I mentioned a couple of months ago I was going to spend time studying these. Prior to that for a few weeks it seemed like a lot of mud had been slung at them around here, some pretty viciously. Right or wrong, I'm going to share what I've gathered.
I think it may be interesting and hopefully instructive to review the Rochester B series before trying to solve any problems. Thousands of these are still on stovebolt engines after all, and things which might be considered basic or even unimportant can have notable effects. I got 5 of these to study so as to have what I figured would be a decent field. Before we begin I’ll tell you the “model” for the images was a Rochester B which probably should be considered a parts carb. Along the years this poor thing suffered a great many atrocities. When I got it, it was covered in grease and mud and the air horn was held to the bowl at 1 of the 4 screw holes by what looked like a paper clip straightened and twisted with pliers. Another screw hole was occupied by a nail which had been bent (possibly with a rock) into an L shape after it was inserted. There is an o-ring used to seal the power valve where a prior artisan managed to strip the threads. But it is fine for visuals and I was surprised at how nice it looks after cleaning. If I had a good throttle body and float bowl mast with a good power valve and power piston, it could be saved. First a view of some parts of the air horn:
http://i1154.photobucket.com/albums/p535/JonGoodman/Air%20horn%20parts.jpg1 is the float/float pin, 2 is the main metering jet, 3 is the power valve cap, 4 is the bowl mast, 5 is the power piston assembly, 6 is something we'll discuss later, 7 is the main nozzle, 8 is the vacuum passage lip for the idle circuit, 9 is an internal vent with an air bleed hole above it.
And next a view of some parts of the fuel bowl:
http://i1154.photobucket.com/albums/p535/JonGoodman/Float%20bowl%20parts.jpg1 is the accelerator pump hole, 2 is the accelerator check ball assembly, 3 is the main venturi area, 4 is the boost venturi area, 5 is the power piston vacuum passage.
And a look down inside the bore to see the cross shaft:
http://i1154.photobucket.com/albums/p535/JonGoodman/Air%20bleed%20holes.jpg1 is two of the air bleed holes, 2 is an internal vent tube.
1. Because of the design of the B series, the fuel level is
very important in determining whether you run rich or lean. As you'd expect atmospheric pressure plays a part here and the level naturally affects the distance the fuel has to travel for the idle circuit and also the main circuit. Higher fuel level = richer. Lower level = leaner.
2. In the B series, the idle scheme is simple and given the right circumstances will work well. At idle there isn’t enough air flow to make the venturi do anything, so the carb relies on the main jet, a vacuum passage, an idle tube in the float bowl, four air bleeds, discharge ports and a mixture needle. Vacuum pulls fuel up through the float bowl via the idle tube and into a cross shaft a couple of inches below the top of the carburetor in which air is mixed with fuel via 3 air bleeds and also an internal air bleed that is sort of hidden. Then the mixture is drawn back down through a passage in the fuel bowl and fed into the engine below the throttle valve. There are only 3 things you can change or adjust in a correctly working idle circuit: the jet, the level of fuel in the bowl and the idle adj. needle. All the air bleeds and idle tubes and everything else are fixed.
Located
above the idle discharge hole are other discharge holes known collectively as the "off-idle" port. They're activated as the throttle is opened, drawing air across them and increasing the same fuel/air stream already in play at the idle discharge hole. As the throttle is opened further and air flow increases, air pressure in the boost venturi begins to drop and fuel starts being sucked out of the main nozzle (which interestingly also works as one of the air bleeds in the idle circuit). This fuel travels through the main venturi down into the engine and this
almost completes the idle/off-idle scheme. I say almost because the idle port will still be feeding fuel into the engine at this time. Rochester engineers believed this would create a smooth transition from idle to driving speed, and it wasn’t a bad thought. Here is a view of the throttle body showing those plus the distributor vacuum holes:
http://i1154.photobucket.com/albums/p535/JonGoodman/Throttle%20body%20ports.jpg3. When air flow increases and causes more fuel to go through the main nozzle, the main metering system begins to take over. Ideally fuel will be metered predominantly by the jet and that fuel will travel through the bowl mast (and still through the idle tube, naturally but with lesser importance now). The fuel once again mixes with air through the same air bleeds that fed the idle circuit and gets discharged into the engine through the boost and main venturi. And
if the Rochester engineers had stopped at this point, things might have been jake. But they didn’t. They took one strange step further and invented...
THE POWER SYSTEM
4. And anyone who has ever worked on one of these should know about this. If they ignored it or just didn’t understand what it was, they
seriously wasted their time. This scheme was designed to provide additional fuel as needed...meaning whenever the vacuum drops to a certain point. This could be going up a hill, when you are passing another vehicle, when you're using your windshield wipers, operating your wolf whistle, etc. And as we'll learn later, it can also provide additional fuel when you don't need it or want it. Here’s how it works: there is a very important vacuum passage going through your carb which routes manifold vacuum up through the throttle body, on through the main well and up into the air horn. When vacuum is high enough (in my observation it could be around 9” to 11” and in some cases more) that vacuum will pull the power piston up and very slightly off of the spring-loaded power valve (basically a ball pushed up by a weak spring). When vacuum falls below 8 to 11”, then a
much stronger spring in the power piston vacuum chamber will force the piston straight down and unseat that ball. When this happens, you get much more fuel in the bowl mast. Oh, I should also tell you the power piston is vented back into the main air horn bore. This is so fuel won’t be sucked past it and back down that vacuum passage I mentioned. Here...you can see what it looks like when the power piston is in the “down” position:
http://i1154.photobucket.com/albums/p535/JonGoodman/6%20no%20vacuum%20wide%20open%202.jpgAnd by contrast it would look like this in the "up" position (away from the power valve):
http://i1154.photobucket.com/albums/p535/JonGoodman/power%20piston%20up.jpgRochester used at least 2 styles of power piston designs and I wasn't able to unscramble that egg. One had an open shaft and one had a closed shaft and otherwise they seemed identical. The one in this carb has been crudely soldered closed solely for illustration purposes and also because when I got it, it looked like somebody had been using it as a toothpick. Possibly the bent nail person. Here are the two types:
http://i1154.photobucket.com/albums/p535/JonGoodman/Power%20piston%20tip%20types.jpgThe extra fuel provided by the power system is available to the idle tube
and the main metering system. It is metered by what Rochester calls a “Power Restriction,” and in the carb I’ll be using for illustration, this restriction hole was 1mm or .0394” in diameter. Here...you can see it:
http://i1154.photobucket.com/albums/p535/JonGoodman/power%20restrictor.jpgWe’ll talk more about that "restriction" in the next post and we'll discover something that might be interesting to you.
I’m going to overlook the choke system, accelerator pump and throttle body. Those don’t seem to me to be classic problem areas, however if the throttle shaft bore is worn to the point you notice wobble it goes without saying it should be bored and sleeved.
more to follow...
