741 Carb Bowl Condition

[JTR249]

I know. I have to have a sense of a sense of humor about these things because my learning process with some of these parts has been by and large trial and error. Sometimes with a lot of error to learn the hard way. But I still find it interesting and very worth-while.
I was familiar with the 3/4' venturi, but that is the standard carb and not mine. So I will need the 13/16" venturi?
Also, I did not know about 7/16" float setting. If this is impossible, what is functionally correct?
are these two features the only differences?
Thank you for the information.
-JR

[T. Cotten]

...So I will need the 13/16" venturi?

Unless you weigh three hundred pounds, and never expect to reach 60 MPH, Yes, JR!

Also, I did not know about 7/16" float setting. If this is impossible, what is functionally correct?

Nobody knew about it; With floats of proper buoyancy, the setting for all Model Ms (except some M53 series) is a level quarter inch.

..are these two features the only differences?..

Without comparing the idle bleeds in my hand, I would have to trust untrustworthy sources. The AI was right though, that the difference would be 'slight'.

....Cotten
Ps: Before ordering a venturi, please inspect the bore for wear from the throttledisc; You may want to repair it, which will require a slightly oversized production (and disc, etc.):
EYEBROW1.jpg

[JTR249]

I doubt if I ever have this one at anything over 45mph. I'll have to check the one on my surplus motor as you have raised a very interesting difference.
Do you think the military surplus powerplants were given the tuned-up version of the carb or the standard?
I might just get a new venturi to avoid the problem. These are among the less expensive parts as I have seen many online and are in reproduction as well.
I don't have my boxes of carb parts yet and Starklite will ship soon. I'll let you know what I find and how it goes.
Thank you for the help.
-JR

[T. Cotten]

I doubt if I ever have this one at anything over 45mph. I'll have to check the one on my surplus motor as you have raised a very interesting difference.
Do you think the military surplus powerplants were given the tuned-up version of the carb or the standard?
I might just get a new venturi to avoid the problem. These are among the less expensive parts as I have seen many online and are in reproduction as well.
I don't have my boxes of carb parts yet and Starklite will ship soon. I'll let you know what I find and how it goes.
Thank you for the help.
-JR

Fact of life, JR.,..

Sooner or later, its gotta be able to get out of the way.

If you have bore wear that extends near to the idle bleeds, you will be spending your money twice on a production venturi.

(Unless you have a lathe and can get creative...)

....Cotten
PS: It wouldn't surprise me if there are NOS powerplants with either model.

[JTR249]

Here's the spec sheet from the manual I am using to learn about the carb. This might sound like a stupid question, but is the dimension 13/16" or 1 and 13/16"? Or am I reading the dimensions for both versions? I understand that it's tapered sort of and that it is larger diameter on one end compared to the other.
-JR
741 Spec Page.jpg

[T. Cotten]

Both Linkert and Schebler made nominal 1" and 1¼" Models, JR,

With various sizes of 'chokes' for each. (Venturies have cylindrical outsides, but the inner bore is an airfoil, or 'choke')

Please beware that there were no M640-B, nor M741-B carbs in other literature, nor in the fossil record!

The 640-B motorcycle used a 1¼" Model Schebler DLX128, normally found with a 1 1/16" choke, although this sheet says 15/16", and another common source cites 7/8". This merely illustrates, again, you can't always trust sources, no matter how legitmate.

....Cotten

[JTR249]

And if no searches produce a 13/16" venturi can a shop just make one?

Wally said his shop can make anything. Should I ask?

Thanks.
-JR

[T. Cotten]

And if no searches produce a 13/16" venturi can a shop just make one?

Wally said his shop can make anything. Should I ask?

Thanks.
-JR

You can buy one reasonably, JR,

From https://www.jerrygreersengineering.com/, for '41-'42 Fours, but if your bore needs honing of its damage, the venturi must then be swaged oversize, and machined to a slip fit (and a 12° disc cut to match.)

Or your friend can cut one from scratch, but it will take him some time... (An oversized 1" Model disc can be cut to match from a spent 1¼" or 1½" Model disc.)

How bad is your bore? (It is best inspected after blasting.)

...Cotten
PS: Note .030" oversized discs on the market might then be lathe-cut down to size, but honing to fit such an oversize would blow through the idle bleeds, and ruin the body.

[pisten-bully]

JR, did you read through this thread about rebuilding my 741-1? Tons of great input from Cotten and others.

https://forum.antiquemotorcycle.org/...m741-1-rebuild

[JTR249]

Thanks, Paul. I will read through that. I did not know.
-JR

[JTR249]

Wait a minute. I checked 13/16 vs 3/4 and they are not equal. Greer's only sells a 3/4" venturi.
Am I misunderstanding you? I realize 13/16 is a little less than an inch.
-JR

[duffeycycles]

It would be easier to modify a 3/4 to make it 13/16" than make a complete venturi. 3/4 is 12/16

[T. Cotten]

Wait a minute. I checked 13/16 vs 3/4 and they are not equal. Greer's only sells a 3/4" venturi.
Am I misunderstanding you? I realize 13/16 is a little less than an inch.
-JR

Post #23, JR!

The 13/16" is offered for '41-'42 Fours; Obviously it would make sense to offer them on the M741 page too, but they don't.

....Cotten
PS: Greer's diagrams are pretty good, but '41 productions had large needle knobs. and the floats are drawn like brass floats with the fat part minimized to the bottom.