Is M35 Carb Correct for 1947 Knuck 74

[partshunt]

My old shop manual staes that the M35 Carb was used on 61 Knuck but also on the 74 Knuck to 1947. Its a bit confusing to me as there were also other carbs used. The M35 came with 1-1/8 Venturi, sounds small for the 74". Any comments or more info on the subject would be apreciated....thanks...Joe

[Cotten]

Joe!

Yes, the M35 is "correct" for either E or F models.

The M35T application is a mystery to me, but the M35TP is predominant on 1948 machines.

There's nothing wrong with playing with different venturies, as long as they seal on their outside diameter, and at the nozzle spigot.

....Cotten

[Bigincher]

I'm running a 1-1/8" venturi in my M74B. Because of the greater pressure drop as air moves across the smaller venturi, there is a stronger signal. Because of this, it has more response than the same carb with a 1-5/16" venturi. The larger venturi will flow more air and offer more power, but at a certain calculated sacrifice in response. The smaller venturi offers easier starting and more snap and throttle response at lower speeds, but may suffer a loss of power at speeds over 70 or 80 mph. So there's a trade-off, one from the other. Also, because of the stronger signal, the emulsified fuel rises up the nozzle sooner, so I suspect the float should be set just a tad low. Mine is now set exactly at 1/4", and I'll be lowering that perhaps 1/32" in an effort to eliminate a slight 'burbling surge' at mid-rpm crossover speeds. (I am not certain of this, but I want to try it. Feels like ignition timing is too advanced, but it checks out dead on the money.)
When I discovered the 1-1/8" venturi in my carburetor, I asked around and did some research. What I found was the small venturi may have been used primarily in Police and Military motorcycles, for the reasons above. I'm very satisfied with the performance of my bike. The motor is a stock Panhead except for a Sifton 412 cam. I have another M74B with the standard 1-5/16" venturi, and I'll swap those carbs some day for a comparison. Right now the weather isn't exactly great for riding.
There is an excellent publication, good technical reading about the Linkert carburetors called (oddly enough) "The Linkert Book", available from the Victory Library. I highly recommend it. There is a table in it which identifies the M-35 as being used on EL models 1941-1948 and FL models 1942-1948.

[59Panman]

Cotten,

Do you keep records on the Linkerts you rebuild? Or do you just record them on the receipt you send with the refurbished carb? Just curious as I have sent you 2 Linkerts to rebuild both of them the M74. You stated that one of mine was the "BIG B" model. What is the difference between the standard B and the BIG B? I changed email accounts and cannot seem to find the answer to this question which you addressed.

[Cotten]

Mark!

Each carb gets a flat-rate form for its model attached to it, and all shop notes are written upon it, or attached to it.
From that, I massage a reasonable invoice.

I can look up invoices by their paid date. Shop notes, however, would be quite a rabbit hunt, but they are there somewhere in a hardwood file box my evil father made for a router case.

Rarely do I remember specifics, but I remember I photographing one of yours, at least, for its condition when I took it apart.

"Big" and regular "B"s pertains to the stamps of course.
The later big Bs had no serifs, and perhaps they originated as early as '57, coincidental with the first foam floats, but I could believe anything, since there's no records at large.

Big Bs have a 'choked' lowspeed needle well (the hole it goes into...)
Don't know why.

My best guess is that it was to prevent the use of the destructive drills supplied in earlier field repair kits.

Now back to venturi size:
Bigger means better breathing allowing more horses.
Smaller means better mixing allowing more torque.

Unfortunately, you can't have the best of both with a classic carb.

Modern highways and traffic demand higher speeds than our machines were designed for.
If our payload can be trimmed, then opening up the throat makes perfect sense.
But if manageability through parkinglots and driveways,.. occasionally un-paved,.. and then jumping into traffic before they climb up on your ass is your real riding experience, then torque is what its all about.

After all, how much of your riding time do you spend over 70 mph?
How fast do you really want to go with a springer?

Sidehacks and heavy dressers with payload logically would stick with stock specs, or investigate a tighter throat.

This is a tuneability aspect that is unique to vintage American motorcycle carbs.

....Cotten

[partshunt]

Bigincher:- My 74 burbles like yours, I agree with you on the float. need to try lowering mine a bit as its also hard on fuel. I need to also check and may have an altered or worn main jet. Thank you and Cotton for your help. I apreciate it as the knuck carb is for a friend with a 47. My 54 has the M74B, Cotton, is that the big "B' model you mention or are you reffering of another version I'v never seen.? ...thanks again fellas...Joe

[RUBONE]

Partshunt,
Go back and READ Cotten's response. Your answer is in his post!
Robbie

[Cotten]

Once again,
the most significant difference between these varieties is that the later larger B stamp without serifs (bottom in attachment) has a lowspeed needle well that is shouldered down to a smaller bore.

And once again, I have no clue as to why, at this time.

....Cotten

[59Panman]

Cotten,

Found my emails for my friends M53A1 you rebuilt along with my 74B. I also have all of the photos you sent of the glass beads in my M74.

[partshunt]

Cotton:-

Thanks for the pics Tom, that explains it well. Need to see what mine is on the bike. My spare is the early M74B, (with seriffs). Interesting., woulda never noticed the difference in the model number, AND, even if I had noticed the diffent "B". I would be thinking its just another empoyee had a diffent number set at the time in the factory.Thanks......Joe