For a while it has been on my mind that the power piston in the Rochester B and BC models is one of the main problems. Well actually for these carburetors I believe it is more of a time bomb than anything. These pistons always get stuck in the down position, and this invariably makes the carb run very rich. Straightening the air horn and bowl helps. Adding a brass tube between the two seems to help strengthen the vacuum passage, but that probably isn't enough. Why? The problem is a chain of events. I believe the worst of things is ethanol gas and infrequent use. The gas when it dries leaves a sticky residue. As it ages, this gas devolves into a fluid which in concentrated form leaves more of a sticky residue...almost rosin-like or something you might use to make flypaper. The power piston is a close fit in a bore in the air horn and pushed ever downward by a spring. When the carb dries out (as it will in about 1 week of non-use), the residue acts as a bonding agent between the piston and the bore. In one of the 5 Rochester carburetors I recently studied, the piston was stuck so tightly it took me quite a bit of time and work to remove it without damage...it was stuck nearly as if it had been glued. So what is the answer? I think it could be as simple as keeping the piston from sitting against the bore by using a small o-ring like this one:
Conveniently the piston has two slots machined into it on top and bottom. But the o-rings must be small enough they don't interfere and yet large enough they are slightly larger than the piston itself like this:
So far, vacuum tests prove this approach works just fine. I'm thinking since the o-rings will more or less seal the center zone of the piston, some light grease (maybe silicone grease) will improve the scheme further.
Thoughts?
PS...Jon (King of Carbs) I know you're scratching your head saying "seems an awful lot of time to spend on a Rochester B." Probably so.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
O rings wear quickly, particularly in an atmosphere dominated by strong solvent vapors. How many times a month are you willing to dismantle the carb and replace them? I'd say that would be needed about once a week on a daily driver. Also, anything that constantly drags against the bore will make the power piston action so sluggish as to be virtually worthless in response to varying load conditions. "Looks good on paper- - - - -sucks in practice!" Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
The grooves were placed in the pistons for two reasons:
(1) To reduce friction because of less area contact of the piston to the cylinder (2) To make it more difficult for restorers to reproduce the pistons
The use of an O-ring would negate number 1 above.
Jerry said it well.
And Jon G, I really do think it admirable of your research. Much is learned by persistant (a.k.a. stubborn ) individuals that continue to research where others have ceased to do so.
And since we are for the most part talking about truck applications; one can always improve the Rochester B by hiring a 15-year-old to stand on the running board (trucks!) and pour fuel into the engine from a leaky boot!
Jon.
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify. If you truly believe "one size fits all," try walking a mile in your spouse's shoes! The Carburetor Shop
First of all, daily drivers are going to have much less trouble with the power piston sticking. It isn't those situations I'm addressing. It is the people who start their engine once a month...those are the people who are going to have the headaches. Their power pistons are in time going to be stuck and "varying load conditions" will have zero effect on anything happening in the carburetor at that point.
There are at least a couple of ways to approach the power piston problem in my mind (and ideally this needs to be something included in a rebuild kit):
1. Devise a way to block any gas from entering the piston and bore. Today's gas is going to guarantee this piston sticks when it evaporates. That is a given for occasional use engines. And I've come up with ideas. Here is one:
The aluminum frame I've made holds something around the piston which seals and keeps it dry and this frame is held in place by the mast. Could be an o-ring, could be a piece of thin neoprene, a nylon ring or could be a leather ring. I haven't found the best way to seal it yet, plus the air horn to bowl gasket would have to be modified. I think the best answer might be an accordion-like seal that would fit on the power valve rod and be held tight at the piston bore but holy cow that little seal would cost a fortune.
Another possibility is to make a power valve that sits outside the carb and run the rod down to the power valve. But there are plenty of challenges there. One for example: what strength for the spring? It won't be the same for all B and BC carbs (I found 3 different spring tensions in just the 5 that I studied). Plus that would not be something you could include in a carb kit.
2. Devise a way to keep the piston from being in constant contact with the bore and seal it a bit in the process so any gas that reaches it will only affect the bottom 2mm of the piston. That's easy and cheap. O-rings. As you pointed out there are some limitations. The maker of these says they're tolerant of gas, oil and even ethanol. How long will they last? I don't know yet, but I'll find out. But I do know the design of these carburetors and the gas being put into them suggests to me occasional use carburetors are going to work best if they're serviced at least once a year. I've thought also of split rings (ptfe coated, nylon, pvc, delrin, etc) and I think those would work but again we're looking at $$. I haven't yet found any existing (as stock) items.
If you have any suggestions, please put them out there.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Hi Jon, 15 year olds in Dallas don't want to be hired. They seem happier to sit in a stupor and play games on their iPads. Do you have any ideas about how to keep the piston from sticking in the bore?
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Do it the way Carter did for years- - - -use a mechanically-actuated metering rod and eliminate the vacuum piston altogether. With a few thousand hours of R&D and a few million dollars worth of tooling up and manufacturing, you could probably convert Rochesters to use mechanically actuated metering rods- - - - -or just run a Carter carb in the first place. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
A stronger spring would make the carb run rich more of the time- - - -not exactly what we're after. Red Green can probably help out with a couple of rolls of duct tape! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Hi Gord. I haven't given up on the o-ring idea, but I want to come to a solution that leaves as much of the carburetor "as is" as possible so that "everyman" could get a kit and use it and hopefully improve things for himself or herself.
At the risk of being boring, please let me wade out a bit more. The choice of o-rings was by no means accidental. I measured everything about 27 times before I ordered them. If you look at this digital image of the piston, you'll see the o-rings appear to be the same size as the piston or possibly a bit less. In fact, they aren't but only .006" of the outer edge of the o-rings touches the bore of this carb I'm using as the test case. A typical sheet of copy paper is .004" for reference.
So as I said, when the piston is in the bore, only .006" of the circumference of the o-ring touches. Admittedly, it does slow the piston's movement a bit, however this is dry and the resistance is not as much as anyone might expect. I haven't tried any lubricant yet and that should make a large difference. It actually seems that the piston operates at the same vacuum range as designed.
Based on Jon's comments, I'll try to look for a hard (but gas resistant and slick) material that could be used in those grooves instead of an o-ring. It will have to be a split ring like a piston ring, but maybe something exists.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
It's sprayed on, cured in a 300-degree oven, and the finished part is slicker than goose poop. Vigorously-applied 4-0 steel wool only dulls the finish a little- - - -it's that hard.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Jon G - if you are determined to go down this rabbit hole, try the Teflon strips Holley used for throttle shaft bearing surface.
Jon
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify. If you truly believe "one size fits all," try walking a mile in your spouse's shoes! The Carburetor Shop
Yes, I would appreciate any possible updates to the Rochester. The thing works just fine just after a cleaning. The problem is it's only run occasionally for short times and it gets gummed up. I use Sta-Bil but I don't really know how much it's helping.
Teflon is a hard slippery material. Could make a split ring out of it. Don't know about how resistant it is to gasoline. Delrin might be another possibility.
Interesting you mention delrin, Gord. I have a square foot by 1/4" thick piece sent by a friend (not a stovebolt guy...another friend). As it turns out it is only fairly resistant to gasoline. Plus it is not easy to work with. Teflon/PTFE is a possibility. Rings of it are available, however I haven't found any in the size I need. There are also a couple of other plastic compounds and there is always nylon, although I wonder if nylon would perform any better than the o-rings. Part of me doubts it.
Jerry...does that stuff add any dimension or thickness? As I figure it, I have around .002~.0025" to play with between the piston and the bore.
Jon...I agree with you and others the Carter one-barrel is a better design from a few important points of view. Really it seems to me the Rochester is in ways fundamentally flawed and I'm surprised they stayed with the design knowing the mistakes (and it is clear to me they did). I wish I could have found more of their bulletins. It isn't so much that I'm enamored with the Rochester. As much as anything I want to see if answers can be had. I'm not familiar with the Teflon strips Holley used. I'll try to give you a ring next Monday or Tuesday.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
You are getting deep in the weeds with your inquiries. So far your investigation is beyond my abilities, but I appreciate your efforts to address issues that plague many of us in this hobby. I hope you keep working with this to find a workable solution.
Jon G - In the mid to late 1960's Holley tried the Teflon strips to reduce throttle shaft wear. Grooves were machined in the throttle shafts where the shafts contacted the throttle body. A thin Teflon strip was wrapped around the groove. The Teflon stood proud of the shaft by a few thousands, and wear was on the Teflon, not the shaft. When the carb was rebuilt, the Teflon strips were to be changed. Of course, none of the FLAPS rebuilding kits contained the strips, and most owners were too cheap to buy the Holley kits and griped about throttle shaft leakage, so the feature was discontinued. It took about 4 hands to insert the shaft into the off-side of the throttle body without losing one of the strips.
And seriously, while I do applaud your efforts with the Rochester B; it reminds me of one episode from the "Walker, Texas Ranger" TV show. In the beginning of the episode, Walker's partner Jimmy was stopped for speeding and an $800. ticket! "This is ridiculous, I'll hire a lawyer". A few minutes from the end of the episode, Jimmy is all smiles, as the court notified him the ticket had been revoked. Then right at the end of the episode, Jimmy got a bill from the lawyer................................................for $1200.
But good luck anyway.
Jon.
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify. If you truly believe "one size fits all," try walking a mile in your spouse's shoes! The Carburetor Shop
Everyone needs a hobby. Very extensive and thought provoking work. I say, if it keeps you out of trouble and makes you happy just keep on truckin.
Martin '62 Chevy C-10 Stepside Shortbed (Restomod in progress) '47 Chevy 3100 5 Window (long term project) ‘65 Chevy Biscayne (Emily) ‘39 Dodge Business Coupe (Clarence) “I fought the law and the law won" now I are a retired one! Support those brave men/women who stand the "Thin Blue Line"! Hug a cop! USAF 1965-1969 Weather Observation Tech (I got paid to look at the clouds)
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Think those 4 saints would help a Methodist, Jerry?
This morning I thought I had chanced upon a possible alternative idea. I was pouring the remaining liquid soap from the old bottle into a new one...you know you never can get all the soap out with the little pump...and I saw the tube used to suck up soap. It looks like common HDPE and seemed the right size:
This tube is .024" thick. If it had been .003" thicker, it would have been the ideal size. I know there is PTFE (teflon) tube available, but I never found any thinner than .030" when searching earlier. HDPE has no trouble with gasoline except above temps of 140 F, so it won't work for this application. But at least now I can search for other FEP/TFE/PFA/PTFE tubing to see if the right size is available. Could be a possibility...don't know at this point, but I'm beginning to doubt it. The problem with a split ring is vacuum will still be sucking gas and vapors up into the bore and that is what I'm trying to stop.
So, I'm back to thinking the o-ring plan may be the best scheme. This after all would be the easiest solution for anybody to use, simple to install with a rebuild, inexpensive and (at least to me) logical. I did put on the piston a small amount of corn head grease this morning to see what would happen and as I expected it works almost like there are no o-rings in place. Digging deeper into the o-ring specs, this particular rubber compound is suitable for static or slow dynamic applications (slow dynamic meaning sliding or rotating slowly with lubricant), so it isn't as if we're putting it to an unintended use..
A few positives about the o-ring approach:
the o-rings will effectively seal the piston from gasoline/vapors...aside from the 2mm area at the bottom
with the piston sealed in this manner the grease will stay in there and the top o-ring will prevent it being sucked into the vacuum passage
these bores / pistons have seen a lot of use...meaning they're worn now. The o-rings and lubricant compensate for that, make the piston much more effective and at the same time stop lost vacuum. The Rochester boys knew lost vacuum was going to be a concern and that is why they drilled a vacuum break passage going from the air horn bore back into the power piston bore. That passage goes into the recessed area of the piston. If they had not done that, they knew vacuum would be constantly sucking up gas and vapor, making the carb run too rich. Remember, they weren't dealing with ethanol gas so rich/lean were their concerns. With the o-ring approach, the vacuum stops at the top of the piston.
I've gone ahead and ordered more o-rings formulated in a slightly harder compound.
Jon, I'll still try to give you a ring on Mon/Tues. I have a question relating to a Carter YF (NOS) I found recently.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Outboard motors use rubber o-rings where the gas and oil lines attach to the engine. (I have had to replace them) So there are gas resistant o-rings (possibly neoprene?) out there.
If you have a friend with a lathe, you can make almost any size ring you want. I made some Delrin bushings on a friend's lathe.
Jon G - you are correct about the bores wearing. This is just one more instance where Carter was more customer-oriented. The first Carter vacuum pistons were brass. Carter found both the brass and the zinc alloy/aluminum bodies would wear; so the material for the vacuum pistons was changed to a soft alloy of aluminum. Thus the pistons were "sacrificial" but the bodies did not wear. Replace the piston, and back into original clearance specification.
Jon.
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify. If you truly believe "one size fits all," try walking a mile in your spouse's shoes! The Carburetor Shop
Jon: I have been following your interesting thread on sealing the piston on your Rochester B carb and your creativity on trying to improve this design with new ideas. Several months ago I refurbed my Rochester B for my 235 and remembering looking closely at that piston in question so this peaked my interest. About 10 years ago in my profession I had to design a pneumatic valve with a similar spool piston that required extreme sensitivity( no seal drag) yet required good sealing. My first attempt was with O-rings but needed even less drag. I opted for using X-Rings (or Quad rings) that offer much less seal drag plus other benefits. These guys will fit into and existing O-ring seal gland. (I attached a catalog screen shot of the benefits). Possibly these rings may help in your endeavor! or Google it to learn more.
The downside is that they are usually in stock available only in the common Buna N ( Nitrile) compound which is not compatible with ethanol gasoline but possibly you can find some of the correct size in neoprene and other compounds which holds up with ethanol gas. They can be molded in any material but in large lot quantities which won't help here but stock may exist from overruns.
In addition, I have also used the Teflon rings in addition to the rubber seals to keep the seal compression the same on the installed diameter of the seal. The standard o ring design seal clearance between the piston and bore should be .001"/.0025" PER SIDE ,which I think is where you are.
Good luck with your determination and thinking outside the box. Remember necessity is the mother of invention. Where would we be without it!
I had the same idea (x-rings) and tried to find these in the right size, but no joy there...at least I couldn't find them in the right center section size.
Honestly I'm pretty certain only the dreamers at GM thought these carburetors would still be in use in 2018 and that anyone of a sane mind would be trying to solve these (known) problems.
What I'm doing now is checking the 5 carburetors I have to see when (from a vacuum standpoint) the power piston lifts off the power valve. As long as the piston will lift off and stay off at a point less than 10 inches, I think it will be ok. If any of these pistons won't lift off until after 10 inches, then it is back to the drawing board. The power valve should be definitely closed while cruising and while this may not be a problem at sea level, at 6000 feet somebody might only have 10 or 11 inches at cruise speed. What I'm saying is nobody wants the piston needle to be bouncing up and down on the valve ball and making it open/close at cruise speed. If that's happening, the gas mileage will go to pot. Actually if you stop and consider this, there is a fine line anyway. Rochester from all the data I've been able to dig up designed these so that the piston would be forced down (by the spring, naturally) at roughly 9 inches. And that applies to the B, BC and BV. I'm positive they had a reason for this, but 9 inches seems on the low side to me. So, when your vacuum drops to 9 inches or less, your power system will be in play and that will happen either at high speed or under load. I think I would have leaned toward different margins. I think I'd have made that power system open around 6 or possibly 7 inches and close up around 8 or 9...and with a weaker spring we could do that. Doing that would mean the power system would only open when you really needed it and it would be disabled sooner to return you to the "economy" setting as controlled by the main metering jet. And it was interesting to me to find springs of different strengths in 3 of the 5 carburetors I have here. One of them is notably weaker and I don't know what we have going on there. I suspect somebody tinkered with it along the way, because it is the same length. But I should tell you that spring was in the newest of all the ones I had.
This whole project reminds me of watchmaking (which I began learning in 1958 and now do only occasionally). Sometimes you have to work for a while and then walk away. Meditate, pray, think, drink a beer, read a good book, cup of tea, whatever. And when your mind is uncluttered other solutions will come to you unexpectedly. When I began this project I decided on an amount of $$ I could spend (some might say squander) and I'm about to the end of that now. Interesting you've mentioned the nitrile compound isn't compatible with ethanol gas. You might want to check that...I found lots of conflicting data out there and at this point it seems the overriding thought is buna-n or nitrile is happily compatible with up to E85 gas. I've noticed no reaction to it here. Viton is fine but not available in the right size, neoprene is in this case not suitable because once exposed to ethanol and allowed to dry begins to become brittle and polypropylene is another one of those compounds you hear conflicting data about.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
You said you couldn't find Viton o-rings in the right size. You might try OringsUSA. I've bought from them for odd sizes before, and they have a wide variety of materials available. Sorry for possibly enticing you to exceed your $$ limit.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
At the level of OCD you're showing, I'd suggest using a chucking reamer in a Bridgeport mill or a VERY accurate drill press to enlarge the vacuum piston bore, then fabricating a piston from soft aluminum like Jon suggested, one that's a very tight slip fit in the new, perfectly-sized bore. A few thousandths' increase in bore diameter will also allow much more sensitivity to small levels of vacuum to keep the power valve closed. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
And I would suggest a modification to Jerry's suggestion in that you bore the cylinder out oversize for a standard sleeve maybe from brass, then make the new soft aluminum piston standard size. That way some folks that maybe find an NOS carb with no wear could buy the soft piston, and eliminate the issue before it happened.
Jon.
Good carburetion is fuelish hot air The most expensive carburetor is the wrong one you attempt to modify. If you truly believe "one size fits all," try walking a mile in your spouse's shoes! The Carburetor Shop
Both of those are really good suggestions, however I'm concerned the cost might be more than many people are willing to pay.
Let's hark back to Jerry's suggestion of the spray on ceramic coating that is baked in the oven. Jerry...do you know how much dimension that adds to the parts you use it on? Do you know if it will work on brass? If the spray on ceramic coat only adds a bit (<.001") to the diameter of the piston, it might work and I'd be willing to buy a can and see.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Midway suggests a 3-coat layer with 15 minutes' drying time between coats before curing in a 350 degree oven for an hour, which adds about .005" in my experience. It's enough that they don't recommend using it on gun parts that have a slip fit like pump action shotgun slides or lever action operating levers. It would probably be necessary to polish the piston bore for a smooth slip fit after coating the piston. I'd use a piece of slotted wooden dowel rod with a piece of 600 grit wet/dry sandpaper wrapped around it, and a cordless drill to fit the hole to the coated piston. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Thanks Jerry. Let me think about this some more. As I've said all along I'm looking for an inexpensive solution that "everyman" could handle without difficulty.
I do appreciate your time and thought on it.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
I haven't run a trotline for catfish lately, but Rochesters have always been pretty useful as anchors for one end of the line. Tie the other end to a tree and leave it overnight. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
You might not catch many fish, Jerry. You might just scare them over to another line where some other fisherman is using a more sensible weight...like a voting machine.
Right after we lost internet for the entire day yesterday (the tech rep in India told me thought their server was knocked out by thunder...or tunder as he pronounced it), my wife solved the problems anyhow. The conversation went like this:
Her...Whatcha been doing? Me...Talking to a man about carburetors. Her...For an hour? Why didn't you ask me? I can tell you all you need to know in 2 seconds. Me...Really? I didn't know you'd studied them. And such a marvelous economy of words you must have. Please illuminate me. Her...They're metal and they don't work. Just like some other parts of the truck. Me...Say goodnight, Gracie. Her...Goodnight.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Tenacity: te·nac·i·ty - the quality or fact of being very determined; determination.
Back in 2012 I found a mostly stock 49 3100 sitting in a barn, been there since 69. Got it running in 2013. The Rochester B was the cleanest part on the engine so I suspect it was a replacement before it's long winter nap. A simple disassemble, cleaning and installation of a cheap rebuild kit later it's still humming along powering that old wind broke 216 at a clip of 13 mpg. (less on 100% gas but thats another story). No fuel or vacuum leaks. If it sit's no more than a week it'll start right up with some choke, year round. Sitting longer requires the fuel pump to pump up enough fuel into the bowl to replace that which evaporated. Is it going to fail? Is it man made LOL......
There's a similar comment about Mercury outboard motors that a fishing guide on the Kenai river in Alaska shared, but I don't dare repeat it here. The PC Patrol would get a case of the vapors! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Day before yesterday I made a small T so I could test (with a known good vacuum gauge) the Rochester carburetors I have with the o-rings on and off of the power pistons. Everything can be condensed into 4 points:
1. The o-ring with grease in the center scheme works and does not alter the designed vacuum points.* In fact, I believe it is the most logical and affordable way to restore the designed vacuum points and make these work as designed. Because of this first and main point I'm not considering any other approaches to the "Power Piston Problem." Good or bad, the o-ring answer is as far as I'm going to take it. However I've made a change or two and I will post that along with some other suggestions soon.
2. The wear of time and use causes these carburetors to run more richly. Why? Couple of reasons. The constant loss of vacuum through the worn power piston bore will be between 2 inches at the best and probably more like 3 or 4 inches. For all I know it could be more in some cases...I can only judge from the ones I have here. This lost vacuum prevents the piston from working as designed. It was designed to drop down and cause the power valve to activate in Rochester's own words "at approx. 9" hg." When it was new, I'm certain it did. Today with the wear I've observed and testing with vacuum, that piston could drop down at anywhere between 11" and 13" and that puts the power valve into play at least 20% sooner than it was designed. The flip side...with the wasted vacuum, your power piston will require more vacuum to lift it back up and off the power valve. And the more the carburetor wears, the worse this will get. The o-ring approach gets us very close to the 9" Rochester envisioned...actually a bit less (which is on the good side if you're hoping for a leaner mixture).
3. Without the o-rings the pistons tend to run a little wonky. The spring and vacuum are opposing forces and the combination of these plus the worn bore seem to set up odd patterns of motion...not fluid motion but jerky motion. With the o-rings, the slack is removed, the motion is dampened and fluidity of motion is restored.
4. As I said, the o-rings dampen the sideways movement of the piston in the bore. They also reduce lost vacuum to a trickle**.
However I will tell you that today I received the other o-rings I bought and I now know there is only one acceptable size. Any larger and the piston sticks. Any smaller and you may as well have nothing there.
* assumes the piston spring was the correct one for the carburetor. Important note: of the 5 I've got one is 25% weaker and some others are weaker by varying degrees. I can't unscramble that egg, but I do know there were different strengths of springs used and obviously this is important as the strength of the spring determines the piston's timing.
** assumes there is no vacuum leak at the air horn where it joins the float bowl. Please see the work on straightening the air horn I posted a few weeks ago.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Yes, but there is more coming, mate. At least that's my plan. I have had to chase down a slight but irritating oil leak in the cam gear cover however so nothing is going on until I sort that problem. And in line with that work I'm also moving the radiator ~3/4" higher (new holes/slots) because in changing to an electric fan I've decided it is wise to add a wee bit more room between that fan and the water pump pulley. Plus there are a couple more things going on in life now, so this may seem to stall a bit.
As soon as possible I am going to be installing a wideband air/fuel gauge. That involves some exhaust pipe work, sorting out the wiring and I must also buy the sensor. I bought the gauge (a new and high quality unit) from a friend who never got to use it because the sensor got damaged. I've decided having more realistic data about how the Rochester (or any carburetor) performs on the 235 is the intelligent next step and would be good information to have here. At least I will say with that it should be easy to see and catalog what difference altering some of these variables makes. From what I've read so far, it is clear I have to study the whole A/F stuff in greater depth. I'm hoping Jerry and Jon can help with that.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Hey, this is topical! I actually just messaged carbking on a different forum about replacing my Rochester B (It's one of those rare ones with a C print but is still a B), and I search for new threads every now and then.
I have all the issues you mentioned because I seldom drive it. About to take it apart, AGAIN. But it has incredible sentimental value to me, plus it's the rare C kind, so if you found some magic bullet to let me keep this thing on... wooeee I'd be a happy guy.
Good on you for doing all this stubborn leg work! Here's hoping you find some solutions!
