For our purposes here though, the venturi sizes are indeed what is most useful. The first one we might note is the 3 bolt M-5/ M-55 of early 61" Knucklehead fame. They featured a venturi bore of 1-1/16". OK, so what, you might say. Well, let me put that in terms of modern carbs: 27mm (rounded off). That friends, is not a lot of area to get much air through. But as we go down the list, the venturi sizes do increase, but not much. While we do find the M-25/ M-75 with its whopping 1-5/16 venturi on 1940 Knuckles and as an option on ‘41-’48 models, most Knuckles left the factory with the 1-1/8" M-35. Finally, with the 74" Panheads a 1-5/16" venturi became the standard bearer for brass bodied Linkerts on Harleys, being the largest Linkert offered. Again let’s put those sizes into terminology which will make it easier to compare with modern carbs. The 1-1/8" venturi M-35 series carbs were only 28.6mm (again, rounded off). The big kid on the block M-74 comes in at a whopping 33.3mm (do I really need to point out that this figure too is rounded off?). As a point of reference, when Harley switched to Bendix carbs in the mid 1970s, they put a 36mm version on the XL models, which just happen to come in at very near the same cubic inch displacement as the 61" Knuckle, and a 38mm on 74" Shovelheads.
Super E on left, M35 Linkert on right
As a further point of reference, let’s mention a couple modern performance carbs. S and S (why does it bother me so much that blogger’s html will not allow the use of the ampersand?) designates their carbs by the diameter at the manifold surface rather than the venturi size. An S and S Super "E" carb which is called 1-7/8" is actually 40mm at the venturi, while the "G" model’s 2-1/8" bore is 44.7mm at the venturi. Right in between these two carbs is the flat slide Mikuni HSR42. Keep in mind though, none of these three performance carbs relies on a choke plate as a starting aid. All of the Linkert carbs employ such a plate, which further limits their air flow beyond what the already smaller venturi does. And the measured airflow through these carbs bears this out. An S and S "E" carb flows a whopping 73% more than an M74 despite the venturi bore being only 20% larger.
Granddaddy Joe Smith still ran Linkerts on his Knuckle just before switching to a Shovelhead
That is not to say that the addition of nitro turns the Linkert into killer performance carb with no other changes. Nitro requires a much, much, much richer fuel to air ratio. For a given amount of airflow, nitro will need about 7-1/2 times as much fuel as the same engine running gasoline. This requires some substantial modifications to the fuel delivery system all the way from the petcock to the float system to the jets. The potential results however, are nothing short of awe inspiring. A switch from gasoline to nitro methane comes with a potential of just over double the horsepower. Do I need to mention that the bike and the rest of the motor needs to be strong enough to survive double the power?
Now it looks like this series will drag out into yet another post. God willing, I plan to wrap it up in a post encompassing my thoughts on what makes a viable street application for vintage style dual carbs.