Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

High Idle at 1000rpm, cannot get it lower. SBC 350, newly rebuild q-jet.

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cosmic_3000:
Hello everyone, just a quick update. I'm currently redoing the intake gaskets with Fel Pro 1256, and also bought a 3/8" secondary shaft bushing kit from Cliff. At the same time I'm gonna install a fuel pressure regulator so I can set it at 5psi.

Hopefully this will fix my issue! Will update once everything is complete. :)

cosmic_3000:
Hello again.

So I redid the intake gaskets, installed a fuel pressure regulator set right at 5psi, and bushed the secondary shaft. I think bushing the secondary didn't do much at all, there is a little bit less slop but I can't imagine much air going past the shaft.

I did spend quite a bit of time trying to get the secondary's to seal as best as I can. I think I got them fairly good but I'm no expert.

So here's what it's doing now. I did manage to get the idle down a bit. Most times it will idle between 700 and 900, surging up and down periodically. Sometimes it will idle down to 300-500 and almost die, and then minutes later be back at 700. It's just all over the place.

Currently I'm at 10 degrees ported vacuum, 3 turns out on the mixture screws. Idle screw is near all the way out. The primary's are bone dry, so nothing coming from the nozzles. I did notice however that the bottom of my secondary air valves are completely wet with fuel.

I'm planning on removing the air valve doors and letting it idle. With no valve doors, the secondary rods should never lift off their seat.

Also, if I pull a manifold vacuum port rubber cap off, the idle increases. Does that mean I'm running rich?

I also tried taping the secondary's shut. That made the engine idle extremely rough and barely stayed running unless I gave it gas.

Interesting thing is, once I get off idle, the carb is super smooth, much better than before the rebuild. It pulls strong, never hesitates or bogs even at low rpm and wide open, and even when cold. Just idle is very frustrating right now and all over the place, seemingly random. Sometimes it's very high and surges, sometimes low, sometimes it's steady. I may try to see if I can get the secondaries to seal better, maybe swap the throttle plates. I may buy a second qjet just to troubleshoot and rule out carb problems. I've also thought about going with an Edelbrock carb, but I really really like to stick with this carb as it is original stock setup. I honestly like the design of the quadrajet.

Here's a video of it idling: https://www.youtube.com/watch?v=zz8zpe0KJKc&feature=youtu.be

Cliff Ruggles:
"I did manage to get the idle down a bit. Most times it will idle between 700 and 900, surging up and down periodically. Sometimes it will idle down to 300-500 and almost die, and then minutes later be back at 700. It's just all over the place."

Check the distributor to see if the timing is moving up/down at idle speed, common problem and it makes idle tuning nearly impossible.

You mentioned that you "replaced it".  Unless the original distributor was worn at the shaft or weight pins I would have cleaned it up and put it back in place.

Where did the replacement distributor come from?  If it's commercially "remanufactured" or one of the Chinese units currently being sold it's probably part of the problem, not the solution.......Cliff

cosmic_3000:
I've got a known good distributor coming from a friend, so I can check that out and see if that improves it.

I took the carb back off and spend about an hour adjusting the secondary throttle plates. I ended up swapping them between the two bores, and they seem to fit better now where they are. I can see light through it but the gap is so very tiny, I doubt even a hair could get through there. Less than a thousands.

So currently it idles about 750-800rpm with exactly 19 inches of vacuum, 10 degrees advanced, and 3 turns out on the idle mixture. Idle speed screw is about a quarter turn from fully close. It still idles very sporatic. One thing I noticed when the engine is hot and I shut it down, when I restart it, it wants to idle really low, like 300-500, until I tap the gas a few times.

Questions: Even though the engine is pulling 19 inches at 750 rpm, could there still be an vacuum leak? Also, since the carb is freshly rebuild, is it possible to reuse the gaskets if I were to take it apart?

My next plans are to look for an internal intake manifold leak but pulling the plugs, and by sticking a video boroscope down the intake and see if one of the intake runners is oilier than the rest. That should show if the intake is sucking in air and oil from inside the engine. Also a friend has a Demon 650 carb I may try and see if that idles smoother. Plus the distributor.

Thanks again.

Cliff Ruggles:
Your symptoms are CLASSIC for a distributor issue.

IF you can't get it to slow down below 750rpm's at idle speed, then shut it off and it's idling WAY too low, the mechanical advance is the problem more times than not.

Rev up the engine and it will not slow back down below 750rpm's or so tells you than when you shut it down the weights overcame the spring tension (or the mechanism is sticking) and the timing is NOT retarding back to the same setting.......Cliff

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