Kz900 won’t rev over 3k

[Nick.s]

Hi guys I need some help, I have just got my Kz900c running after a total rebuild. It runs and ticks over at 900 revs , and revs freely in neutral. But when I ride it I can only rev to 3k, then it gets all lumpy and won’t go much further. The carbs are not standard, they are 28mm pilot fuel type, no air screw ( off of a 1000 I think )
Main jet is 102.5, pilot 15.
Any ideas where to start would be much appreciated.
Thanks.

[zed1015]

Could be quite a few things and carbs are always the first to blame but mostly not the cause.
Usual suspects are -
Incorrect ignition timing.
ATD sticking.
Restricted fuel supply.
Partial blockage in fuel tap..
Restrictive inline fuel filter..
Float heights set too low..
Inlet cam timing a tooth out ( This is the main cause of your symptoms on 8 out of 10 zeds I get in, they always count back from the wrong pin and set it a 27 instead of 28 )
Restrictive pattern 4/4 exhaust.( Pull the baffles to see if there's any improvement)
Faulty ign /coils / condensers.
Electrical short under load in kill switch or elsewhere In ignition circuit.
Start with checking the above and once ruled out then go to the carbs and airbox..

I'll keep adding to the list if I think of anything else..

[Nick.s]

Ok thanks for the suggestions, the timing is correct and the advance seems to be working. The exhausts are new, and I have checked the cam (but will check again). The air box is off a 1000 with a new filter.
Anyway thanks again, I’ll start checking tomorrow.

[z1bman]

is the air filter fitted the correct way round ?

[000zeds]

102.5 main jet is too small

[Nick.s]

Do you know the correct standard main jet, for 4x4 pipes? Considering the carbs are from a 1000

[zed1015]

Don't go changing mains until you have sorted the 3000rpm issue.
There is no std main jet size for 1000 carbs and 4/4 pipes but 102 won't be far off for a 900 it all depends carb spec ,exhaust type and intake flow.
Stock mains on a GS1000 with VM28's is only 96 ( and the same 96 main for the Katana 1000 with 32 smoothbores )
112.5 is the average main on a stock Z1000.
There's no need to start changing settings that don't relate to the problem you have.
You wouldn't go buying specs because you are going deaf would you..

Also check the carb mouths for air corrector jets..
If the carbs have previously been run with pods on this bike or another then they may have the corrector jets still fitted.
This will also cause the bogging you describe due to over fueling with the airbox in situ.

[Nick.s]

Ok thanks for that, I’m going to start with the cam timing first as you suggested, but I can’t do that until Friday. If that’s correct I’ll try the carbs. The bike came with 28mm pumper carbs but the air screws were seized and rounded off. So I got a set from USA which we’re supposed to be standard 26mm from a Kz900, so I don’t know what setup they had.
Thanks for your advice, I’ll keep you posted

[zed1015]

The bike came with 28mm pumper carbs but the air screws were seized and rounded off. So I got a set from USA which we’re supposed to be standard 26mm from a Kz900, so I don’t know what setup they had.

So what carbs are fitted ?
In the first post you say 28's from a 1000 with fuel mix screw but that must be a typo as there are no fuel mix 28's only air mix ( unless they are Suzuki which have both fuel and air on the same body ) .
So I presume you meant VM 26's with fuel mix from a 1000 but now you mention 26's from a 900 ?
Which is correct ?
Either way makes no difference to solving the issue but it's helpful to know exactly which carbs we are dealing with if that is where the trouble lies.

[kev edwards]

Your jets are standard size for stateside thou of 1978, my thou had them in as well, I now run a 4 into 1 with 107.5 mains and 20 pilots, but as Rob says don't change jetting until the running is sorted out.

[Nick.s]

The carbs are 28mm mikuni but they don’t have an air mix screw, just a fuel screw underneath on the engine side, I got an overhaul kit for them for a kz1000 1977. when I bought them the advert for them said they were 26mm from a 900 but they aren’t. So I thought I would use them anyway instead of sending them all the way back to USA. The middle pic shows the pilot screw (circled in red) underneath, engine side.

[zed1015]

The carbs are 28mm mikuni but they don’t have an air mix screw, just a fuel screw underneath on the engine side

I see the fuel screw but Mikuni NEVER made a VM28 with just a fuel mix screw.
What did the boot side measure.
26's measure closer to 27 and 28's closer to 29 .
If they are dead on 28 all the way then you have a set of 26's bored to 28.

[8 Valve Mark]

nick, surely you would have been better off getting the air screws out of your 28mm pumpers, but hey ho! as rob has stated already, your current carbs are 26mm. ex, z1000 a1/2 which came with 102.5 mains/15 pilots so they'll be ok. if they have been bored to 28mm that's a bonus. check everything already mentioned and you'll be ok. good job with the bike so far mate, i remember the start... oh and if you want carbs again, don't buy from the states!! save money and gimme a shout! good luck m

[Nick.s]

Thanks all, I’ll start checking Friday, I hope it’s only a tooth out on the cam. Fingers crossed.
I’ll let you know.
Nick.

[Nick.s]

I just had a look at the carbs and there is a number on the engine side flange, it’s hard to see but the offside carb has 299 04 or 01 it’s hard to see. Does this help with id?