Re: 1966 c20
Hello Hantke,
My Rochester carb experience is limited to the "Model B". Your "Model 3M" shares the same "brilliance" in the design of the joint between airhorn (bowl cover/top) and bowl (bottom). Older Rochesters are known to leak at this joint.
Following is a discussion regarding this problem with the "Model B". Hope it helps you deal with your "Model 3M".
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ChevyAD-Carb-RochesterB-LeakRepair-CarburetorShop
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Subj: [oletrucks] Re: Leaking Rochester 1bbl. Any cure?
Date: 1/27/04
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(Dave)
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Re: 1950 GMC 1/2 ton.
We just rebuilt the head on the 235 six in the truck and got a replacement rebuilt carb from BFCI in Burbank. As soon as we bolted it together and idled the motor for several minutes, the main body gasket began getting moist and slightly seeping.
This is the third remanufactured carb we put on here. I checked the fuel pump pressure - 6 lbs. Shouldn't be too excessive for a brand new needle and seat. I spoke with Jim Carter's about it, and they admitted that most of the remanufactured carbs they sell will leak there too. Almost ever one of those old carbs have been rebuilt several times already, and over-tightened and warped slightly at the mating surfaces. This allows those leaks.
Any solutions, short of sealing it up with JB Weld??
Dave Destler
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
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Subj: RE: [oletrucks] Re: Leaking Rochester 1bbl. Any cure?
Date: 1/27/04
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(Larry R. Kephart)
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(Oletrucks)
This is a posting from Jon (The Carburetor Shop) on the stovebolt site from last year that may help with the leaky Rochester problem:
(Posting from "Carbking"
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on stovebolt.com forum)
Had Rochester only used 8 screws instead of 4 to hold the airhorn/bowl cover to the bowl on the "B" series carburetors, this would not be a problem. As they didn't, warpage from heating/cooling cycles causes an internal vacuum leak in the passage controlling the power valve, causing it to stay in the open position (too rich).
Double gaskets may work for awhile, but will hasten better repairs.
The procedure we use is very time-consuming, but also very simple and I will post here for do-it-yourselfers.
First obtain some scrap iron plate at least 3/8 inch thick. Drill holes in the plate to align with the airhorn/bowl screws; also machine the plate so that it can be placed flat on either the bowl or the air horn (or use two plates).
Now, screw the airhorn to the plate (snug, but do not tightened. With a feeler guage, measure the clearance between the airhorn and the plate. Take another plate and place on top of the airhorn; and using a rather sophisticated fixture (C-clamp), clamp the two plates together, squeezing the airhorn. Tighten the C-clamp to remove 0.003 inches; NO MORE. Set the assembly in an oven and heat to 400 degrees F. Allow to cool naturally. When cool, the C-Clamp will probably be loose.
The heat and tension reverse the warpage process. Repeat until warpage is gone. Repeat for the bowl.
Of course, in another 15 years, the problem will return.
Filing or milling the castings flat obviously removes metal, and the repair will be very temporary, and ruins the castings for proper repair. Best to take the time and do it right the first time.
Oh, one other thing, best to not use the oven belonging to your significant other!
Jon.
Larry Kephart
1955 1st 3100 Chevy (BillyBob)
1991 Jeep Wrangler (Renegade)
Boca Raton, Florida
e-mail:
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Date: 1/28/04
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(Brian Davis)
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Someone suggested: after a rebuild, soak the carb in a coffee can full of gas overnight before installing it.
I did just that with a leaking Rochester carb off my '49. It has been two years now and it still does not leak.
Brian
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Subj: Re: [oletrucks] Re: Leaking Rochester 1bbl. Any cure?
Date: 1/28/04
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(tim)
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(Ole Truckers)
Brian Davis wrote:
> ...after a rebuild to soak the carb in a coffee can full of gas overnight before you install it... <
It's been a while since I took a class on Materials, but I'm having trouble seeing just what that would do.
Tim Lloyd,
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1954 Chevy 3100 Pickup "Peanut"
1954 Chevy 3100 Panel "Being paid for"
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Date: 1/28/04
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Hello Ole Truckers,
On the Rochester "B" overnight gas soak matter, I asked Steve, who operates our neighborhood service station. (He repeatedly says the worst mistake he ever made was to sell his 1949 AD 3100. In my book, that puts him in about the top 5 percentile of all service station operators. He'd be in the top 2 if he had kept it.)
He suggests three things could happen:
1) The gas, full of ethyl alcohol and butane additives, not to mention molasses mistakenly pumped into the railroad tank car somewhere up the line, eats through both the carburetor and the coffee can. In the morning it's all gone. If you're like me, you figure the thief needed what's gone more than I do: you buy a new carburetor and try to get on with your life.
2) The same gas, with the same stuff in it, softens the gaskets and saturates them with a good dose of gas varnish. The soft gaskets conform to the Rochester's twists. The varnish seals things up. No more gas leak.
3) It's called luck--kind of like hitting the oletruck's frame with a two-pound hammer to fix an electrical short you can't find. When it works you think the hammer is the best electrical tool ever invented.
Anybody know if Steve is on to something?
Regards,
Culver Adams
1951 Chevy 3100
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Date: 1/28/04
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I had leaky Rochester's.
The ultimate cure was to switch to 1bbl Carters (Stovebolt).
However, I was able to dramatically reduce--and on a couple of occassions completely stop--leaks by:
1) installing a fuel pressure gauge (NAPA) and running the pressure at about 2-1/2 to 3 lbs. (made a big diffence on leakage and no problem on performance).
2) disassembling the carb body and machining absolutely flat, and then polishing, the mating surfaces--both tops and bottoms. Then I put them back together and torqued the screws to get an even pull on the gaskets. It was expensive but it worked.
Steve G.
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Date: 1/28/04
From: Cadamsarch
To: IDPHR
Hello Steve,
I like your thought--and results--about limiting fuel pressure to the carb.
Is there an easy way and are there off the shelf parts to do this?
From your experience, what is it about Carter carbs that makes them less leaky than Rochesters?
Thanks and Regards,
Culver Adams
1951 Chevy 3100
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Date: 1/28/04
From: IDPHR
To: Cadamsarch
I have a '51 3100 which initially had a '54 full pressure 235.
I went with the Offy dual intake w/ Fenton headers, dual exhaust and a Mallory dual point ignition. Everything was great except the carbs were leaking and sometimes they would load-up and actually overflow. I rebuilt them and had pros rebuild them but still I had problems.
I asked the AD world for help: the fuel pressure suggestion seemed reasonable so I went to my local NAPA store and bought an adjustable fuel pressure gauge and intalled it between the fuel pump (I have a stock fuel pump) and the carbs. It reduced my problems immediately.
The other idea was just the result of deductive reasoning. Why does a carb leak? It leaks because fuel is able to seep through the mating surfaces or other holes in the carb. I studied mine and determined that the primary leak was between the air horn and the carb body. I had it taken apart machined absolutely flat and polished then put it back together taking care to properly align and torque the screws. It stopped the leaking. I've also considered epoxying the mating surfaces instead of gaskets. They do that a lot on Quadra-jet 4bbls.
As far as Carters are concerned, I have a lot of respect for the guy who runs Stovebolt Engine Co. He's a former automotive engineer for GM. His contention is that the Rochester's are not properly sized for the 235: they're to big. His solution was Carter 1 bbls: smaller, same performance, better gas mileage and no leaks.
Well, I had just purchased a rebuilt "58 235 for my truck and decided to try the Carters' (relatively inexpensive about $65. each). A great decision: all the things he said would happen did: the truck runs great, no leaks and I get about 18mpg (I got 12 with my 2 Rochesters).
Hope you're successful in solving your problems: let me know how you do.
Steve G.
1951 3100 PU
1952 Suburban Carryall
1970 El Camino SS 396
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Regards and Hope this helps,
Culver Adams
Minneapolis, MN
1961 GMC K1500
PS: On my 1951 Chevy I replaced Rochester with Carter and have been happy since.
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