Quadrajet or Aftermarket?

[86LK]

YouTube is loaded with Q-jet videos. But first watch the videos that explain how a carburetor works. Because they all work the same way. Unfortunately they break the many videos down to rebuild, choke, bog, idle, fast idle, fuel starving, etc. I have not found a good video that really explains the inner workings very well. The ones that begin well, usually devolve into a rebuild without explaining what they are doing.

a well-explained video is gold. I still remember Nuclear Power school when they were talking about converting water to steam when it hits phase change.
"Chief, what do you mean by phase change? what exactly is it and at what temperature does it occur? I know water boils at 212F but how is that different from phase change?"
Chief says "great question! let's break it down!" and if you know military training they don't usually do that but he recognized some of us had a need for the breakdown instead of accepting it on faith. after all, if we're going to operate nuclear power plants, what exactly are the conditions we are monitoring for?

 

[Longroof79]

It depends on what the factory wanted to achieve. Something a simple as 200 more pounds due to an option could call for a different metering rod. I used to think there was a mathematical ratio between rods and jets. But after taking so many apart- that did not hold up. I have seen the same jets that a 400 would use, be used on a 301. But with a radically different metering rod.

That would tell me you have a very street-able cam in there. Now you have me curious about that carb too. Damn I wish we lived closer!

Mark,
Had we lived closer, like if I still lived in Brooklyn, I would've been a frequent visitor. 😉
yeah, the cam is a mild Comp Cams High Energy 268 grind with 454 lift in and ex. which keeps it very streetable. even using a stock convertor and 3.08 gears. It's not a neck snapping combination, but it still performs pretty well and fun to drive. .

 

[Bonnewagon]

268 grind with 454 lift in and ex.

Wow Jack. We think alike. The cam I used in my 1968 Firebird and then later my '77 Trans Am, was a Crane 262°/272°- 427"/454". Super street-able, plenty of vacuum, a bit of a grumble in a 350, but butter smooth in a 400. More lift and duration on the exhaust side which Pontiacs need. I chose it because stock Pontiac heads flow great up to about 450" of lift. It was advertised as a off road/ towing/bottom end cam. It sure was. All the power was below 4000 rpm, which is where you need it on the street. The Q-jet I used for both was very close to stock because that was what worked best. I believe I used a #72 jet and a #44 rod.

 

[Longroof79]

Wow Jack. We think alike. The cam I used in my 1968 Firebird and then later my '77 Trans Am, was a Crane 262°/272°- 427"/454". Super street-able, plenty of vacuum, a bit of a grumble in a 350, but butter smooth in a 400. More lift and duration on the exhaust side which Pontiacs need. I chose it because stock Pontiac heads flow great up to about 450" of lift. It was advertised as a off road/ towing/bottom end cam. It sure was. All the power was below 4000 rpm, which is where you need it on the street. The Q-jet I used for both was very close to stock because that was what worked best. I believe I used a #72 jet and a #44 rod.

Mark,
It makes you wonder how many guys over cam their engines. Bigger isn't necessarily better. My exhaust is a bit on the quiet side. No loud mufflers. I first had a set of turbo mufflers on it but didn't like the drone that they provided. I put stock type Walker mufflers on it. It has a slight rumble, but very tolerable and pleasing to the ear.
One of the Q-jets I had given away was for a Pontiac 400 or 455...I can't recall which.
It was probably the first Q-jet I took apart to find out how it's put together.

 

[Longroof79]

Mark,
I think I may have found my notes on my present carb. They were inside the Doug Roe Rochester book. Looks like the main jets are #67, primary metering rods are 48K, and the secondary metering rods are #DP with an M- hanger... Does that seem to make sense?

 

[Bonnewagon]

guys over cam their engines

Absolutely. I see that all the time. They grab some speed shop sale item and think that a racing cam is going to make their car unbeatable. Instead, they can't get it to idle, it wastes gas, is loud, and performs worse that the stock cam. What I really don't understand is the resistance to admitting the mistake, backing up, and re-grouping. They whine "I spent money on that", and they throw more money at it in vain. Same with carburetors. If the factory used a two barrel, is was because that is all that engine needed. It probably had a low rpm cam, small valve heads, dead highway rear gears and a four barrel would be a waste. Yet, they throw a huge Holley on and the problems follow. Don't get me started on headers. Headers are for racing. Period. They are useless on the street, cause more headaches than anything, and turn simple maintenance into a chore. I may get flamed for that statement, but it is true. How often are you at WOT, where headers are needed? Probably not much. If you think you need them, great. But when I am asked to solve engine problems, usually the first thing I do is get rid of the headers. Example: my buddy has a 1980 TA, the 301 was replaced with a 1978 400. I found him a dead stock 1978 400 Q-jet and it just purrs. The 301 headers stayed on the car, and have roasted three aftermarket HP starters so far. Because the 301 has a one inch shorter deck height than a 400, so the wrong headers are rubbing against the starter. No one figured this out until now and the car is stranded 50 miles away from me at his house. I have a set of stock cast iron log manifolds and a complete Dynomax dual exhaust here ready to go on. But the only starter that will fit is a small 301 unit. We would have to drag a jack, stands, starter, and tools out there to perform the swap, and it is winter now. If he flatbeds it here the first thing I am going to do is sawzall them headers right off.

 

[Bonnewagon]

main jets are #67, primary metering rods are 48K

Now that makes a LOT more sense. If the engine burns fine I would leave the jets alone. I think I ran #69 jets in my Pontiac 350 for long time. But I would try a different rod. Something like a #44 or #45. This is where Roe's book is invaluable. Some rods are tapered straight, some are curved, and that taper is what affects take-off. A rod that tapers fast is what you want, to give more fuel off idle and mid-range. Sometimes it takes a magnifying glass to see the taper, or a micrometer, but that is where the fine tuning comes in.

 

[Bonnewagon]

Jack refer to the https://www.newagemetal.com/ site. Look what Chevy used in their 1972 350's. Low compression, but still healthy four barrel V-8's. #74 jets, #45 rods. Your rods are pretty fat for #67 jets.

 

[olds307 and 403]

With that vacuum, it must be a very mild cam. It should be tunable enough to get a decent idle as is. My current modified Qjet is running a bit shitty and lean on my rebuilt Olds 358. I am surprised much adjustment is needed with 204/214 cam and 9.5 to 1. I have it so a 1/4 to 1/2 turn made a noticeable difference, only out a couple of turns before I adjusted them. It was stalling and barely idled once the choke came off. I'm not 100% sure the O2 sensor on my AEM wideband is reading correctly, have a new sensor. On a stock carb like yours, expect 5 to 6 turns out on the idle mixture screws.

 

[Longroof79]

With that vacuum, it must be a very mild cam. It should be tunable enough to get a decent idle as is. My current modified Qjet is running a bit shitty and lean on my rebuilt Olds 358. I am surprised much adjustment is needed with 204/214 cam and 9.5 to 1. I have it so a 1/4 to 1/2 turn made a noticeable difference, only out a couple of turns before I adjusted them. It was stalling and barely idled once the choke came off. I'm not 100% sure the O2 sensor on my AEM wideband is reading correctly, have a new sensor. On a stock carb like yours, expect 5 to 6 turns out on the idle mixture screws.

My mixture screws are set at about 3-turns out and I haven't even tried to readjust them. It seems to be happy at that adjustment.