Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: FatFreddy on March 26, 2007, 10:51:35 PM
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Now that i've got the old girl firing on all four cylinders at the right time, it's time to sort out the carbs. (no groaning at the back please).
The engine in my bike was a replacement for a severly knackered F2 (conrod for cylinder 1 came away at 90mph and did a very good impersonation of a certain gut-busting scene from Aliens). It's a F1, and it didn't come with any carbs. So after some time perusing ebay, i got hold of a set.
However, i'm not entirely sure i've got hold of the right type, ie i think the ones i've got are somewhat too old for the engine and thereby i'm not going to get the best performance. Do you mind having a look at the pics below and helping me out. Should i replace them with something more recent/better?
(http://www.honda-sohc.co.uk/components/com_joomlaboard/uploaded/images/CB750_carb_2.jpg)
Post edited by: FatFreddy, at: 2007/03/26 22:53
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Hi Freddy,
Those are of the same type as the F1 in fact all the CB750 used that type up untill the F2.
The way I see it, if it idles and runs up to 70/80mph without ant flat spots, chances are the're fine.
Check your plugs - See chapter 3 in the Haynes,
The main problem with eBay carbs I found is that you never know if the last own tweaked any thing.
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Hey there,
I agree, carbs are the same on the F1 and the K models, only the K7 and F2 have other carbs, although the same innerdia, these have accelerator pumps and pressed in primary jets. Be happy with the ones you've got, because they are easier to adjust.
Good luck.
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Cheers chaps.
Just thought i has better make sure before i start sorting them out. The bikes running rich on 3 cylinders, doesn't rev out like it should, not as powerful as it should be, and sounds rough despite sorting out the timing. You can hear occasional 'popping' in the exhaust when she's warmed up and ticking over. I guess that'll be excess fuel not being burnt up in the cylinder.
Well then, i've got meself some carb balancers, so it's time a learnt how to use them. Any tips or things to look out for when sorting these carbs out?
Cheers
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I found Balancing with a set of vacuum guages a pain, and couldn't get it right, in the end I had to remove the carbs and manually setup the float heights. After that it all settled down.
Good louck and remember :S it's only a hobby :S
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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
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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
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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.
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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??
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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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Wwll i've found out why the bike was running so rich and rough.
have a look at the picture below and see if you can spot the problem!
(http://www.honda-sohc.co.uk/components/com_joomlaboard/uploaded/images/CB750_carb_dodgy_float_1_email.jpg)
Yes i did fix it, and the bike is now running like a dream. Or rather it will just as soon as i get the old front wheel bearings out and replace them with some shiny new ones. Got a problem with the removal of the retainer ring.
Post edited by: FatFreddy, at: 2007/05/04 16:57
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Here's another picture of the main offender,
(http://www.honda-sohc.co.uk/components/com_joomlaboard/uploaded/images/CB750_carb_dodgy_float_2_email.jpg)
I haven't a clue how i/it managed to end up like that! Good job i quite skillful with two pairs of pliers.
Post edited by: FatFreddy, at: 2007/05/04 17:00
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:) I bought a set of K4 USA Spec carbs off eBay USA as my F1 ones were full of crud & crap & glued up with old petrol plus the carb rubbers were very bad \"mice had eaten holes in them\". I was shocked by the price off new carb rubbers
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Well spotted that man, if you are having problems in the wet try sealing the HT leads where they go in the coil, I used Araldite in the old days but nowadays some doorframe sealant would do a better job.
If it works I've posted a photo of the wheel bearing retainer removal type tools I use, very low tech.
Cheers
Den (http://www.honda-sohc.co.uk/components/com_joomlaboard/uploaded/images/Bearing_tools.jpg)