Topic: cb550 Jetting

[bonkey88]

 I understand how the taper of the needle impacts fuel delivery. 

Regarding float levels, I am very fixated because while I understand the ease of fuel delivery created by raising the fuel level in the bowl (and its ability to richen across the entire jet needle range), it seems arbitrary to just raise fuel levels to their highest point and tune from there. 

I guess what I don't understand is if raising the fuel makes tuning easier (at least in my case), why wouldn't Honda have raised the fuel levels in the bowl to as high as possible and then tuned with smaller jets straight out of the factory?  Another way of putting it is, how does Honda determine the appropriate float height?  What is the science behind float height?  I know fuel level can be too low and cause the engine to starve, but could raising the fuel level too high create a problem (maybe atomization of the fuel is more difficult no matter the jet size because more leaks past the needle?)?  If so, I would want to avoid this.

There is an optimal float level for my setup (I assume).  I have absolutely no way of determining what it is.  So why potentially set them high and tune out the flat/troublesome spots as opposed to tuning out the flat/troublesome spots with the float levels where they are now (assuming they deliver enough fuel to support WOT)? 

Not trying to be difficult, but I guess I just need the science behind the float height to put an end to my tuning questions.  The answers are probably somewhere in this forum. 

Either way, after the discussion below (thanks to everyone for the knowledge), I believe I know enough about carb basics to tune the bike. 



 

[flatlander]

don't go too crazy with the fuel level. set it to 2-3mm below gasket surface with the clear tube method. the main thing is that it's within that range, and even across all 4 carbs. then determine your main jet, needle setting and slow jet and that order.

i don't know why honda picked a certain fuel level but these were mass produced bikes so they most likely picked something that on average would render decent performance, and have enough tolerance either side of the setting to compensate for manufacturing and assembly tolerances. mind you, clear tube method shows you what's actually going on inside the carb bowl and will give you a more reliable result than the float height measurement that they used.

[TwoTired]

The transition between idle and 1/4 throttle, or from the pilot circuit to the throttle valve, is mixture controlled by the throttle slide cutaway shape.  The bottom of the slide is forming a venturi and the shape determines the air velocity and therefore the lift of fuel out of the bowl. This trades off slide bottom shape with distance of lift. Higher cutaway increases air velocity and fuel lift force from both pilot and throttle valve orifices.
I believe this is why Honda changed the fuel height between 77 and 78' as raising the fuel height to improve transition and reduce acceleration stumble was considerably easier than having new slides made with a different cuttaway shape.

Your pods have lowered the fuel lift force, if you don't want to raise the fuel level, you can make the compensations for every fuel delivery orifice, making them bigger, and then experiment with the slide cutaway shapes to aid throttle transition.

Best of luck!

[bonkey88]

Ok, got it.  Thanks to all that posted. 

[Deltarider]

mind you, clear tube method shows you what's actually going on inside the carb bowl and will give you a more reliable result than the float height measurement that they used.

Doing the clear tube method whilst idling is probably even better, as it would show an intermittent sticking floatneedle and no, you don't need a fan blowing for this test.

[Gene]

for "WOT" plug chops - IIRC there is a stretch of Canal st. from Roosevelt south to 18th with no lights and no stop signs. Kind of a little drag strip - IIRC.

[bonkey88]

Gene - Thanks!  Found a little drag strip over by humbolt park as well! 

[bonkey88]

Clear tubed the carbs this weekend.  The fuel level in all carbs is over the connection point by about 3.5MM except #4 which is right on.  So the bike is getting enough fuel. 

Is it ok for floats to vary up to 3.5MM from carb to carb? 

[flatlander]

no, you will get unequal A/F mix between the carbs. also, these carbs are quite susceptible to changes in fuel level. a mm or two goes a long way.

get them all the same (actually, you should have done that while at it!).
standard, they should be 2-3mm below gasket surface (upper edge of the bowl). as TT said, for you it can be a bit higher but i would start at gasket surface or 1mm below. 3.5mm above seems pretty drastic. don't know now how high exactly the overflow tubes sit but you must be close.

[bonkey88]

Ok.  To clarify, all the float heights are set the same (were set the same way and to the same height).  However, the fuel level varies up to 3.5MM from #1 float bowl to #4 float bowl.  Can I just monkey with the float height in each carb until the clear tube shows the correct fuel level, even if the floats will be set at significantly different heights?

[flatlander]

in principle, yes. you can carefully bend the tang that the float needle rests against, up or down a bit. especially if aftermarket parts are used, the height prescribed by honda can result in a different level than intended, so you need to tweak the actual fuel level to get things right.

although, it would be good to understand the reason why they are so different. you say no.4 is the odd one out. is there something different there? an aftermarket float, or needle being used? i don't remember: did you set the heights yourself, or could that one be misadjusted? are the needle valves still good?

[TwoTired]

Ok.  To clarify, all the float heights are set the same (were set the same way and to the same height).  However, the fuel level varies up to 3.5MM from #1 float bowl to #4 float bowl.  Can I just monkey with the float height in each carb until the clear tube shows the correct fuel level, even if the floats will be set at significantly different heights?

Yes, you can do that.  But, I'd rather figure out what causes the difference and correct that first.  Suspect someone has replaced a float, float valve, float seat, not seated the valve seat the same as the others, wrong orings, etc.

Most important to achieve the same fuel bowl level.  But, I strive to make all the carbs behave identically, rather than have one custom out of the four.

Just to be clear, you are checking the levels while on the center stand, right?  Or, alternately, with the carbs level on the bench?

Cheers,


P.S. I see flatlander has similar ideas.  Faster on the post button than me.   

[bonkey88]

When I rebuilt the carbs, I only used genuine Honda seals.  As far as float height, I set them myself.  I did not replace the floats, float valve, or float seat .  They all looked the same so I assumed they were genuine Honda.  I have a spare set of the same carbs to rob genuine parts if need be.  So, next time I open up the carbs, I will double check all the parts carefully to make sure they are genuine.

I was checking the fuel levels with the bike on a lift but the carb bank was level (similar position to center stand). 

What is the best way to check if the float valve and seats are in good order (float valves are the rubber tipped kind)?  I understand they can leak and distort fuel levels.  I made sure all the float valve pins pushed in and out.  I guess I could take the bowls off and hold the floats up to check for leaks but that seems like it might be a little messy.  Is there better way?