Topic: CB750 Carb bowl Sight Glass Mod. Amazing Discovery!

[Spanner 1]

I'm feelin' lucky.... so I'll respond ... The round type 750 floats ( early carbs )  need to be set at 26mm.
Measured from the upper body gasket surface to the top of the round float, BUT the carb must be held at a shallow angle from horizontal so as the float tang is touching the needle valve but not compressing the needle valve spring/plunger..... easy to make a tool from an old credit card, notched at the bowl body end to sit on the gasket surface and cut-out suitably with a notch that fits over the float, 26mm between .

[OneWheelDrive]

What I've never understood is how people measure the fuel/float height with a site tube at all.  The factory measurements are from the top of the float to the bowl lip.  Obviously the fuel level lies somewhere in the middle of the float, depending on buoyancy.  Not unless you measured how much of the float is submerged, then subtract that number from the factory adjustment could you determine where the fuel level should be relative to the lip of the bowl.  I would think the float site tube would only serve to verify that the bowls were even to each other, not to measure if the fuel level was to factory spec.

[BobbyR]

What I've never understood is how people measure the fuel/float height with a site tube at all.  The factory measurements are from the top of the float to the bowl lip.  Obviously the fuel level lies somewhere in the middle of the float, depending on buoyancy.  Not unless you measured how much of the float is submerged, then subtract that number from the factory adjustment could you determine where the fuel level should be relative to the lip of the bowl.  I would think the float site tube would only serve to verify that the bowls were even to each other, not to measure if the fuel level was to factory spec.

People have found that the proper spec brings the fuel level to a reference point. The reference point is the float bowl lip. If you hook up your tube and hold it next to the carb, the fuel level should be at the bottom of the lip and not at the gasket level.

[chewbacca5000]

What I've never understood is how people measure the fuel/float height with a site tube at all.  The factory measurements are from the top of the float to the bowl lip.  Obviously the fuel level lies somewhere in the middle of the float, depending on buoyancy.  Not unless you measured how much of the float is submerged, then subtract that number from the factory adjustment could you determine where the fuel level should be relative to the lip of the bowl.  I would think the float site tube would only serve to verify that the bowls were even to each other, not to measure if the fuel level was to factory spec.

People have found that the proper spec brings the fuel level to a reference point. The reference point is the float bowl lip. If you hook up your tube and hold it next to the carb, the fuel level should be at the bottom of the lip and not at the gasket level.

Much Thanks!

[flybox1]

4mm below bowl gasket seam is the goal, per Hondaman 
your tube will attach to your bowl at either your overflow tube, or your drain screw hole, depending on which type of carbs you have and how they drain.

if you have a separate drain screw,  heat and stretch some of the tubing to taper it, and cut it so you can twist it into the drain screw. 

[cmonSTART]

I thought it was a cool idea at first, but I'm not sure I want to see what's in my carb bowls on a daily basis.  I would probably go absolutely nuts and feel the need to clean them weekly. 

[phil71]

cleanliness is next to godliness?

[brewsky]

Posted by: OneWheelDrive:
"Not unless you measured how much of the float is submerged, then subtract that number from the factory adjustment could you determine where the fuel level should be relative to the lip of the bowl."

You mean like this?

[OneWheelDrive]

Exactamundo! :-)

[chewbacca5000]

Thanks guys for all the great info and those pics from flybox1 and brewsky.  If the fuel level is border line or low in the bowl will this create a hard start when cold? 

I think Dave is right probably would not be able to see the sight glass on the inside two anyway. 

[BobbyR]

Thanks guys for all the great info and those pics from flybox1 and brewsky.  If the fuel level is border line or low in the bowl will this create a hard start when cold? 

I think Dave is right probably would not be able to see the sight glass on the inside two anyway.

It could have an effect on the cold start. It can also result in exhaust popping on deceleration. The closer you can get your carbs to optimal condition, the better your bike will run.

[chewbacca5000]

https://www.scooter-attack.com/shop/#!/en/Carburettors/Accessories/ASCHWIMMERKAMMERSTR8PWK-Coloured-Float-Bowl-STR8-Keihin-PWK.html

EDIT:  link fail - thank flash-based websites.  hold on...

Do they have a bowl that matches the cb750.  I can't tell from the pictures on the site.

[chewbacca5000]

So I have been struggling with carb issues for the past week and I think most of it boils down to fuel level.  My main symptoms are hard starting and leanness in the mid range.  So I got the chopper from an ex marine and this thing runs like a champ and I got to thinking maybe there is a difference in the fuel levels.

Here's what I found.

Carb 1 is 3 mm from the gasket surface
Carb 2 is 5 mm from the gasket surface
Carb 3 is 11 mm from the gasket surface!!
Carb 4 is also 11 mm from the gasket surface.

What I am thinking is since 1 is the 1st in the firing order and it has plenty of fuel that is the reason this bike starts so easily compared to my other two that are bears to start.

Here is a pic using a clear tube and a MM slide rule.  Pic is of Carb 4 and is not the best, but gives you an idea of what I did.

What do you guys think is my assumption correct that a higher fuel level on 1 and 2 is responsible for this bikes easy starting?

[lucky]

I see no need for it.

The float bowl material is very thin and it expands and contracts.
A potential source for another leak.


The older Datsun B210 float bowls had a sight glass.

isnt that what the gasket does? i didnt know the float bowl itself expands and contracts...........

There is heat coming up from the engine cases and there is heat coming off of the cylinder head.

[chewbacca5000]

I see no need for it.

The float bowl material is very thin and it expands and contracts.
A potential source for another leak.


The older Datsun B210 float bowls had a sight glass.

isnt that what the gasket does? i didnt know the float bowl itself expands and contracts...........

There is heat coming up from the engine cases and there is heat coming off of the cylinder head.

Lucky,

What do you think does this old chopper start better due to the high fuel level on Carb 1 and 2?

[lucky]

Hi Lucky,

Can you snap of a pic of the right way to use the gauge and where the measurements are taken from.

I found this, but am not sure if it is correct http://www.salocal.com/sohc/tech/carb/asmpg_mgs/flthgt.htm

Here it is ...The big secret.

END of story.

[lucky]

4mm below bowl gasket seam is the goal, per Hondaman 
your tube will attach to your bowl at either your overflow tube, or your drain screw hole, depending on which type of carbs you have and how they drain.

if you have a separate drain screw,  heat and stretch some of the tubing to taper it, and cut it so you can twist it into the drain screw.

This is incorrect!!!

Almost every Honda motorcycle has the float adjusted in such a way that the float bowl ends up being about 3/4 filled NOT that close to the top edge. Look in your Honda workshop manual.

[chewbacca5000]

4mm below bowl gasket seam is the goal, per Hondaman 
your tube will attach to your bowl at either your overflow tube, or your drain screw hole, depending on which type of carbs you have and how they drain.

if you have a separate drain screw,  heat and stretch some of the tubing to taper it, and cut it so you can twist it into the drain screw.

This is incorrect!!!

Almost every Honda motorcycle has the float adjusted in such a way that the float bowl ends up being about 3/4 filled NOT that close to the top edge. Look in your Honda workshop manual.

26mm the Manuel.  Just wondering what that translates to real terms, and your saying 3/4 full.  I will reset to 26 mm and see what happens.  Where do you think the fuel level will show in the tube?

[Retro Rocket]

4mm below bowl gasket seam is the goal, per Hondaman 
your tube will attach to your bowl at either your overflow tube, or your drain screw hole, depending on which type of carbs you have and how they drain.

if you have a separate drain screw,  heat and stretch some of the tubing to taper it, and cut it so you can twist it into the drain screw.

This is incorrect!!!

Almost every Honda motorcycle has the float adjusted in such a way that the float bowl ends up being about 3/4 filled NOT that close to the top edge. Look in your Honda workshop manual.

Lucky, Hondaman {Mark Paris} is a world renowned expert on these bikes, he has even written awesome books on these bikes, if he says 4mm, then its 4 mm.... Maybe before you start trying to correct the man you should send him a PM and find out why he says 4mm, even you can learn something new....

[chewbacca5000]

4mm below bowl gasket seam is the goal, per Hondaman 
your tube will attach to your bowl at either your overflow tube, or your drain screw hole, depending on which type of carbs you have and how they drain.

if you have a separate drain screw,  heat and stretch some of the tubing to taper it, and cut it so you can twist it into the drain screw.

This is incorrect!!!

Almost every Honda motorcycle has the float adjusted in such a way that the float bowl ends up being about 3/4 filled NOT that close to the top edge. Look in your Honda workshop manual.

Lucky, Hondaman {Mark Paris} is a world renowned expert on these bikes, he has even written awesome books on these bikes, if he says 4mm, then its 4 mm.... Maybe before you start trying to correct the man you should send him a PM and find out why he says 4mm, even you can learn something new....

Guys I am not trying to start an argument just get some understanding.

Will higher fuel level make for easier starting, my main problem and what I wanted a sight glass.

[Retro Rocket]

No argument Chewy, just stating what i know to be the better source of information.... PM Honda man and see what he has to say, or buy his book, it is the bible on these bikes....

[chewbacca5000]

No argument Chewy, just stating what i know to be the better source of information.... PM Honda man and see what he has to say, or buy his book, it is the bible on these bikes....

Got the book and sleep with it under my pillow.  I want to experiment with this a little see how it goes and will PM Mark if I get stuck.  Thanks Guys for all the help!

[Spanner 1]

Float level is not for experimentation IMO... it needs to be @ spec.  Starting issues are something other than float level IF you can verify levels are correct ( clear tube method is very good ) and fuel delivery across all 4 carbs is 'fast' and able to maintain correct float level under actual riding conditions ( long periods of high fuel demand, running fast, in other words  ). This is not easy to verify, obviously, while riding so a good fuel flow test thru' the carbs with all 4 drain screws open is a start. Fuel must flow evenly from each drain hose and stay flowing over time !.... drain the whole tank into a 5 gal gas container and do NOT open the gas cap at any time during the drain to try to force a tank vacuum ( test for free tank vent and clear petcock which can have blocked passages ). A long drain time here is the key, a quick ' Well the flow to the carbs looks fine' doesn't prove the flow rate. There are venting passages in the upper carb body too which are connected by that black rubber hose between carb 1 and 2 and 3 and 4 to 'equalise' the air pressure above the fuel in each carb and allow for fast fuel intake..... any blockage in that system = low/slow fuel intake even if your carb drain test is good and fast  .... a lot of s**t to consider/ check isn't there ! ....

[chewbacca5000]

Float level is not for experimentation IMO... it needs to be @ spec.  Starting issues are something other than float level IF you can verify levels are correct ( clear tube method is very good ) and fuel delivery across all 4 carbs is 'fast' and able to maintain correct float level under actual riding conditions ( long periods of high fuel demand, running fast, in other words  ). This is not easy to verify, obviously, while riding so a good fuel flow test thru' the carbs with all 4 drain screws open is a start. Fuel must flow evenly from each drain hose and stay flowing over time !.... drain the whole tank into a 5 gal gas container and do NOT open the gas cap at any time during the drain to try to force a tank vacuum ( test for free tank vent ). A long drain time here is the key, a quick ' Well the flow to the carbs looks fine' doesn't prove the flow rate. There are venting passages in the upper carb body too which are connected by that black rubber hose between carb 1 and 2 and 3 and 4 to 'equalise' the air pressure above the fuel in each carb and allow for fast fuel intake..... any blockage in that system = low/slow fuel intake even if your carb drain test is good and fast  .... a lot of s**t to consider/ check isn't there ! ....

Thanks Spanner!  You gave me alot to think about.  I will make sure I check flow when setting the float back to factory spec and report back tomorrow.  Thanks Again!

[Really?]

Check both floaties too. Not just one.