Kz900 won’t rev over 3k

[zed1015]

Hi, Indie, I'm Rob who does the plungers and air jets.
I was also zed1015 on FB till they banned me for going by a user name that everyone knows me by .
Remind us of you bike spec, airbox or pods fitted etc and we'll see if we can make a difference.
Most of the checks etc are already mentioned in this thread though.
Sorry that the new seats didn't cure your particular bike but it eliminates them as the culprit.

[Nick.s]

Hi indie
I still haven’t been out on mine yet, the tank is away being sprayed and the small tank I use for setting it up isn’t up to running it on the road. I’m hoping the tank will be back in a couple of weeks.
Please keep us informed how you get on, I’m hoping mine is ok now but I wouldn’t bet on it, so any other information could come in handy. Good luck.
Nick.

[Indie500]

Hi Guys,
Ok 76 Z900 A4 VM26 Carbs everything is standard although the pipes are made by Doremi but standard in appearance, Wiseco pistons to 1013 i seem to remember was the capacity.
Jets are 115 main and 17.5 pilots. Airbox and filter std Z900 A4
When starting it sometimes runs on 3 with the 4th cylinder coming in within 10-15 seconds, ok so slow gentle riding to get her up to temperature then throttle on gently and it hits around the 3k mark and it feels like its tieing itself in knots, stumbling and the power drops off, open the throttle and its off like a scolded cat. Its most certainly the transition between the pilot jet and the main jet. Ive done some plug chops at 4-6k revs and its burning ok on the plugs, slow riding sub 3k and hitting the flat spot and the plugs are getting darker sometimes black.. Its been mentioned in this thread that the air screw seats may be warn due to over tightening this is something I'm going to check later with some engineers blue to see how the air screws are seating. I have the carbs off and totally stripped again now and using a thread tap holder I've removed the pilot pick up tubes that go down to the bottom of the float bowls for cleaning and they were fairly scaly. Im also wondering if the pilot screws have been over tightened that maybe they are compressing the springs fully and not allowing the screw to seat. My air screws have no effect on slow running which is not right. Ive tried everything.. big jets , small jets, dyna s ignition and coils a new reconditioned set of carbs from Wired George which were worse than the ones i started with. I will find this fault its what i do and hey guess what i have a standard Z1A with the same fault so I'm thinking 40+ year old carbs are bound to need some investigation and checking of things like the pilot screw seats. I have also tried using the choke to identify rich or weak in the flat spot and to be honest I'm not sure and think firstly i need to know that the pilot system is working properly using a pro vac to see the resistance that should differ when adjusting the pilot screws. And to ZED1015 the work on the plungers is first class and no it didn't cure the fault but it must have helped as after 40 years or so the rubbers do go hard and so will leak as proved with the pro vac when comparing the old with the new so thank you all i can say is everyone should have this work completed by yourself.

[zed1015]

The black plugs and stumbling certainly sounds like bogging due momentary over richness.
Dropping the pilots to 15 or the needle a notch should have made an improvement but it sounds like you have tried that and it should have even been able to be tuned out on the mixture screws.

[Indie500]

Thanks zed1015 ill let you know how i get on..

[Indie500]

Ok, more progress.. As I've posted i have the carbs completely stripped down to bare bodies to double check everything air/fuel ways etc and with the Mighty Vac I've tested the refurbished choke plungers by zed1015 and they hold vacum unlike the old ones so no leaks there and no open choke circuit when the choke is off.. The aircrews all leaked to a point where the vacuum gauge dropped like a stone no resistance at all !! the reason was the aftermarket airscrew tapers are different to genuine and the new springs that came with the screws looks like a thicker gauge wire which cause the springs to bind when the screws are seated closed so I've got some NOS screws with new orings and used the old springs now they all hold vacuum so will be adjustable.
Even if the above points don't cure the problem i have the carbs working properly in those areas. Just got to finish up with checking float levels etc and ill give her a run..

[Nick.s]

Good luck with that Indie, I hope it works after all that. I’m still waiting to try mine, the tank has been delayed until the end of March I just hope it will be ok, at least my air screws make a difference when I adjust them. I have noticed that when I weaken the mixture the engine overruns longer before dropping to tick over, when I set it to 1 1/2 turns out or less the revs drop ok.

[Mr Bump]

Ok, more progress.. As I've posted i have the carbs completely stripped down to bare bodies to double check everything air/fuel ways etc and with the Mighty Vac I've tested the refurbished choke plungers by zed1015 and they hold vacum unlike the old ones so no leaks there and no open choke circuit when the choke is off.. The aircrews all leaked to a point where the vacuum gauge dropped like a stone no resistance at all !! the reason was the aftermarket airscrew tapers are different to genuine and the new springs that came with the screws looks like a thicker gauge wire which cause the springs to bind when the screws are seated closed so I've got some NOS screws with new orings and used the old springs now they all hold vacuum so will be adjustable.
Even if the above points don't cure the problem i have the carbs working properly in those areas. Just got to finish up with checking float levels etc and ill give her a run..

That rings a bell, what brand of carb kit was it?

Olly

[Nick.s]

Hi Indie, have you checked if the needle jets are worn? I’m not sure how you check them but if they are too big they will let petrol suck straight into the engine, so with closed throttle you will get over rich mixture. As I said in my post I didn’t have any at all, so I was getting your symptoms to the extreme, and it would affect the mixture screws. I maybe wrong about this but it would be worth checking, I’m sure zed1015 would know for sure.
Nick..

[Indie500]

Hi Guys, The carb kits were Keystar and in my opinion there rubbish.. ok needle jets, the aftermarket ones are made oversize so useless. Ive been advised by Steve at MotoCarb that the original Mikuni data sheets contained incorrect data regarding the needle jets so i guess thats why the aftermarket ones are to big. NRP have new ones that are reamed to the correct size but trying to get NRP to send me some has taken about a week so far and still no joy, originals are out there but hard to find so I still need to find them but I'm sure they will make a difference..

[Nick.s]

My carb kit are keyster as well, the needle jets may not be perfect but they seem to work. I’ve got mine ticking over quite nicely and adjusting the air screws up and down makes a difference. I still can’t get it out and try it properly, no tank yet, this corona virus isn’t helping

[Nick.s]

Ok to finish this thread, I’ve fitted Dyna S ignition system while I was waiting for the tank, I got the tank back today and it works, it runs lovely with no bogging at 3k revs. So it looks like I’ve cracked it.
Thanks for all the advice you’ve given me, I don’t think I could have done it otherwise. I think it looks a bit better now
Nick

[Bill P]

Credit to you and glad you got to bottom of your problem perseverance generally pays of

[Pigford]

Yeehaa

Well done lad (and everyone advising)

[Nick.s]

Quick question if anyone is still watching this thread, the bike is running ok now but I have noticed the vacuum caps between the carbs and the engine are full of petrol. I have seen this before but thought it would stop when I started riding it. Is this normal? Or is there still something wrong? Maybe float level too high?
Thanks
Nick