GPZ900R flat spot on acceleration??

[lauriep]

Hi
Constructive Suggestions/help wanted, please.

From idle On sudden throttle opening (thats = normal quick acceleration) I have a flat spot and almost stalls, but if you open (twist) the throttle slowly (very slowly) it revs just fine all the way upwards.??

The bike is a GPZ900R (89 model) and has been sitting for about 2 years.
Pulled down carbs and cleaned out and including all jets/holes.
Replaced bowl gaskets and orings, so no leaks.
Checked fuel float levels, procedure as per manual, and adjusted them and they all seem ok.
The jets and needles looked ok to me, no obvious ridges or worn grooves but I dont really know what I am actually supposed to be looking for anyway.
Idle Mixture screws were nearly 3 turns out, so left them at that setting.
Cleaned fuel tank and sealed inside etc. etc.
I think the problem might be still to do with the carbs somewhere?
Would worn needle jets do this?
Would the rubber connection tubes (which are fairly hard) between carbs and engine or between carbs and airbox be leaking a little and sucking a little bit of air and therefore give this result?

Thanks
Laurie

[mike-s]

Quite possible, although replacing the boots in general is a good idea if they are that stiff.
I would suggest looking at the diaphragms to make sure they aren't torn. Aside from that i don't have any idea.

[Strika]

Quite possible, although replacing the boots in general is a good idea if they are that stiff.
I would suggest looking at the diaphragms to make sure they aren't torn. Aside from that i don't have any idea.

If the Diaphragms are holed, it will exhibit very different symptoms to a flat spot. It is more likely to run on three off the bottom and clear up into the top end with a holed diaphragm. I'd check the cleanliness of the carbs once more.

[lauriep]

Update

Thanks for your comments.

Well, got it to run fairly ok, enough to be able to take it for a ride.
I couldnt find any apparent leaks after spraying around the inlet rubbers, but I am pretty sure it is sucking air from somewhere.
The rubbers are quite hard!
I adjusted the iddle mixture screws to richen it up a bit. (A little at a time)
Now they are now out nearly 4 turns!
Idles a little better too (constant) and the flat spot is all but gone now, just a slight hessitation.

I have decided to bite the bullet and replace all 8 inlet rubbers.
After this I will then try and find someone to take it to for a carb balance, mixture adjust and tune.

Dont suppose anyone can suggest a good tuner for the GPZ900 in WA?

Also;
I have installed a K&N air filter.
Thought this is resulting in a more improved air flow so now I am thinking if I should consider to go a slightly larger Main jet, up from the standard 95?
Maybe a 100?

Thanks
Laurie

[grego.77]

Check ignition timing and change plugs. Ngk's dont clear themselves up like they used to they might be fouled from the old carbie settings, engine should have a slight lope to it rather than having a constant sound and be snappy when stabbing the throttle. dont think your problem lies with the carbies anymore.. hope that helped

[seiko1]

God I'm old
I remember way, way back.....the GPZ900R (when it first came out) had trouble with the diaphragms being "sucked" in

[bonester]

My GPZ900r would behave just like yours after I had the carbs disassembled. It took a couple of kms of riding before it would come good. Never worked out why...

[lauriep]

update 2
Thanks for your suggestions, will think about and check those items.

Finally bit the bullet and spent more money.
got new carb rubbers, all 8 of them.
now its a lot easier to pull carbs on/off esp. compared to before with the old rubbers.

just finished working on it yesterday.
got the bike sort of nearly there, bout 90%.
Downloaded some 'Carb tuning' instructions off a UK website.
From that worked out that the main jets seem to be the correct size.
Adjusted the float levels (yet again) more accurately.
Did not realise how critical the float levels are!
You have to be spot on with these levels as the instructions mention that even 1mm make a difference.
Used the idle mixture screws (now out to about 4 turns) to try and sort out throttle/accelaration and running response.
I think the next move if I want to go any further is to replace the needles and holders (they dont seem to me to be looking worn but hey how do I really know im no carby expert).
Its all ok at thigh revs and high revs acceleration, just a little sluggish(flat) down low between 2k - 4k ish.
Will tinker bit more and see if I can improve it slightly, then I will find somewhere to take it to get the carbs balanced etc.

Regards
Laurie

[Strika]

Stretch the diaphragms out against a bright light. It only takes a pin hole and your last post described your problem a little differently which points me back to the diaphragms. Running on three or roughly off the bottom, clearing into the top end.

[Gosling1]

.....Stretch the diaphragms out against a bright light. It only takes a pin hole and your last post described your problem a little differently which points me back to the diaphragms. Running on three or roughly off the bottom, clearing into the top end.....

I did exactly that yesterday arvo, chasing a similiar problem with the 34mmCV's I had been using on the little z650.....

3 of the 4 diaphragms had pinholes !! well, 1 of them you could more accurately describe as 'small rips' !!

This bike had been getting harder to start, running rough down low, but clearing up once you got the revs on board......but it did take a bit to get those revs up ! But overall, it would not idle cleanly and just basically was running pretty bad....I think those carbs were a bit out of balance as well.....

Laurie - if you had leaking manifold rubbers, you would hear 'popping' out of that cylinder, and also the revs would not drop as quickly as they should when you close the throttle - air-leaks in the manifold always do that. But as you have replaced all the manifold and airbox rubbers - this should no longer be a problem....

How freely do the slides move up and down inside the carbie bodies ? If any of these are in the least bit 'sticky', you will get a slow response from a quick throttle movement....

Check out those rubber diaphragms as soon as you can.

[lauriep]

yes thanks.
since posting my last, and going for a decent ride, I have add some more thought on the subject and Im not all that happy with such rich idle mixtures.
I dont like the sound at idle, its not a crisp smooth sound like I believe it should be.
and the bike I believe is still running lean in the mid range.
I think I have only tried to sort out one problem, with adjustments for another area (idle mixture), and have created another issue in doing so (too rich at idle).
I have written down on a piece of paper my next steps to look at.
and from what you are saying sort of makes sense as I have written down and I am starting to look in that region now, of the needles and lift etc.
So the diahprams will now need closer examination.
Yes the slides are nice and free.

I would like to ask if anyone knows, in case I do need new diaprams, where is the best place to get these from?
Or is there a repair proceedure?
They look kind of expensive to buy new!

Thanks

[phear8me]

heya could you let me know where you got the carby tuning stuff from as i should have a look at it before i spend money on someone else doing it as at idle its inconsistant so i know they need to be tuned/balanced

and how goes it all as there is no update for a while

cheers

[Gosling1]

..... in case I do need new diaprams, where is the best place to get these from?....Or is there a repair proceedure?.....They look kind of expensive to buy new!.....

They are expensive as buggery from Kawsaki Oz, and slightly less expensive from US suppliers. The diaphragms for the 34mmCV carbs I have, are US$128 each ! I have heard stories about diaphragms being repaired with rubber glue and pieces of condoms - but have not tried this myself.....

[lauriep]

Hi Guys
Actually got around to doing some work on the bike sooner than I thought.
Now in middle of trying to find somewhere to live, as house is sold.

In posting to this forum and with the help I read from you guys, I am trying to describe the symptoms and what I am discovering along the way together with your help.
The findings/solutions/issues will hopefully be helpful to anyone else who may be having similar problems, as in my searching and browsing I found no real answers or help myself.
What was most helpful was the UK forum website (from GPZ zone) out of anything else.
So hopefully one day when the next person does a search on the net they may come across this thread and hopefully it will provide some insight, help or even more confusion, who knows.
Many times I have nearly given up and got frustrated, but my advice is to keep perservering and dont be afraid to ask for help, like on this forum
So….
Update No:3
The more I investigate and tinker the more I learn and the more I realise I didn't know before!
Ok.
I have it running about 90%.
But I like things close to perfect.
I am treating this as a challenge now and want to get it 100% before I decide if I will keep the bike or not.
Nice bike, easy to ride and enjoyable but I really need more of a tourer, and I don’t need 3 bikes and a car in the garage!

The rubber diaphragms are ok, ie. No holes.
The needles look ok, ie. No apparent wear.
Inserted a small washer under the needle head to try and lift them up slightly to see if this richens up the mid range a bit.
What I know now;
Fuel levels-
Now Critical (note this, its important, more than I initially realised) is the fuel bowl level setting. and doing it correctly, ie. I have used both the mechanical and tube method (to confirm setting), as per the manual instructions.
If measuring by ruler the float height, make sure it is as per manual and the tang of the floats are contacting but not compressing the pin of the fuel shut off needle, then measure float height.
This gets me in the ball park then I check with a tube, with fuel in the carbs.
I have it at 17mm now (as stated in manual) which for me results in a fuel level of approx +1mm above the carb bowl gasket surface.(I think the manual states = 0.5mm)
The next chance I get I am going to try and raise this 1mm more, to try and richen it a bit more.
I believe it is still running too lean in the mid range.
Idle mixture-
I have the idle mixture screws out 3.5 turns to compensate but I am not really that comfortable with them that far out. (manual states 2 turns)
I want to fiddle with these a bit next time and see what difference 2 turns Vs 4 turns gives me, curiosity!
Also I found that I need to go for a longish ride (about 20-30mins) for the bike to sort itself out and settle down.
Well that’s my excuse anyway for a ride.
Idles ok at about 900-1000rpm.
Only slight flat-ish spot between about 3.5k to 4.5k.

Engine a bit rattly in the top end.
Think I am up for a new Timing chain soon.
Did check valve adjustment before, but will re-check again soon when I do the chain and maybe lessen the gaps slightly as I left them in the higher gap spec range.
Or maybe these engines when they get old are noisy by nature?
Oh and new rubbers, get new inlet rubbers for the carbs. pay the money and get them!
So much more easier.

The carby tuning proceedure.
to 'phear8me':
If you would like to message me directly I can send the details to you.

Thanks

[phear8me]

mine i have idling between 1000-1500 as it needs adjusting/balancing but i found it best with all my bikes to ride it and wait for it to warm up before adjusting it a little at each set of lights once warm

but yes i do agree you should take it out and run through a full tank of juice going right through the rev range before you go any futher with the carbs so they gat used opened up and get any crap blown out of them or the fuel lines

mine was running like a pig for the first 3 days i got it back on the road as it had been in storage for 10 years before i got it i bolted it together started it and registered it and did nothing till i rode it for a few days and is was leaking fuel from carbies fuel tap etc until i took it out for a squirt through the hills then a nice long 4 hour ride then it all came good did an oil change coolant plugs leads and it ran like a dream except the still inconsistant idle which its done for the last year ive been riding it and ive taken it to queensland and back and to tassie and back with no problems

also the timing chain ive been told not to worry too much about it although mie only rattles at idle when cold as thats a normal thing for the older kwakas so the last 3 mechanics have told me