Linkert M74B

[T. Cotten]

Going back to the first page of this thread, Jerry Wieland said that he didn't have good results trying to run an M74B on Knuck's. I had problems with my '49 Pan with low compression pistons & M74B. I changed the jet from 19 to 17 to get the high speed needle in an operable range.

We better define 'operable range', Duffey!

Let us review the manual:

74B trimmer.jpg
That means the needle can be set just about anywhere, with the OEM fixed jet.

Please beware the early 74s (no B) with the smaller jet, had an even more restrictive HS needle:

M74POINT.jpg

The later B series used larger jets and larger needles, presumably so there would be more than ample fuel, yet the needle could completely shut it off when necessary: Like when racing up Pike's Peak.

In actual practice, I sent them out (with spec'd jets) at three-quarters of a turn, and suggested the customers just left them there.

Until they rode in the mountains.

...Cotten
PS:

Fixed jets have always been an idiot switch.

Beware modern ethanol fuels have a reduced volumetric efficiency, compared to 1950 fuels. It takes more now.

If I must error, and error I did, I was happiest to do it to the rich side of things.

[duffeycycles]

Thanx Duke& Cotton, Both of your experience is greatly appreciated

[dukekleman]

Going back to the first page of this thread, Jerry Wieland said that he didn't have good results trying to run an M74B on Knuck's. I had problems with my '49 Pan with low compression pistons & M74B. I changed the jet from 19 to 17 to get the high speed needle in an operable range.

Duffeycycles,

I have always found the #19 to be out of range as you stated.

Based on the condition of the engine, #18 and smaller have been required to put the adjustable needles taper into a more useable range.
Well built tolerance engines commonly require the #17 just as you mentioned.

Sharing my observations and experiences,

Duke Kleman

[T. Cotten]

Going back to the first page of this thread, Jerry Wieland said that he didn't have good results trying to run an M74B on Knuck's. I had problems with my '49 Pan with low compression pistons & M74B. I changed the jet from 19 to 17 to get the high speed needle in an operable range.

Everybody's results differ, Duffey!

I built a '39 EL (already had an added 4th gear) for an associate with an M74B, and it always jumped like a scalded dog.

Then a new owner wanted a 'correct' M35, and with all other conditions the same,.. it de-tuned it.

Would never have noticed if it was only ridden on gravel.

....Cotten

[duffeycycles]

Going back to the first page of this thread, Jerry Wieland said that he didn't have good results trying to run an M74B on Knuck's. I had problems with my '49 Pan with low compression pistons & M74B. I changed the jet from 19 to 17 to get the high speed needle in an operable range.

[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!

[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]

...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*).

[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

[Iacovos]

Thank you Cotten,

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

THANKS

[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 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]

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

[BigLakeBob]

Original Paint 1958 Harley Davidson Duoglide Overview and preservation

15 min to 18 min might help

[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.