Finalizing the Carbs

As the Knuckledragger nears completion, here is a little info on the Linkert carbs it is fitted with:

Alcohol in a Linkert carb? Yes, it has been done before. As it happens, world record setting motorcycle drag racer from the mid '50s, Lloyd Krant, ran Linkerts on his Knuckle drag bike. And thanks to a blog post in which I mentioned Lloyd, his son Guy contacted me, revealing priceless technical details of his father's bike "The Giant Killer." Some of this info includes such tidbits as the fuel mixture Lloyd preferred (30% nitro/70% alcohol) right down to the jets sizes he used.

Keeping in mind that you need to be able to flow roughly twice as much volume for Alcohol as you would running gasoline, a few modifications were in order. When I got the "Whizzer" gas tank, I had a pair of bungs with 3/8" pipe thread welded in, and installed the largest brass petcocks I could find. (BTW - these were donated by Kevin at Bass Metal Craft - thanks again Kevin)


Next up were the needle and seat. I started with a couple of generic aftermarket seats, and machined the round holes that allow fuel from the needle to the bowl into rectangular ports. The picture below shows the stock one on top and the modified one below.

From there I chucked them up in a lathe and turned down the inlet end to take a 3/8" hose instead of the stock screw on fitting. I also chose to lap a pair of old stock steel needles to the new seats rather than take the chance that the aftermarket "Viton" tipped needles would deteriorate from the alcohol.

The floats were up next. I happened to have a pair of used brass floats left over from when Kokesh Motorcycle was manufacturing them. These provide the added advantage that I know they don't leak. If you detect a hint of my distrust of aftermarket parts, let me assure you that it is well founded.

Since alcohol also has a different specific gravity than gasoline, the float level needed to be addressed. The float level for a stock (cork) float is 1/4" for gasoline. Due to its heavier weight, a brass float needs to be set at 5/16" to 3/8" for gas. I chose to do a mock up on the float bowl assembly to be sure that 1) I got the fuel level right, and 2) the needle and seat actually seal. As you can see from the picture below, no expense was spared on the test fixtures. (that was sarcasm, in case you hadn't noticed)

As it turns out, the specific gravity difference and the weight difference of the float very nearly cancel each other out. I wound up with a float level of 9/32" (compared to the stock 1/4"). Increased fuel flow in the carbs are via addition of a fixed jet (like an M74) along with the stock adjustable mains. Below are the finished carbs mounted on the bike.

If these carbs look exotic, it is mostly due to the mods performed on the bodies. These started as run of the mill M35 Linkerts, from which the choke assembly was removed. In its place a steel ring which acts as an air inlet guide was installed. This necessitated the use of Flathead style side fill float bowls. Each carb also received a new, larger M74 veturi from Colony.

How do they work? Hopefully I can tell you soon.