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Old 02-17-2017, 03:00 PM   #1
40cpe
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Default Rochester small base on a 276"; Jet size?

I put a Delco rebuilt Rochester 2G and a bored out Mercury manifold on my 276"/L100 engine. It ran a little ragged in lower RPMs but seemed OK at highway speeds. I checked the plugs and they are paper white. Decided to try a little choke and it smoothed out great, I could tell the difference at highway speeds, too. I pulled the top and it has 47 jets in it. My 94 had 51 jets and the plugs looked good, a tan color. What do you that run the Rochester use for jet size?
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Old 02-17-2017, 03:35 PM   #2
flatheadmurre
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Default Re: Rochester small base on a 276"; Jet size?

Hereīs a little reading on what was in which setup.
http://www.carburetor-parts.com/Roch...ns_ep_525.html
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Old 02-17-2017, 10:12 PM   #3
Bob/Ohio
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Default Re: Rochester small base on a 276"; Jet size?

I have rebuilt many early 2G'S, and don't ever recall one having that jets that small, most are 51 or 52's, which would be stock for a 265, or a 283 Chevy.

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Old 02-17-2017, 10:55 PM   #4
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Default Re: Rochester small base on a 276"; Jet size?

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Originally Posted by Bob/Ohio View Post
I have rebuilt many early 2G'S, and don't ever recall one having that jets that small, most are 51 or 52's, which would be stock for a 265, or a 283 Chevy.

Bob
According to the link flatheadmurre sent, they came with 51-58 in 283" engines from '55-'67. I had an old core here that had 56 jets in it, so I put them in my carburetor. It seems to run pretty good. I've found that a test drive doesn't always reveal all the buggers. I called the guy I bought it from, he is sending me some jets around 52. I'll just have to see what works best.

How many miles does it take spark plugs to reveal the lean/rich condition? I'll need to check them to see how the mixture is.
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Old 02-18-2017, 01:49 AM   #5
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Default Re: Rochester small base on a 276"; Jet size?

Reading sparkplugs with modern fuel isnīt to easy...if itīs etanol and other additives in it they lie to you.
I would get an A/F meter.
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Old 02-18-2017, 07:28 AM   #6
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Default Re: Rochester small base on a 276"; Jet size?

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I put an A/F meter on my engine test stand. It's kind of neat and helpful for getting your engine close. It's a 12 volt unit which is fine for my test stand, but I don't think they are available for 6 volt operation. Anybody know?
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Old 02-18-2017, 09:40 AM   #7
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Default Re: Rochester small base on a 276"; Jet size?

Do you put a bung in both sides of the dual exhaust header pipe, or does the meter only use one side?
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Old 02-18-2017, 12:21 PM   #8
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Default Re: Rochester small base on a 276"; Jet size?

Since there is no reason to have one of these mounted permanently (as they are for tuning purposes only), it might be best to have a temporary setup. On my stand, the sensor is mounted on a "sniffer" extension that I can move between the 2 ft pipe extensions I made. These are available for just this purpose.

I may have just answered my own question above; I think I'll build a box mounting the gauge and 2 6V lantern batteries with the "sniffer" extension to make a complete independent setup that can be moved from car to car.

Last edited by tubman; 02-18-2017 at 12:28 PM.
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Old 02-18-2017, 01:23 PM   #9
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Default Re: Rochester small base on a 276"; Jet size?

Running an AF meter on a test stand, doesn't help the cruise ratio. The cruise ratio is the most important one as the engine spends 90% of it's time there, both at low speeds and hi. The power valve is the next thing the AF meter will help. What some people don't realize is: the ignition timing is very important when adjusting the ratio. For example, in cruise I like to get at least 15.5 for best economy. Thisn requires more advance. At present I'm running 53 jets, but it's too rich, didn't have any smaller so I have to call Datona carb and get some. +
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Old 02-18-2017, 02:47 PM   #10
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Default Re: Rochester small base on a 276"; Jet size?

I have the early 2G from a 283 on a Merc manifold on a 239 that is +.060, with an air/fuel monitor installed. I am at high altitude but have driven it extensively down low. #53's were the best compromise.

One thing not mentioned on the 2G, the power valves frequently have plastic shim washers under the head of the plunger (bottom of spring). I found the PV was coming in way too early, took out all but one shim. Much better! It doesn't come in until about 1/2 throttle on the interstate, for big hills, yet at lower speeds when accelerating it is right there. There are also two sizes of PV jets, see if yours has a number on it.
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Old 02-18-2017, 03:37 PM   #11
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Default Re: Rochester small base on a 276"; Jet size?

Ross thank you very much, I didn't know that. I was told to remove a few coils from the return spring. I'll check that out.
Thanks again. My engine is a stock 280 ci engine.
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Old 02-18-2017, 10:39 PM   #12
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Default Re: Rochester small base on a 276"; Jet size?

One other thing to consider on Rochesters. There were two float needle valves, small and large. Physically the same size, but the hole fuel comes thru is different. Rebuilders commonly put the Large size in, I suppose just so they don't need to guess. Large valves were used in the later models made for bigger engines. But Large bore needles having the same float as Small ones tend to be less tolerant of fuel pressure. (more area times same pressure = more force trying to open the valve) Several vendors sell the Small orifice valves, for our engines they are the better choice.
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Old 02-19-2017, 11:43 AM   #13
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Default Re: Rochester small base on a 276"; Jet size?

Quote:
Originally Posted by flatheadmurre View Post
Hereīs a little reading on what was in which setup.
http://www.carburetor-parts.com/Roch...ns_ep_525.html


Beware of this chart on jet sizes. There are some typo's. For example the '65 GTO tripower carb (7024178) is listed a couple times. One says #68 main jet, and one says #88 main jet. The correct size is #68. May be other typo's.

Also, the Rochester carb manual is on the Old Car Manual Project if you want it from the horses mouth.

Sal
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Old 02-27-2017, 03:31 AM   #14
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Default Re: Rochester small base on a 276"; Jet size?

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Originally Posted by 40cpe View Post
I put a Delco rebuilt Rochester 2G and a bored out Mercury manifold on my 276"/L100 engine. It ran a little ragged in lower RPMs but seemed OK at highway speeds. I checked the plugs and they are paper white. Decided to try a little choke and it smoothed out great, I could tell the difference at highway speeds, too. I pulled the top and it has 47 jets in it. My 94 had 51 jets and the plugs looked good, a tan color. What do you that run the Rochester use for jet size?
I'm going through this right now also. Apparently the "47" jets are found in a Buick / Olds 215 engine. If you look at Mikes Carburetor Parts link that flatheadmurre posted you will find the "47" jets are for a 215 Buick & the 215 Olds albeit there are listed under 1963 Chevrolet Truck but noted as F 85 215 engine.
I was excited that my 2GC from a 215 Buick carb that I had laying around not only bolted on but the throotle plates actually opened up without boring the Merc 4 bolt manifold out. Then I started looking a little closer at it and realized the 215 2GC had a tiny 1.0245''(approx. 1 3/128'') venturi and the throttle bores were 1.3025(approx. 1 121/400'') instead of the regular 283/327/307 ci. small base 2bbl carbs sizes of 1.09375(1 3/32'') venturi & 1.4375 (1 7/16'')throttle bores.
Hope this helps and it would explain the lean mixture that are showing up on your plugs. I included an edited chart that I found on the internet but as you see it doesn't mention anything about the 215 size carb measurements.
Attached Images
File Type: jpg Rochester 2bbl CFM Ratings.jpg (97.6 KB, 15 views)

Last edited by 22rdstrguy; 02-27-2017 at 03:53 AM.
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Old 02-27-2017, 08:52 AM   #15
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Default Re: Rochester small base on a 276"; Jet size?

Thanks for that information. I forget the throttle bore size on mine and it is bolted on now. We are at about 400' elevation and I'm running 56 jets in it now and it runs good. I took it on a 20 mile run and the plugs had a very light gray color, but I'm running ethanol fuel with Marvel Mystery oil. I'm going to run it normally for a while without the MMO and recheck.
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Old 02-27-2017, 11:42 AM   #16
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Default Re: Rochester small base on a 276"; Jet size?

Did you have to bore or radius the manifold inlet so that the throttle plates would open or did the carb fit without any modification?
If you didn't have to do any mods then probably a 215 carb especially since it came with #47 jets.
But due to the size of the 215 carb size you are only going to only gain a little more airflow than 94 carb.
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Old 02-27-2017, 12:03 PM   #17
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Default Re: Rochester small base on a 276"; Jet size?

I had the manifold bored. The carb bores measured just under 1 7/16", something like 1.420. The carb is a Delco-Remy rebuild and the seller said it was 278 CFM.
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