Fuel Consumption

[T. Cotten]

Cotton,

The Venturi effect is a direct relationship to Bernoulli's principal.

It's not necessary a speed (mph) , mph is derived from blade angle, load and gear selection.
The Venturi effect is directly connected to a Throttle position (throttle blade angle)

Duke

The way I read the Armored School Handbook, Duke,

The throttledisc doesn't make vacuum, just atomizing turbulence within the bleed chamber (or "well") when almost closed. Once its opened past the bleeds, disc angle (9º,12º,13º etc.) has little effect.

Only the motor produces 'suck', until the venturi kicks in upon the nozzle at moderate speeds. The idle circuit then apparently flushes like a toilet, to meter air instead of fuel.

A great tuning concern is when the throttledisc wear into the bore over the bleeds confuses a smooth transition from idle to power circuits, as the disc no longer efficiently drives air into the front bleed hole.

BOREWEAR.jpg
EYEBROW1.jpg
And obviously, any leakage elsewhere around the disc cannot help mileage..

...Cotten

[dukekleman]

Cotton,

That is correct, the engine creates the vacuum. The throttle disc creates the initial Venturi and it moves the initial Venturi along to approximately 25% throttle which then the fixed Venturi everyone refers to takes over from there.
Highway cruising speeds occur below 25% throttle unless going up hill or extreme frontal winds...

A worn throttle body bore area as you have shown in the photo is a very bad thing...trying to tune around that is a waste of time and must be remedied first.

Carburetors are very smart, arguably smarter than EFI. However, for a carburetor to work correctly and efficiently, EVERYTHING must be correct.
I've seen many carburetors adjusted and tune oddly due to the technician masking and band aiding the actual problem because carburetors are also fairly forgiving...

Duke Kleman

[dukekleman]

Otis,

Have you ever measured the transitional circuit on your carburetor to verify they have not been modified by someone over the years?
Sometimes people will use things they shouldn't to clean passage ways and permanently enlarge them. This would adversely affect fuel economy.
Just a thought...

Duke

[T. Cotten]

Cotton,

That is correct, the engine creates the vacuum. The throttle disc creates the initial Venturi and it moves the initial Venturi along to approximately 25% throttle which then the fixed Venturi everyone refers to takes over from there.
Highway cruising speeds occur below 25% throttle unless going up hill or extreme frontal winds...

A worn throttle body bore area as you have shown in the photo is a very bad thing...trying to tune around that is a waste of time and must be remedied first.

Carburetors are very smart, arguably smarter than EFI. However, for a carburetor to work correctly and efficiently, EVERYTHING must be correct.
I've seen many carburetors adjusted and tune oddly due to the technician masking and band aiding the actual problem because carburetors are also fairly forgiving...

Duke Kleman

So we 99.9% agree, Duke!

But I just can't call the turbulence at the bleeds a venturi effect.

Yet absolutely: "EVERYTHING must be correct".

(I confess I did a lot of 'masking and band aiding', but it was all cosmetic.)

...Cotten

[otis71]

No I have not measured the transitional circuit. I would need guidance on that. Incidentally I’m running the correct M51L which if memory serves has a bigger Venturi then the 51. For the time being I’m going to inflate the tires to more modern specs and try leaning it out. That said I inflate the tires and tune the carburetors on my pan and knuckle the same with much better mileage.

[T. Cotten]

No I have not measured the transitional circuit. I would need guidance on that. Incidentally I’m running the correct M51L which if memory serves has a bigger Venturi then the 51. For the time being I’m going to inflate the tires to more modern specs and try leaning it out. That said I inflate the tires and tune the carburetors on my pan and knuckle the same with much better mileage.

Its all about the idle bleed holes and slot, Otis,...

...Under the large gallery plug on the idle side of the carb body, back by the manifold flange.

Normally, cleaning and inspection to be certain nothing is stuck in the slot is enough:
BEADS.jpg
BLEDBLOK.jpg
BLEDBURR.jpg


If you wish to measure the holes and slot, feeler gauges must be ground quite narrow for the slot, and only the butts of drills should be used for the holes.

Please never attack any metering orifice with a cutting edge.

Palmer's lists the front hole as #55, the rear as #70, and the slot as .009".

Note also that the M51 series was occasionally sloppy:
51SLOTS.jpg


....Cotten

[DEEBS47Chief1991]

That is fascinating. I had a look at an M344 I have and it appears good. No pronounced wear ring or damage at the bleed hole. I would imagine it would take a long time to wear a carb body in that way. Maybe the wear could also be caused by rough treatment at rebuild time.


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[T. Cotten]

That is fascinating. I had a look at an M344 I have and it appears good. No pronounced wear ring or damage at the bleed hole. I would imagine it would take a long time to wear a carb body in that way. Maybe the wear could also be caused by rough treatment at rebuild time.

Its hard to imagine any wear other than abuse, Deebs...

(The obvious duty wear is in the bore from the disc.)

This mixing chamber is an enigma wrapped in a riddle; The M344 (and some other Indian Linkerts) had a 'tit' occupying the mixing chamber directly adjacent to the slot:

BLEDPLUM.jpg

Can anybody explain that?

....Cotten

[DEEBS47Chief1991]

Yes, we Indian guys like tits in our carbs.

[otis71]

Thanks Tom for the concise information! Very helpful.

[T. Cotten]

Thanks Tom for the concise information! Very helpful.

But I only raised more questions, Otis...

Meanwhile, Indians also had a tit on their late oilpump plunger caps, and the suggestion on the VirtualIndian mailing list was that it was a "bubble-buster".

Linkert made both Indian's and HD's carbs, but this was unique to some '41 models, and all later Indians.

HD never used it. (Unless M17L&R did,.. I dunno, never done any.)

....Cotten
PS: Anybody still call Linkerts simple?

[DEEBS47Chief1991]

"....Cotten
PS: Anybody still call Linkerts simple?"

Well, yes ...kinda.

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[T. Cotten]

"....Cotten
PS: Anybody still call Linkerts simple?"

Well, yes ...kinda.

So then tell me Deebs,

How did they cut the idle bleed slots?

(A fellow at D-port said he had the answer, but he never emailed it to me!)

....Cotten

[DEEBS47Chief1991]

With coal-fired lasers...? . ( I admit that is a good question) The two holes are not too hard to figure but perhaps the slot was cut from the inside with a tiny diamond disc in a jig to join the two holes together. I'm surprised there isn't documentation on that but maybe that was proprietary information the company kept to themselves.

BTW, I searched the internet to ponder this subject and I keep coming up with lots of Linkert information in several different forums that was written by an individual named "Cotten". Perhaps you have heard of him?

[T. Cotten]

...perhaps the slot was cut from the inside with a tiny diamond disc in a jig to join the two holes together...

They produced hundreds and hundreds of thousands of Linkerts over more than three decades with slots, Deebs!

Diamond tools shouldn't be necessary for bronze, but The Golden age of American Motorcycles depended upon this design.

Yet nobody remembers how?

I was told the equipment used is still in production, from tiny ones to missile silo sized!

....Cotten
PS: I asked the artificial intelligence 'co-pilot' on my wife's PC, and it directed me back to me.