Float adjustment

[Kurt in S.A.]

I understood what you were saying. Most of the time, we're riding down the road straight-and-level. In turns, we've leaned the bike over and the forces keep the fuel more or less level. But if the fuel level is too low, it's harder for the carb to draw the fuel into the throat of the carb. Clearly you can be too low or too high on the fuel level. I'm not saying that the fuel level must be at a specific point, but we've all taken steps to adjust the floats to meet some kind of spec. If it didn't have some importance, why are we even doing it?? It's best to be near that spec, not drastically far away from it.

Kurt

[gspd]

....the forces keep the fuel more or less level...

hmmmm....?????

OK. We need a close-up video of a clear float bowl in actual action to credibly and constructively continue this discussion.
My prediction: There will be no discernible level per se., just a bunch of bubbly liquid shaking around allover.

[Kurt in S.A.]

Let me get right on that! I wasn't trying to say that it was perfectly level, but given that the forces balance out when in turns, and that the bike has a decent suspension, it's not going to be all that "bubbly" in my estimation. Sure, maybe some ripples here and there. But you want the mean height of the fuel to be within a good range.

Kurt

[barryh]

My bike runs rich thought the range. I weighed the floats and they both seem to be around 12.5 (ISH) gms on cheapy electronic kitchen scales. I put a clear float bowl on with the carbs fitted and the levels are a couple of mm below the gasket face on the forward (lower) edge of the float bowl.

Does anybody have any comparison to that? As they are both the same I am loath to start bending float tags yet.

Your level sounds near enough right to me - see my earlier post with the sight tube picture showing the fuel level just below the forward edge of the float bowl. Only variable would be the degree to which the carbs are inclined from horizontal and I think R45/R65 carbs might be more inclined that type 247 airhead carbs. Better to make the comparison at the centre line of the bowl which is what mostly matters as that is where the main/needle jet is drawing fuel from. That's why I had the sight tube positioned pretty much centrally.

[barryh]

... the exact level isn't as critical as many people would like to think.

+1.
Just had to chime in here. The fuel(float) level has to be within the range between too high (overflowing when parked) and too low (starving for fuel at full throttle in top gear). Picture what's actually happening in the float bowl when you lean right or left in a curve, accelerate, brake, go uphill or downhill, or ride on a washboard or pot-holed road. Then factor in engine vibration. In this specific application, Bernoulli's principle has no effect on the mixture. The well intentioned albeit misguided people on this thread that believe floats must be set with surgical precision and directly affect the mixture are basically chasing unicorns. Good luck to them. Literally millions of carb bikes are running perfectly in stock form with their float levels set within the acceptable 'ballpark' without modified floats, add-on level viewing tubes and clear float bowls. K.I.S.S

I look at it slightly differently.

First I would agree that the exact level doesn't seem to make much apparent difference in the way the engine normally runs which we've all demonstrated by turning of the fuel and riding on for a mile or two, The level will be quite low before the engine stumbles. Although there is more sensitivity to fuel level at idle and particularly at full throttle.

I also agree that the exact level is affected by the angle of lean and vibration etc and that this is also not apparent in the way the engine runs.

However I'm persuaded that the exact level is still important for two reasons:

Everything about tuning an airhead for smooth running is to do with trying to get both cylinders the same, so in my mind you have to start with both fuel levels as near the same as possible and it needs to be measured reasonably accurately to achieve that.

Secondly while I'm accepting small changes in fuel level produce no easily apparent difference in the way an engine runs, there is a post somewhere (which I will try to find) where a guy puts sensors in an airhead exhaust pipes and found the the mixture ratio was in fact very sensitive to fuel level in the float bowls. Perhaps another way to put it would be that an airhead isn't overly sensitive to an exact fuel ratio.

[jackonz]

You cannot look at float level alone you have to look at the entire carburettor as a whole, if you look at it there will be an upper and lower limit for the float height and in no way can you set the float level via how high the level is in the float bowl.
Out of tune carburettor settings can come from worn needles and jets as well as leaks as in from the seal on the butterfly spindles, then there is perforated diaphragm.
Unless you get this all within recommended specifications you are simply chasing your tail.

My advice is start from scratch and set it up as per specification and use a full carb rebuild kit.

I do have in my possession a Bing service manual and am only too happy to look up the relevant specs for anyone who needs it.

[barryh]

You cannot look at float level alone you have to look at the entire carburettor as a whole, if you look at it there will be an upper and lower limit for the float height and in no way can you set the float level via how high the level is in the float bowl.

As far as I know there is no specification for an upper and lower limit, the only thing that exists as a specification is "set new float parallel". Any other method, for example fuel level in a removed bowl has been derived by users and has no official status.

It's reasonable up to point to have derived a level specification in a removed bowl providing it was done after having set new floats parallel and it could be done accurately. In principle that's all I've done. I set new floats parallel and from that produced a benchmark for the actual operational fuel level using a sight tube, a clear plastic float bowls achieves the same result.

I would argue that a benchmark of the operational fuel level is best because it will highlight any future changes in float weight. Also with the best will in the world, the removed bowl method is prone to error because of spillage and the additional fuel remaining in the feed lines. I went to the trouble of calculating what the additional volume in the feed lines would be if it all drained into the bowls and it came to 9.6cc per carb. I then measured the volume of 22mm of fuel in a removed bowl which was 46cc. So that's a scope for 20% error. I suggest that's hopelessly inaccurate and because of the arrangement of the fuel lines would not be consistent between the two carbs. I concluded that the removed bowl method was only viable if the fuel lines were removed before dropping the bowls.

Of course setting an accurate level is only the beginning but there is no point in doing anything else in the way of adjusting a carb until the fuel level is first known to be correct.

[Kurt in S.A.]

I think Barry has stated an important aspect of adjusting the floats. It's a starting point for future assessment. But I'll also add that it seems that with the float body parallel to the body of the carb, that puts the most pressure on the needle and seat to ensure it stays sealed. With the float parallel, the needle will be 90 degrees to the float body and this would be the best position for sealing off the gas flow. It may be that with the float adjusted this way, and with a "good" float installed, the height of the fuel might be the numbers that have been published. I don't know...I wasn't around when the experiment was run. It's safe to say that if the float was going bad and actually sinks a bit, it surely seems that when the float is adjusted parallel and it floats up to seal the inflow, there's going to be more gas in the bowl than needed. So maybe the height measurement could be an indicator that something is wrong.

Additionally, I think my thoughts on "lifting" the fuel are really not a factor here. I was thinking about mid-throttle operation, where we spend most of our time. In that situation, the fuel is drawn in through the jet stack, the main jet at the bottom, the needle jet, the emulsifer, and past the tapered needle. As long as the fuel level is always above that lowest point of the jet stack, then the engine will run just fine. So setting the precise position of the float might not be so critical as long as the carb is never starved for fuel.

Still, setting the float to a known location is a good idea. One never wants the demand for fuel to exceed the supply and if the float is set too low, this could happen. Or if there are vibrations causing rippling of the fuel level, the proper float height would prevent that situation from affecting fuel availability. On the other extreme, setting the float too high will result in excess fuel which eventually will find its way to the outside through the vents.

So setting the float height with the owners manual parallel method likely puts the float system in the middle of its operating range.

Kurt