Linkert M74B Purpose of hole in carb and pins in bottom ?

Carb is not machined flat

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Raytag
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Linkert M74B Purpose of hole in carb and pins in bottom ?

#1

Post by Raytag » Tue Feb 11, 2020 11:18 am

Linkert M74B 408

I looked thru too many KB-articles and wonder wether I`m the only one who doesn`t know :lol:

What is the purpose of this large hole in the carb ?
what is this_atmosphere is somewhere else.jpg
what is this_atmosphere is somewhere else.jpg (50.91 KiB) Viewed 369 times
The vent bore for the bowl is 1 inch towards the manifold.
Is this a sort of invers hood scoop for the main nozzle ?

The carb has 2 steel pins in the bottom of the carb.
Looking at other carbs showed that these 2 pins are OEM.
Anyone knows the purpose of them ? Maybe a different float bowl for the same carbbody?

Another question is if anyone has removed them ?
If so how ?
The reason I ask is because the carb body is not machined flat.
step.jpg
step.jpg (85.66 KiB) Viewed 369 times
I cannot machine that surface cause of these 2 pins.
and because it seems OEM I did not want to simply cut them off.

I have quite a big step on it and therefore a plain gasket is useless.
bowl is straight but carb is not.jpg
bowl is straight but carb is not.jpg (119.66 KiB) Viewed 369 times
The former owner sealed it with some RTV sealant.
As a matter of fact it wasn`t sealed just smeared

Thanks
Ray



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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#2

Post by RooDog » Tue Feb 11, 2020 2:16 pm

I'll let the resident Linkert experts comment on the carb, but let me say this about RTV for all those reading this thread. It is bad stuff, a sure sign of a tech who knows not what he is doing, and much of it is not fuel proof, will swell and disintegrate when in contact with many common fuels. That stuff, RTV, has no place on a Harley.
Just my opinion form years of correcting other peoples errors, and my own too, on these machines....
....RooDog....

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#3

Post by 1950Panhead » Tue Feb 11, 2020 2:33 pm

Is this a sort of invers hood scoop for the main nozzle ?
The purpose is to atomize the fuel as it comes out of the nozzle
Anyone knows the purpose of them ? Maybe a different float bowl for the same carbbody?
It is a crude alignment for the float boal.
Another question is if anyone has removed them ? If so how ?
I cannot machine that surface cause of these 2 pins.
You could heat the pins and body with a torch and pull them out, not necessary.
You need two 90 degree plates on carb ends to hold level, then machine flat, circle around pins with end mill.
Cotten may have plates for machineing, check with him if you want.

Raytag
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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#4

Post by Raytag » Tue Feb 11, 2020 3:25 pm

Exactly my thinking Roo!
Sure there are sealants around , fuelproof, bridging large gaps etc but man what a mess ... :?
1950Panhead wrote: The purpose is to atomize the fuel as it comes out of the nozzle
I blew inside a couple of times already and I`ve had the impression that the air escapes everywhere lol
1950Panhead wrote: It is a crude alignment for the float boal.
:shock:
1950Panhead wrote: You could heat the pins and body with a torch and pull them out, not necessary.
You need two 90 degree plates on carb ends to hold level, then machine flat, circle around pins with end mill.
Cotten may have plates for machineing, check with him if you want.
The jigs for the carb is quite a job and carrying and lifting up my 100lbs rotarytable in order to mill the surface for the round bowl ... oh man ...

Thanks a lot!!
Ray

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#5

Post by Larry » Tue Feb 11, 2020 5:39 pm

How did this happen?

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#6

Post by Raytag » Wed Feb 12, 2020 2:28 pm

How this happened ?
It was definitly machined on a mill . You can see the circular marks.
I guess the depth of cut for the end mill used was too shallow and when the cutting edges of the end mill become loaded they start scratching in periodic steps. To avoid that you need airpressure or better massive coolant to blast away the chips in order to keep the end mill clean. The depth of cut is also not equally deep. IMHO this was done by one of the previous owner.

Why ?
No idea but nobody does that without a reason

When I took off the bowl I wondered about that very thick layer of RTV. The float level was adjusted acc to the RTV layer: Instead of 1/4 it was less than 1/8. Well done :lol:
The manifold and aircleaner surface was tanked with this sealant as well. The spacer of ~ 3/16 was a paper gasket and several layer of RTV. Maybe cause the flange was warped ? :mrgreen:

Ray

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#7

Post by awander » Wed Feb 12, 2020 4:47 pm

I could be wrong, but I am pretty sure that there is no real need for a perfect seal between the carb body and the float bowl.

I think it just needs to keep the gas from spilling out when the bike is tilted, not be a perfect vacuum seal.

Others will correct me if I'm wrong. I know mine works with just a paper gasket and moderate tightness, no goop or anything like that.

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#8

Post by RooDog » Wed Feb 12, 2020 6:31 pm

If we are seeking a perfect match between the profile of the bowl and the bottom of the carb, it occurred to me that it would be simpler to file the rim of the bowl to match the surface of the carb, rather than trying to mill the carb. Easier to do by hand, and of lesser expense should something go wrong. It's Just a thought....
....RooDog....

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#9

Post by Andygears » Thu Feb 13, 2020 3:13 am

Could you not take a Knuckle style bowl (M-??) and just remachine a circle and ignore the pins? Just a thought.

Andygears

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#10

Post by Raytag » Thu Feb 13, 2020 12:04 pm

Thanks all for your comments and your practical approach is telling me that you guys have certainly covered considerable milage.

Another bowl? I believe that my carb and the carbbowl is just the right one for those years: Linkert 408, Drainplug, screw for bearing. I like that.
Perfect seal at the carbbowl ? Yes. I want it to be sealed.
Correcting the rim of the carbbowl ? Typical answer of a millwright. Focus on get her going LOL! However now I`ve got the time despite facing tons of work on this bike.
Presently I don`t care at all about the tins on the bike however I want to have that feeling that I can depend on this and that. And the carb is one
of the items I don`t want to question in the near future.

Thanks again, I really appreciate your knowledgeable input
Ray

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#11

Post by Frankenstein » Thu Feb 13, 2020 1:39 pm

Roo Dog,
I just thought I'd pass on a tidbit about silicone seal. The MoCo currently (well twincams) uses a clear version as the sealant between case halves. Also, after years,(decades) of finding 2 smooth machined surfaces at the case joint, they now Knurl those surfaces, I presume to hold the silicone in place or give it a place to go when squeezed. :D
Doesn't make your statements invalid, but there ya go
DD

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#12

Post by RooDog » Fri Feb 14, 2020 5:37 am

I'm sure the MoCo is not using some off the shelf stuff from The Wall Mart on their case halves. Most back yard mechanics tend to use the worse crap possible to cobble something together that they otherwise couldn't get to seal properly. too used a grey RTV sealer on my last pair of case halves, only because I didn't have YamaBond available....
.....RooDog....

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#13

Post by RooDog » Sat Feb 15, 2020 3:14 pm

Frankenstein wrote:
Thu Feb 13, 2020 1:39 pm
Roo Dog,
I just thought I'd pass on a tidbit about silicone seal. The MoCo currently (well twincams) uses a clear version as the sealant between case halves. Also, after years,(decades) of finding 2 smooth machined surfaces at the case joint, they now Knurl those surfaces, I presume to hold the silicone in place or give it a place to go when squeezed. :D
Doesn't make your statements invalid, but there ya go
DD
DD...
Back in the 1930s, Permatex, to advertise their gasket sealers, assembled and ran an engine, I assume a V-8 FORD, without gaskets. I would also assume that all the mating surfaces were machined to perfection. The point being that properly fitted parts often only need minimal sealer application. And this is just a story from the back of my foggy memory banks....
Best wishes.....RooDog....

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#14

Post by kfelt » Sat Feb 15, 2020 3:23 pm

My Linkert M74B is machined the same as yours. Always wondered about that but it does not leak using the normal gasket so I don't worry about it.

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Re: Linkert M74B Purpose of hole in carb and pins in bottom ?

#15

Post by panhead_kicker » Sat Feb 15, 2020 9:44 pm

Frankenstein wrote:
Thu Feb 13, 2020 1:39 pm
Roo Dog,
I just thought I'd pass on a tidbit about silicone seal. The MoCo currently (well twincams) uses a clear version as the sealant between case halves. Also, after years,(decades) of finding 2 smooth machined surfaces at the case joint, they now Knurl those surfaces, I presume to hold the silicone in place or give it a place to go when squeezed. :D
Doesn't make your statements invalid, but there ya go
DD
What I was recently told was used to join cases:
99650-02
99650-02
IMG_1388.JPG (33.78 KiB) Viewed 54 times

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