Carburetor and Fuel System
Title: Linkert
M74-B question: hole in body
Thanks
Post by 1 star on Apr 23, 2005, 10:24pm
Me-thinks you have a vent there.
There is info about it here, in past posts.
I'm sure others will know for sure..
Post by Cotten on Apr 24, 2005, 12:39am
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.
Post by ES175 on Apr 24, 2005, 1:20am
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.
Post by Cotten on Apr 24, 2005, 1:49am
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.
