school me on Rochester four Jet carbs

I've searched and found plenty of banter about Q Jet carbs but I have identified the carb on the 327 I just bought as 4 Jet and am wondering what the pros and cons of it are. Should I run the 4 Jet or put on the Edelbrock 600 AFB I've got.

 

I'm curious too. I have about 5 or 6 of them and they are more "vintage" than the Q-jets.

Here's what I got for responses when I asked the same basic question:

http://www.jalopyjournal.com/forum/showthread.php?t=161440&highlight=rochester+4+jet

 

I found a manual for 4GC carbs posted on the web. It gives me the techinal info, but I'm wondering what you folks have in the way of experience and advice on these carbs.

Here is the site for all interested: http://carbs.partschoice.com/tech/Rochester/4G-index.html

 

They were the early performance carbs and most of the early OHV GM v8's came with them. Chevy used them right up to and including the 250 hp 327 (the 300 hp got an AFB). A 265 or 283 with a 097 Duntov cam ansd a Rochester 4 jet is still a pretty neat powerplant for a light weight coupe or roadster.

Frank

 

Are they the small bolt pattern like the Carter WCFB ?

 

Only my thought, but;
When you rebuild 4-Jet with new gaskets, check float height, and do other basic stuff, it will be just fine on the street. Ofcourse, if you're racing, and second means everything, you might want to use your almost-new Edelbrock.
But I quess most guys will take shortest way, and bolt on carburetor which will be ready to bolt on right now, and they'll promise to rebuild that more worn one "someday"...

 

The thing about the 4 jet is its got real small primaries and real big secondaries. they are a pretty good milage carb and when you stand on it look out. Ive ran a couple on my street rods and i liked em. My model a sedan with the 350 had one for a while and it was the only car ive ever had that i could run 75 on the eway and kick it down and it would downshift and the secondaries would open and i was gone.. Thats all kinda fun........
Dave

I got thinking and i may be wrong.. the q-jet was what i was playing with im thinkin now the 4 jet was different..

 

Yep.

 

Lucky Strike,
The Rochester 4 jet was/is a good carb.I worked on them for years.The only thing is somewhat complicated choke systems if you try to run it as designed.
They are a one for one swap with carter wcfb carbs.
The 600 Edelbrock probably has holes that will line up on your manifold but in my experience the bores in the manifold are so small the carb butterflys can't open up all way.A 62/65 300hp 327 manifold was made for AFB's,the cast iron one.
later,glider.
ps replacement jets/power valves etc. are scarce for the 4 jet.

 

yeah, but if i remember right, you can scrounge jets from a 2bbl....

 

Tomslik, you remember just right. 2gc jets fit to 4jet.

 

if I remember correctly the big problem on the 4 jet was the throttle shafts would get loose in the throttle plate crating vacuum leaks. Seems to me Brad Urban's Carb Shop in California made a kit to fix this as well as stocking most of the replacement/tuning parts for Rochester's in general. It has been years since I did business with them. They were great help then. I am not sure if they are even around anymore.

 

One of the best for trouble free daily drivers. Both base sizes are found on mid '50s GM cars. I changed the 2bbl Carter & intake on a '50 Cad 60S to a '53 Cad 4GC, & it went from 15 to 22 MPG (at 55 !)

 

yeah, it'd been a while since i ran one.
mine ran outta breath ay about 6500....motor was good for 8000+
still ran low 14's with a stock powerglide in a 65 chevelle...

 

The 4GCs that came on small block Chevies had four throttle bores of equal size, but some Oldsmobiles, Buicks, and Cadillacs came with larger 4GCs that had a slightly spreadbore setup similar to a Carter AFB. The 340hp 409 also had a high flow 4GC.

 

Float Height is important, but most folks forget about "Float Drop". With four floats, unlike the AFB with two, the 4 Jet is a blast to set up. You will be cross eyed before you are through, but it's worth it.

 

Does this carb have an accellerator pump?

 

I think so. I recall the guy "Chuck" telling us someting about him thinking the accellerator pump was broken, but I have not fired the thing up or looked closely at the carb since we pushed it into the engine room.

This weekend I'm taking it to the car wash and giving it a nice power washing to blow 40 years worth of oil, crud, and mud off the block.

 

Yeah, that's why I brought it up, hoping someone would chime in with some wisdom.

 

Lucky/Rich,
Ya, 4 jets have acc. pumps.Maybe not wisdom but experience concerning them,the pump piston will fail to seal properly, very common, in the bore= major pain in ass.Also in the bottom of the acc.pump bore, on some, is a small check ball,if it's missing or corroded shut again MPA.
glider

 

Make sure the carb kit you buy has a neoprene pump plunger, not the leather type. Hone the barrel for the pump with a wheel cylinder hone to remove scoring. Set the linkage up for instant squirt. If your intake has the exhaust crossover running up thru the carb base, do any stud repair needed & dig the carbon out of the passages. If running a heat riser valve, free it up & lube ..

Edit .... 2022 - we have returned to leather on the pumps quite a while ago.

 

302GMC,

Thanks that's the kind of advice I'm looking for. I would like to put this sucker back in service if I can, to try it out. Anyone else out there have experience rebuilding a Four Jet, with wisdom to share?

 

Just finished a rebuild on a 4 jet. Watch the threads where the fuel line inlet attachs. Lots of them are pretty messed up from years of replaceing the filter that lives behind the fitting. There is a place that will rethread them at about $65. I chose to pick up another core and use the top off of it. I haven't been able to find the replacement filter so will run an inline between the pump and carb. Be careful to keep the primary and secondary venturi parts separated. There are two check balls you'll run in to. They are different. One goes to the bottom of the accelerator pump well and the other at the bottom of the primary veturi cluster beneath a spring that is held in place by a "T" shaped retainer. I recall one of the "balls" is magnetic and the other not. Sorry I can't find where I read that so you might wnat to check on disassembly and find out then. The kit I got included new ones so you need to find out which goes where. The rebuild otherwise is just lots of cleaning and blowing out passages. Most of the outside adjustments consist of bending parts to the specified clearances. The instructions do a pretty good job of explaining. Look carefully all around the top piece,air horn, to be sure you have ALL the screws out. I spent a long time trying to figure out why the top wouldn't come off just to eventually find a screw that I missed buried under a bunch of crud.

 

Hey, sorry to be a thread-dredger-upper.

I have a 64 Nova 283 4-jet.
I've had a bad left turn stumble once warmed up.
Took the top off and adjusted the floats. They were off according to a kit spec sheet.
1-17/32 pri float level
1-19/32 sec float level
2-1/4 float drop pri & sec
Adjusted and thought it was better until this last left turn before the house that has a bit of a hard rise to it.

Any 4 Jet gurus out there?
Should I just throw this iron blob in a river and get a real 500 AFB with an adapter?

 

A major 4-jet guru is Doug Roe, who wrote a book 'Rochester Carburetors" with full information, rebuild tech, and performance tuning tech. Roe was a GM carb development engineer.
You MUST track down the early edition, circa 1973. You do NOT want the later edition, 1980's, which has deleted most of the good stuff on pre-Q-jet carbs.
This book (the early one, remember) is also excellent on fixing and tuning 2G and the several one barrels.

 

The Rochester 4G series carburetors (a.k.a. 4 jet) are excellent carburetors. They came in sizes from about 385 to 625 CFM.

The biggest problem with the 4G is Dr. Goodpliers (you know, the evil twin of Mr. Goodwrench). Rochester made many DIFFERENT 4G series carbs. Like anything else, using the correct parts is important.

Try to identify the exact 4G series, and get a rebuilding kit specific to the carb. As an example, there are at least 3 different diameter accelerator pumps that have at least 7 different lengths. Check the existing pump in your carburetor. A 30 year old leather pump is far superior to the modern neopreme often seen in todays kits. A leather pump in the kit is a plus; but if you get a neopreme pump in your kit; take the old pump and place in a baby food jar overnight soaking in a light machine oil (3 n 1, Singer sewing machine oil, etc.).

Also, pay very close attention to the throttle body to bowl gasket. MANY WERE DESIGNED WITH VENT PASSAGES!!!!! If your carb was designed to use the vented gasket, use of the non-vented gasket will cause issues in hot weather.

And a tip you can search a lifetime for: if your carb has the secondary airvalve, the spring is the same as used in a Q-jet, and use 1/2 turn past touch for the windup adjustment.

Great carbs if treated properly.

My opinion, others will probably differ.

And Blaster - there is a factory patch for the turn bog. It does require machine work. It consists of inserting a bushing in a vacuum passage.

Jon.

 

I just bought two nailheads, one a 64 and the other a 65 and both of them have 4Jets on them. Leads me to believe that they were a pretty decent running carb cause those big cars would shit and git. Like all old carbs they all will have worn throttle shafts which can be bushed. The 65 401 Nail in my '50 Sedan Delivery came with an AFB so until it thought all the 65's would have AFB's so I was suprised to see the 4jets on both of these.

 

Thanks guys! Killer info, carbking!

I adjusted the float levels so there's an 1/8" lower (from kit specs) fuel level in the bowls and raised the float drop slightly. Seems like the left turn problem may be gone. No stumble after a quicker than normal stop. There's longer test drives to be did.

Really like the accelerator pump info. The car runs strong and smooth under normal acceleration, but I have had some bad flat-spots at times, or at least a little, when I stick my foot in it hard.

No OG leather pump. Would an old Hygrade Jiffy kit have a leather pump?

I like old carbs. This is my first Rochester of any kind. Throttle shaft bushings seem fine. This carb is welcome to hang out for a while yet.

Thanks again! There may be more to report.

 

I have a 283 with a 4 Jet carb. There's a vacuum inlet opening at the base just in front of, and below, the choke assembly. What is that for, and does it require any attention?

 

It's a 1/8 pipe fitting? That's where the vacuum advance was hooked up..What's there now?