Float adjustment

[jagarra]

I am a bit confused. Was reading about how another rider solved his stumbling problem by raising the float level. He indicated that he set his float level so that the bowl filled with 24mm of fluid. He got that information from Snobum's site. Well I tried the same, my level was 17.4 mm when I started and I was approaching 21mm when the float started getting really close to bottoming on the base of the body. Reread Snobum found the 24mm article and another article where one adjusts the float so that the top is parallel to the base, about where I started from. What's correct???

[Rob]

I always went with the parallel method.

Have you weighed your floats?

I could not find the post with this photo, but you want them to be in the neighborhood of 12.5 grams.

[jagarra]

These floats are a repetitively new set, year or two old. When I checked the floats levels by the amount of fluid in them, they were different from each other. Left was running 17.2mm or so and the right was running 18.2mm. Raised the level for each as 19.3 and 19.5. The only time the bike stumbles is when I ride briskly up a hill, like going up a winding mountain road and pushing it.

[Kurt in S.A.]

When riding briskly, you're asking for more fuel. If the fuel level is too low, it can be used up faster than it comes into the bowl plus it's harder from a venturi standpoint to pull the fuel up the jet stack if the fuel is lower in the bowl. Thus, the bike runs out of fuel when demanded.

Kurt in S.A.

[barryh]

I've always thought 24mm was a little too much and have reasons for preferring. 22mm.

It's worth bearing in mind that measuring depth in a removed float bowl is not an official BMW method. All that happened was some dude set his floats parallel and removed the bowl to come up with a fuel depth figure. It has no more status than that. One of the problems is that two floats of different weight will produce different fuel levels even if both set parallel. To that extent it's a flawed method and at best only valid for floats of a particular weight and buoyancy. But what weight ?

Do you think new floats today are the same weight and buoyancy as the originals nearly 40 years ago. I doubt it and that might explain why I had a similar experience to you. when I put new floats in several years ago and set them dead parallel the engine was clearly running weak and I found barely 19mm in the bowls. I reset them higher than parallel to increase the fuel level to 22mm and the engine ran as it should. Measured on high grade lab scales the floats were 12.495 g

There is only one fuel level that really counts and that's the operational fuel level with the bowls on the carbs. That level is much higher than what you find in a removed bowl due to displaced fuel by the carb castings and the floats. Given the forward inclination of the carbs the operational level is close to lapping over the front edge of the bowl. Now if we had a benchmark dimension for that it would be something worth having and we would be able to make sense of the effects of different float weights.

[Kurt in S.A.]

I look at it a different way. If you use the parallel method, that assumes that the floats are not compromised. If they are, then you'll get the wrong amount of fuel. I think there is a general consensus that a certain height of standing fuel in the bowl is necessary. Too little, and the carb "suction" process can't raise the fuel to the throat of the carb; too high, and there's too much fuel in the throat. So somewhere in there is a happy medium. Snowbum has written that for 32mm carbs, that height is 24mm; for 40mm carbs, it's 28mm. People can quibble about the numbers...seems like the numbers we're tossing around is in that ballpark.

I don't know where I've seen it, but I think someone tried an experiment by putting a plastic window on the side of a float bowl to get a better sense of what the "operational" level should be with the bowl installed. Granted, once you drop the bowl, things change...as I understand it, that's where the 24/28 numbers come from...they try to account for the different between the bowl on and off.

Seems to me that if you think you're floats are good and you do the simple static test by dropping the bowl, then take that fuel height measurement as your "standard"...always shoot for that number. If your float is parallel to the carb base, all the better. Then in the future, if that height begins to get higher and higher, must be an issue with the floats...or the valve seat. If your bike is running fine with that given height of fuel, what's to change? Good performance, acceleration, gas mileage...I'd use that height value for future measurements.

Kurt in S.A.

[andyleonard]

Does anyone else think the alcohol in the fuel affects the float level?

[barryh]

If you use the parallel method, that assumes that the floats are not compromised. If they are, then you'll get the wrong amount of fuel.

And for compromised (heavy) floats even if you correct the fuel level in a removed bowl it will still be wrong due to Archimedes principle.

For example:

New floats 12.5g set to give 24mm fuel in the bowl

Older floats 14.5g set to give 24mm fuel in the bowl

Those two examples will produce different operational fuel levels as the heavier floats will displace more fuel and produce a higher level. If the floats are heavy you need to aim for a lower level in the removed bowl.

[Kurt in S.A.]

Barry -

All the more reason to start with good floats, measure the amount of fuel, and use that as a check point in the future. If the level is changing, then something's wrong. For our bikes, it's all about observation over time...when things don't add up or have changed, time to dig in.

Kurt in S.A.

[minuteman]

No question ethonol eats away at every thing in our airheads. The tank lining erodes ,the fuel lines rot. The rubber seals swell. Put in line fuel filters, use rec fuel only (none ethonol). I've gone 3 years with no bing issues. Now have ,but a 31 year old bike I'll give her a pass.