range.

Easiest way to check is:

Connect a bit of clear plastic tube (available from hardware stores) - about 3mm bore - to the drains on the bottom of the float bowls. Tape the open end of the tube to the top of the carb body.

Set the fuel tap to "on" or "prime" or what ever is needed for fuel flow without the engine running. Make sure the bike is upright & level and open the drain screw.

The fuel will rise up the tube to the same height as in the carb body and you can measure it's height relative to the top of the float bowl - it's usually meant to be somewhere around 1-2mm below the joint between the float bowl and carb body.

Do this for each float bowl. If the fuel height's wrong then you'll need to have the carbs off and adjust the floats, but at least then you'll know you've got a sound base to work from.

19/08/2012 at 07:53

picking up poorly of idle just as you twist the throttle


19/08/2012 at 18:58

In that case raising or lowering the needles won't make much difference and instead you need to sort the fuel height & pilot mixture first.

20/08/2012 at 12:31

all done got acceleration float levels spott on all balanced but num 4 is slow too drop back too the same as the others got a slight hunting but no where near as bad as b4 thanx for all your help your a diamond geezer lol

20/08/2012 at 16:09

Glad it's moving in the right direction! How did you adjust the pilot mixture? Might be related to No 4 misbehaving...

Edited: 20/08/2012 at 16:10
20/08/2012 at 16:56

all adjusted too 1n 3/4

shud i ad or turn back a 1/4

20/08/2012 at 20:28

Although that's what the manual quotes, in my experience manufacturers set them up to run a bit lean for emissions reasons so I'd just use it as a starting point.

A common method is as follows:

Run the engine at a fast idle (about 1600rpm) by screwing in the throttle stop slightly.

Turn the pilot screw on No 1 slowly out until the engine revs start to drop (they may rise a fair bit in between), counting the number of half or quarter turns. Reset it to 1+3/4 turns out. This time turn it IN until the engine revs start to drop, again counting the number of quarter turns. Note the numbers are re-set it to 1+3/4.

Repeat for the other cylinders.

For each cylinder, take the average of the 2 numbers (furthest in & furthest out) and set the screw to the average, then screw it about 1/4 to 1/2 a turn in.

After you've set them all, re-start the engine and re-set the idle . Blip the throttle to test pick up, and go for a test ride.

If you take too long doing the initial experiementation then the engine will get hotter, affecting the readings. If there's a slight trend towards leaner running for the last cylinders you do, then they can probably be set closer to the first cylinders you do, alternatively if you started with a cold engine then you might want to set the first 2 closer to the last 2, and so on.

If any of the pilot screws need a setting very different to the others then there may be a problem with that carb, or the inlet stubs.

The other issue with all this - that I forgot to mention earlier - is that there's little point balancing the carbs or setting the idle mixture if the valves are out of adjustment. So it's worth checking those too - especially if No 4 cylinder persist in misbehaving.

20/08/2012 at 20:39

went for a sceam down the duel carriage way had some popping and banging but all seems ok now just the very slight hunting on tickover

its ticking over at 11000 going up too 12 n down too 10000 before going back up too 11000

21/08/2012 at 13:09

water cooling pipes in the way too be able too do them on the bike and only e inches of space too get hands under real nightmare lol


21/08/2012 at 19:40

Ha ha - I'm blessed with small hands and a long tool.

By which I also mean - you can buy pilot-screw adjustment tools specifically for this job. They're about £20. If you're anywhere near Bristol you're welcome to borrow mine.

21/08/2012 at 23:03

im in weston super mare lol

thanx for the offer

22/08/2012 at 01:45

Ah OK, I'm down that way occasionally to Weston MAG - almost certainly will be for the GWR Rally in September. If you fancy a quick fiddle with my long tool let me know

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