Rochester 4GC Carb-factory defect?

[highcking]

I pulled the 4GC from my recently-acquired 1958 Buick, which runs poorly at part throttle but idles OK and runs fine at wide-open throttle. Looking carefully at the throttle bores, I found that only one of the two primary barrels has an idle jet showing. The secondary barrels both have idle jets. This certainly could explain why the car runs lean at part throttle. Anyone ever seen this? Surely there should be an idle jet on each barrel?

Bill Shields

Luray, Va

[NTX5467]

Go to the www.wildaboutcarsonline.com website and do the free registration (if you haven't already done so). Click on the "Auto History Library" icon, then "Factory Publications". Then head over to the Buick section and find the "Service Manuals" section. The earliest service manual is 1964, but they still used the 4GC Rochester that year, too. When you finally get to the section with the download segments, look for "Tune Up" and such, then on p41, you'll find the 4GC Rochester section.

On many .pdf downloads, when you move the cursor near the bottom center of the page, a new "menu" will appear, with page numbers , print, and save icons. These are better to use than the normal ones on the top of the screen . . . at least that's what I've discovered. Put the cursor next to the "1" of "__" number in the box, click on it, then make that number "41" to get directly to p41 of the section on carbs.

I suspect there might be a few detail differences, BUT your carb should be pretty similar in basic operating systems. The illustrations might be a little "low resolution", but they are there. Once you know what you're looking at, it can make more sense, though.

I didn't know those carbs had 4 idle screws and circuits in them! But you'll find how things are supposed to work and hopefully can figure out what's gong on with your carb.

Take care,

NTX5467

Edited July 27, 2013 by NTX5467 (see edit history)

[highcking]

Very helpful, thanks! The carb has four idle jets but only the two primaries are adjustable with screws. I've done more research and decided that I was slightly mislead by the vacuum port that feeds vacuum to the vacuum advance - it looks like the idle jet but isn't. The idle jet is much smaller on the primaries. I also see why later versions went to a slit to add fuel flow as the throttle moves away from idle. The 4GC has a bunch of TINY holes and I'm pretty sure they are what is wrong with this carb.

Go to the www.wildaboutcarsonline.com website and do the free registration (if you haven't already done so). Click on the "Auto History Library" icon, then "Factory Publications". Then head over to the Buick section and find the "Service Manuals" section. The earliest service manual is 1964, but they still used the 4GC Rochester that year, too. When you finally get to the section with the download segments, look for "Tune Up" and such, then on p41, you'll find the 4GC Rochester section.

On many .pdf downloads, when you move the cursor near the bottom center of the page, a new "menu" will appear, with page numbers , print, and save icons. These are better to use than the normal ones on the top of the screen . . . at least that's what I've discovered. Put the cursor next to the "1" of "__" number in the box, click on it, then make that number "41" to get directly to p41 of the section on carbs.

I suspect there might be a few detail differences, BUT your carb should be pretty similar in basic operating systems. The illustrations might be a little "low resolution", but they are there. Once you know what you're looking at, it can make more sense, though.

I didn't know those carbs had 4 idle screws and circuits in them! But you'll find how things are supposed to work and hopefully can figure out what's gong on with your carb.

Take care,

NTX5467

[1939_Buick]

Look here Rochester Model 4G - The Carburetor Doctor

A link from the The Old Car Manual Project