Quadrajet or Aftermarket?

[Bonnewagon]

Jack what do your spark plugs look like? If sooty then you know it is too rich. If not then look to the rods. I tend to try and run as lean as possible without pinging. That is a carryover from the old emission days when we had to pass tailpipe smog checks. I would actually re-jet a carb to run stupid lean just to pass, then change it back for the rest of the year. One thing Roe said was that to actually 'feel' a jet change on the butt dyno, you needed to go at least three jet sizes in either direction. I found that to be true. Changing one jet size is a waste of time. But metering rods are a different story altogether. The shape of the rod dictates off idle and midrange performance. The tips are all the same size [.026"] so once they are raised all the way out of the jets, they have no affect. Then jet size becomes more important. I would suggest removing the metering rods and miking them up to see how they are configured. The taper is what you want to identify. A change from a fat rod to a thin one would maybe remove that flat spot. I hope all this discussion is convincing the OP to dig into his Q-jet and get it functioning. Once you understand them you can make them do what you want. Be the envy of your friends!

 

[Clone TIE Pilot]

Another issue is the throttle blade angle at idle which is controlled by ignition base timing and by idle air bypass if the carb has it. Not all Qjets have idle air bypass but higher performance sub models tend to have it.. If the throttle blades are too far closed at idle, it causes an off idle stumble, too far open causes main nozzle drip at idle. Too high fuel pressure can also cause nozzle drip.

Is the 1,000 rpm idle in gear or park? In gear idle speed should be lower than 1,000 rpm.

 

[Longroof79]

Another issue is the throttle blade angle at idle which is controlled by ignition base timing and idle air bypass if the carb has it. Not all Qjets have idle air bypass but higher performance sub models tend to have it.. If the throttle blades are too far closed at idle, it causes an off idle stumble, too far open causes main nozzle drip at idle. Too high fuel pressure can also cause nozzle drip.

Is the 1,000 rpm idle in gear or park? In gear idle speed should be lower than 1,000 rpm.

Thank you for your advice and input. The 1000 rpm idle is in park. I haven't really checked the rpm in drive. I usually put in neutral when I'm stopped at a light.

 

[Longroof79]

Jack what do your spark plugs look like? If sooty then you know it is too rich. If not then look to the rods. I tend to try and run as lean as possible without pinging. That is a carryover from the old emission days when we had to pass tailpipe smog checks. I would actually re-jet a carb to run stupid lean just to pass, then change it back for the rest of the year. One thing Roe said was that to actually 'feel' a jet change on the butt dyno, you needed to go at least three jet sizes in either direction. I found that to be true. Changing one jet size is a waste of time. But metering rods are a different story altogether. The shape of the rod dictates off idle and midrange performance. The tips are all the same size [.026"] so once they are raised all the way out of the jets, they have no affect. Then jet size becomes more important. I would suggest removing the metering rods and miking them up to see how they are configured. The taper is what you want to identify. A change from a fat rod to a thin one would maybe remove that flat spot. I hope all this discussion is convincing the OP to dig into his Q-jet and get it functioning. Once you understand them you can make them do what you want. Be the envy of your friends!

Mark,
I haven't looked at the plugs in awhile. The last time I looked at them they were a nice tan color. I will pull a plug or two and look at them.
I think I should've recorded the numbers of the jets and rods to at least know what the baseline is before attempting to make any changes.
Thank you for all your good advice and information. A few years back I had a few Q-jets laying around and spare parts. Like an idiot I gave them away to a friend when I was still running a Holley. Thinking I'd never have a need for them. I could see now how beneficial it is to keep spare parts.

 

[olds307 and 403]

While the jetting sounds generous and is adjustable, the idle circuits may be an issue. The Canadian non CCC Qjets have ridiculously restricted idle circuits. Talking you need to get out the drill bits. It depends on what cam you have on whether it will idle properly.

 

[Bonnewagon]

they were a nice tan color

Sounds like you are burning clean Jack. Maybe you changed out those rich jets for something tamer? That is a high idle. Are you using a vacuum can for the power brakes or do you make enough vacuum as is? When I was running the Crane 278° cam in my 1970 Pontiac 400, it only made about 10" of vacuum. I ended up running full manifold vacuum for the timing advance, 1000 rpm idle, drilled out idle ports, and the vacuum can. Best I ever got was like 17" vacuum which is piss poor. But it made my Malibu wagon scream. Whenever I come across a dead stock carb I write down the numbers. That way if I need to use it again in stock form all I have to do is replace those parts.

 

[Longroof79]

Sounds like you are burning clean Jack. Maybe you changed out those rich jets for something tamer? That is a high idle. Are you using a vacuum can for the power brakes or do you make enough vacuum as is? When I was running the Crane 278° cam in my 1970 Pontiac 400, it only made about 10" of vacuum. I ended up running full manifold vacuum for the timing advance, 1000 rpm idle, drilled out idle ports, and the vacuum can. Best I ever got was like 17" vacuum which is piss poor. But it made my Malibu wagon scream. Whenever I come across a dead stock carb I write down the numbers. That way if I need to use it again in stock form all I have to do is replace those parts.

Honestly Mark,
I think I used the jets stock jets that were already in that carb. I don't need to use a vacuum canister, I get decent vacuum. like 18-20" at idle which is adequate for power brakes. I'm really curious as to what jets are in there now. Also, when replacing the primary rods, I'm guessing the jets would also have to correspond with the rods.

 

[Bonnewagon]

the jets would also have to correspond with the rods.

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.

I get decent vacuum. like 18-20" at idle which is adequate for power brakes

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!

 

[86LK]

Jack what do your spark plugs look like? If sooty then you know it is too rich. If not then look to the rods. I tend to try and run as lean as possible without pinging. That is a carryover from the old emission days when we had to pass tailpipe smog checks. I would actually re-jet a carb to run stupid lean just to pass, then change it back for the rest of the year. One thing Roe said was that to actually 'feel' a jet change on the butt dyno, you needed to go at least three jet sizes in either direction. I found that to be true. Changing one jet size is a waste of time. But metering rods are a different story altogether. The shape of the rod dictates off idle and midrange performance. The tips are all the same size [.026"] so once they are raised all the way out of the jets, they have no affect. Then jet size becomes more important. I would suggest removing the metering rods and miking them up to see how they are configured. The taper is what you want to identify. A change from a fat rod to a thin one would maybe remove that flat spot. I hope all this discussion is convincing the OP to dig into his Q-jet and get it functioning. Once you understand them you can make them do what you want. Be the envy of your friends!

does anyone have a link to a youtube animated video that shows how the Qjet operates?

 

[Bonnewagon]

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 of a Q-jet specifically very well. The ones that begin well, usually devolve into a rebuild without explaining what they are doing.