I bought a Cater YF 966S to replace the Rochester on the 235. The Rochester is manifold vacuum, the Carter is ported. I will be using the stock distributor.
Would a simple solution be to just plug the port on the carb and run the distributor vacuum line to the manifold? If so, could I use that manifold port for both the distributor and the vacuum operated wipers?
You need the vacuum set up as with the Carter. If the Rochester is set up to send manifold vacuum off to the vacuum advance unit on your distributor, it is good you're replacing it. I don't recall seeing any of the Rochester "B" types that weren't designed for ported vacuum, but I could be mistaken. The 235 distributor uses centrifugal weights to provide any needed advance at idle (and at 500 rpm, it won't be much) and the vacuum canister will kick in around 850~900 rpm when you reach off-idle status. The manifold vacuum at idle ought to be around 16~18 and if it is, the vacuum canister will provide much more advance than you need. Hope this helps. Please don't hook your vacuum canister to manifold vacuum.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Jon, maybe I'm mistaken about the Rochester, but it looked like there was maybe 3 (???) small ports on the manifold side of the butterfly when it's closed. I'll take a look at it again.
Here is how the Rochester on my engine was set up. (see pics) It was a running, operating engine, but I have not run it yet. The port holes are exposed on the manifold side with the butterfly closed, and with the screw adjusted as shown in the other picture. Of course I can cover the port holes with 2 turns (or less) of the screw, but this is how it was when I bought the engine.
Maybe I'm overthinking all of this as usual? Can I run it with either ported or manifold vacuum? The 966S is ported, but I think the 2100S (replacement for the newer 235's) was set up for manifold vacuum, which is wondering if I should use the 966 as-is or run the line to manifold vacuum. Not looking to debate which is better, just trying to figure out how it "should be".
Try it both ways, if the idle is better one way over the other, run it. GM used both ways over the years, mine idles best with full time vacuum. The only difference it will make is idle quality.
As I recall you had a 1959 235 out of a passenger vehicle. Here's what I can tell you from GM data for that year/model: Carburetor would have been a Rochester BC, model 7013003 for std transmission and model 7013000 for auto transmission...both types auto choke. Timing...5 degrees btc at idle. Spark advance at 450 rpm = 0 Centrifugal advance begins at/after 450 rpm and the maximum advance is 24 to 28 degrees at 3500 rpm. Vacuum advance begins at 6 inches Hg (which will provide 4 degrees of timing advance) and the max is 22 degrees of advance at 15.5 inches Hg. (I think with a vacuum gauge you can determine at what rpm you will have 6 inches Hg just for point of reference)
The holes in the B model throttle body (from Rochester data I was able to find...dated 1964) are identified below. I was not able to find the scheme for the BC model, but it seems clear GM did not intend for the 1959 235 to operate with any vacuum supplied to the advance canister at idle. Jon H (CarbKing) may have this info. I would be inclined to give him a call and see. Good luck. Hope this is helpful. If full idle manifold vacuum is connected to the vacuum canister, you're going to get a large spark advance (maximum, in fact) and it won't increase much beyond that unless you're going downhill.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
A couple of other points...GM has spark advance curves shown on the Heritage pages for each year and while they are not all the same for the 235, they don't deviate by much. The 1959 235 represented changes that may not be completely understood now. The camshaft was changed and certain other things as well. There was a copy of an interesting internal memo explaining the changes to dealerships. I wish I had saved it as it is gone now...it disappeared when they cleaned up the on-line pages. Mostly it had to do with GM's corporate decision to help eliminate smog and a move to make certain engines produce greater torque at lower rpms so less fuel would be necessary. The torque curve for the 1959 235 makes it easier to understand. Torque reaches a near-max at 1600 rpm and stays a nice flat line all the way over to 2800 rpm. If you study the data for other 235s (say the 1955 model), you'll see differences in the content and how it is presented. For one point the spark advance is shown with rpm data and it is clear no spark advance happens before 500 rpm...and vacuum advance doesn't kick in until around 1800 rpm. But again that's for a different design.
Another point. The Carter YF3211S (which is an auto choke model) was the one Carter engineers came up with for the 1957 through the end of production of the 235. As you might expect it runs a little leaner and sits a little lower (because of hood/body style changes). Also it used a 2 stage metering rod. They're not always easy to find these days, but worth watching for I think.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
A trick I recently borrowed from a YouTube video, that works pretty well on my 235: Rather than trying to dance between idle mixture, idle speed, and timing when doing a tune up….
….Use a vac gauge hooked to the ported vacuum to set the idle speed. Adjust idle speed screw until you just barely start to see vacuum at the gauge. Then leave it alone. Takes one variable out of play. It’s pretty easy to get too aggressive with the idle speed screw, and then you aren’t getting the proper response from idle mixture or timing adjustments.
Then use timing and idle mixture to optimize idle speed, quality, throttle response, and vac reading.
You can also do this by just eyeballing that port with the carb off an upside down, this is just simpler if the carb is already mounted.
