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Z900 A4 Carb issues

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sprint
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Z900 A4 Carb issues

#1 PostAuthor: sprint » Wed Oct 30, 2019 7:44 pm

I have a UK Z900 A4 with std carbs, air box and exhaust.

Bike starts OK and ticks over at 1000 revs. Carbs have been striped, cleaned and balanced. I have electronic ignition fitted which uses the original mechanical adv/ret mechanism.

However, I find that it momentarily stalls/hesitates particularly after closing the throttle and then opening it again from the closed position from either coasting or changing gear.

Any suggestions as to what the likely cause is and if there is a solution?

Thanks

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CAT3
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Re: Z900 A4 Carb issues

#2 PostAuthor: CAT3 » Thu Oct 31, 2019 5:05 pm

My initial suspicions would be that your problem is carb related.
I know you say you've had them cleaned, etc, but carbs would be my first point of call.
Ideally if you could swop some major components, carbs, ignition & advance & retard, for know good ones that would quickly pin-point the culprit.
Does this issue change as the engine warms up or is it the same whether it's cold or up to normal operating temperature ?

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Re: Z900 A4 Carb issues

#3 PostAuthor: sprint » Thu Oct 31, 2019 5:14 pm

Problem remains much the same whatever the operating temp.

I do not have any other items to swop about, carbs etc.

Was hoping that others may have experience similar problems and have identified the cause with a possible solution?

Does anybody else suffer from this?

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Re: Z900 A4 Carb issues

#4 PostAuthor: warren3200gt » Thu Oct 31, 2019 7:08 pm

Give us some background. Has this come on recently on its own and prompted rhe carb strip or since some head work was done. Kinda sounds like the symptoms for cam being a tooth out but obviously not if youve touched nothing since it was running well.
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Re: Z900 A4 Carb issues

#5 PostAuthor: z1bman » Fri Nov 01, 2019 7:00 am

sounds like one of the coils has a weak spark & is not burning the fuel correctly as the throttle increases

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Re: Z900 A4 Carb issues

#6 PostAuthor: CAT3 » Fri Nov 01, 2019 8:35 am

Remove all the spark plugs & see what they look like. This may well indicate things like a defective coil on one or two plugs, faulty mixture on one or more cylinders.

If your HT leads are long enough you should be able to swop over the two coils & see if that affects anything.
Can you swop the ignition pick-ups over, no idea how feasible that is on your ignition ?

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Re: Z900 A4 Carb issues

#7 PostAuthor: Pigford » Fri Nov 01, 2019 8:46 am

Check +12v to coils when running.
Standard carbs are basic bits of kit & not uncommon for "off-idle" hesitation.
Could enrichen pilot system a bit (maybe up a size on pilot jet) the wind air screw out a bit?
Always good fun trying to get this right :D
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Re: Z900 A4 Carb issues

#8 PostAuthor: sprint » Fri Nov 01, 2019 12:45 pm

Thanks for the replies.

It has always had this problem and I know that the cam timing is correct.

I fitted the relay mod to get 12V directly to coils some time ago, but again it made no difference to to the hesitation.

Std original coils which look to be OK with correct primary and secondary resistances and spark jumps around 8 to 10 mm. Carbs have been balance but mixture may be out? Don't really notice any real change in engine revs by adjusting air screws a tick over. What is the normal procedure for adjusting the air screws?

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Re: Z900 A4 Carb issues

#9 PostAuthor: warren3200gt » Fri Nov 01, 2019 1:24 pm

Colourtune.
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Re: Z900 A4 Carb issues

#10 PostAuthor: Ultim8pc » Fri Nov 01, 2019 2:06 pm

Pigford wrote:Check +12v to coils when running.
Standard carbs are basic bits of kit & not uncommon for "off-idle" hesitation.
Could enrichen pilot system a bit (maybe up a size on pilot jet) the wind air screw out a bit?

Got to agree with Pigford.

But you have to rule out everything else as well.
Compression & leakage
Airbox system leakage
Valve clearances
Coil voltages
Coil resistance
Ignition timing
Plugs
Reg/rec, charging and battery

All of those can affect how the bike runs.

Colortune is the way to go fr setting the airscrew.

Also, some cheap aftermarket airfilters can also give a low speed stumble.
http://www.airevalleyclassics.co.uk
10% discount code Z1OCTEN
Keyster rebuild kits & carburettor spares - airscrews, jets, needles, starter plungers & gaskets.
Also petrol/ethanol proof float bowl gaskets. IMD pistons preferred seller.

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Re: Z900 A4 Carb issues

#11 PostAuthor: sprint » Sat Nov 02, 2019 4:03 pm

Ultim8pc wrote:
Pigford wrote:Check +12v to coils when running.
Standard carbs are basic bits of kit & not uncommon for "off-idle" hesitation.
Could enrichen pilot system a bit (maybe up a size on pilot jet) the wind air screw out a bit?

Got to agree with Pigford.

But you have to rule out everything else as well.
Compression & leakage
Airbox system leakage
Valve clearances
Coil voltages
Coil resistance
Ignition timing
Plugs
Reg/rec, charging and battery

All of those can affect how the bike runs.

Colortune is the way to go fr setting the airscrew.

Also, some cheap aftermarket airfilters can also give a low speed stumble.


All of the above have been checked and are OK.

I have an old Colortune kicking around somewhere. Will see if I can dig it out and see if that helps. But as some have indicated, it may well just be a characteristic of the bike/carbs particularly it's now well over 40 years old?

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Re: Z900 A4 Carb issues

#12 PostAuthor: Ultim8pc » Sun Nov 03, 2019 4:54 pm

sprint wrote:it may well just be a characteristic of the bike/carbs particularly it's now well over 40 years old?

Indeed, getting them 100% may not always be possible on a 40 year old bike.
http://www.airevalleyclassics.co.uk
10% discount code Z1OCTEN
Keyster rebuild kits & carburettor spares - airscrews, jets, needles, starter plungers & gaskets.
Also petrol/ethanol proof float bowl gaskets. IMD pistons preferred seller.

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Re: Z900 A4 Carb issues

#13 PostAuthor: zed1015 » Sun Nov 03, 2019 5:26 pm

To determine if it is a fueling issue try feathering the choke at the point the bike hesitates.
If this smooths it out you may have a problem with the pilot circuit.
If it isn't a partial blockage you may need to go up the pilot or lift the needle a notch.


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