Tiger69

Hello, my first posting but have read this site for years. I have my grandfather's 1965 Oldsmobile 98 four door Holiday Sport Sedan that has 425 cu.in 10.5 high compression with a Rochester 4GC. Engine is stock and rebuilt by machine shop 25,000 miles ago. Rochester no good. Had rebuilt, still no good. My Rochester is big jetted and is a 692 cfm carb based on 1965 high octane gas. I was advised to go with Edelbrock 750 cfm model 1411. Edelbrock recommends a 600 cfm model 1406. Can't adapt Edelbrock carb to my stock manifold as can find no adapter and have tried 3 different ones. Thinking I put on Edelbrock aluminum 2151 intake and put on an Edelbrock carb. Any ideas on what cfm to go with? Thanks for any ideas and glad I finally joined ClassicOldsmobile.

47 Convertible

Welcome to CO. Others here smarter than me on carbs and you will get some good advice in that regard. There is a decided preference for Rochester Quadrajets on this forum.

I followed their advice when building a 455 for my 47 convertible and got the Q-jet and Edelbrock 2151. Q-jet was rebuilt by Cliff Ruggles who seems highly regarded here. Cliff wanted all the engine and drive train specs and built the 7028251 carb accordingly. I just recently fired the engine up for the first time and so far it seems to be working just fine.
Jerry

BangScreech4-4-2

Shouldn't a '65 be using a QuadraJet rather than a GC? If it is a Q-Jet, you can hardly do better, as long as it's built by somebody who knows what they're doing. If it actually came with a GC originally, I bet a Q-Jet would be an easy and rewarding swap.


Oh, and also: welcome!

aliensatemybuick

A 65 will use a 4GC. Best bet if you are not slavish to originality. is to swap the intake for a 66 or later one, and then you can use a Q-Jet. You'll need a 66 or 67 air cleaner base too (the factory 65 top should work). The 4GCs are quirky. I have on one my Jetstar 1, and have thought about this swap myself...but it runs OK at the moment. It could be that you 4gc was not rebuilt properly or they used the wrong gasket between the bowls and base (there are apparently a number of different styles). Other issues to check for are the two ball valves (or for the accelerator pump and one for the power valve (I think)) are installed properly and also float heights (these carbs are VERY finicky when it comes to that).

BangScreech4-4-2

My bad. AAMB is absolutely right, 4GC = square bore, while Q-Jet = spread bore, so you are indeed looking at a manifold swap unless you can find some sort of adapter.


If you want to leave the manifold, maybe there's some sort of Holley you can use, if you can't get the 4GC to run right.

joe_padavano

HUH????

The E-brock bolts directly in place of a 4GC. No adapter is required, which is why you can't find one.

Tiger69

I appreciate all the responses. And, "Joe" is legendary as I have read in the past. An honor to get your input. This frame off on the 65 Olds 98 has had many hurdles. Finding the body bushings, converting master cylinder to dual bowl, finding a flex plate, to name a few. Parts are not easy to find but I understand a 65 98 was a one year model and certainly not a car desired by anyone. The Edelbrock I bought does bolt onto my stock 425 intake manifold as the Edelbrock has 8 holes, but it has to have a spacer as the throttle linkage can't travel as it hits the manifold. It has to be raised. I have called Edelbrock and Trans Dapt and the part numbers they suggest do not work. None of the spacers match the contour of the factory manifold machined part where the carb sits and there are air gaps on the sides. I attach a photo of the manifold facing and in the picture at 9 oclock (which is actually the driver's side) there is an air gap. Same thing at 3 oclock (opposite side). But the edelbrock carb has to be raised for linkage clearance. If I use the factory intake, it has to have a spacer for the Edelbrock carb linkage clearance. I am thinking Edelbrock 2151 intake and put a Quadrajet or Edelbrock on it. I bought an Edelbrock 1411 (750 cfm) but Edelbrock says it is too big and will run lean??? I don't undersand that. I am certain the car had a 692 cfm Rochester 4GC. Edelbrock said gas is not what it used to be so you need smaller like a 600 cfm. Thanks.

joe_padavano

What's wrong with this one?

Tiger69

Joe, what is that spacer? looks like it would work.

joe_padavano

Google edelrock carb spacer. That one is from Summit.

BangScreech4-4-2

Sorry, Joe. I got in my head that he was swapping from 4GC to Q-Jet. Apologies to Tiger 69 as well, for muddying the water.

Tiger69

Update: Joe Padavano was correct. Edelbrock techs said 2151 intake manifold would accept a Carb 1411. It won't and that combo needs an adapter which will jeopardize hood clearance. Sending that back. What did work was an Edelbrock 8723 adapter which allowed me to put a new Edelbrock 1411 onto my factory 425 intake manifold. $28. Thanks Joe.

Jetstarjim

I got to this string by search, and was wondering how the set-up was working for you, Tiger69.


I've got a 4GC on the 65 425/370, and it's driving me insane. I went through it, an low and behold, it did have the wrong bowl to plate gasket. I figured that out almost by accident, as I was matching up the old gaskets with the dozen or so, that came in the kit. So, at least one vacuum leak taken care of.


Basically, I've got a lean idle problem. The adjustment on the right side (passenger) has no effect. In addition, when checking for vacuum leaks, I seem to have an issue around the mixture screws. No other vacuum leaks. The needles are good, and all the passages were cleaned and clear. I'm wondering if there is a seat that might be damaged.


To make a short story long, I was wanting to keep this car as original as possible. The 4GC seems simple enough, but I seem to be missing something. Some seem to feel that those carbs are finicky, and if that is the experience many here have, maybe I should just put that carb on the shelf, and go with the Edelbrock set-up for sanities sake.


I may have to bite the bullet and bring in a professional, lol! Any thought's appreciated.

BZZBRand

Interesting conversations here! I have a stock 65 JS1 w/4GC, rebuilt recently and seem to have a rich fuel situation going on (spark plugs black-sooty) I have the stock R43S plugs in it and might try a set one step hotter to see if this helps. Car runs great but a bit rich.

Thoughts?

BZZBRand

I made the switch to an Edelbrock just recently and posted a thread in Big Blocks the other day with some basic swap info

olds 307 and 403

I am glad to hear a few have had luck with the Edelbrock carb, there has been many threads rooted to Edelbrock carb issues. Personally, I would give the Street Demon 625 or 750 a try. It is either square or spread bore being a 3 barrel design. It is basically an improved version of the Carter Thermoquad that replaced the AFB/AVS. A phonelic spacer like Joe showed is a good idea, especially if you live in a hot climate. They had hot startstart i when Chrysler ran those carbs factory.

BZZBRand

I went into this conversion knowing there would be a performance loss, but the gain in overall drivability was more important. I still have the 4GC and will be on the hunt for some jets, etc. to try to salvage it as when it worked it was a very good performing carb.

65B01Delta

I never had a problems with the 4GC on my 65 Jetstar 1, 4 speed car...

BangScreech4-4-2

Why would you expect a performance loss?