SOHC.co.uk Forums > CB750

Have i got the right carbs?

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leonardo269:
Hi, I can't count the times I have changed the float valves on bikes as they tend to intermittently leak and make you mixture rich. My advice is strip the float bowl and clean out any crud, change alll the float valves and set the float heights to correct setting when you refit the floats. They should give you years of touble-free service after that. Next make sure that the mixture screw on each carb (underneath the float bowls) are set to the exact number of turns stated in the book. Also make sure that  your cam chain and tappets are correctly adjusted (easier to get to the cam chain adjuster with no carbs in the bike).

Check by eye that all the throtle slides should be opening at the same time. Next fit your carbs and fit a new Honda air filter. (I carnt stress this enough).

Hook up your Vacuum carbs and rig up a temporary fuel supply (in my case it was an upturned fabric conditioner bottle with a sealed-in nozzle and a long length of pipe. hang the fuel supply only something strudy so that you dont knock it over (like the door of the garage in my case). Next start up the bike and adjust the ide speed to a low, regular tickover.  

I might as well add at this point that that contact breaker points and ignition timing is cruitial to getting any subsequent tuning right. (I have ftted an electronic ignition which makes it a lot more accurate & reliable). If you havent got a stobe timing gun, then get one. They are easy to use and you will wonder how you ever got by without it.  Set the timing on both sets of points (1-4 and 2-3).

Adjust 2 carbs at a time until they are all registering the same vacuum. when they are all the same blipping the throttle will give you a nice fast response.
I have always found that checking the mixture on each cylinder with colortune to be affective (but often the factory mixture screw settings will give a nice bunsen blue anyway). If you adjust any settings check your balance again, it wont be far out, but it it is worth checking it twice and getting it spot on if you want ripping performance.     :woohoo:

If you go through these things methodically, the balancing should be straightforward.

Regards,

Ray

leonardo269:
Hi, I can't count the times I have changed the float valves on bikes as they tend to intermittently leak and make you mixture rich. My advice is strip the float bowl and clean out any crud, change alll the float valves and set the float heights to correct setting when you refit the floats. They should give you years of touble-free service after that. Next make sure that the mixture screw on each carb (underneath the float bowls) are set to the exact number of turns stated in the book. Also make sure that  your cam chain and tappets are correctly adjusted (easier to get to the cam chain adjuster with no carbs in the bike).

Check by eye that all the throtle slides should be opening at the same time. Next fit your carbs and fit a new Honda air filter. (I carnt stress this enough).

Hook up your Vacuum carbs and rig up a temporary fuel supply (in my case it was an upturned fabric conditioner bottle with a sealed-in nozzle and a long length of pipe. hang the fuel supply only something strudy so that you dont knock it over (like the door of the garage in my case). Next start up the bike and adjust the ide speed to a low, regular tickover.  

I might as well add at this point that that contact breaker points and ignition timing is cruitial to getting any subsequent tuning right. (I have ftted an electronic ignition which makes it a lot more accurate & reliable). If you havent got a stobe timing gun, then get one. They are easy to use and you will wonder how you ever got by without it.  Set the timing on both sets of points (1-4 and 2-3).

Adjust 2 carbs at a time until they are all registering the same vacuum. when they are all the same blipping the throttle will give you a nice fast response.
I have always found that checking the mixture on each cylinder with colortune to be affective (but often the factory mixture screw settings will give a nice bunsen blue anyway). If you adjust any settings check your balance again, it wont be far out, but it it is worth checking it twice and getting it spot on if you want ripping performance.     :woohoo:

If you go through these things methodically, the balancing should be straightforward.

Regards,

Ray

FatFreddy:
Thanks Ray. Duely noted. You've reminded me of few things there, and introduced to some new ones too. Thanks.
I'll let you know how it goes.

jimshea:
Freddy,
One thing mentioned a lot is the use of bell mouths and the effect it has on balancing the carbs and the way the bike performs... you may find that the carbs work better with the original (ugly) air box??

FatFreddy:
Hmmm...yes i see what you mean. Had a bit of fun riding her in the rain the other day. :S

But it looks nicer like this though! ;) :unsure: :dry:

Post edited by: FatFreddy, at: 2007/04/05 11:26

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