65 Rochester

[bmac]

I picked up a 65 rochester carb 705122 that looks like the 705123 and 705124 in the reference pics. Couple questions: Can I put in larger jets and run this on my 340 hp 409 (once, if ever) it's in? If so, does anyone know what the jet sizes were in the 409 Rochester or a good site that references Rochester jets on the web? And, still a larger question, is there anywhere you can still purchase Rochester jets?

I'm just keeping my options open on this carb and it's fun to figure out while I'm waiting for my block back.... finally, should be next week.

thanks, bmac :cheers

[bobs409]

Can't help on your question Bob but I'd like to get the next size up jets for my 72 Malibu's 2 bbl Rochester too. I'm sure they should be available somewhere. One of our knowledgeable members are sure to enlighten us. ;)

[64ss409]

Bob

Are you sure about that 6 digit number? They are usually 7. I would think the jets that are in it are going to be pretty close. I would try it first and then re-jet if necessary.

Glad to hear your block is about done.

Ron

[MK IISS]

7025122 would be a '65 327/250 carb

[bmac]

Man you'd think I'd catch that on the preview, especially three times. Of coure Richard is correct on the number.

[MK IISS]

bmac:

Isn't the throttle bore size smaller on the 327/250 carb then the 409/340 carb.? I always thought it was but maybe not.

[fatride]

http://www.hotrodcarbs.com/parts_list.html
;)

[65Impala409]

BMAC,

I had my 65 409 rochester rebuilt by chicago corvette company. I would think that they would have all the pieces you would need for your carb... Good luck..
Ned
65Impala409

[Rockfish39]

Bob,
Regardless of what car it went into , Rochester 4GCs come in 3 basic configurations... Small, Medium and LARGE bores.

What you have in your hand is a medium-bore 4GC. IT wont work properly for your application. No matter how you jet it, you will find that there is insufficient pump shop to feed that big brute, so as soon as you take it 'off-idle' it will stumble. Guarenteed... BTW, what casting # intake are you planning on using????

Some books sugguest that you can open the accelerator pump cylinder to accomodate a Q-Jet (larger) pump cup, but Ill tell you this right now. There is insufficient material in your carb to do this correctly... You can only take out a little material out of its bore. The bigger accelerator pump swap trick will ONLY work in a large-bore 4GC. This technique is where you put the larger Q-jet pump into a large-bore 4G body

OK...lets talk about jets for a moment...

From the factory, a medium bore 4GC (327) uses a size 60 Primary Jet (tapered) with a size 66 secondary

a 409 uses the same size 60 Primary, but uses a size 80 secondary.

Simply swapping up to a size 80 secondary jet into the medium-bore 4G isnt the answer either.

The medium bore 4G is a true square flange/square bore carb. It has a 1.462" bore on all four throttle plates and they are (all 4) on the same centerline in 2 axis.

NOT TRUE of the large bore 4GC. That carb is a spread bore. It may have a square flange, but trust me, IT'S A SPREAD BORE. The secondary throttle plates are much larger at (1.750) and ARE NOT on the same centerline (in any axis) as the primary bore are.... The volumetric air flow capacity on the secondaries is significantly more than the primaries too (about 33% more) that why you need a size 80 secondary jet to feed that much more fuel.

What to do with a Medium Bore 4GC to make it usable ???

1) you will still stumble due to insufficient pump shot, but if you can live with that annoyance, then ...

2) modify the primary and secondary venturi clusters and open up the fuel discharge ports a little ... TIP: (you need a pin vice and really small drill bits to do this)

3) use a size 74 Q-Jet secondary jet.

SLICK TRICK: Chevrolet large bore 4GC...Hmmmm...:scratch: difficult to find, also expensive....

Buy a 4GC from a 57-67 Cadillac.... YES, Cadillac !!
The CAD 4GC is exactly that, a large bore 4GC. The only difference between the CAD carb and the Chevy carb is the air horn is cosmetically different. IE The fuel inlet points downward on the Chevy carb, and doesnt on the Cad carb.
Under the lid, they are identical...

:cool:

[MK IISS]

Rock:

Great post & explaination. Didn't Oldsmobile and Buick also use the large 4GC on some applications?

[bmac]

Thanks a ton Lou,

I really appreciate the explanation!. I'll shelve this one and keep my eye open for a cadallac one. I have some other carb options so I'l just hold onto this one for some other application.

again thanks, bmac :bow

[SSpev]

There are more than 3 Throttle bore sizes/combos. I have 5. Top - Caddy, 2nd row 283 marine, odd (Pontiac?), 300 buick. Bottem, Olds 364, 65 327, 283.

Throttle bores respectively top 1.555/1.685, middle 1.555/1.685, 1.430/1.430, 1.300/1.430. bottem 1.430/1.685, 1.430/1.430, 1.300/1.300.

All my carbs, the blades are on center front to back making them square. There are 2 different squares. Primary to secondary is 1.8125 side to side 1.960 or 1.700

I only know of 2 4gc's Chevy used with fuel inlets pointing down 409 and marine (my 283) All other Chevy 4gcs point out stright.

Don't know about putting a Q-jet pump in one, but you can use a 2-jet pump in a q-jet. About 1/16 bigger (memory?)

Second pic is a 409 4gc

[Brian64SS]

I am in the middle of the same search for an affordable 4GC for my '64 340 h.p. I don't claim to be any sort of expert but this is what I believe to be true:

All 4GC's look basically like the correct 340 hp 4GC but they are not all the "low silhouette" type that came on the '64 340 hp. Many 4GC's are about 1/2" taller than the low silhouette type.

I bought a '62 Cadillac 4GC 7019030 off e-bay last winter and found that it is the low silhouette type but doesn't have the correct passage in the base for the heat riser exhaust gasses. It looks almost perfect except I would need a plate to cover the holes on the intake to use this carb. Also the primaries are smaller than the primary bores in my correct '64 340 hp intake manifold.

So I made a template from my intake of the bores and and heat riser passage and took it to an old junkyard. There I found a 7023143 from a '63 Buick which was a match for the heat riser exhaust passage and bore sizes. But the Buick choke thermostat mounts on an extended piece of the cast iron carb base. The Buick intake is really wide so there's room for it but on the 409, the choke thermostat will hit the pass. side valve cover.

I looked at the '60's Cadillacs but didn't find one with the right heat riser passage. Not saying they don't exist, but I didn't find one.

All of the low silhouette type I have seen have the fuel inlet angled down including the Caddy's. The Cadillac carb I have uses a 5/16" flare fitting and the Buick has a 5/16" barbed fitting for a hose. Neither has a 3/8" fuel line connection - I don't know if the correct 409 carb does. The taller, high silhouette carbs I've seen (I have one of those too) have the fuel inlet going in horizonatally, not angled.

I don't know if the low silhouette type is needed for the air cleaner to clear the hood or not but the big-engine GM's seem to have gone to the low silhouette by '62. The "high silhouette" 4GC I have doesn't have a tag but has much smaller bores than 409 or Caddy carbs. It came on the '58 348 intake that came with my QC engine. By the gasket material left on the heads, intake and carb, that small bore 4GC and small port 348 intake were being used to run my 340 hp 409 before it was taken apart and sold to me.

[SteveD409]

I have a 4GC that I picked up a few years ago thinking it was a 409 carb. It has the slanted gas inlet, approx. 1 9/16 inch (maybe slightly smaller) primaries with 1 3/4 secondaries and the bottom is just like SSpev's picture. But I just found the number 7026667 cast into the main housing, just visable between it and the throttle plate. Anyone know what this one is? It sure looks like it could pass for a 340 horse carb!

SSpev-your pic link says 6 pics of the first and 5 pics of the second link, but I only get one of each.

SteveD

[SSpev]

SteveD, I think you are looking at how many times the pics have been "viewed"

[SteveD409]

DUH :bang

SteveD

[roaminchariot]

Well, the 4GC I got from Walden Automotive, which was supposed to be from a 340hp 409 looks exactly like the one in your picture. The bores are 1.562 primary, 1.687 secondary which is supposed to flow 690CFM, according to the carb manual I have. The fuel inlet is angled somewhat downward from horizontal. I've had it rebuilt twice and its still not perfect but I like the factory auto choke and I got it for free, so I'm trying to make it work :)


All my carbs, the blades are on center front to back making them square. There are 2 different squares. Primary to secondary is 1.8125 side to side 1.960 or 1.700

I only know of 2 4gc's Chevy used with fuel inlets pointing down 409 and marine (my 283) All other Chevy 4gcs point out stright.

Don't know about putting a Q-jet pump in one, but you can use a 2-jet pump in a q-jet. About 1/16 bigger (memory?)

Second pic is a 409 4gc

[SSpev]

Sounds like you got it right. 340 '09 carb. Heres a pic of the top. There is a couple in the 348-409 poto referance page too. I have a rochester book that only list 3 sizes with cfm. Obviously there are more than 3. Primary/secondary 1-7/16--1-7/16 486 cfm 1-7/16--1-11/16 553 cfm 1-9/16--1-11/16 692 cfm. Keep pluging away at it.

Just to point out. All kinds of people are always referencing cfm, usually with four barrels. 2 and 1 barrels are rated differently. 4's at 1.5 in.Hg pressure drop, 2's and 1's at 3 in.Hg. Like comparing oranges to grapefruit. Close but not the same.

[stew]

Hello,,, I,m trying to find a suitable carb for a low horsepower "409" also. I have one question about the smaller 4GCs they used on trucks. They had the small carb with the 1 7/16" bores. Did the trucks use a different accelerator pump to allow using this carb on a truck "409" , without getting a stumble off idle?? Thanx Stew