Z900a4 rebuilt but missing

[Jfc]

Bikes fully rebuilt, all new bits, engine reconditioned and carbs clean rebuilt. All new wiring plus new Dynatek bits with coils.

Starts fine, when at riding small throttle openings misses, open the throttle wide and no issues.

It’s bored to 1015 and has z1r carbs

Any pointers at where to start ?

[warren3200gt]

First two things I would check are pilot jets in straight and not loose and secondly the Dyna timing. Did you set it up at full advance or stitically on the f mark?
Dyna say time on full advance but I have always found they run better when timed to the f mark statically.

[Garry.L]

Another one for Pilots. Ultrasonic cleaning etc doesn't always clear the pathways even though the jet might be clear.

[Jfc]

Thanks both, I set the timing statically and checked with a strobe, I’ll check the pilot jets

[Jfc]

Carbs stripped, and all pilot airways blown clean with carb cleaner, 117.5 pilots. Timing done with strobe at idle and 2500+ rpm. Set the carbs up with a 4 way manometer, all within 2cm ish Hg at 1500 rpm

Bike starts and runs much smoother and is now drivable, really smooth to around 2500. Hesitates around 2500 - 3000rpm but throttle is opened wide runs fun.

Could it be wrong pilot size, could it be too much/little air on the top end of the pilot jet range. Any thoughts on what to try would be much appreciated.

[kev edwards]

Are you running a stock air box and filter, 26mm or 28 mm carbs, are they pumpers, air or fuel screw type or both, main jets, needle position, ?s

[Jfc]

Stock air box and exhaust, they are 28mm carbs no pump, no fuel screw

[Jfc]

17.5 pilot, 120 main jet, not sure on needle position and standard air filter

[kev edwards]

From info I have, your jetting is out, 115, pilot 102.5 mains, this is based on Stateside bikes though, your main is way to big.

[warren3200gt]

Should that read 15 not 115 Kev?

[Jfc]

17.5 pilot, 120 main jet, not sure on needle position and standard air filter

[kev edwards]

Should that read 15 not 115 Kev?

Yes 15, thanks for noticing

[zed1015]

120 mains is miles too big with airbox and will be drowning the motor.
Should be 102.5 to 107.5 depending on year so start at 107.5 and work down.
Once fitted if the problem still persists and the pilot circuit is definately clear then the issue might be tuned out on the pilot air screw.
To determine whether you have a rich or lean condition causing the hesitation simply feather the choke (enrichener) at the point of hesitiation and if it improves you are weak and if no change or it worsens you are too rich.
Also having 120 mains fitted means the carbs may have previously run with K&N filters or stacks.
This may mean they are also fitted with air corrector jets which will over richen the bottom end in this situation and will need removing.
If this is the case they will clearly be visible in the mouths of the carbs in the form of a slotted brass screw.

[moizeau]

My ST had the same problem when I bought it.Due to the copy KnNs and a Marving that had the baffle cut in half some muppet put Solex 130's in the mains. Went like the clappers with all air filters removed on wide open. Now on standard airbox, still got the Marving, went up a main jet size from standard, all is good.

[Jfc]

Lifted the needles one notch (from 3 from top to 4 from top) and the bike runs great - smooth right through the rev range (to 5000 ish, running in after engine rebuild)