Quadrajet lean spot.

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gnicholson
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Re: Quadrajet lean spot.

Post by gnicholson »

Explain how changing the main jet alters idle on a qjet. Pre fuel injection gm lock up transmissions use vacuum switches for the lock up to disengage at low manifold vacuum. I dont know how quick your mech adv is but 4 degrees initial is to low. Get the timing right 1st then tune the engine to run correctly hot. Use the coke and pull off to get it to run right cold. If you run in a cold climate heated air cleaner and efe valve will help
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Re: Quadrajet lean spot.

Post by chevyfreak »

gnicholson wrote: Wed Nov 13, 2019 1:11 am Explain how changing the main jet alters idle on a qjet.
Sorry my bad. Were in a rush and didnt proof read. Thanks for pointing it out.
Probably put it there since the last qjet i struggled with had a issue with main jet and idle.

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rfoll
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Re: Quadrajet lean spot.

Post by rfoll »

According to Tuner via another discussion about this engine, viewtopic.php?f=1&t=54046&hilit=screw+p ... s&start=15 , he stated that the swirl port heads require much less total timing, about 25 degrees total. The carb I had originally on this motor didn't behave this way, it didn't mind the low initial timing, but the throttle blades tended to stick. I swapped the 305 carb on because it ran so well on the 305. It clearly isn't as happy on this engine and I am trying to find out why. I am starting to wonder if something in the carb got jiggled when I pulled the 305 out of the car. To be clear, the engine runs very well. Most people would never notice, but car nuts like us are always paying attention to eccentricities of everything we drive.
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Re: Quadrajet lean spot.

Post by mag2555 »

You can't escape the fact that since that Carb is now on a 350 it's going to want to near 10% richer across the board just for starters!
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Re: Quadrajet lean spot.

Post by dannobee »

Agree with the timing on the swirl port heads. When I was working at a chevy dealership, we ran into the same problem with a Frankenstein engine we put together for the shop truck. It would rattle like a skeleton jerkin' off with a standard HEI dizzy at base timing.

Regarding the Q-jet, is the float level too low? Do you have the float level plastic stick to check with the engine running? Did the power piston pop out of the bore? That little plastic piece gets brittle with age and sometimes won't hold it in place. If you think it happens when as the secondaries start to open, make sure the two tip-in holes are not plugged near the front edge of the secondary air valves. Another thing we'd do was slip a piece of thin wire under the secondary metering rod hanger to space up the metering rods. If you do that and it runs better, obviously it's too lean. Quick and easy thing to check.

Man, this is going way back. Haven't touched one of those in decades. There were a bunch of different tricks we'd use to make those things driveable back then.
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Re: Quadrajet lean spot.

Post by rfoll »

This carb was born on a 350. I don't remember changing anything on it when I rebuilt it. That being said, the current engine is a world apart from the smog motors the carb was intended for. The carb I took off was an over the counter rebuilt unit for a truck with a 400 engine. It was stupid rich on the 400 In my truck, but that engine is a warmed up 400 with Vortec heads. There are many avenues to solve the problem, I have a wall of carbs to choose from, but I like to stay with the stock stuff.
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Papastoy1
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Re: Quadrajet lean spot.

Post by Papastoy1 »

Does this carb have large main air bleeds (.120) or the small ones (.046 or .070)? The 17057204 should have the large main air bleeds with 77 main jet and 52 needle jet stock.
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Re: Quadrajet lean spot.

Post by rfoll »

Papastoy1 wrote: Wed Nov 13, 2019 7:34 pm Does this carb have large main air bleeds (.120) or the small ones (.046 or .070)? The 17057204 should have the large main air bleeds with 77 main jet and 52 needle jet stock.
You are correct, I read the needle/jet combo off the wrong column.
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econo racer
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Re: Quadrajet lean spot.

Post by econo racer »

I would definitly start off with a stock air cleaner with the heat flapper hooked up.
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Re: Quadrajet lean spot.

Post by turbo camino »

econo racer wrote: Thu Nov 14, 2019 6:36 pm I would definitly start off with a stock air cleaner with the heat flapper hooked up.
So, tune the car to match the carb, instead of the other way around?
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Re: Quadrajet lean spot.

Post by rfoll »

I had a dual snorkel Corvette air cleaner on it but the heat tube was right over the radiator inlet hose, no chance of getting near the heat can on the passenger side. I put the stock cleaner back on and hooked up the heat tube and it helped. I adjusted the choke pull off so it would only open the blade half way on startup and the problem is nearly gone. Unfortunately, electric chokes tend to close every time the engine is turned off. Every place you go in this city is never more than 2 miles away, so the pull off adjustment has the choke half on nearly all of the time. I have tried in the past to find faster opening electric choke thermostats, but I have never been able to find any spec for opening rates and these things start out at about $35 ea.
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Re: Quadrajet lean spot.

Post by Firedome8 »

Gm puts dual rate electric chokes on some quadrajets. Find a chassis dyno shop that has q jet experience and do a couple of pulls with a wide band. The choke pull off has 2 functions, pull off and a counter balance effect with the secondary spring the stock metering hole in pull off is .010 start with .014 and 3/4 turn from 0 setting on spring. And yes the hanger height does effect tuning I have a fixture to check height and modify as necessary,, easy to build.
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