M-74 Linkerts

[King]

Hi All

While working on my Linkerts I noticed differences in the M-74 "Big B" (a Capital B with the serifs off the top and bottom) and the M-74 "Small B" (a Capital B without the extra marks) brass body castings. The POWER side of the castings are virtually identical but the IDLE sides are a bit different. The M-74 Big B has a deeper recess for the Idle Needle washer and much cleaner idle circuit castings than the little B.

I believe the change to the Big B occurred in the middle of the 51 production year. My FL is numbered 50XX and it originally had a Small B so I believed it is an "Early 51". Although, with a production run of 6065 FLs you would think it would be a"late 51". But as we all know the Motor Company just kept using parts till they ran out.

Is there any difference in the functioning of the two M-74B styles??

Here are some pics.

King

[Cotten]

King!

Another difference is the later B (no serifs, which I have previously called "big B" for its larger stamp) has a choked bore (or "well") for the lowspeed needle: a shoulder reduces its volume.

Heights for the spring collar seat vary widely between individual carbs as well as models,
(and even more after I cut them flat!)

And there was yet another serif'd B casting: Photos to follow later.

My impression is that the un-serifed B was introduced much later, late '50s at the earliest, however I have no hard evidence.

Those with un-molested, un-restored machines,.. Please speak up!

...Cotten

[Cotten]

King!

Pictured below from top to bottom are:
M74 (no B)
M74B with serifs
M74B with serifs
M74B with no serifs

M74CASTS.jpg

Note the second B with serifs has a boss over the springcollar seat and a dimple on the idle gallery which are identical to 'bombsight' castings.
Note the B with no serifs loses the boss, but keeps the dimple.

I suspect that they are in chronological order, with the bombsight conformity for '52-'54, and the serifless B appearing n '55.

That would make the second from the top late '51 only!

Just guessing.

...Cotten

[King]

Cotten

Thanks for the interesting post. Sounds like I got it backwards the serifed Bs were the earlier and the unserifed Bs later. The serifed B that I have is very close to the one you identified as being late 51. That would make sense as my engine number 51FL50XX indicates it is a late 51. It is the one I am rebuilding so I might be “correct” for a change.

Linkerts are a fascinating subject as they are so simple and subtle at the same time and function so well. I’m curious, have any been used on auto engines?? I’m fantasizing a Buick straight 8 with four M-74s.

One question what do you recommend to secure the shoulder nut screw to the float pivot lever? In the old days it was shellac but with today’s caustic gas that wouldn’t work.

King

[Cotten]

King!

L&L made auto and other carburetors of course, but I doubt any resembled anything we would find familiar from the motorcycle assemblies.

The need for an adhesive upon the lever is not so much to lock the screw (which should be firmly bottomed out upon the 'pivot nut'), as it is to lock the lever to the float itself.

I use an isocyanate urethane, but many have used JBWeld sparingly.
A very good adhesive that has resisted my local fuels is the contact cement "Pliobond", available at common hardware outlets. I am considering switching over to it entirely.

....Cotten

[Cotten]

Just for clarity,

Here is a '52-'54 M53 "bombsight".

BMBSITE.jpg

Note the detente ball above what would have been a springcollar seat, and the venturi set screw though the dimple cast upon the idle gallery.

Once again, the timeline I suggested is pure conjecture, as it is highly unlikely that L&L nor H-D started and ceased runs of production on a date convenient for anyone else but themselves.

Apparently the serifless Bs may have
appeared about '55, as a complete relatively-unmolested motor just landed on my benches with a serif-less B, with the boss over the springcollar seat.

And it does NOT have the choked LS needle bore!

Dave Swanson!

Your machine is a priceless reference.
Does your Linkert have the boss over the LS needle, and a serif-less B?

We all appreciate your data!

....Cotten

[King]

Hi Cotten

The M-53 is a different one for sure. What models employed it? It appears to have another plug in the idle needle shaft and an external vent to the nozzle chamber on the IDLE side. The dimple seems to be an odd place for the locator screw as it appears to be in the idle passage. Did the “bombsight” improve performance?

Two other questions come to mind. Is an insulator block between the carb and manifold needed for a bike in routine service? And where would one obtain the gasket that goes between the nozzle flange and the venturi?

Thanks,

King

[Cotten]

King!

Last things first:
The phenolic spacer is not only a heat barrier, but it spaces the carb outward properly for the proper carb support, lest it exert upon the manifold.

OEM nozzle gaskets were thick, and intended for military assemblies as well as the "bombsights".

NOZLGASK.jpg

I cut thin ones from PEEK for worn venturi holes and nozzle spigots,

NOZLSEAL.jpg

although very worn spigots should be soft-soldered and re-cut to size. With a little taper, the solder will provide a seal.

NZLREPAR.jpg

The M53 was a K model unit, and there were other 'bombsights' for competition machines. I know little about them, but as you can see, they needed to vent the lowspeed needle 'well', since there was no metering spring collar. My memory fades, but the plug had to be drilled as a metering orifice. That casting boss appears on the '52 (I presume...) and later OHV model castings un-drilled.

The setscrew that retained the venturi just misses the idle gallery. The external vent upon the idle side angles downward to the nozzle like no others.
Their performance must not have been worth the effort, as it was a short-lived production.

Yet many won't let it go!

...Cotten

[1450]

King!

Last things first:
The phenolic spacer is not only a heat barrier, but it spaces the carb outward properly for the proper carb support, lest it exert upon the manifold.

OEM nozzle gaskets were thick, and intended for military assemblies as well as the "bombsights".

NOZLGASK.jpg

I cut thin ones from PEEK for worn venturi holes and nozzle spigots,

NOZLSEAL.jpg

although very worn spigots should be soft-soldered and re-cut to size. With a little taper, the solder will provide a seal.

NZLREPAR.jpg

The M53 was a K model unit, and there were other 'bombsights' for competition machines. I know little about them, but as you can see, they needed to vent the lowspeed needle 'well', since there was no metering spring collar. My memory fades, but the plug had to be drilled as a metering orifice. That casting boss appears on the '52 (I presume...) and later OHV model castings un-drilled.

The setscrew that retained the venturi just misses the idle gallery. The external vent upon the idle side angles downward to the nozzle like no others.
Their performance must not have been worth the effort, as it was a short-lived production.

Yet many won't let it go!


is the main nozzle gasket used on the m74b cab? i do not see it in the books?
...Cotten

[King]

Again Many Thanks Cotten

My spigot to venturi fit was as you described, a light press fit, so I'm OK there. I'll put the spacer on my shopping list.

King

[Cotten]

1450!

The books do not show a gasket for M74* nozzles, but it raises the very good question of where one really wants the spigot: Fully to the center of the venturi bore, Slightly below it?

I cut thin PEEK seals for every rebuild, as a decent seal between spigot and ventury is critical to the nozzle's "accellerator effect".

Often I cut thicker ones to adapt later spigots to earlier models, or by request.

....Cotten