SOHC.co.uk Forums > CB750
Idle and Starting Set Up
Chris400F:
Came across this article (referenced from elsewhere on these forums, I searched for 'float height') on setting float height which may be of use:
http://www.salocal.com/sohc/tech/carb/asmpg_mgs/flthgt.htm
mgy66:
Thanks Chris,
This is actually the post that triggered my question initially... The post clearly debates about whether the measurement should be from the lip or from the gasket surface but the below text between the pictures seems to indicate the measurement should be from the valve in open position ''so the valve pin tip is just barely touching the float tang''. Doing the work on the spot with the carb mounted is not easy as you would not see when the tip is barely touching the float tang and contrary to the description, in my carbs it does not seem that I have any spring on the valve (it looks like it is gravity). I would be interested to know if the 750K1 should have a spring in the float valve. That may explain why it looses rev as I wait at red light.
Wrt the approach I have taken and which I am sharing with the forum to get some feedback, I am now pushing the float at the highest point so when the valve is in close position I make sure I still have some little gap between the float and the top of the carb (i.e. the float does not bang onto the top of the carb which of course would prevent full closure of the valve). I may have a level a bit higher but I think as long as it is not over the gasket (which I will notice as it will leak from here). The assumption I am making is that high level (but no leak) is not affecting the carburation as what is moving the full up into the jets is the suction pressure and the diameter of the jets (i.e. it does not depend on how much of the jets are immerged into the fuel)... Any further experience to share on that one?
Wrt to Dennis post and table, do you know what means 4 for the needle jet? Is it a length, diameter or just the level where the pin should be attached at the top?
Best,
Marc
Chris400F:
I was actually searching for a different thread when I came across that link, which I have now found.
http://www.sohc.co.uk/index.php/topic,7617.msg48072.html#msg48072
The thread relates to 350/400 models but methinks the principles should be the same.
I did my own float heights around that time so it was useful information.
From my experience I would think trying to set heights with the carbs in situ would be vey difficult; I wouldn't even attempt it.
Working with the carbs off the bike and the bank of carbs vertical (as suggested somewhere) seemed a good scheme to me.
I don't know enough about the operation of carburettors to know how crucial float height is or what effect it has but guess a value is specified for a reason.
On the 400, and other carbs I have come across, the float valves have an internal spring which I assume is to prevent excess pressure on and consequent damage to the valve seat.
Spitfire:
--- Quote from: mgy66 on January 14, 2016, 02:55:37 PM ---Thanks Chris,
This is actually the post that triggered my question initially... The post clearly debates about whether the measurement should be from the lip or from the gasket surface but the below text between the pictures seems to indicate the measurement should be from the valve in open position ''so the valve pin tip is just barely touching the float tang''. Doing the work on the spot with the carb mounted is not easy as you would not see when the tip is barely touching the float tang and contrary to the description, in my carbs it does not seem that I have any spring on the valve (it looks like it is gravity). I would be interested to know if the 750K1 should have a spring in the float valve. That may explain why it looses rev as I wait at red light.
Wrt the approach I have taken and which I am sharing with the forum to get some feedback, I am now pushing the float at the highest point so when the valve is in close position I make sure I still have some little gap between the float and the top of the carb (i.e. the float does not bang onto the top of the carb which of course would prevent full closure of the valve). I may have a level a bit higher but I think as long as it is not over the gasket (which I will notice as it will leak from here). The assumption I am making is that high level (but no leak) is not affecting the carburation as what is moving the full up into the jets is the suction pressure and the diameter of the jets (i.e. it does not depend on how much of the jets are immerged into the fuel)... Any further experience to share on that one?
Wrt to Dennis post and table, do you know what means 4 for the needle jet? Is it a length, diameter or just the level where the pin should be attached at the top?
Best,
Marc
--- End quote ---
The JN related to the needle position 4 being the 4th from the top, if I remember correctly, have a read of the following for more details
http://forums.sohc4.net/index.php?topic=63177.msg687840#msg687840
Cheers
Dennis
mgy66:
Thanks...
@Chris, it looks from the post that they are referring to a spring in the float valve (pin 'contacted but unloaded'). I am pretty sure I do not have any spring in the float valve. I am not sure this can be done with the carbs on the bike. Interestingly, there is also a sketch referring to the adjusting procedure (from Bodd). This should be done with the needle screw sufficiently engaged. I think that was my mistake when doing the sync.
@Spitfire, I will try to take out the carbs and do a full cleaning. The post you copied also refer to some white calcification on the main jet holes (from HondaMan). I don't think it is my case as I have cleaned them with a carb cleaning drills but I am pretty sure I have some calcification or some kind of deposit in the starting chamber or channel where the slow jets are connected. I see there is a small hole at 3 or 9 o'clock when you face the inlet.
Does anyone can recommend some product to use in a ultrasonic bath? Is it the same product sold in spray as 'carb cleaner'?
All good stuff which should take me to a much more usable bike.
Thanks,
Marc
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