Thursday, December 18, 2014

Vintage Dual Carbs, Part 3

In part three of this treatise on dual carb Harleys, I would like to put the venerable Linkert under the proverbial microscope. First stop will be an unlikely source for an avowed Knucklehead fanatic such as myself, that being the ’48 to ’57 Panhead Service Manual. The last page of the carburetor section has a handy-dandy chart listing most (all?) of the carbs used from 1936 to 1957. Along with applications, throttle disc angles and even transfer port dimensions, we find venturi sizes listed. Hats off to those technical writers of yesteryear; if only their modern day counterpart would follow their lead.

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.

Knuckledragger Carbs
Right about now you may be thinking, "What about all those old pictures I’ve seen of vintage drag bikes with a pair of Linkerts fitted?" In fact, those paying close attention may even wonder why my own vintage drag bike, "The Knuckledragger" breathes through a pair of them. Speaking for my own situation, I had two reasons to use Linkerts. Nostalgia and pragmatism. The nostalgia portion is self evident. The pragmatic stems from the simple reason that the heads I had in my possession for the project were already set up with Linkert 4 bolt flanges. Had it been a more serious performance effort, I would have reworked the heads.  As it is, I still cut the choke assemblies off, added radius inlets and swapped to the somewhat larger 1-5/16" venturi's on the M35 carbs to maximize air flow.

Granddaddy Joe Smith still ran Linkerts on his Knuckle just before switching to a Shovelhead
The reason you see Linkert in old pictures of early drag bikes is also pretty straight forward, though maybe not so self evident. Many of the drag bikes you see outfitted with a pair of Linkert carbs were run on nitro methane fuel . "Nitro" as it is commonly called among racers, is quite different than gasoline in that it has oxygen in its chemical makeup. That means that rather than relying on airflow to get enough oxygen into the combustion chamber, with nitro a goodly portion of that oxygen is supplied by way of fuel flow. Remember back in part 3 when I wrote this? "The problem is very seldom getting enough gas into the engine for high performance, the challenge is in getting enough air in." Perhaps I should have more specifically used the word oxygen rather than air and fuel rather than gas, but I think you get the picture.

 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.


Mark said...

Hi Lee, Are you still running E-85 in your bike? and are you thinking of going to fuel.

St. Lee said...

Hi Mark - Yes, still the E-85 & no plans for nitro in the foreseeable future. That was a decision I made at the beginning of the build when I opted for cheap Panhead cylinders rather than stronger custom cylinders. I might add that this was a lesson many racers from the '50s learned the hard way. Stock cylinders are really not up to the task of withstanding the pressures generated by nitro. said...

Looking forward to the next installment of the dual carb series, hopefully you'll impart some wisdom that will help me get my setup working well. I have dual MR-4 carbs on an 84" stroker panhead with a Lieneweber J4 cam. It seems to need copious amounts of gas to start, but once up and running it runs like a scalded dog, albeit very rich (fouls plugs unless you stay into the higher RPMs). Any tricks or tips on starting and tuning these MR-4s? Thanks!!

St. Lee said...

I'm no expert on the MR-4's in particular, but a common drill for starting a bike with a Linkert is choke on, throttle closed, 3 to 4 kicks w/o ignition to prime and then kick with ignition turned on (some motors seem to like the choke one click open for the kick with ignition. Other than that there are a lot of variables which can effect starting.

As for the rich running at lower rpms; if you have one carb on each cylinder you may want to confirm that the bores of the carbs are not grooved at the throttle plate which will make them impossible to tune. In a similar vein, if you have the two carbs on a common manifold, then the idle circuit on one carb really should be disabled, plus some sort of progressive linkage so that the second carb works somewhat like the secondaries on an automotive 4 barrel will be very helpful. More on that in part 4. said...

The MR4s don't have choke plates, which I imagine contributes to the hard starting issue. The carbs are set up one per head (modified heads with individual runners right into the chamber). I believe the MR4s have a 1 and 1/4 inch venturi. Is that within the ballpark for operating with the stroked engine? I have heard about adapting a tickler like the amals, or adapting an accelerator pump from a bendix...

St. Lee said...

Not sure how well a tickler will work with the dual carbs on that side of the bike angled down toward the air cleaners while on the kickstand for starting. The fuel from the tickle will tend to just run out into the air cleaner.

Accelerator pumps for Bendix carbs are notorious for how badly the leather plunger fits. Not sure of any good quality replacement for them.

An add-on accelerator pump was once available for Mikuni carbs (back in the old round slide days). Maybe someone still makes something like that. Of course, to some extent an accelerator pump will suffer from the same problem of gas running out into the air cleaner, but you really need to find some way to enrich the mixture for starting.