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Carburettor Jets

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Wayne1960
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Carburettor Jets

#1 PostAuthor: Wayne1960 » Mon Aug 13, 2018 9:48 am

Should I upgrade the jets in my Mikuni 26 carbs now that the engine has been rebored to 1015cc from standard 903cc?

If so with what?
Just purchased a 1976 UK Z900 A4 for renovation. Wish me luck.
Bike now finished what a joy!

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Re: Carburettor Jets

#2 PostAuthor: kev edwards » Mon Aug 13, 2018 7:22 pm

I should imagine you may need to up the sizes by a bit, my thou runs 20 pilots 107.5 mains 5cn7 needles on groove 3 with pilot screws at 1 and a 1/4 turns out give or take a little. i run stock air box and a Harris style exhaust.

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Re: Carburettor Jets

#3 PostAuthor: Wayne1960 » Tue Aug 14, 2018 9:18 pm

Thanks Kev i also have the original air box and the original 4 into 4 exhausts. I seem to recall being told no need to upgrade but i just feel something is missing.
Just purchased a 1976 UK Z900 A4 for renovation. Wish me luck.

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Re: Carburettor Jets

#4 PostAuthor: ZedHead » Wed Aug 15, 2018 8:43 am

Good question ... Everything being equal, the air fuel ratio would remain the same. However, at high revs there is going to be some loss in volumetric efficiency resulting in less efficient combustion and unburnt fuel. It may actually mean a leaner mixture is needed at full throttle.

I would be interested to see the result of a a Dyno run on this.

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Re: Carburettor Jets

#5 PostAuthor: zed1015 » Wed Aug 15, 2018 8:57 am

Just having a normal 1mm rebore won't affect jetting but going up 100cc (10% capacity increase) will need a slight up jet on the main.
Especially if you have gone with higher comp Wisecos etc too.
More air sucked in means more fuel needed.
Go up 2 sizes on the main and work down.
Leave everything else as is.
You will likely end up one size up from what you have in now.

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Re: Carburettor Jets

#6 PostAuthor: ZedHead » Wed Aug 15, 2018 4:22 pm

zed1015 wrote:More air sucked in means more fuel needed.


I am probably over thinking, or over simplifying the science on this. Does an engine that is 10% larger in capacity not draw air at the same rate as the smaller engine revving 10% faster? If this is the case, the rate of air flow though the carb bores will have the same venturi draw and mixture ratio as with the smaller engine?

Major factors to combustion efficiency and subsequently jetting choice are the compression pressure, as with high compression pistons and the engines ability to refill with fresh charge, which is governed by air intake, carb size, cylinder head/valves and exhaust back pressure.

I am probably talking b*ll**ks but it is an interesting discussion for anyone to chip in.

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Re: Carburettor Jets

#7 PostAuthor: Philippe » Wed Aug 15, 2018 5:16 pm

Hi Wayne
why don't you try to find Z1000 A1 or A2 carbs? Fit those after a clean up and a refurbishment with standard jets and needles for a Z1000 A1 or A2. They look more or less the same and I don't think you'll have to worry about the mixture. Those should do the job I think.
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Re: Carburettor Jets

#8 PostAuthor: zed1015 » Wed Aug 15, 2018 5:42 pm

ZedHead wrote:
zed1015 wrote:More air sucked in means more fuel needed.


I am probably over thinking, or over simplifying the science on this. Does an engine that is 10% larger in capacity not draw air at the same rate as the smaller engine revving 10% faster? If this is the case, the rate of air flow though the carb bores will have the same venturi draw and mixture ratio as with the smaller engine?
I am probably talking b*ll**ks but it is an interesting discussion for anyone to chip in.


Partially right but the key factor here is the maximum amount of fuel the main jet can flow at max revs which is why you need a bigger main when increasing the capacity.
The main jet is sized for the engines maximum air demand and not for the carbs maximum flow rate.
Once you get to that point the carb becomes the restriction and you will need a larger venturi.
You can usually get away with leaving the pilot and needle etc alone but leaving the main "as is" risks running lean when that extra capacity starts pulling the extra air and the current main can't keep up..

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Re: Carburettor Jets

#9 PostAuthor: ZedHead » Thu Aug 16, 2018 6:01 pm

I am not totally convinced that main jets have a maximum fuel flow, theoretically, I think this is determined by the pressure differential before and after the venturi?

I have to bow to your practical experience on this one before my head explodes :sta :fire2

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Re: Carburettor Jets

#10 PostAuthor: Wayne1960 » Thu Aug 16, 2018 6:24 pm

Buzzard seems to think that the 1000 has the same Mikuni 26 carbs so upgrading the jets might be the way forward. Fascinating stuff though.
Just purchased a 1976 UK Z900 A4 for renovation. Wish me luck.

Bike now finished what a joy!

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Re: Carburettor Jets

#11 PostAuthor: zed1015 » Fri Aug 17, 2018 8:09 am

Wayne1960 wrote:Buzzard seems to think that the 1000 has the same Mikuni 26 carbs so upgrading the jets might be the way forward. Fascinating stuff though.


Yes! they are the same bodies just jetted to suit each different models airbox and exhaust etc.

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Re: Carburettor Jets

#12 PostAuthor: zed1015 » Fri Aug 17, 2018 8:36 am

ZedHead wrote:I am not totally convinced that main jets have a maximum fuel flow, theoretically, I think this is determined by the pressure differential before and after the venturi?


The only pressure differential which causes fuel to be drawn up through the main jet is between the low pressure in the venturi and the atmospheric pressure in the float bowl, not either side of the venturi..
The rate of flow is determined by the pressure differential (and governed by the jet needle) as the throttle is opened to increase the air speed but the main jet size limits the maximum fuel volume possible.
Every orifice has a maximum flow rate and the main jet is not exempt.
If they had no max flow rate there would be no need to fit bigger or smaller jets in any circumstance..
Last edited by zed1015 on Fri Aug 17, 2018 10:08 am, edited 1 time in total.

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Re: Carburettor Jets

#13 PostAuthor: ZedHead » Fri Aug 17, 2018 9:04 am

My head really hurts now :kod

Wayne1960 what colour are the plugs out of interest.

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Re: Carburettor Jets

#14 PostAuthor: ZedHead » Fri Aug 17, 2018 7:26 pm

I hate not understand stuff so I have been doing a bit of reading and thought I would share what I have discovered for interest

The number stamped on a main jet cooresponds to the flow rate of fuel in cc/sec when poored from a height of 50cm. So a 120 will pass 120cc/sec in this calibration test.

In operation the flow through the jet is directly proportion to the pressure difference between the float bowl and the venturi. A 10% increase in venturi pressure will draw 10% more fuel. However .....

The flow and pressure rate relationship for a venturi is different. This time the flow rate is directly proportional to the square route of the pressure difference. Meaning that the 10% increase in air flow demanded by our bigger engine will only produce just over 3% increase in pressure difference and a proportion increase in fuel through the main jet. So we need around 7% more fuel flow to balance things up.

So if this is correct. A standard Z900 main jet size of 115 needs to increase by around 7%. By my calculation making it around 120 - 122.5 range.

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Re: Carburettor Jets

#15 PostAuthor: rickm » Sat Aug 18, 2018 6:52 am

Fascinating stuff. Not questioning the wisdom of folk who know better than me, but just to continue the discussion....

"The number stamped on a main jet cooresponds to the flow rate of fuel in cc/sec when poored from a height of 50cm. So a 120 will pass 120cc/sec in this calibration test."
That's a heck of a lot of fuel to flow through such a tiny hole. It must be under more pressure than just a 50cm header tank to do that?

"A 10% increase in venturi pressure will draw 10% more fuel. "
I presume that's increase in pressure difference?

Would this all work in a similar way with CV carbs, as on the J ?
I'm guessing it probably would at full throttle.
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