Rebuilding Rochester model B carburetors

Regarding the Rochester model B carburetor, it is best not to pre-soak the leather accelerator piston in oil or gasoline.  Leave it in its paper sleeve and slide it into the cylinder.  Being dry and pre-formed in the paper cylinder, it slides in more easily.  Once the carburetor fills with gasoline, it will soak well.

It is best, prior to installing the accelerator piston, that you blow up through it to see if the check ball is seating properly or not.  Often aftermarket companies install a rough aluminum ball that doesn't seat properly.  If the check doesn't seat well, acceleration might be rough.  The check ball retainer is often a knurled brass dowel with a hole in the middle.  You can remove this and install a new check ball, or polish the seat if the seat is faulty.  It is best to use an aluminum check ball (light weight), but a steel one will do.  Adjust brass dowel so that anything more than the slightest breath through the vent seats the check ball.  That way the ball will be sure to seat during acceleration.  Hesitation can occur even after a new accelerator piston is installed, all because the check valve assembly was faulty.

Regarding jetting, I have found that with E10 gasoline you must jet richer.  Ethanol is twice the viscosity of gasoline, has a lower stoichiometric ratio, and has less energy. I actually have a mathematical formula for finding a new jet size, but I forgot it currently.  Anyway, I use a #56 jet in my 216.5.  It works well.  A #55 works alright, too, but the plugs are extra clean.  A #57 is too rich.  The plugs start to soot slightly.  My formula came up with 56.5, and a #56 seems right on.  A good way to monitor mixture is to install an oxygen sensor in the exhaust pipe just after the manifold.  The voltage should be in the .80s....possibly high .70s, though I find that the engine runs a bit lean with misfires when the voltage is in the .70s.

You cannot increase the jet directly by number, since circumfrence is not linear with area.........but I'll save that for when I remember the formula.  My mileage was regularily 19 in the city with regular gasoline and a #51 jet.  Then, when they changed to E10, my engine ran like garbage, and the mileage went way down.  Now, with the carb rejetted, I get 16-17 in the city, and 18-19 in the country.  I used to get 20 in the country.  The mileage will go down since ethanol has less energy.  (If anyone knows a secret for getting mileage better than this, I'd like to know it, though even my brother's Jeep inline 6 gets 18 mpg with fuel injection!  One fault I see with the 216.5 is that the intake valve is way down compared to the spark plug.  I can see the intake charge leaving all of the unvaporized fuel down low, away from the spark plug.  I assume, though, that the spark plug is by the exhaust valve because as the exhaust gasses go out, they'll suck the intake charge up to the spark plug during the valve overlap period.....who knows....???!!!)

Jet for smooth performance slightly on the lean side rather than the rich side (for clean plugs and economical operation, but don't jet too lean...that'll ruin economy).  The enrichment jet will take care of full throttle conditions, thus preventing engine damage due to a lean condition.  Since the enrichment jet is not removable, you can lean it down with thin pieces of wire until you get the right mixture with good power, should that be necessary.  Use small thin pieces of wire, and bend over in a J so that the wire cannot fall through the jet.  Once the check ball plug is installed, it will keep the wire in place.  Be sure that the plug doesn't mash down on the wire, but rather that the wire rests on the side of the plug.  I did not find that it was necessary to enlarge the enrichment jet.  It was actually too rich as it was, so I installed a couple of thin wires, and now power is much better.  You don't want the enrichment jet to be too lean, though, because at full throttle engine damage will occur.

Be sure that the vacuum power piston slides smoothly, and that it unseats the power valve at 5 inches of mercury vacuum. Use a hand operated vacuum pump to check this...it comes with attachments, one which will fit into the vacuum passage in the bowl cover.  Oil the piston with engine oil during the test, and have cover in the upright position so that piston weight is aimed in the direction it would be during normal service.   Polish the piston and the cylinder if operation is rough.  Use a fine polish, and don't remove too much metal.  Twist the corner of a rag into a finger-like extension.  Coat with polish and force into the cylinder.  Turn rag in cylinder to polish.  You can also use a torn off piece of cloth wrapped around a small drill bit.  Run the drill at a slow speed, carefully.  Remove the polish residue with gasoline.

I find that acceleration and economy is best when the piston starts to unseat the check ball at about 8 inches of mercury, and fully opens it at 5 inches of mercury.  Do what works best for you for best acceleration, economy, and smoothness of transfer.  If the valve doesn't open at the right time, all three items will suffer.  

Best fuel vaporization occurs under high vacuum, and best acceleration and economy will occur when the carburetor delivers proper fuel ratios during acceleration making it unnecessary to fully open the throttle for power.  There should be fairly good power at all throttle positions.

I have a replacement spring on my heat riser valve.  I find that when I turn the spring half-way around as the shop manual suggests, the heat riser never shuts off.  It gets really hot, and both economy and power suffer.  Making the valve turn off too early affects smooth acceleration and economy.  I found that the engine was most stable when I allowed the engine to idle for a number of minutes on a moderate day (about 65 to 70 degrees), and then bend the outer part of the spring so that the heat riser just comes to its full on position.  For all driving conditions other than idle, the valve will be at various positions other than full on (since exhaust gets hotter as engine speed and load increase), and I find that the manifold temperature stays more stable, meaning that I can come to a stop and then idle for a while, and the idle doesn't change due to a changing intake temperature.  A more stable intake temperature means that the carburetor is more likely to deliver a more consistent mixture, since proper mixture is highly dependent on intake temperature and air density.

Finally, regarding acquiring spare carburetors for parts, you must be sure not to mix throttle bodies, float bowls, and covers from carburetors with different numbers.  Internal jets and passages are different, and the conglomerate carburetor won't function properly.  Be sure that the replacement part number is the same as the original.  

lot of good info here,  thanks.. wish I had a good one to tweak at this level

I have purchased 2 carbs, 1 Roch and 1 Carter.. traded the core, wish I still had it

both rebuilds have had issues right out of package.. vacuum leaks, springs in bottom of the bowl, bad gasket etc.

Would like to start with a fresh carb for my 216.5
any recommendations?  
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1950 Chevy 3600

1942 Ford Jeep