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I'm using the carbs off a '95 CBR900RR on a custom intake manifold for a Ford 289 V8 in my '85 Ranger. Don't hate on me yet, I AM a Honda owner, but it's a 2003 400EX quad (I need my training wheels, don't do good on just two :))

I've got it up and running, and a wideband O2 gauge to aid with calibration. Right now it runs well up to about 1/4 throttle, then as you give it more throttle it starts going way lean. I've upped the main jet with very little improvement, I'm pretty sure my problem is the needle doesn't have nearly enough taper. I raised the needle as much as possible using a .040" washer, that helpe dramatically, but still won't do much past 1/4-1/3 throttle.

I've been trying to find info on needle part numbers, figure out what I can order that will run better, but information is scarce on these carbs! Whenever I look for needle info, I only come up with stuff for FCR carbs, with the adjustable clip. I'm about to go to the local Honda dealer and start looking at and measuring what they have in stock to try to find something that will work better. Does anyone here have any information, or can recommend anyone to contact, that would be able to give me some information on what needles will work in this carb, and some of the specs like diameter, taper, etc.?

BTW the needles I have now are marked "J5DC" and "J5DB". Google comes up with nothing, and when I search for Keihin CV needle I get a bunch of Harley Davidson sites, but their needles are different numbers (all start with 'N', but are four digits like these). So far I've done this project pretty low-buck, I don't want to spend a fortune trying different needles that don't run or don't even fit. Any help is much appreciated!
 

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I'm using the carbs off a '95 CBR900RR on a custom intake manifold for a Ford 289 V8 in my '85 Ranger. Don't hate on me yet, I AM a Honda owner, but it's a 2003 400EX quad (I need my training wheels, don't do good on just two :))

I've got it up and running, and a wideband O2 gauge to aid with calibration. Right now it runs well up to about 1/4 throttle, then as you give it more throttle it starts going way lean. I've upped the main jet with very little improvement, I'm pretty sure my problem is the needle doesn't have nearly enough taper. I raised the needle as much as possible using a .040" washer, that helpe dramatically, but still won't do much past 1/4-1/3 throttle.

I've been trying to find info on needle part numbers, figure out what I can order that will run better, but information is scarce on these carbs! Whenever I look for needle info, I only come up with stuff for FCR carbs, with the adjustable clip. I'm about to go to the local Honda dealer and start looking at and measuring what they have in stock to try to find something that will work better. Does anyone here have any information, or can recommend anyone to contact, that would be able to give me some information on what needles will work in this carb, and some of the specs like diameter, taper, etc.?

BTW the needles I have now are marked "J5DC" and "J5DB". Google comes up with nothing, and when I search for Keihin CV needle I get a bunch of Harley Davidson sites, but their needles are different numbers (all start with 'N', but are four digits like these). So far I've done this project pretty low-buck, I don't want to spend a fortune trying different needles that don't run or don't even fit. Any help is much appreciated!

Have you got the carbs mounted so the float bowls are level as they would be when installed on the bike?
 

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Discussion Starter #3
Yes, as "level" I can determine. I don't have a bike to look at so I had to go by the float bowl top edge/gasket. The carbs are at about a 30-35 degree downturn.

 

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Yes, as "level" I can determine. I don't have a bike to look at so I had to go by the float bowl top edge/gasket. The carbs are at about a 30-35 degree downturn.


Looks like a good setup.
My guess then would be they're struggling to flow enough air and fuel for the five times greater engine capacity.
What size mains are you running?
 

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Discussion Starter #6
By air leak do you mean manifold vacuum leak? Those typically have greatest affect at idle, not above 1/4 throttle where the airflow through the carb is far greater than the leak would be.

They had 115 mains, I've jetted up to 160 with minor improvement, but I think right now it's not getting enough airflow to really get on the main, it's still in the needle when it goes lean.

Yes the engine is five times larger, but at any given moment only 1 cylinder is drawing air at a time (ok, maybe 1.5 as the cam has more than 180 degrees of intake duration). On the bike each carb only feeds one cylinder, and each cylinder only draws air on the intake stroke, so each carb is not flowing any air 25% of the time. With the carbs feeding the intake plenum, all four can feed the 1-2 cylinders drawing air at once, and all carbs are flowing air 100% of the time.

Long story short, the carbs are appropriate size for the engine, with any luck it's just a matter of recalibrating the carburators for the different engine and constant flow instead of pulsed flow they are made for on the CBR.
 

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I'm using the carbs off a '95 CBR900RR on a custom intake manifold for a Ford 289 V8 in my '85 Ranger. Don't hate on me yet, I AM a Honda owner, but it's a 2003 400EX quad (I need my training wheels, don't do good on just two :))

I've got it up and running, and a wideband O2 gauge to aid with calibration. Right now it runs well up to about 1/4 throttle, then as you give it more throttle it starts going way lean. I've upped the main jet with very little improvement, I'm pretty sure my problem is the needle doesn't have nearly enough taper. I raised the needle as much as possible using a .040" washer, that helpe dramatically, but still won't do much past 1/4-1/3 throttle.

I've been trying to find info on needle part numbers, figure out what I can order that will run better, but information is scarce on these carbs! Whenever I look for needle info, I only come up with stuff for FCR carbs, with the adjustable clip. I'm about to go to the local Honda dealer and start looking at and measuring what they have in stock to try to find something that will work better. Does anyone here have any information, or can recommend anyone to contact, that would be able to give me some information on what needles will work in this carb, and some of the specs like diameter, taper, etc.?

BTW the needles I have now are marked "J5DC" and "J5DB". Google comes up with nothing, and when I search for Keihin CV needle I get a bunch of Harley Davidson sites, but their needles are different numbers (all start with 'N', but are four digits like these). So far I've done this project pretty low-buck, I don't want to spend a fortune trying different needles that don't run or don't even fit. Any help is much appreciated!
Why not just run Ford 289 carbs? After all, you are using carbs that were designed for a high reving street bike engine. Not a low rev, torque monster.
 

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Discussion Starter #8
Why not just run Ford 289 carbs? After all, you are using carbs that were designed for a high reving street bike engine. Not a low rev, torque monster.
It's a little closed minded to think these carburetors were "designed for a high reving street bike engine". These same design carburetors are used on Harley Davidson motorcycles, for example, which don't rev as high as my 289. The carburetor doesn't care how big your engine is, or how many RPM it turns, or if it's a 2-stroke or 4-stroke, it just cares about air flow. It's a device designed for mixing a metered amout of fuel with an air stream, with enough adjustability to work on a wide range of applications.

And this is the first time I've ever heard anyone refer to the 289 as a "low rev torque monster". :rotfl:
 

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It's a little closed minded to think these carburetors were "designed for a high reving street bike engine". These same design carburetors are used on Harley Davidson motorcycles, for example, which don't rev as high as my 289. The carburetor doesn't care how big your engine is, or how many RPM it turns, or if it's a 2-stroke or 4-stroke, it just cares about air flow. It's a device designed for mixing a metered amout of fuel with an air stream, with enough adjustability to work on a wide range of applications.

And this is the first time I've ever heard anyone refer to the 289 as a "low rev torque monster". :rotfl:

Perhaps measure the manifold pressure to get a clue as to whether they're flowing enough air to feed the engine.
The 289 makes _peak_ power at around the same RPM that the bike engine is _just_ starting to breathe.
I'm curious why you put four carbs onto a manifold so they essentially operate as one rather than feed paired cylinders, or even better why you didn't go with two banks of carbs?
I think that's what is causing the problem.
 

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Have you tried the time tested modification of shimming the needles higher?

I really doubt it's the needles as they are for fine tuning once you have the right main jet.
Have you tried it without the needles already?
You will need to put something inside to block the hole though or the slides won't lift.
 

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It's a little closed minded to think these carburetors were "designed for a high reving street bike engine". These same design carburetors are used on Harley Davidson motorcycles, for example, which don't rev as high as my 289. The carburetor doesn't care how big your engine is, or how many RPM it turns, or if it's a 2-stroke or 4-stroke, it just cares about air flow. It's a device designed for mixing a metered amout of fuel with an air stream, with enough adjustability to work on a wide range of applications.

And this is the first time I've ever heard anyone refer to the 289 as a "low rev torque monster". :rotfl:
My main point is that the 289 is going to put out more power compared to the cbr900. More power means more fuel is needed. Therefore, the cbr900 carbs which are designed for engines with less than 200 HP are too small for the 289. Which is why I think you are running lean. You are just creating more problems for yourself by using motorcycle carbs on a car engine. Buy some Holly carbs, and you are done.
 

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Discussion Starter #13
Bladeracer: pressure gauge would do no good, in order to test if the engine is flow limited I need to be able to run at full throttle. As I mentioned, it's leaning out and falling on it's face before I even get the throttle blades half open. Does that sound like air flow limited to you?

Gang two cylinders per carb: a 38mm carb is too mall for an IR on a 289, you need at least a 42-44mm to not be flow limited. Also ganging two cylinders is tough, to do it RIGHT you want to gang cylinders that are 180 degrees apart in the firing order, which on a V8 is tough due to the odd firing order.

Two banks of carbs: again 38mm not big enough to do IR, and twice as many carbs means twice as much tuning, synchronizing carbs is a hassle, lots of reasons to not go IR period.

Four carbs feeding a plenum, think of it as an inline four-barrel Holley. After all, the four barrels of a Holley all have their own metering circuits, it's effectively like four carbs with each pair sharing a fuel bowl.

I've considered taking the needles out, but seems like a pointless exercise, as it won't run right (will be way too rich down low) so why try it?

Rustedroot: Yes I shimmed the needle as much as possible, it did make an improvement, but not nearly enough. This helps lead me to believe that the needle is the problem.

Slothman: Yes the 289 makes more power than the CBR900, meaning more fuel and air. One important limiting factor here is needle and seat area on the fuel inlet valve. I have already considered that problem, and concluded it's not a problem. The CBR900 is gravity fed, the fuel level being 1ft above the carbs would yeild about 0.25PSI of fuel pressure at the carb. Liquid flow is proportional to the square root of the pressure difference. If 0.25PSI is enough for 200HP, then four times that (1PSI) will flow enough fuel double the power (400HP). I've tested these carbs up to 4.5PSI with no leaks/problems, so I'm running them at 2 PSI to have a margin of safety and still have plenty of fuel flow. If I do have fuel starvation problems, they will not show up until I've had my foot to the floor some time, not as soon as I tip the throttle in.

The second part which again you don't seem to be grasping is that on the bike each carb is only feeding one cylinder, and each cylinder is only drawing air on the intake stroke, and doing nothing the other 75% of the engine revolution. Therefore, these four carbs can feed a 200HP engine (your number) while only flowing air and fuel 25% of the time. With a common plenum intake like my tunnel ram, they are feeding air 100% of the time, so they can feed 4x the power (800hp). I think judging by the size (38mm) and some other research I've done, these are well suited for up to 400hp. If that doesn't click, think of this, on a V8 each cylinder draws air 90 crank degrees apart, with an intake stroke duration of 180 degrees, so at any given moment only two cylinders are drawing air, and when one is at peak airflow the other is just getting started or finishing. So I effectively have four carbs feeding 1-2 cylinders, that's effectively over 2:1 carbs per cylinder, that's over double the 1:1 carb per cylinder when it's on a bike.

I've had a Holley, I know more about Holley, Edelbrock, and Quadrajet carburetors then most people know about any one carburetor. Because of what I know about these carbs, their limitations, their 60 year old design, is one of the main reasons I picked these more modern carburetors.

My main reason for starting this project is I wanted to run a tunnel ram intake manifold, but didn't want to cut a hole in the hood and deal with two Holley carburetors. I started looking at side-draft carburetors, most of what's typically used on cars like Webers are uber expensive, and still ultimately old designs. I picked up these carbs from eBay for $20 with shippping, and they were in perfect working condition.

Trust me, I've put far more thought and research into this than any of you guys, so how 'bout you stop questioning my intentions and just answer the question, if you can that is. Your arguments are all things I have considered, thought through, and concluded that there is no fundamental problem with what I'm doing. I'm open to honest curiosity, but if all I'm going to get is nay-sayers then obviously I've come to the wrong forum for technical information.
 

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Bladeracer: pressure gauge would do no good, in order to test if the engine is flow limited I need to be able to run at full throttle. As I mentioned, it's leaning out and falling on it's face before I even get the throttle blades half open. Does that sound like air flow limited to you?

Gang two cylinders per carb: a 38mm carb is too mall for an IR on a 289, you need at least a 42-44mm to not be flow limited. Also ganging two cylinders is tough, to do it RIGHT you want to gang cylinders that are 180 degrees apart in the firing order, which on a V8 is tough due to the odd firing order.

Two banks of carbs: again 38mm not big enough to do IR, and twice as many carbs means twice as much tuning, synchronizing carbs is a hassle, lots of reasons to not go IR period.

Four carbs feeding a plenum, think of it as an inline four-barrel Holley. After all, the four barrels of a Holley all have their own metering circuits, it's effectively like four carbs with each pair sharing a fuel bowl.

I've considered taking the needles out, but seems like a pointless exercise, as it won't run right (will be way too rich down low) so why try it?

Rustedroot: Yes I shimmed the needle as much as possible, it did make an improvement, but not nearly enough. This helps lead me to believe that the needle is the problem.

Slothman: Yes the 289 makes more power than the CBR900, meaning more fuel and air. One important limiting factor here is needle and seat area on the fuel inlet valve. I have already considered that problem, and concluded it's not a problem. The CBR900 is gravity fed, the fuel level being 1ft above the carbs would yeild about 0.25PSI of fuel pressure at the carb. Liquid flow is proportional to the square root of the pressure difference. If 0.25PSI is enough for 200HP, then four times that (1PSI) will flow enough fuel double the power (400HP). I've tested these carbs up to 4.5PSI with no leaks/problems, so I'm running them at 2 PSI to have a margin of safety and still have plenty of fuel flow. If I do have fuel starvation problems, they will not show up until I've had my foot to the floor some time, not as soon as I tip the throttle in.

The second part which again you don't seem to be grasping is that on the bike each carb is only feeding one cylinder, and each cylinder is only drawing air on the intake stroke, and doing nothing the other 75% of the engine revolution. Therefore, these four carbs can feed a 200HP engine (your number) while only flowing air and fuel 25% of the time. With a common plenum intake like my tunnel ram, they are feeding air 100% of the time, so they can feed 4x the power (800hp). I think judging by the size (38mm) and some other research I've done, these are well suited for up to 400hp. If that doesn't click, think of this, on a V8 each cylinder draws air 90 crank degrees apart, with an intake stroke duration of 180 degrees, so at any given moment only two cylinders are drawing air, and when one is at peak airflow the other is just getting started or finishing. So I effectively have four carbs feeding 1-2 cylinders, that's effectively over 2:1 carbs per cylinder, that's over double the 1:1 carb per cylinder when it's on a bike.

I've had a Holley, I know more about Holley, Edelbrock, and Quadrajet carburetors then most people know about any one carburetor. Because of what I know about these carbs, their limitations, their 60 year old design, is one of the main reasons I picked these more modern carburetors.

My main reason for starting this project is I wanted to run a tunnel ram intake manifold, but didn't want to cut a hole in the hood and deal with two Holley carburetors. I started looking at side-draft carburetors, most of what's typically used on cars like Webers are uber expensive, and still ultimately old designs. I picked up these carbs from eBay for $20 with shippping, and they were in perfect working condition.

Trust me, I've put far more thought and research into this than any of you guys, so how 'bout you stop questioning my intentions and just answer the question, if you can that is. Your arguments are all things I have considered, thought through, and concluded that there is no fundamental problem with what I'm doing. I'm open to honest curiosity, but if all I'm going to get is nay-sayers then obviously I've come to the wrong forum for technical information.
oh that's funny, if you're so smart why is it still running lean ? :rotfl::clap:

again: why are you busting your nutz trying to make these carbs work when you can EASILY fix the issue with regular 289 Holly's ?????
 

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Bladeracer: pressure gauge would do no good, in order to test if the engine is flow limited I need to be able to run at full throttle. As I mentioned, it's leaning out and falling on it's face before I even get the throttle blades half open. Does that sound like air flow limited to you?

Gang two cylinders per carb: a 38mm carb is too mall for an IR on a 289, you need at least a 42-44mm to not be flow limited. Also ganging two cylinders is tough, to do it RIGHT you want to gang cylinders that are 180 degrees apart in the firing order, which on a V8 is tough due to the odd firing order.

Two banks of carbs: again 38mm not big enough to do IR, and twice as many carbs means twice as much tuning, synchronizing carbs is a hassle, lots of reasons to not go IR period.

Four carbs feeding a plenum, think of it as an inline four-barrel Holley. After all, the four barrels of a Holley all have their own metering circuits, it's effectively like four carbs with each pair sharing a fuel bowl.

I've considered taking the needles out, but seems like a pointless exercise, as it won't run right (will be way too rich down low) so why try it?

Rustedroot: Yes I shimmed the needle as much as possible, it did make an improvement, but not nearly enough. This helps lead me to believe that the needle is the problem.

Slothman: Yes the 289 makes more power than the CBR900, meaning more fuel and air. One important limiting factor here is needle and seat area on the fuel inlet valve. I have already considered that problem, and concluded it's not a problem. The CBR900 is gravity fed, the fuel level being 1ft above the carbs would yeild about 0.25PSI of fuel pressure at the carb. Liquid flow is proportional to the square root of the pressure difference. If 0.25PSI is enough for 200HP, then four times that (1PSI) will flow enough fuel double the power (400HP). I've tested these carbs up to 4.5PSI with no leaks/problems, so I'm running them at 2 PSI to have a margin of safety and still have plenty of fuel flow. If I do have fuel starvation problems, they will not show up until I've had my foot to the floor some time, not as soon as I tip the throttle in.

The second part which again you don't seem to be grasping is that on the bike each carb is only feeding one cylinder, and each cylinder is only drawing air on the intake stroke, and doing nothing the other 75% of the engine revolution. Therefore, these four carbs can feed a 200HP engine (your number) while only flowing air and fuel 25% of the time. With a common plenum intake like my tunnel ram, they are feeding air 100% of the time, so they can feed 4x the power (800hp). I think judging by the size (38mm) and some other research I've done, these are well suited for up to 400hp. If that doesn't click, think of this, on a V8 each cylinder draws air 90 crank degrees apart, with an intake stroke duration of 180 degrees, so at any given moment only two cylinders are drawing air, and when one is at peak airflow the other is just getting started or finishing. So I effectively have four carbs feeding 1-2 cylinders, that's effectively over 2:1 carbs per cylinder, that's over double the 1:1 carb per cylinder when it's on a bike.

I've had a Holley, I know more about Holley, Edelbrock, and Quadrajet carburetors then most people know about any one carburetor. Because of what I know about these carbs, their limitations, their 60 year old design, is one of the main reasons I picked these more modern carburetors.

My main reason for starting this project is I wanted to run a tunnel ram intake manifold, but didn't want to cut a hole in the hood and deal with two Holley carburetors. I started looking at side-draft carburetors, most of what's typically used on cars like Webers are uber expensive, and still ultimately old designs. I picked up these carbs from eBay for $20 with shippping, and they were in perfect working condition.

Trust me, I've put far more thought and research into this than any of you guys, so how 'bout you stop questioning my intentions and just answer the question, if you can that is. Your arguments are all things I have considered, thought through, and concluded that there is no fundamental problem with what I'm doing. I'm open to honest curiosity, but if all I'm going to get is nay-sayers then obviously I've come to the wrong forum for technical information.

I haven't seen any nay-sayers comments here, just a bunch of questions and suggestions. I'm sure you've thought it all through but that doesn't mean your conclusions are automatically correct. They may well be but it's also possible you've missed something or made an error. I would take our questions at their face value, we are only trying to help after all.

Yes, I meant running each carb to the two cylinders that draw from it :)
But I would still have gone with one carb per cylinder myself. Synchronisation is not difficult or time consuming and only needs to be done once, unless you have some problem later on and need to adjust them.
Do the plugs indicate a rich or lean mix?
Have you measured the O2 to see if it's running rich or lean?
Are the slides lifting as you open the throttles?
Are the headers all running at similar temperatures?
Was the engine running fine before you swapped the carbs onto it?
 

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oh that's funny, if you're so smart why is it still running lean ? :rotfl::clap:

again: why are you busting your nutz trying to make these carbs work when you can EASILY fix the issue with regular 289 Holly's ?????

I think he explained his reasons.
I prefer seperate carbs for each cylinder every time. Being able to tune and diagnose each cylinder as a seperate engine with its own induction and exhaust is an absolute god-send compared to working on cars with one carb and an exhaust manifold.
But this is the first V8 I've seen attempting to run bike carbs so I am quite curious about it.
I had similar discussion with a guy some years ago that put a bank of semi-down-draught bike carbs on his car but he mounted them vertically so the float bowls were at almost 45-degrees. I'm certain they won't work at all in that position but he was adamant they would. He never did manage to provide any evidence that they ever did though.
 

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Discussion Starter #17 (Edited)
oh that's funny, if you're so smart why is it still running lean ? :rotfl::clap:

again: why are you busting your nutz trying to make these carbs work when you can EASILY fix the issue with regular 289 Holly's ?????
Ok, I will try to spell this out as clearly as possible and use small words so you can follow.

It's running lean because the needle does not have enough taper, I need a different needle. This carburetor is not tuned to my engine when I received it. So I am in the process of tuning the carburetor. Carburetors are made for this, that's why they have easily replaceable parts like jets, needles, and air bleeds. It can be challenging especially when starting with a carb that is way out of tune for your application. It's not something you get perfect on the first try, or even the second, or third. It's mostly trial and error, and the right tools, knowledge, and experience can help speed the process significantly. I wouldn't expect someone that's never done this to understand.

I am trying to make this work because A: I've ran Holley carbs, they work OK, these carbs have the potential to work better, and B: using side-draft carbs will allow me to run a tunnel ram intake under the stock hood. Tunnel rams are known for unbeatable torque and top-end power, which is exactly what I want from my 289.

I've already obviously committed to this project, and am in the last stages of it, so I am not looking for any input on what I should do different or why someone that knows nothing about it thinks it won't work. I'm asking for help finding information on any different needles that are available for this carburetor. If you comprehended my original post you would already know this. I thought a bunch of guys that own CBR900RRs might have experience tuning their bikes, so this seemed like a good place to ask.

Also, it's spelled "Holley", and there is no such thing as a "289 Holley".
 

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Discussion Starter #18
I haven't seen any nay-sayers comments here, just a bunch of questions and suggestions. I'm sure you've thought it all through but that doesn't mean your conclusions are automatically correct. They may well be but it's also possible you've missed something or made an error. I would take our questions at their face value, we are only trying to help after all.

Yes, I meant running each carb to the two cylinders that draw from it :)
But I would still have gone with one carb per cylinder myself. Synchronisation is not difficult or time consuming and only needs to be done once, unless you have some problem later on and need to adjust them.
Do the plugs indicate a rich or lean mix?
Have you measured the O2 to see if it's running rich or lean?
Are the slides lifting as you open the throttles?
Are the headers all running at similar temperatures?
Was the engine running fine before you swapped the carbs onto it?
I think slothman is clearly a nay-sayer here. The other questions have been honest and genuine, and I don't mind answering those, especially if they help get answers to original question. I'm not saying my conclusions are automatically correct, just that I've put a lot more thought into this than anyone else. Definintely not saying I didn't miss something, but I'm at the last 1 yard line here, the project is already done, this is just the final tuning phase.

I have a wideband O2 gauge (Innovate LC1), so no need for plug reads, at least not at this stage when the gauge is indicating the engine goes lean off the chart beyond 1/3-1/2 throttle.

Yes the slides all open simultaneously and equally when I crack the throttle open, so I know at least that parts working right, although it's possible at some point I may need to adjust the opening rate with different springs.

The engine ran perfect before starting the swap, it's still running perfect now at below 1/3 throttle, so I think all is good there.

I'm fairly certain I'm going down the right path with needle and main jet adjustments. I could be wrong, but the easiest way to find out is to try it. I've already upped the main jet to 160, I think now it's at the point where the needle is a bigger restriction than the main jet at the throttle opening that I'm having trouble at, so I need help finding information on different needles.
 

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I think slothman is clearly a nay-sayer here. The other questions have been honest and genuine, and I don't mind answering those, especially if they help get answers to original question. I'm not saying my conclusions are automatically correct, just that I've put a lot more thought into this than anyone else. Definintely not saying I didn't miss something, but I'm at the last 1 yard line here, the project is already done, this is just the final tuning phase.

I have a wideband O2 gauge (Innovate LC1), so no need for plug reads, at least not at this stage when the gauge is indicating the engine goes lean off the chart beyond 1/3-1/2 throttle.

Yes the slides all open simultaneously and equally when I crack the throttle open, so I know at least that parts working right, although it's possible at some point I may need to adjust the opening rate with different springs.

The engine ran perfect before starting the swap, it's still running perfect now at below 1/3 throttle, so I think all is good there.

I'm fairly certain I'm going down the right path with needle and main jet adjustments. I could be wrong, but the easiest way to find out is to try it. I've already upped the main jet to 160, I think now it's at the point where the needle is a bigger restriction than the main jet at the throttle opening that I'm having trouble at, so I need help finding information on different needles.

Possibly, but I think Sloth is just going for the easy fix :)
I haven't messed with quad-bore carbs in 25 years since the Carter and Holleys on my old Ford and Holden V8's. With a single manifold regardless of how many bores or carbs it has you can only tune to make the worst running cylinder run optimally. On bikes you can jet each cylinder optimally which is incredibly helpful.
The major difference here between the two engines is that each carb was designed for flowing enough air to feed a 225cc four-stroke cylinder doing 12000rpm with an almost straight intake tract only four-inches long. You want it to feed two 590cc cylinders at 5000rpm with perhaps twelve-inches of tract, half of them having to turn the airflow ninety-degrees.
Running some quick numbers.
At 1250rpm idle the bike engine is wanting 320 litres of air per minute through one carb.
At 500rpm idle the car is wanting 590 litres per minute through four carbs.
At 5000rpm the flow requirement is 1280lpm for the bike and 5900lpm for the car, each carb needing to flow 1475lpm - 15% more air than it needs to on the bike. On a bike that's a big difference in flow and would require significant tuning.
Obviously the airflow is what determines how much fuel is drawn through the jets so the jets need to be significantly bigger.

Are you checking O2 on each cylinder or at least on each bank? The turn in airflow bothers me a bit. Are you sure the left bank is not stealing all the flow from the right bank due to the more-direct flow?

Have you got some clear hose you can connect to a float bowl so you can actually see the fuel level? Perhaps it's drawing all the fuel in that first instant and drastically lowering the level so the air flow is not able to draw any more fuel up the jet.
I really can't see it being the needle as the needle is for very fine tuning once you have the right size main.
Have you got some bigger mains you can try or some old ones you can drill?
Will it pull revs while you're spraying starter fluid into the carbs?
 
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