Linkert M74-B question: hole in body

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ES175
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Linkert M74-B question: hole in body

#1

Post by ES175 »

I have a Linkert M-74B that I am rebuilding for use on my pan chopper project. There is hole in the body that is on the high speed needle side, above the float bowl flange. It is approx 1/4" dia and it intersects with the groove in the venturi right above the hole where the main nozzle comes through the body. I cannot find any reference to this hole in my Harley manual and there is no plug for it in my rebuild kit. Does anyone know the purpose of this hole and should it be plugged?

Thanks
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Re: Linkert M74-B question: hole in body

#2

Post by 1 star »

Me-thinks you have a vent there.
There is info about it here, in past posts.

I'm sure others will know for sure..
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Re: Linkert M74-B question: hole in body

#3

Post by Cotten »

Yes, the hole is the air supply to the main nozzle.

Plug it, and you won't get off the idle circuit, although most Linkerts have venturies so shrunken as to allow air in around them anyway.
(Just one of the reasons why they run forever, although increasingly cantankerous and inefficient. I have received more than a few carbs with the venturi installed backwards, yet the owners always asserted that it ran "fine".)

The cure is a matter of swageing back to size. Some modern venturies are worthless, so inspect them closely. Aluminum or steel venturies cannot be re-sized easily.
ES175
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Re: Linkert M74-B question: hole in body

#4

Post by ES175 »

Thanks Cotten. If I plugged the hole you are right, I wouldn't get off idle. The carb I have is minty, no wear anywhere, including the venturi or body. The venturi is a nice tight slide fit. Even the throttle shaft has no wear and the bushing fit is so good I may not even install the the bushings from my O/H kit. There is an "M-4" stamped in the mounting flange. Do you know what this means.
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Re: Linkert M74-B question: hole in body

#5

Post by Cotten »

Actually the venturi should be a light press fit, or tighter. When cleaned, most vintage venturis show daylight around them when held to illumination for inspection. A blueprinted carb would have a nearly perfect seal, and enough press to allow for a decade or two more shrinkage.
Fortunately it takes a great deal of daylight to confound tuning, but any shakeing of the venturi commonly wears a groove around the base of the nozzle spigot, and opening its hole into the venturi for an internal airleak. This defeats the 'accellerator pump" action of the nozzle.

Which also brings to mind, M74Bs had a phenolic gasket between the nozzle and the venturi. Few have survived the years or the changing fuels. If I cannot find a quality source, I shall have to commission them.

Are you sure its an "M" and not something like HD before the 4? Most of it ususally gets ground off to remove warpage. Scrub it on some emery and you will probably see what I mean.
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