Linkert M51 set up for Stroked Scout

Hi guys
Its been a long while since I've needed advice, most problems with the bike have been pretty straight forward to sort out and for the most part I've been thoroughly enjoying riding the pants off of her.
Dirtyss has a 741b bottom end with harley crank and flathead power 45cu in jugs and pistons giving 830cc. I've fitted peek rings to the manifold and cured the leaks that were from the nipples and joints.
The Linkert m51 is fitted with a brass float - 1/4" level gave way to rich mixture with constant plug fouling, changing it to 3/8" gave smooth running from idle right through 3/4 of the rev range, although she's a little bit rich at the lower revs, the motor is leaning out in the top rpm range (plugs coming out clean)
The power needle is 1 3/4 turns out, a couple of clicks short of too much fuel, the scout 640 main nozzle has five holes - top three are 41 thou the bottom two being 76 thou, the correct externally vented 1 1/16" venturi is used.
I cannot see that raising the fuel level or adding a main jet is going to help other than bring back the unreliability issues with plugs etc.
Would changing the main nozzle to one with the larger top two or three holes and smaller lower holes alter the mixture curve to suit my bike better. ie the scout 641 nozzle ? or am I way off track here ?
Best regards to you all
Paul

Paul,
first off get rid of that crap brass float and get one that actually floats, from moldthread or Cotten! By changing the bowl level so radically you are also changing the characteristics of the fuel flow. Until you correct the float it will be near impossible to get the rest right.
JMHO
Robbie

Paul!

I have no conversion chart handy for #drills to thousandths,
but your descriptions of 640 and 641 main nozzles are obviously different than what I have logged.

The "Standard" nozzle, which may have only become standardized after '42, had a # 36 bore, with a #44 bottom hole, #46 above it, then three #57 holes on top.
(The Bonne nozzle was a #28 bore, with all holes a #54.)

I'd cut to the chase with a Bonne nozzle, and bore the venturi another 1/16, unless you are more of a payload than most.

....Cotten

OK, not to try to start anything here, but I think there are some misconceptions about brass floats out there. For many years there was little in the way of options for float replacement for Linkerts except for brass. One company that manufactured them was Kokesh Cycle, of which I have personal experience. I know they made many hundreds, if not thousands of these floats over the years. Generally speaking I believe that about the only issues there were with those floats were when the installer ignored the included instructions to reset the float level to between 5/16 and 3/8". The reason for this was because the heavier brass float, when set to the lower setting, would supply the SAME fuel level as the stock float did at 1/4".

Now, if your float provides a fuel level that is the same as stock, about the only problem it can have is that it does not continue to float. From what I understand, that is the problem that we see with the Chinese made brass floats on the market today. Apparently they crack pretty quickly. I am pretty sure that was not the case with the old American made brass floats.

All that being said, I heartily agree that the new Rubber Ducky floats (and others of similar material) are the way to go if you are in the market for a new float.

Lee!

Attached is a photo comparison with a Kokesh float and a float of normal buoyancy.

The extinct Kokesh brass float was the heaviest of all floats ever produced, averageing four times more than any original Linkert (or Schebler DLX) float.

(Competitor's brass floats that often suffered leakage weighed only three times as much. They were NOT made in China, but India, as if that makes the slightest difference, as if from Detroit or Indianapolis.)

A float is supposed to float. Duh.

Any compromise sacrifices performance and fuel mileage:
The purpose of a bowl is to supply a reserve for accelleration among other things.
The limits were found quite early, and that is why flat-trackers often enlarged their bowls, as illustrated at http://beautyofspeed.com/gallery/linkert/linkert.htm.

Reserve is WHY you have a bowl.
Brass floats were greatly enlarged in an attempt to gain buoyancy at the expense or buoyancy.
Setting your float to 3/8" (or more accurately for a 12g Kokesh .410") robs you of just that much more reserve.

Fuel mileage is robbed by the sluggish inertia of a heavy float.
The amplitude of the wave of shutting off and opening up is greatly magnified, leaving the motor operating under longer periods that are over-rich, or over-lean.
Your motor craves a steady-state, with a very low amplitude of change: a sensitive float.

Langsenkamp and Linkert never produced a brass float for a Model M carb, because it would be futile.

On to the Rubber Ducky and other "similar" materials, ... THEY ARE NOT ALL SIMILAR!
Beware of off-patent obsolete "Nitrophyll" floats that are imposters,
and beware that the current lull in USA fuel digestivity may be temporary.

And if-and-when your "similar" float swells, please consider whether your source can replace it with anything better.

....Cotten

OK, not to try to start anything here

OK - guess I failed there.

The Kokesh floats were heavier than the import brass floats ...Hmmm - wonder if that could be why they didn't fail quickly (if that is indeed the case)?

Brass floats were greatly enlarged in an attempt to gain buoyancy at the expense or buoyancy.
Setting your float to 3/8" (or more accurately for a 12g Kokesh .410") robs you of just that much more reserve.

If you are saying that setting the float to 3/8" robs you of reserve, I would respectfully disagree. The 3/8" figure is the float level, not the fuel level. The reason it is lower than a stock float is to compensate for the weight and provide a stock fuel level. As to the brass floats floats being physically larger, I will take you word for it (though I had not noticed it) and granted, that would slightly reduce the amount of fuel reserve in the bowl. Would it be enough to make an appreciable difference for non-racing applications? Again, not that I have noticed. (Your results may vary)

The amplitude of the wave of shutting off and opening up is greatly magnified, leaving the motor operating under longer periods that are over-rich, or over-lean.
Your motor craves a steady-state, with a very low amplitude of change: a sensitive float.

We are still talking about early V-Twins here aren't we? I personally have a tough time imagining that a wave of enough amplitude from the float opening and closing would be noticeable in a float bowl subject to the vibrations of a V-Twin in a rigid frame.

As to using the term "similar material, that was simply my attempt at diplomacy. Since I am familiar with the Rubber Ducky floats, but not with yours, I did not want to make it look like I was plugging the one and snubbing yours. Guess I failed there too.

On a lighter note though:


Stupid brass floats - with 'em I can barely make enough power to spin the tire in 4th gear with a 5" slick! Oh, wait ...maybe I can.

Lee,

Think about it: The bigger the float, the less room in the bowl.
The deeper it must be set to get the same fluid level, the more volume of fuel is occupied by the float.

Reserve is WHY you have a bowl, and some folks found the need for even more, as shown here:
http://beautyofspeed.com/gallery/linkert/linkert.htm.
Here's my favorite:

fatbwl1.jpg

The cycle of the float not only governs replenishment of fuel to the reserve, it affects fuel mileage if it spends too much time away from the optimum centerline.

One well-logged and documented Chief went from an average of 29.4 mpg with a brass to an average of 37.7 mpg by changing the float only.

Delhi brass floats were brittle because of their "English" brass composition, but their fine solderwork made them as much as three grams lighter than the the heavily soldered Kokesh offering.
Kokesh's averaged 12 grams.
Delhi floats averaged 9 grams.
Original floats, both cork and "Armstrong", were three grams.

Think about it. Kokesh floats were four times the weight of what the carb was designed to use.

Rubber Ducky floats are equisitely molded and of very accurate buoyancy.
Some "nitrophyll" floats on the market are of the same size and weight as Delhi brass!
But both are of obsolete compounds that have suffered swelling from digestive modern fuels.

There is only one new material that has proven swell-resistant, and it is the one that actually IS used by all major automotive manufacturers today,including the Host of the forum. Such material as floats for vintage American motorcycles is only available in machined form.

But if burning rubber in the back of a truck is your measure of performance, you certainly don't care.

....Cotten

I apologize Cotten ...I guess I just didn't quite understand your sense of humor.

But if burning rubber in the back of a truck is your measure of performance, you certainly don't care.

Good one!

Now I think I am beginning to understand some of the other jokes you've made that I completely missed before. Thanks!