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Discussion Starter · #1 ·
Hello, first post on this forum!



My carb on my zxr750 J1 1991 may need replacing, and found a reasonably priced set from the H2 model year 89 or 90, and was curious whether they fit? Or if any of the later models will?

Thanks

Rick
 

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H models carbs will NOT fit. Totally different spacing due to location of cam chain in centre.
1991-1995 will be the same spacing. Jetting will need to be matched to your motors needs though.
 

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H models carbs will NOT fit. Totally different spacing due to location of cam chain in centre.
1991-1995 will be the same spacing. Jetting will need to be matched to your motors needs though.
Ninjanut is 100% right.


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Why would they need replacing?

Welcome to the forums btw
 
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1993 Zx-7 L Model, 2006 ZX-10R
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Discussion Starter · #6 ·
Why would they need replacing?

Welcome to the forums btw
Valid question. I'm curious too
Thanks for the welcome. Well i bought the bike private as it was stood doing nothing for 3-4 yrs and it does start, but the previous owner tried messing with the carb balancing and it is now way out of sync. I plugged in a set of balancer gauges, not the liquid style ones. And, even though it seems easy enough on youtube videos etc, I have tried to balance them and just cannot for the life of me get the carbs to sync up.

So thought to hell with it and was just going to replace with a used/serviced carb which has not had the balancer screws messed with.... Taking the easy and lazy way out i guess!
 

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Discussion Starter · #7 ·
H models carbs will NOT fit. Totally different spacing due to location of cam chain in centre.
1991-1995 will be the same spacing. Jetting will need to be matched to your motors needs though.
So if i was to buy a carb off any ZXR750 released between 1991 and 95 it'd fit straight on with no issues? Sweet. And I would just need to swap the jets out of my existing carb, and put them into the "new" carb?
 

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They are the same size and spacing, only the jetting is different. Hopefully the set you buy is not the exact same condition as your current ones! Tough buying used carbs unless you are willing to work on them. You will likely need to go through them to make sure they are clean and in good condition.
 

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Thanks for the welcome. Well i bought the bike private as it was stood doing nothing for 3-4 yrs and it does start, but the previous owner tried messing with the carb balancing and it is now way out of sync. I plugged in a set of balancer gauges, not the liquid style ones. And, even though it seems easy enough on youtube videos etc, I have tried to balance them and just cannot for the life of me get the carbs to sync up.

So thought to hell with it and was just going to replace with a used/serviced carb which has not had the balancer screws messed with.... Taking the easy and lazy way out i guess!
There's 3 sync screws in between the carbs. From left to right, we'll name them #1, #2, #3.

#3 syncs carb 3 to carb 4 (carb 4 is your baseline carb)
#1 syncs carb 1 to carb 2 (iirc)
#2 syncs 1&2 as a pair to carbs 3&4

I was able to sync them using a single carb sync tool. Like the ones that are used on the old vw bug engines.

I'm open to any corrections. It's been awhile since I had to sync them.
 
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Discussion Starter · #10 ·
There's 3 sync screws in between the carbs. From left to right, we'll name them #1, #2, #3.

#3 syncs carb 3 to carb 4 (carb 4 is your baseline carb)
#1 syncs carb 1 to carb 2 (iirc)
#2 syncs 1&2 as a pair to carbs 3&4

I was able to sync them using a single carb sync tool. Like the ones that are used on the old vw bug engines.

I'm open to any corrections. It's been awhile since I had to sync them.
This is the carb sync tool i own. Obviously each gauge a reading for each carb with the needle. So with carb 4 being the baseline carb, would turning screw #3 cause both carb 3 and 4's reading to fluctuate up and down? Or would carb 4 always stay the same, and i just match number 3 up to it?

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This is the carb sync tool i own. Obviously each gauge a reading for each carb with the needle. So with carb 4 being the baseline carb, would turning screw #3 cause both carb 3 and 4's reading to fluctuate up and down? Or would carb 4 always stay the same, and i just match number 3 up to it?

View attachment 20429
#3 screw will adjust #3 carb to #4 carb
 

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What you will notice is that for example, #3 has more vacuum than #4. When you turn the adjuster screw, one carbs vacuum will go up while the other will drop. You will see that if you go the wrong way, #3 will go higher while #4 will go even lower. Turn the other way and you will see the two come to a common value. This is where they are in sync. Then do #1 and #2, then balance the 1/2 bank to the 3/4 bank using the centre adjuster in the same way.
 

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What you will notice is that for example, #3 has more vacuum than #4. When you turn the adjuster screw, one carbs vacuum will go up while the other will drop. You will see that if you go the wrong way, #3 will go higher while #4 will go even lower. Turn the other way and you will see the two come to a common value. This is where they are in sync. Then do #1 and #2, then balance the 1/2 bank to the 3/4 bank using the centre adjuster in the same way.
Thank you for the better explanation
 

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Also, there is no master carb with CV units. They list #4 as master on FCRs.
Learned something new. I had applied that rule to CV carbs as well because I thought (and assumed) the carb with the butterfly valve actuator was the master carb. Thanks @ninjanut
 

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I have never understood how you could keep a master carb after you strip them for cleaning. FCRs you simply adjust all the slides to the same height and voila! I think when the manual discusses this, it is based on routine servicing not complete strip, clean and rebuild.
 

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Discussion Starter · #17 ·
What you will notice is that for example, #3 has more vacuum than #4. When you turn the adjuster screw, one carbs vacuum will go up while the other will drop. You will see that if you go the wrong way, #3 will go higher while #4 will go even lower. Turn the other way and you will see the two come to a common value. This is where they are in sync. Then do #1 and #2, then balance the 1/2 bank to the 3/4 bank using the centre adjuster in the same way.
Thanks mate. And how crucial is it to blip the throttle during sync? I'm also guessing when turning the screw to balance, we are talking not even quarter of a turn at a time.... even less

I am 99.9% sure they need re-syncing, but what are the major signs to look for in order to be absolutely certain that a re-sync is needed....
 

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Poor idle is the most obvious symptom. You are correct, quarter turns is about right. You will see as you slowly turn the screws how far and which way to turn.
You are simply trying to have all 4 carbs drawing the same vacuum. The number is not a factor just that they are the same.
 
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