Carb float leveling conundrum

I'm going to be leveling the floats over the weekend, thought I would have it done today but need to rethink how to do it where I have to work (not my place & no workbench available).

I'll figure that problem out but here's something I'm curious about; since the bike is constantly not only moving, going up hills, down hills, but is being jarred constantly and the gas in the float tube is jostled like water in the agitation cycle in a washing machine, how is it that it matters exactly how precise the float level is and that all clear tube measurements reflect that precise matching height +- 1mm?

As so many people with excellent experience are saying it's necessary, I'll do it of course but I'd like to have a better understanding why something has to be leveled & measured so exactly on a jig and then once in operation, the carbs will never be level again except by random momentary chance when the bike is actually being used.

At least the bike is now running on all 4 cylinders!

Niice!

 

The pilot, and especially the main fuel jet, are more or less centered within the float bowl confines, and thus any tilting (forward/back or side-to-side or any combo thereof) tends to also "rotate" or allow the fluid to pivot around those "centered" fuel jets, thus maintaining the fuel level at the proper height.............plus, all those jostles and bumps were accounted for by Hitachi when designing and specifying what the proper fuel level is to be.

Needless to say, at certain times the fuel levels will not be at their "static" setting while in motion, and the air-fuel mixture will vary off of "ideal", but carbs are not really "ideal" devices.........

 

I have also wondered the same question. Seems like the carb float levels should be set while on the bike as the carbs are tilted at an angle when mounted to the engine. I have played a lot with float levels in many different types of engines and unless they are way out of adjustment, I find that it is not a very critical adjustment on several machines. Just my experience.
Now, don't shoot me down you guys. I am old and feeble.

 

well i guess if it's a little low and your going up a steep hill you won't be able to keep it open for long enough, and if your on a down grade maybe you'll run rich, or vice versa whatever. But, KA1J did you know that the needle in the bowl stops the fuel from entering when either the floats float high enough to push it closed and when the fuel level is low = the needle closes the circuit also from the jostle. so in respect to the two above comments i guess Yamaha found a happy medium. I guess w/ dirt bikes it doesn't seem as important since you can't be on the throttle for as long as a street machine.

 

Consider a lowered rear end. It seems like that would effect it also.

Just a random thought

 

Thanks for the replies! I'm definitely going to have them properly aligned by tonight, I do believe in following learned advice when I'm a novice and I am that...

I also like to understand things that don't make sense that I have to do. Nothing bugs me much more than telling someone in the future who is doing the same procedure, that they have to do "something" and then when they ask me "why", I can only say "because I was told to". I really like being able to explain truisms and why they are real. Helps everything fall into place...

 

If the fuel level is too low, it takes a lot more air rushing by the venturis to pull the fuel out of the bowls, and you end up with a mixture that is too lean across the board, and you will lose top-end power. (pilot mixtures can be compensated with the mixture screws).

If the fuel level is too high, it takes very little air to draw the fuel up, and you end up with a much richer mixture, so your bike will run great when it's cold, but as it heats up, it will start to show rich symptoms, like stumbling on take-off.

I set my 750's carbs to a baseline "factory" level, but ended up having to drop it about 1.5 mm all the way across to get the most power without running too lean. There does not seem to be a magical number that is right for everyone.

This may not be as noticeable on a 1 or 2 carb setup, but on a rack of 4, it makes a huge difference in how the bike runs... Well, at least it did on mine. YMMV.

 

What is difficult for me to grasp is the jostling that goes on with liquid when on the road's bumpy surface vs precision adjustment of a fluids container.

If you attached a float bowl on the top of the carbs, protected from wind blowing into it and filled it to the proper level and left it uncovered, my "guess" is while the bike is stationary you might not loose much but as soon as you get on the road, the bumping would fling out the liquid and very little would remain.

I do get the idea of a proper float height being important in a stationary engine, it's the road conditions that seem like it would negate such precision in real time.

I will have them balanced by tonight though...

 

I'm amazed they work on a stunt bike while someone is doing a twelve o-clock, or on a dirt bike, while flying thru the air.

 

The only time that the fuel will drastically move around the bowl is during acceleration, deceleration or when you go over some big bumps. The rest of the time it's pretty calm.
Take a look at the soda you just picked up at a drive-through... Even as you go down a bumpy road, the soda, for the most part, stays in the same place, aside from a few ripples. As Chacal pointed out, the sloshing from acceleration or deceleration doesn't change the level a whole lot in the middle, so the jets aren't affected much by that, unless you're WOTing up a steep hill.

Now, if you have to negotiate miles of washboard and stop signs every 30 feet, that's a different story... :mrgreen:

 

The answer you are looking for is this: There has to be a "reference point" for a baseline adjustment that takes into consideration all of the effects you're talking about.

That "base point" is the Yamaha-specified float height with the carbs level.

Yes, in practical use, the fuel level changes all over the place, but there has to be a "starting point."

 

As I have said in a previous thread, and as jamcam has noted in this thread, the float level is not that critical. It is there in order to ensure that fuel is available for immediate delivery as needed. As long as the float levels are in the ball park the carbs will function just fine as chacal and bigfitz have noted. The minute differences related to pressure differentials pertaining to fluid levels in the float bowls will not change the performance of the carbs in any distinguishable way. The change in fluid level necessary to cause a discernible difference in the function of the carbs would have to be measured in several inches - a half a millimeter or even a millimeter is just not going to do that.

If you are racing (at the professional level) then you are going to tweak for every last smidgen (is that a word) of performance. But that is all the difference you are going to get out of fluid levels in the float bowls - a smidgen.

Think about the effect air pressure has on our carbs. You can go up and down 100 or 200 hundred foot hills all day long without any noticeable difference in performance. But if you take a trip like our friend misni (?) did last year where he road from Cleveland OH to California, he noticed some substantial performance issues as he went over the Rockies. But there you are talking about changes of 1000's of feet in air pressure.

The negative air pressure that our carbs will experience in the float bowls due to the slight differences in fluid levels is just not going to do that.

This is my humble opinion and not necessarily shared by anyone else on this board.

 

Re: Carb float leveling conundrum - Resolved

As per a post from several years ago, I set the float bowls to 14mm. This measurement was with the carbs upside down and measuring from the deepest part of the body where the gasket rests (there are ridges that extend up maybe .5mm but I used the flat area below the ridges). I used a depth gauge set to 14mm and measured from the top of the floats (in normal operational position, the bottom of the float) I did this with all 4 floats. FWIW, I was using chacal's float bowl rebuild kits...


The service manual says the clear tube should be 3mm below the junction of the carb body & the bowl. Every one of my floats were set exactly right., maybe .5mm away from perfect.

To get this proper height I make a jig, attached the carbs via aluminum angle, attached to a 1x4 wooden hinge and elevated it via a hydraulic jack. I used a spirit level to assure the tops of the carbs were level and then used a second jack (the scisor one from my subaru to keep the front of the carbs steady). When forward and sideways were balanced 100% I checked the clear tube level and the meniscus was 99% exactly 3mm below the junction of bowl to body.

So it looks like as far as the Hitachi carbs in the XJ650 go, 14mm as described above is a precise setting if you're using chacals rebuild kit.

I did not have to remove one float bowl and adjust the tab, all 4 were perfect after this initial dry setting.

Now to balance the carbs & then colortune....