It has been about 4 years since I did the Rochester B study and since it never became a tech tip nor even a sticky all that work has become harder to locate now. I understand that, so I'm going to resurrect that work and correct a few things in the process. Besides that the images were hosted by photobucket and that service has become virtually extinct it seems...at the very least you will not be able to see images on the prior work. So here’s a repeat:
I think it may be interesting and instructive to review the Rochester B series before trying to solve any problems. Thousands of these are still in use after all and things which might be considered basic or even unimportant can have notable effects. When this study was done I purchased 5 of these in varying condition. The carburetor that served as the “model” for the images was just a parts carburetor. When I got it, it was covered in grease and mud and the air horn was held on to the float bowl at one of the 4 screw holes by a nail which had been bent (possibly with a rock) into a crude L shape. It did clean up ok, however. There is an O-ring used to seal the power valve where a prior genius had stripped the threads. But it is fine for visual use.
So, first a look at some parts of the air horn: (please see image one below)
1 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: (please see image two below)
1 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: (please see image three below)
1 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 lever = richer, lower level=leaner.
2. In the B series, the idle scheme is simple and given the right circumstances will work well enough. 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 main jet, the level of fuel in the bowl and the idle adj. needle. All the air bleeds and idle tubes and everything related to those 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: (please see image 4 below)
3. 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: (please see image 5 below)
Okay, must move to another post...have used our 5 image limitation here.
