Linkert M74B

[T. Cotten]

This is fully closed and fully opened

when the disc is closed is it normal to have the gap with the idle screw all the way backed ouf?

When the disc is fully opened the lever is on the carb body stop

With the idle screw backed out, Jack,

The disc should close completely against the bore wall at the idle bleeds. With the bleed plug removed, the disc should be visible in the middle of the idle slot:

DISKBLED.jpg
(Model shown is other than M74B, but the position is the same.)

The idle screw, when turned in to slightly open the disc for initial starting, becomes the 'closing' stop.

I must note that to inspect for daylight around the disc, the choke must open; I inspected with even the venturi removed for certainty.

....Cotten

[Iacovos]

With the idle screw backed out, Jack,

The disc should close completely against the bore wall at the idle bleeds. With the bleed plug removed, the disc should be visible in the middle of the idle slot:

DISKBLED.jpg
(Model shown is other than M74B, but the position is the same.)

The idle screw, when turned in to slightly open the disc for initial starting, becomes the 'closing' stop.

I must note that to inspect for daylight around the disc, the choke must open; I inspected with even the venturi removed for certainty.

....Cotten

Well noted! Thanks for your reply Cotten!

[Iacovos]

The “damn” thing fired up on the first go!! This 64 did not run for 10 years now!

Big Thanks for all the input for the linkert rebuild! Much appreciated everyone!

No gas leaks (was worried as was my first carb rebuild …..)

Oil pressure switch not working (had to check oil circulation hemce the seat being up)

[Iacovos]

QUESTION FOR KNOWLEDGE:
Upon rebuilding an M74B if the venturi is stuck in the carb bore and one does not remove it (or fix it being stuck) will it cause any problems with the running / starting etc ?
What are the consequences?
Why go to the point of pressing it out (by force). Also i read about a "light gap".
I have never got my head around this and could not find an answer in the forum
THANKS

[T. Cotten]

QUESTION FOR KNOWLEDGE:
Upon rebuilding an M74B if the venturi is stuck in the carb bore and one does not remove it (or fix it being stuck) will it cause any problems with the running / starting etc ?
What are the consequences?
Why go to the point of pressing it out (by force). Also i read about a "light gap".
I have never got my head around this and could not find an answer in the forum
THANKS

Did the main nozzle come out easily, Jack?

Fuel residues can combine with moisture to 'cement things together, but disassembly is manditory for cleaning the 'air correction' chamber (extreme example shown).

SNDYVNT.jpg

It is common that once the 'crust' is removed, original venturies will be quite loose, and show daylight around them;

DAYLIT88.jpg

CLNYVENT.jpg

Most break loose with penetrant and a heat lamp, but I've had plenty, especially Scheblers, that sounded like a firecracker in the sixty-ton press.


....Cotten

[duffeycycles]

I use a paint stripper heat gun until it is pretty hot to the touch. Never need the big pressure just patience but Cotton has done waaay more than me.

[BigLakeBob]

Original Paint 1958 Harley Davidson Duoglide Overview and preservation

15 min to 18 min might help

[T. Cotten]

I use a paint stripper heat gun until it is pretty hot to the touch. Never need the big pressure just patience but Cotton has done waaay more than me.

I got all the sick puppies, Duffey.

Heat guns risked warping out potmetal Scheblers, and igniting solvents, so I played it safe with heatlamps:

CHEMSOAK.jpg

The important part was cutting accurate drifts.

I tried to scroll to view the video, BLB, but as soon as I saw him pick up an eight-inch farmer screwdriver, my blood pressure flared, and I had to click off for my own good.

....Cotten

[Iacovos]

Thank you everyone for sharing all these helpful hints based on experience!

T. Cotten let’s assume main nozzle came out easily. What does the air correction do in the venturi operation and why it needs to be there? Does it affect priming/starting? Or the running of the carb?
can it create imbalances with the LS or HS circuits?

Thank you for the feedback in advance

[T. Cotten]

The "air correction circuit", Jack,

Supplies air to the main nozzle, critical for speeds over ~35mph, and rapid acceleration.
In this case, Air enters a hole through the side of the casting to the venturies' annular void (shown full of crud in my previous post), where it can then purge the nozzle's cutaway upon demand,






The Armored School Handbook best explains the function:

accltxt2.jpg

The idle circuit is even more inscrutable.

.....Cotten

[Iacovos]

Thank you Cotten,

Now it starts to make sense! Will make some more reading of course, just for the knowledge of this.

THANKS

[dukekleman]

As Cotten stated, removing the Venturi is extremely important in servicing the carburetor correctly. The entire carburetor needs attention given while performing the task.
Here are a few photos of current carburetors I'm servicing. I see this very often...

IMG_20260313_080448024.jpg
Above is a M36


Below is from a M35
IMG_20251107_144740661.jpg IMG_20251107_144631600.jpg

The corrosion left on my press plate after removal

IMG_20251107_144517608.jpg

Hope this helps encourage the removal of the Venturi,

Duke Kleman

[T. Cotten]

...Here are a few photos of current carburetors I'm servicing. I see this very often...

I'll bet its toxic, too, Duke!

CRUD5.jpg

Let's inspect the cleaned venturi and nozzle closely, to see if vibration has marred the mating nozzle-to-venturi surfaces:

wornvent.jpg

A good seal at this interface is necessary for the 'accelerator sump' effect; My quick fix for all was a paper-thin PEEK seal, but wear upon the nozzle spigot itself required soft solder and the lathe.

The remedy for the distortion and shrinkage of the venturi is not so simple.
Before any attempts at swaging the venturi oversize and then cutting to fit, it must be determined what the outside diameter shall be, after any borewear repair.

....Cotten
PS: Although fuel-related issues are usually geographic, with the USA suffering from peculiarly nasty additives, this similar sort of deposit showed up on my benches from the Phillipines:

PHILCRUD.jpg
(This was a modern assembly, as it has a modern Euro repop floatvalve.)

Note how fuels aged to 'varnish' many decades ago:

VARNISH.jpg (Although the float was post '50 'Armstrong', the machine was a remarkably un-touched '47, eventually lost to the *collector market*).

[PaulCDF]

I have been following Laurie's thread/post with interest. I have been "working" on a 344 the nozzle was wrong but the venturi came out without too much coaxing. (Ziamax I think) cleaned up pretty good but think I will order a new one and the proper nozzle.





344 Venturi and tube IMG_2446.JPG

[T. Cotten]

I have been following Laurie's thread/post with interest. I have been "working" on a 344 the nozzle was wrong but the venturi came out without too much coaxing. (Ziamax I think) cleaned up pretty good but think I will order a new one and the proper nozzle.

All 'potmetal' is one form of ZAMAK or another, Paul,

And I wouldn't throw money at it unless you think a repop is going to fit better than the original; Please inspect for borewear, as it is far easier to swage an original venturi than modern billet aluminum, if it doesn't fit your finished bore.

....Cotten
PS: The OHV nozzle shows just how forgiving this design can be!